Glossary 261 terms

A standing wave 정상파 #

Physics

Pluck a guitar string and it does not sound just any note — only the notes that string will give. Shorten it and the note changes.

A string fixed at both ends cannot carry just any wave. Only wavelengths that fit its length survive; the rest fall out of step with their own reflection and cancel.

This is where whole numbers come from — 1, 2, 3 appearing suddenly in a continuous world.

The seats of an electron in an atom are fixed for the same reason. A matter wave must meet itself after one lap, so only orbits holding a whole number of wavelengths remain.

See alsoWaveFrequencyMatter waves

Absolute temperature and absolute zero 절대온도와 절대영도 #

Physics

Keep cooling something. How far down does it go? Minus a hundred, minus two hundred — and then?

Temperature is a measure of how vigorously particles move. Cooling is taking that motion away, and eventually there is nothing left to take. That floor is minus 273.15 degrees Celsius: absolute zero.\n\nCelsius counts from where water freezes and boils — a human choice. Absolute temperature counts from nature's floor, which is why science uses it; it is called the kelvin and has no minus sign. It is the same move as taking the basis of a unit from an object and giving it to nature.\n\nOne caution. Everything does not stop at absolute zero. Matter is simply in its lowest possible state (the ground state), and even there some trembling remains (zero-point energy). It is the most settled state, not a still one.

See alsoHeatThe ground stateZero-point energy

Ad hoc hypothesis 임시방편 가설 #

Method & instruments

Phlogiston theory was caught by metals gaining weight when they burn: nothing that loses something gets heavier. The reply was that phlogiston has negative weight.

Adding one is not itself a fault. Theories have been saved this way and proved right: when Uranus's orbit disagreed with Newtonian mechanics, astronomers assumed an unseen planet — and Neptune was there.

The fork is whether the added assumption makes a new prediction.

The Neptune hypothesis said look there, and it was there. Negative weight said nothing new; it existed only to cover what was already known. When such assumptions accumulate and offer nothing to test, the theory is dying.

See alsoFalsificationUseful wrongsHypothesis

Air pressure 기압 #

Earth Science

Carry a bag of crisps up a mountain and it swells. The air inside has not grown; the air outside has stopped pressing so hard.

Air feels like nothing, but it has weight. A column of it stands above us all the way to the top of the sky, and that column presses down — roughly the weight of a grown adult on an area the size of a palm. We are not crushed because the body pushes back just as hard from inside.\n\nAlmost the whole of weather starts here. In some places the air piles up heavier, in others lighter, and air flows from where there is more to where there is less. That flow is wind.\n\nThose closed curves on a weather map are lines joining places of equal pressure. The closer together they run, the stronger the wind — the same way water runs faster down a steeper slope.

See alsoDensityConvectionThe Coriolis effect

Algebra 대수학 #

Mathematics

Your brother is five years older, and your ages add to thirty-one. How old are you?

You could guess and check. Or you could let your age be a box: box + (box + 5) = 31. Tidy that by the rules and the box is 13.

Algebra is putting the unknown into the calculation while it is still unknown — carrying the answer around before you have it.

The word comes from al-jabr in the title of al-Khwarizmi's book, and his own name became algorithm. Rare, for one man's name and one book's title both to survive as everyday terms.

See alsoZeroAlgorithmFunction

Algorithm 알고리즘 #

Mathematics

Think of the instructions on a packet of noodles: boil 550 ml of water, add the noodles and seasoning, cook four and a half minutes. Know nothing about cooking and you still get noodles.

That is an algorithm: a procedure that yields the answer whether or not you understand it.

Long division is one. Follow the steps without knowing why they work and the answer still comes out right — which is why the work can be handed to a machine rather than a person. What a computer does is follow algorithms very quickly.

Which raises a question: does every problem have an algorithm? The answer was no, and the idea of the computer came out of establishing that.

See alsoThe Turing machineThe halting problemAlgebra

Amino acid 아미노산 #

Biology & Medicine

A Lego set has perhaps twenty kinds of brick. From those twenty come castles and ships alike.

Our bodies use about twenty amino acids. What they become depends entirely on the order in which they are strung together — that order makes one protein rather than another.

Once the order is set, the chain folds by itself, and the folded shape decides the job.

The order is written down in a gene. That is why a gene is called an instruction: it really is a written sequence.

See alsoProteinGenePolymer

Anatomy 해부학 #

Biology & Medicine

For fifteen hundred years physicians learned from Galen's anatomical drawings. But Galen had dissected monkeys and pigs, not people — human dissection was forbidden.

Vesalius opened bodies himself, and found more than two hundred places where the books were wrong.

What mattered was not the errors but the decision to trust the body in front of him over the text. Until then an assistant did the cutting while the professor read aloud from a book on a dais; Vesalius took the knife. When authority and observation disagree, which do you follow? Science parts from what preceded it at that choice.

See alsoCirculation of the bloodHumoral theoryThe cell

Antibiotics 항생물질 #

Biology & Medicine

Fleming came back from holiday to find mould on a culture dish — and a clear ring around it where no bacteria had grown.

The mould was giving off something that killed bacteria. Extracted, it became penicillin.

Before this, an infected wound left little to be done. Now dying of a common infection is rare. But a problem has followed: bacteria carrying a mutation that lets them survive the drug are the ones that breed, so resistant strains keep increasing. Natural selection happening in front of us.

See alsoGerm theoryMicroorganismsMutation

Antibody 항체 #

Biology & Medicine

Some illnesses do not come back. The body has remembered something.

When something unfamiliar arrives, the body builds a protein shaped to fit its surface. That is an antibody: it grips and marks the intruder so immune cells can deal with it.

The fit is the point. An antibody against a cold does nothing against measles.

A vaccine makes the body do this in advance. And antibodies are now made as medicines in their own right, using exactly that habit of sticking only where intended.

See alsoThe immune systemVaccines and inoculationProtein

Antimatter 반물질 #

Physics

In 1928 Dirac wrote an equation for the electron and got two answers. One was the electron; the other was identical but with the opposite charge.

Such an answer is usually discarded. Dirac insisted it must be real, and four years later it was found.

Stranger still is what followed. Feynman and Stückelberg showed that an electron going backwards in time is mathematically identical to a positron going forwards; the equations do not tell them apart. In Feynman's diagrams antiparticles are drawn with the arrow reversed.

This is a working method of calculation, not a metaphor. But it is a statement about single particles, not about a person running backwards.

See alsoThe arrow of timeQuantumEnergy

Atom 원자 #

Chemistry

Water splits into oxygen and hydrogen — but always in the same proportion. By weight, always one of hydrogen to eight of oxygen. Never a little more or less.

If water were a continuous porridge, any ratio would do. Splitting only in fixed proportions means it is joined in units. That was Dalton's argument for atoms.

An atom is unimaginably small — a drop of water holds more of them than there are grains of sand on Earth. And inside, an atom is nearly empty: a nucleus at the centre and space around it.

See alsoElementMoleculeThe atomic nucleus

Avogadro's number 아보가드로 수 #

Chemistry

Eggs are counted by the dozen. Atoms are so small that their dozen is this big.

Twelve grams of carbon hold exactly one bundle of carbon atoms; eighteen grams of water, a sip, hold one bundle of water molecules.

One breath shows how large this is: it holds about a 1 followed by twenty-two zeros of air molecules, enough that each breath now contains hundreds of millions of molecules breathed out by someone centuries ago.

See alsoAtomMolecule

Axiom 공리 #

Mathematics

Ask why of any claim and you get a reason. Ask why of the reason and you get another. Where does this end?

If it never ends, nothing can be proved. So at some point you must say: this we simply accept. Those starting points are axioms.

Euclid began with five — one straight line through two points, things equal to the same thing are equal to each other — and drew 465 propositions out of them.

What matters is that an axiom is not true because it is obvious. It is a chosen starting point. Change the fifth one (about parallels) and you get a different geometry, equally free of contradiction. Which one fits our universe is not for mathematics to say but for physics.

See alsoProofRiemannian geometryThe incompleteness theorems

Bacteria 세균 #

Biology & Medicine

Soup left out in the evening is sour by morning. Overnight, one became billions.

A house with one room. Eating and the blueprint share that room. Under good conditions it splits in two every twenty minutes.

Three shapes: balls, rods, spirals. Kimchi's lactic bacteria, E. coli and the tuberculosis germ are all bacteria. Antibiotics break the bacterial wall, so they work only on bacteria. Viruses have no wall.

See alsoVirusFungusMicroorganismsAntibiotics

Basic and applied research 기초연구와 응용연구 #

Method & instruments

There is making a drug, and there is finding out what happens inside a cell. Both are science, and they are not the same kind of work.

Applied research begins with a goal — a drug for this illness, a battery that lasts longer. Success and failure are clear and there is a deadline.

Basic research begins with a question: why does it behave this way? Where the answer will be used is usually unknown.

That is not ignoring use. It is declining to fix the use in advance, and the distinction matters.

The two feed each other. Primes were long the purest and least useful mathematics, and are now the pillar of internet cryptography. Magnetic resonance was an enquiry into the properties of nuclei and became the hospital MRI.

Yet most basic research bears no such fruit. Many seeds are sown because nobody can tell which will take — which is also why the field is perpetually argued over.

See alsoSerendipityHypothesisPeer review

Bell's inequality 벨 부등식 #

Physics

A group of people, each with three properties: tall or not, wearing glasses or not, left-handed or not. Suppose all three answers are fixed for each person.

Then this must hold.

(tall and not bespectacled) + (bespectacled and not left-handed) ≥ (tall and not left-handed)

Anyone counted on the right either wears glasses or does not; if not they join the first group, if so the second. The right cannot exceed the left. This is arithmetic, not physics.

Run the same count on entangled particles at three detector angles and the inequality breaks — meaning the answers were not fixed in advance. What Bell did was turn what looked like philosophy into a question an experiment could settle.

See alsoEntanglementHidden variablesLocality

Biological classification 생물 분류 #

Biology & Medicine

A whale lives in the sea and looks like a fish, yet it is not a fish: it bears live young and nurses them.

The result depends entirely on the criterion. Sort by habitat and the whale joins the mackerel; sort by body structure and how the young are born, and the whale is a mammal like us.

Aristotle first set criteria and sorted living things; in the eighteenth century Linnaeus firmed up the framework by giving every organism a two-word name. Today the yardstick is how alike the genes are, more than how things look.

See alsoGene

Bits and information 비트와 정보 엔트로피 #

Mathematics

Think of twenty questions. Twenty yes-or-no answers can pin down one thing out of a million, because each question halves the field.

One yes-or-no is one bit. The amount of information is measured by how many questions it takes to find out.

From this came Shannon's surprising point: the obvious carries no information. That the sun will rise tomorrow is 0 bits, since nobody needs to ask. The less predictable something is, the more information it carries.

Which is why compression works. In English, a 'q' is nearly always followed by a 'u' — the predictable can be written short. Photographs, music and video are compressed on this principle. The formula for the quantity turned out to have the same form as entropy, and took the same name.

See alsoBoolean algebraEntropyProbability

Blackbody radiation 흑체복사 #

Physics

Iron in a forge turns dull red, then orange, then yellow, then white. The blacksmith reads the temperature from the colour alone, without a thermometer.

What kind of iron it is does not matter. Same temperature, same colour. Temperature alone sets it, not the material.

Reading a star's temperature from its colour works the same way: red stars are cool, blue stars are hot.

When late nineteenth-century physics tried to calculate this distribution, the answer came out infinite. The quantum came out of that dead end.

See alsoLightFrequencyQuantum

Blinding 맹검 #

Method & instruments

Give people a pill made of flour and call it a new drug, and a good number genuinely improve — less pain, better sleep.

Expectation affects the body. So participants must not know whether they are getting the real thing.

The harder half is the observer. Someone measuring outcomes who knows which patient had the drug will, without meaning to, read things favourably. Hence double blinding, where neither side knows.

Physics uses it too. For the black hole image, four teams processed the data separately without showing each other their results. When the sealed results were opened, all four matched. The design existed to stop anyone drifting towards the picture they expected.

See alsoControl groupReproducibilityError and uncertainty

Bloodletting 사혈 #

Biology & Medicine

In 1799 George Washington, ill with a sore throat, was bled four times in a single day and died that night. He was not the victim of quacks. The best physicians of the day were giving him standard care.

Under humoral theory illness is an excess of one humour, from which it follows logically that removing the excess should cure it. Bloodletting was not superstition but the theory's prescription.

It lasted because it appeared to work. Taking blood really does lower a fever and slow the pulse. And if the patient recovered the bleeding had worked; if they died they had come too late. The theory was safe whatever happened — the shape described under falsification.

What ended it was not a new theory but counting. In the 1820s Pierre Louis compared pneumonia patients bled heavily and lightly and found no benefit: the point at which the control group entered medicine.

It has not disappeared entirely. Where blood grows too thick, or iron accumulates in the body, drawing blood is still standard treatment. The procedure is the same and the reason is different.

See alsoHumoral theoryFalsificationControl groupUseful wrongs

Boolean algebra 부울 대수 #

Mathematics

A light switch is on or off, with nothing in between. Wire two of them so the lamp needs both, or so either one will do.

Boole's idea was to treat logic as arithmetic: let true be 1 and false 0, and use 'and' and 'or' like multiplication and addition. Reasoning that had been done in words became equations.

When it appeared in 1854 it was pure logic. Eighty years later Shannon noticed that a switch in an electrical circuit is also two-valued — so Boolean algebra can be built directly out of circuits.

That is what happens inside every computer: logic becomes circuitry, and circuitry becomes calculation.

See alsoBits and informationAlgorithmThe Turing machine

Buoyancy 부력 #

Physics

Step into a bath and the water rises — and you feel lighter. The two are the same event.

Your body pushes water aside. The water wants its place back, and that push lifts you. The lift equals the weight of the water displaced — a thought that came out of a bath two thousand years ago.

That settles what floats: if the displaced water weighs more than the object, the object floats.

A lump of steel sinks; a steel ship floats because it is hollow and displaces a great deal of water. Load it and it sits lower, displaces more, and gains more lift. The line painted on its side marks the limit.

A submarine adjusts this: take in water and sink, blow it out with air and rise.

See alsoDensityDragMass

Calculus 미적분 #

Mathematics

A speedometer shows the speed at this instant. But speed is distance over time, and an instant is zero time — and you cannot divide by zero.

Differentiation sidesteps the contradiction. It never puts zero in; it watches where the value heads as the interval shrinks (see limit). If the averages over one second, a tenth, a hundredth close in on 60, the answer is 60.

Integration runs the other way. To find the area of a crooked plot, slice it into near-rectangles and add them; the finer the slices, the better.

The remarkable thing is that the two undo each other. Think of a bank account: adding every deposit to get the balance is integration; reading today's deposit off how fast the balance rises is differentiation. Two directions through one ledger.

See alsoLimitFunctionInfinity

Carbon 탄소 #

Chemistry

Pencil lead, diamond and our bodies are made of the same element.

Six protons, six electrons. It shares its four top-floor electrons with neighbours, so it has four bonds, and with them builds chains, rings and branches: fats, proteins, DNA.

The same carbon can be soft graphite or hard diamond depending on how it is arranged. The arrangement, more than the atom, sets the properties.

See alsoValenceCovalent bondAtom

Carbon dioxide 이산화탄소 #

Chemistry

Our exhaled breath holds about a hundred times more carbon dioxide than the air we breathe in.

There is very little in air, less than one part in a thousand, yet it does two big jobs: it is the material plants build with, and it traps heat leaving the ground, keeping Earth warm. Too much thickens that blanket and changes the climate.

Air's carbon dioxide falls in spring and summer as forests breathe it in and rises in autumn and winter: the whole planet breathing.

See alsoPhotosynthesisThe greenhouse effectCarbon

Catalyst 촉매 #

Chemistry

Two substances will react eventually. The trouble is that eventually can mean a century.

A reaction usually has a hill to get over. A catalyst lowers the hill, so the same temperature gets far more molecules across.

The point is that the catalyst is not used up. It helps and steps out, so a very small amount suffices.

In our bodies this job is done by enzymes. In industry a better catalyst changes what a product costs, which is why hunting for them is a large part of chemical research.

See alsoPolymerMolecule

Cepheid variables 세페이드 변광성 #

Astronomy

Some stars brighten and dim over and over — some in days, some in months.

Leavitt found 1,777 such stars in the Small Magellanic Cloud and timed them, and found a rule: the brighter the star, the slower it blinks.

Why that is decisive: the stars in that cloud are all at roughly the same distance. At equal distance, differences in apparent brightness are differences in true brightness — so period and true brightness pair up.

Which means that for any such star anywhere, timing the blink gives its true brightness. That produced the standard candle, and with it the size of our galaxy and the distance to Andromeda. Leavitt held a job with no access to a telescope, examining photographic plates.

See alsoStandard candleRedshiftParallax

Chaos 혼돈 #

Physics

However carefully you repeat a break at billiards, by the third or fourth collision the balls go somewhere else entirely. Not for want of rules — the rules are exact.

Chaos does not mean a mess. The laws stay exactly as they are; it is the speck of difference at the start that grows as time passes.\n\nLorenz met it while computing weather. He restarted a run with a number trimmed a few decimal places, and the weather months later came out entirely different.\n\nThe point is this: a better computer does not break the wall. The initial state can never be measured with infinite precision. However densely we observe, the gaps between stations are empty, and a small difference hiding there becomes a large one within days.\n\nSo forecasting has a horizon, somewhere around a fortnight, beyond which it means nothing. The method that came after accepting that limit is running the sums many times and speaking in probabilities.

See alsoNumerical forecastingProbabilitySimulation

Chemical bond 화학결합 #

Chemistry

Hydrogen and oxygen are both gases. Put them together and you get water — a liquid. The result has nothing in common with the ingredients.

Because how they join sets what they are. It is not only what is joined but the manner of joining.

The holding force comes from electrons. Share the outer electrons of two atoms, or let one hand them over, and both settle into a state they will not readily leave. Graphite and diamond are the same element; one is soft and one the hardest thing there is, because the joining differs.

Cooking is the handling of chemical bonds. Roasting and boiling break and remake them, and flavour and texture change.

See alsoAtomMoleculeElement

Chromosome 염색체 #

Biology & Medicine

Among the flies, one male turned up with white eyes. Bred on, white eyes appeared only in males. Why should it travel with sex?

Because sex is also set by a chromosome. If a characteristic travels with sex, its gene must ride on the chromosome that determines sex.

Think of a chromosome as a thread with genes strung along it rather than scattered loose. Things on one thread are usually inherited together — and when they do come apart, closer neighbours come apart less often. Measure that rate and you get a distance, which is how genetic maps came to be drawn.

See alsoGeneDNA and the double helixTraits, dominant and recessive

Circulation of the blood 혈액순환 #

Biology & Medicine

Multiply the blood pushed out by one heartbeat by the number of beats in a day. The total comes to hundreds of times a person's body weight.

You cannot manufacture that much blood daily. So there is only one answer: the same blood is going round.

With that one calculation Harvey demolished a fifteen-hundred-year-old theory, which had the liver continuously making blood for the body to consume. He did not see it; he reasoned it from numbers. The fine vessels linking artery to vein were confirmed under a microscope after his death.

See alsoAnatomyThe cellMicroorganisms

Cleanroom 클린룸 #

Method & instruments

Sunlight through a window shows dust drifting in the air. What is visible is the least of it.

Ordinary room air carries hundreds of thousands of dust particles in the volume of a fingernail. Normally this matters not at all.

But where patterns are drawn at the nanoscale, one speck severs a circuit outright, because the speck is far larger than the lines being drawn.

So the air is filtered continuously and made to flow downwards, and rooms are graded by counting what remains.

The largest source of contamination here is not the machinery but the people. A person standing still sheds skin and fibres without pause. The suits are worn not to protect the person but to protect the room.

See alsoWaferNanometre and the nanoscaleSemiconductor

Collapse of the wave function 파동함수 붕괴 #

Physics

An electron is described as a wave spread through space — yet it lands on the screen as a single dot. Where did the rest of the spread go?

The Copenhagen interpretation answers: at measurement, what was spread folds to a point. That folding is called collapse.

But nobody has settled why it folds, or when the moment of measurement is — when the detector touches, when a record is made, when a person looks. That question is the measurement problem.

Some interpretations remove collapse altogether: nothing folds, the branches merely divide. Which is right remains undecided after a century.

See alsoThe wave functionSuperpositionThe Copenhagen interpretation

Complementarity 상보성 #

Physics

In the double slit you can watch the stripes, or you can watch which slit the electron used. You cannot do both.

How the apparatus is arranged decides what can be seen, and the two pictures cannot be laid over each other.

Bohr took this for a condition of nature rather than a limit of skill: we have only two vocabularies, and nature fits neither of them cleanly.

A coin comes close. Heads and tails are both the coin, and only one faces you at a time.

See alsoThe double slitThe uncertainty principleThe Copenhagen interpretation

Condensation 응결 #

Earth Science

Pour a cold drink and beads of water form on the outside of the glass. The glass is not leaking; that water came out of the room's air.

Cool the air and it has less room for vapour. Whatever overflows returns as droplets: the sweating glass, the dew at dawn, the fog on spectacles — one and the same event.\n\nCloud is that event happening in the sky. Rising air cools; past the limit the vapour beads into very small droplets, and those droplets in their millions are what we see.\n\nSo a cloud is water in the air, not smoke. The drops are simply too small to fall. Let them collide and grow and they can no longer hold up. That is rain.

See alsoHumidityConvectionThe greenhouse effect

Conservation law 보존법칙 #

Physics

Firewood burns down to ash, and the ash weighs far less than the wood. Did the rest simply vanish?

No. It left as smoke and vapour. Weigh the wood plus the air before, and the ash plus smoke plus air after, and the totals match.

Some quantities behave this way: the form changes, the total does not. Energy does, momentum does, and so does the amount of matter. When such a sum fails to balance, it usually means something has been left out rather than that the arithmetic is wrong.

See alsoEnergyMomentumSymmetry and Noether's theorem

Conservation of mass 질량보존의 법칙 #

Chemistry

Stir sugar into water and it vanishes before your eyes. But put the whole glass on a scale first: the reading does not move.

It has stopped being visible, not stopped existing. Chemical reactions are the same: forms change, things become invisible, and the total weight stays put.

This law made chemistry a science. Once the books had to balance on a scale, you could account in numbers for what goes in and what comes out. If they do not balance, the arithmetic is probably fine and something has been left out — usually an invisible gas.

See alsoOxidationPhlogistonConservation law

Continental drift 대륙이동설 #

Earth Science

Open a world map and look at the east coast of South America and the west coast of Africa. They fit like puzzle pieces. Coincidence?

Wegener thought not, and piled up evidence: the same fossil species on both sides, strata that continue across, mountain ranges that line up. Fossils of creatures that could not have swum the ocean appear on both coasts.

The field pushed back hard, and not unreasonably: he could not say what moved them. Continents sliding like ships across solid bedrock made no physical sense.

The evidence was there; the mechanism was not. The answer came fifty years later from the sea floor (plate tectonics). Wegener did not see it — he died on an expedition in Greenland.

See alsoPlate tectonicsSea-floor spreadingFossilsStrata

Control group 대조군 #

Method & instruments

You take a cold remedy and recover in three days. Did it work? Colds also clear up in about three days.

Looking only at people who took it tells you nothing. You have to compare them with people who did not — and that untreated group is the control.

Semmelweis established the effect of handwashing this way: two wards in one hospital, death rates differing several-fold, and one difference between them.

What matters is that everything else be alike. If one group is young and the other elderly, no comparison is possible. This is where blinding comes in — knowing who is in which group lets expectation and bias into the result.

See alsoBlindingHypothesisReproducibility

Convection 대류 #

Physics

Watch a pot come to the boil: water climbs in the middle and sinks at the edge. Nobody stirred it, yet it turns.

Warmed matter expands, its density falls, and it rises. Cooling above, it grows heavy and sinks. That loop is convection.\n\nIt is why a heater goes on the floor and an air conditioner near the ceiling: cold air sinks, warm air lifts.\n\nThe sky does the same. The sun does not heat the ground evenly — tarmac, paddy, hill and sea all take it differently. Air above the warmer patch lifts first, cools as it climbs, and becomes cloud.\n\nThat is the summer afternoon cloud swelling upward: the ground has been heating all day, so the air goes up hard.

See alsoDensityHeatCondensation

Covalent bond 공유결합 #

Chemistry

In water, hydrogen's electron does not move over to oxygen. The two homes knock down a wall and share a room.

When two atoms' electron clouds overlap, both count the electrons in the overlap, so both end up with a full top shell. The shared region belongs to neither atom; it spans the two.

When one atom gives an electron away outright and the other takes it, as in salt, the bond is called ionic. Water, sugar, proteins and DNA are held together mostly by covalent bonds.

See alsoChemical bondValenceOrbital

CRISPR 크리스퍼 #

Biology & Medicine

Bacteria are invaded by viruses too. A survivor files a scrap of the invader's DNA in its own genome, and when the same intruder returns, matches it against the file and cuts it up.

It was a bacterial immune memory. What researchers noticed was that the 'what to look for' part can be swapped out.

Supply the DNA sequence you want and it goes there and cuts. Precise, cheap and fast. Gene editing used to take years and large sums; now it takes weeks in an ordinary laboratory. Which is exactly why the questions have grown — what can be done and what should be done are different matters.

See alsoGenomeDNA and the double helixGene

Current and the battery 전류와 전지 #

Physics

Static electricity gives one sting and is done — it all flows at once. A torch, though, stays lit for hours.

The difference is whether something keeps pushing. Volta stacked alternating discs of two metals separated by brine-soaked cloth, and got that continuity: a chemical reaction that keeps driving charge one way.

That is a battery. Until then electricity was a momentary burst — lightning, a spark. Now it flowed, and because it flowed it could be studied. Within twenty years the whole of electromagnetism was built.

See alsoElectric chargeElectromagnetic inductionEnergy

Cyanobacteria 남세균 #

Biology & Medicine

The green blooms that cover rivers in summer are these bacteria. The same crowd filled the sky with oxygen 2.4 billion years ago.

They catch sunlight, split water, and add carbon dioxide from the air to make food. Splitting water leaves oxygen, and to them oxygen was rubbish.

What they dumped for hundreds of millions of years rusted all the iron in the sea and then spilled into the sky. It was poison to most life of the day and vast numbers died — but it built the ozone layer that let life move onto land. The chloroplasts inside plant cells are their descendants.

See alsoBacteriaThe ozone layerMicroorganisms

Decoherence 결어긋남 #

Physics

An electron can be in superposition; a cat cannot. If size is the issue, where is the boundary?

Anything cat-sized is struck constantly by air molecules, photons and its own atoms. Each collision leaks which-path information outwards, and once information leaks, interference is gone.

It happens unimaginably fast — superposition has no room to persist. It is also why quantum computers are kept very cold and isolated.

This is only half an answer, though. Decoherence explains why superposition is not seen; it does not explain why exactly one outcome remains (the measurement problem).

See alsoWhich-path informationThe measurement problemSuperposition

Deep time 깊은 시간 #

Earth Science

Compress the Earth's 4.6 billion years into a single day. Life appears around four in the morning. Dinosaurs arrive at 10:40 pm and are gone by 11:40. Humans turn up two seconds before midnight.

Our senses cannot hold this scale. A century already feels long; a hundred million years is just a large number.

Yet some things are only explicable with that much time: a river cutting a canyon (uniformitarianism), continents parting to open an ocean (drift), tiny differences accumulating until species separate (selection).

Hutton first felt the scale at a Scottish cliff where two sets of strata meet at different angles — the time needed for the lower set to be laid down, tilted, eroded, and then buried again. He wrote that he could see no vestige of a beginning and no prospect of an end.

See alsoStrataUniformitarianismHalf-life

Density 밀도 #

Physics

A steel ball and a polystyrene ball the same size. Lift them and the weight is nothing alike.

Same size, different weight: different density. Steel is packed tight, polystyrene is loose.

Density decides what floats and what sinks. Denser than water sinks; less dense floats. Steel sinks, yet a steel ship floats because it is full of air and its overall density is below water's (buoyancy).

The sea works the same way. Cold salty water is heavy and sinks; warm water rides above. The great current that circles the world's oceans begins where polar water sinks to the bottom.

Ice floats because water, unlike almost everything else, becomes less dense when it freezes.

See alsoBuoyancyIce coreIsotope

Diffraction 회절 #

Physics

Shine light on the back of a CD and a rainbow appears. There is no pigment; the closely spaced grooves have split the light.

A wave passing something finely spaced splits and overlaps, and where it overlaps it makes bright and dark bands. The pattern depends on the spacing.

Run that backwards and it becomes a tool: read the pattern, learn the spacing.

Atoms in a crystal are lined up very closely. Fire X-rays with a wavelength as short as that spacing and a pattern appears; unravel it and out comes the arrangement of the atoms. The double helix of DNA was found this way (X-ray crystallography).

It is a way of reading, as a pattern, what the eye could never see.

See alsoWaveWavelengthX-ray crystallographySynchrotron light

DNA and the double helix DNA 이중나선 #

Biology & Medicine

To carry heredity a substance must do two things: hold information, and be copied faithfully. DNA has only four letters — it looked far too simple.

The structure answered both. Two strands twist together, and once a letter on one side is fixed, so is the letter facing it — the pairs are set.

So open it down the middle, use each strand as a template, and you get two identical copies. The method of copying was built into the shape. And four letters are plenty: rearranged, the combinations are effectively endless. It is length, not alphabet size, that sets how much can be said.

The X-ray photograph that made the structure legible was taken by Rosalind Franklin, and passed on without her consent.

See alsoGeneChromosomeGenome

DNA profiling DNA 감식 #

Biology & Medicine

Human DNA is 99.9 percent identical from person to person. So how is one person singled out?

Not by reading the whole thing; that would take far too long.

DNA has places where the same short spelling repeats over and over — ten times in one person, thirteen in another. The count differs between people and is inherited.

About twenty such places are chosen and only the repeat counts are read. A match at one place is quite likely; a match at all twenty is far rarer than one in the world's population. That is what makes it an identity card.

Its strength is how little it needs — a single hair, saliva on a cup — thanks to a technique that copies one fragment millions of times.

Its limit is just as clear: it says a person was there, not what they did.

See alsoDNA and the double helixGeneFingerprint

Drag 저항 #

Physics

Push your open palm through water and it is hard; slice with the edge and it is easy. Same hand.

A moving object must push water aside in front and rub against water alongside. The cost is drag.

It comes in three parts: friction with the water, the pressure difference between front and back, and one part that belongs to ships alone — a ship also spends energy making waves. Every ripple left in its wake is energy the ship paid for, and the share grows sharply with speed.

So a rounded bulb is fitted under the bow. The wave the bulb makes cancels the wave the bow makes. That alone cut fuel use by more than a tenth.

Aircraft and fish solve the same problem, and the answer is often a teardrop: blunt in front, tapering behind.

See alsoFrictionBuoyancyMomentum

Electric charge 전하 #

Physics

Pulling off a jumper in winter gives you a shock. Rub a balloon on your head and your hair follows it up.

Rubbing moves tiny grains (electrons) from one thing to the other, leaving one with a surplus and one with a shortfall. That surplus and shortfall is charge.

Like repels like; opposites attract. And charge is never destroyed, only moved. Franklin named the two kinds positive and negative — and guessed the direction backwards, which is why conventional current is still drawn opposite to the way electrons actually go.

See alsoCurrent and the batteryAtomElectromagnetic induction

Electrolysis 전기분해 #

Chemistry

Dip electrodes in water and pass a current: bubbles rise at both. Hydrogen at one, oxygen at the other.

Electricity has pulled apart what was joined. Since a chemical bond is held by electrical force, electricity can undo it.

Right after the battery appeared, this method poured out new elements: sodium, potassium, calcium, magnesium — all locked inside compounds and never before seen pure. One new tool, and six elements in six years.

Aluminium is still made this way, which is why most of what aluminium costs is electricity.

See alsoCurrent and the batteryElementChemical bond

Electromagnetic field 전자기장 #

Physics

There seems to be nothing around a magnet, yet iron filings sprinkled there form lines.

Like temperature on a weather map, it has a value at every point. Nothing is filling space; it is a property space has, and even the emptiest vacuum does not lose it but trembles faintly.

Ripples spreading through this field are electromagnetic waves, light among them, as Maxwell showed.

See alsoElectromagnetic waveVacuumThe aether

Electromagnetic force 전자기력 #

Physics

Press a desk and your hand does not pass through. What meets is not nuclei but electrons.

Opposites attract and likes repel. Electrons held by nuclei make atoms, atoms sharing electrons make molecules, and the electrons of hand and wall pushing apart keep us from walking through walls: all this force. Chemistry is almost entirely its work.

Maxwell showed electricity and magnetism are two faces of one force.

See alsoThe electronCovalent bondGravity

Electromagnetic induction 전자기유도 #

Physics

A bicycle dynamo lamp needs no battery — but it only lights while you pedal. Stop, and it goes out.

The wheel turns a magnet, and the moving magnet raises a current in the wire beside it. Leave it still and nothing happens. Motion is the whole point.

When Faraday demonstrated it he was reportedly asked what use it was. Nearly all the world's electricity is now made this way: a power station burns or drops something in order to spin a magnet.

See alsoCurrent and the batteryElectric chargeMaxwell's equations

Electromagnetic wave 전자기파 #

Physics

Radio, sunlight and hospital X-rays look nothing alike, but they are the same ripple, differing only in how closely packed it is.

Widely spaced ripples are radio, a little closer infrared, closer still visible light, then ultraviolet and X-rays. All travel at 300,000 kilometres a second.

Unlike sound it needs no material to travel through, so starlight crosses empty space to reach our eyes.

See alsoElectromagnetic fieldLightInfrared

Electron shell 전자껍질 #

Chemistry

Board 6 of the element tool calls these the floors of an apartment block.

Each shell holds a fixed number of electrons: 2, 8, 18, 32, twice the floor number squared.

Inside, a shell divides into groups of differently shaped rooms, each room an orbital. A row of the periodic table ends not when a whole shell fills but when the top shell's ball-shaped and dumbbell-shaped rooms are full.

See alsoOrbitalPauli exclusion principleValence

Element 원소 #

Chemistry

Boil water and it becomes steam; cool it and it is water again — still water. But run electricity through it and it splits into two gases, and those do not simply turn back.

Some things split and some do not. What cannot be split further is an element.

The ancients counted four: earth, water, air, fire. We now know 118, some ninety of which occur naturally. Every substance in the world is a combination of those ninety-odd — wood, people, rock, stars. But chemistry cannot turn one element into another. That is why alchemy failed, and the wall can only be crossed at the level of the nucleus.

See alsoAtomMoleculeThe periodic table

Encryption 암호화 #

Method & instruments

As children we passed notes with each letter shifted one along: A becomes B, B becomes C.

That is a cipher in earnest — the same one Rome used two thousand years ago.

Two things are in it: a method and a key. Shifting each letter is the method; how many places to shift is the key.

For a long time people tried to hide the method. But methods always leak: a machine is captured, or somebody talks.

So today the opposite is done. The method is published to the world and only the key is kept. Publishing it means everyone hunts for its weaknesses on your behalf, and a lock the whole world has hammered on is sturdier than a hidden one.

What remained was how to hand the key to the other side, and the answer to that is public-key cryptography.

See alsoPublic-key cryptographyHashAlgorithm

Energy 에너지 #

Physics

A ball held up high has done nothing yet, but let go and it falls and strikes the floor. Something was stored in it while it was up there, and spent as it fell.

That something is energy. It can be stored in height, become motion, become heat, become light.

The point is that it only changes shape — it never disappears. When the ball stops on the floor its energy has not gone; it has scattered as sound and heat. Once scattered, though, it is hard to gather back.

See alsoHeatConservation lawThe second law of thermodynamics

Entanglement 얽힘 #

Physics

Split a pair of gloves between two boxes and send one to Seoul, one to Busan. Open the Seoul box, find the left glove, and the Busan one is right-handed. Obvious — it was decided from the start.

Entangled particles are not like that. At the moment of sending, neither was left or right. The instant the Seoul box is opened, both are settled together.

It sounds like word-play, but experiment can decide it: pre-set and not-pre-set give different statistics across measurement angles. When measured, nature came out on the not-pre-set side. Quantum cryptography and quantum computing now use this property as material.

See alsoSuperpositionLocalityHidden variables

Entropy 엔트로피 #

Physics

A cup falls off a desk and shatters. The pieces never gather themselves back into a cup.

Hot water left in a room cools. Cooled water never heats itself back up.

Why only one way? Because there are few arrangements that look like a cup and countless ones that look like scattered pieces. Left to chance, things go where the arrangements are many.

The hot water is the same. There are many ways for the lively motion of water molecules to spread through the room's air, and almost none for that motion to gather back into the cup.

Entropy puts a number on how spread out things are. So it only ever grows — which is also why time appears to run one way.

See alsoHeatThe second law of thermodynamicsMolecule

Enzyme 효소 #

Biology & Medicine

Chew rice long enough and it turns sweet, though you added no sugar.

Something in saliva has cut the long chains in the rice into sweet-tasting fragments. That something is an enzyme.

An enzyme is a catalyst that the body has built out of protein: it speeds a reaction without being used up.

The remarkable part is its fussiness. One enzyme does one job, because it only grips what matches its shape. That is how thousands of reactions run side by side in us without tangling.

See alsoProteinCatalystFermentation

Epicycles 주전원 #

Astronomy

Watch Mars for months and something odd happens: moving eastward, it stops, doubles back westward, then turns and goes east again.

If the Earth sits still at the centre and planets circle it, this cannot happen. So a small circle was set on the big one: ride the large path while turning on the small one and you sometimes appear to go backwards.

It is often said that epicycles multiplied into hundreds until the system collapsed under its own weight. They did not. The whole Ptolemaic system used about forty circles, and that number did not swell steadily through the Middle Ages. Stacking circles can imitate any repeating motion as closely as you like — a precise instrument, which is why the tables stayed useful for fourteen centuries.

On the heliocentric view the backward loop is simply the Earth overtaking Mars on the inside track. No device needed at all.

See alsoRetrograde motionGeocentrismHeliocentrismKepler's laws

Epigenetics 후성유전 #

Biology & Medicine

Every cell in your body holds the same DNA, yet a liver cell and a nerve cell look and behave entirely differently. How does one blueprint build such different things?

Because marks sit on the DNA deciding what is read and what is kept closed — methyl groups attaching, or the proteins the DNA winds around tightening and loosening. The sequence is untouched; only the switches change.

The concept arose to explain development rather than evolution. Waddington named it in 1942.

On the claim that Lamarck was right. Famine and severe stress alter these marks, and some marks have been found again in the next generation. From this came the widespread claim that epigenetics has revived Lamarck. It has not.

1. There is no direction. Lamarck's point was that use develops a thing and the development is passed on. Epigenetic marks do not supply an advantageous trait to order; the traces of famine appear as disordered metabolism, not as a body hardened against hunger

2. They are mostly erased. Mammals wipe the marks on a large scale twice, after fertilisation and again when germ cells form

3. The sequence does not change. A mark gone within a few generations cannot be raw material for evolution

4. The machinery is itself genetic. The enzymes that add and remove marks are written in DNA; the whole system is a product of natural selection

The most cited case, the Dutch famine of 1944, needs care. A foetus in the womb experienced that famine directly, and inside it the germ cells of the next generation were already forming — two generations exposed at once. Genuine transmission would have to reach a generation never exposed, and in mammals that remains disputed.

The accurate sentence is this: Lamarck was not right; the Weismann barrier is very slightly less absolute than had been supposed.

See alsoThe Weismann barrierInheritance of acquired characteristicsGeneDNA and the double helixUseful wrongs

Error and uncertainty 오차와 불확도 #

Method & instruments

The scale says 68.4 kg. Step off and on again and it says 68.2, then 68.5. Which is right?

None of them exactly. Every measurement has a spread. So science writes not '68.4 kg' but '68.4 ± 0.2 kg', stating the spread alongside.

Why state it? Because it lets you judge whether a difference means anything. If two readings are 68.4 and 68.5 and the spread is ±0.2, they are the same value. If the spread is ±0.01, they are different.

Kepler abandoned circular orbits on exactly this judgement. The discrepancy was eight arcminutes, and Tycho's observations were better than that. A residual larger than your uncertainty is not error but signal.

See alsoStandardisationKepler's lawsProbability

Exoplanets 외계행성 #

Astronomy

Planets do not shine. The star beside them is overwhelmingly bright — it is like spotting a firefly next to a searchlight. Finding one by looking is close to impossible.

So astronomers watch the star instead. Two methods.

First: the planet pulls on the star, so the star wobbles slightly. As it moves towards and away from us its light shifts red and blue in turn, and the period of that shift reveals the planet.

Second: when a planet passes in front, the star dims a little. Regular dimming and brightening means something is going round.

Since the first detection in 1995, thousands are known. It turns out that stars usually come with planets.

See alsoRedshiftSpectral linesStandard candle

Extinction 멸종 #

Earth Science

Enormous bones turn up that match no living animal. So where is this creature? Living somewhere nobody has been?

Until the eighteenth century, yes. It was held impossible that anything could vanish from a perfect creation, so people expected further exploration would turn it up.

Reading the strata settled it. Some fossils disappear entirely above a certain layer, and sometimes many species vanish at the same layer together.

Extinction happens, and it cannot be undone. There have been five mass extinctions in Earth's history, and there is a view that a sixth is under way. What differs from the previous five is that the cause is not an asteroid or a volcano.

See alsoFossilsStrataDeep time

Falsification 반증 #

Method & instruments

Consider the claim that all swans are white. A million white swans do not make it certain — the next one might differ. But one black swan settles it.

Confirmations never add up to proof, while one refutation is enough. That asymmetry is the point.

So a good theory says in advance what would sink it. Einstein specified exactly how far starlight must bend at an eclipse; had it not bent, his theory was finished.

A theory that accommodates any outcome is not strong but empty. By this standard you can also see why a theory that adds a device for every exception, like epicycles, grows weaker as it grows.

See alsoHypothesisUseful wrongsPeer review

Fault 단층 #

Earth Science

Press your palms together and push them in opposite directions. Nothing moves — and then suddenly it slips.

The same happens underground. Plates press on each other and stress accumulates while friction holds. The moment friction gives and the rock slips is an earthquake.\n\nSo earthquakes happen at breaks, and a break is a fault. Ground that has slipped once tends to slip again, so mapping where the faults are is where preparation begins.\n\nWhen it will slip, though, is unknown — as unknown as the moment your palms will give.

See alsoPlate tectonicsSeismic wavesFriction

Feedback 되먹임 #

Method & instruments

Try picking up a cup with your eyes shut. It goes badly. Open them and it is easy.

With your eyes open you can see where your hand is and how far off the target it is — and correct by that much. Repeat this very fast and you have feedback.\n\nIt is not calculating the answer once and hitting it. It is measuring and correcting, over and over.\n\nRobots work this way, so does the boiler holding a room's temperature, and so does your own balance. The method exists because the world does not behave as calculated.

See alsoAlgorithmFriction

Fermentation 발효 #

Biology & Medicine

Open a kimchi jar and it fizzes. Open a bottle of makgeolli and it foams over. Both are the sound of someone eating inside.

Fermentation and rot are not two different things. They are two human names for one event — microbes eating. If we can eat the result, fermentation; if not, rot. Kimchi is well ripened; spoiled milk is badly ripened.

What differed was which microbe got there first. So kimchi uses salt, and makgeolli uses nuruk, to let the wanted guest in first.

See alsoYeastMicroorganismsLactic bacteria

Fingerprint 지문 #

Biology & Medicine

Hold a glass and let go, and a print remains. Hard to see head-on, but tilt it to the light and there is a pattern.

Fingertip ridges are set before birth. They are the folds skin takes as it grows in the womb, so even twins differ.

Touching leaves a mark because of sweat. Pores sit along the ridges, so every touch stamps the pattern in sweat and oil. Dust with powder and it clings only to that oil, and the pattern shows.

The method is over a century old and still the most used. Computers sift millions of prints for similar ones; the final call is made by a person.

A fingerprint says only that this person touched here. It does not say when.

See alsoDNA profilingThe cell

Force 힘 #

Physics

Push a stationary trolley and it starts moving. Grab a rolling one and it stops. Push from the side and it turns.

All three are what a force does. A force does not make something move; it changes how it is moving.

So a body with no force on it does not stop — it carries on as it was. That is why an object thrown in space keeps going. A ball stops rolling on Earth because friction, a force, is holding it back.

See alsoInertiaMomentumUniversal gravitation

Fossils 화석 #

Earth Science

A stone shaped like a seashell turns up on a mountaintop, hundreds of kilometres from any sea. How did it get there?

For a long time there were two answers: a flood carried it there, or stones grow underground and this one happened to grow shell-shaped.

Neither. That place had once been sea. A shell died, was buried in the strata, and over immense time the ground rose into a mountain.

What makes fossils unsettling begins here. Fossils in lower layers differ from those above. Below are creatures that no longer exist; higher up they grow more like the present. Life has changed over time. And some creatures never appear above a certain layer at all (extinction).

See alsoStrataExtinctionNatural selection

Frame of reference 기준틀 #

Physics

You walk down the aisle of a moving train. To a passenger you are doing four kilometres an hour; to someone on the embankment, three hundred and four. Which is right?

Both are. Speed is always speed relative to something, and that something is the frame. A speed quoted without a frame is only half a statement.

Older physics knew this and still supposed there was one frame truly at rest — an absolute backdrop that everything else moved against. The aether was the substance appointed to be that backdrop.

What follows if there is no such backdrop is special relativity: no frame is privileged, the laws take the same form in all of them, and so does the speed of light. Time and length are what differ from frame to frame instead.

See alsoSpecial relativityThe constancy of light speedThe aetherInertia

Frequency 진동수 #

Physics

Tighten a guitar string and the note rises. Thick strings give low notes, thin strings high ones.

What sets the pitch is how many times the string shakes per second. That count is the frequency. Shake 440 times and you get the note we call A.

Anything that is a wave has a frequency, light included. Low-frequency light looks red, higher looks blue, and higher still becomes ultraviolet, which the eye cannot see.

See alsoWaveLightSpectral lines

Friction 마찰 #

Physics

You cannot walk on ice. You push with your foot and nothing moves you forward.

We walk because the foot pushes the ground back and the ground pushes the foot forward. That push is friction.\n\nWe tend to think of it as a nuisance to remove, but without it nobody walks, grips anything, or stops a car.\n\nThe awkward part is that it resists calculation. A slightly different surface gives a different value, so it never tidies into a clean law the way gravity does — which is also why robots fall on slippery floors.

See alsoInertiaForceUniversal gravitation

Function 함수 #

Mathematics

Put a coin in a vending machine and press button 2, and the same drink always comes out. Same input, same output — and one input never yields two things.

That is a function: one input, one output, by a fixed rule.

A taxi fare is a function — put in the distance, out comes the price. So is ice-cream sales as a function of temperature. Drawing one is a graph, and asking about a graph's slope is differentiation.

See alsoLimitAlgebraProbability

Fungus 곰팡이 #

Biology & Medicine

Leave bread a few days and fuzz appears. Every strand of fuzz is a fungus growing as a thread.

Unlike bacteria, its cell has a separate room for the blueprint — a nucleus. In that sense it is closer to us than to bacteria, which is why fungal infections are harder to treat: drugs that kill fungi tend to bother our own cells too.

Makgeolli's nuruk, the meju block of doenjang, the blue mould that gave penicillin, and athlete's foot are all fungi. Yeast is a one-room fungus that makes no threads.

See alsoYeastBacteriaMicroorganisms

Gene 유전자 #

Biology & Medicine

Eight years of counting peas revealed a rule. Cross tall with short and the next generation is all tall — but cross those and short reappears, in exactly one case out of four.

If they blended, this could not happen — mixed paint does not separate again. Because they pass on whole, like grains, one can hide and return. That grain is a gene.

Mendel did not know what it was made of. He knew only that something behaved this way. That it sits on a chromosome took fifty more years; that it is DNA, eighty.

See alsoTraits, dominant and recessiveChromosomeDNA and the double helix

General relativity 일반상대성이론 #

Physics

Put a heavy ball on a rubber sheet and it dents. Roll a marble past and the marble curves towards the dent.

The ball did not pull the marble. The floor was bent.

That is what gravity is. The Earth circles the Sun not because the Sun tugs on a rope, but because it is going straight through a bent region the Sun has made.

The test came at the 1919 eclipse: starlight passing close to the Sun was measured to bend by the predicted amount. The geometry the theory needed had been built sixty years earlier by Riemann, with no idea what it was for.

See alsoUniversal gravitationSpecial relativityRiemannian geometry

Genome 유전체 #

Biology & Medicine

Written out, a human's genetic text runs to some three billion characters — thousands of books' worth.

Reading it from end to end was the Human Genome Project: thirteen years then, a day now.

The reading held surprises. The gene count was far lower than expected — around twenty thousand, fewer than a rice plant has. And most of the text does not code for protein at all. Once dismissed as junk, much of it is now turning out to regulate. Having read every letter is not the same as knowing what it says.

See alsoDNA and the double helixGeneCRISPR

Geocentrism 천동설 #

Astronomy

Look outside. The sun rises in the east and sets in the west, and the ground under your feet does not so much as tremble. Going by what you can see, the Earth is still and the sky turns.

Geocentrism was not a product of ignorance. It was a theory faithful to observation — and it calculated well. Ptolemy's system predicted stellar positions to useful accuracy for over a thousand years.

There were objections to the alternative, too: if the Earth moves, why is a thrown stone not left behind, and why is no stellar parallax visible? Both were good questions, and each took centuries to answer. A wrong theory lasted not because people were foolish but because refuting it was hard.

See alsoEpicyclesHeliocentrismParallax

Germ theory 세균설 #

Biology & Medicine

The same illness spreads through a village. Bad air? A curse? An imbalance in the body?

Koch made the question decidable by laying down rules. To call a microbe the cause of a disease you must show four things: it is present in the sick, it can be grown apart, introducing it makes a healthy subject ill, and the same thing can be recovered again.

He put conditions on the word 'cause'. With that rule, culprits could be identified disease by disease — and handwashing, disinfection and antibiotics followed.

See alsoMicroorganismsVaccines and inoculationAntibiotics

Gravitational lensing 중력렌즈 #

Astronomy

During the eclipse of 1919, a star that should have been behind the sun appeared slightly to one side. The sun had bent its light.

In general relativity, gravity is not a force but curved space. Light goes straight through that curvature, which from outside looks bent.

So what lies behind a massive body is not hidden but displaced, or multiplied, or drawn out into a ring.

It is now a tool. Measuring the bending gives the mass of whatever is in front, and this is how it emerged that galaxies hold a great deal of mass nobody can see — one pillar of the evidence for dark matter.

See alsoGeneral relativityLightThe event horizon

Gravitational waves 중력파 #

Physics

Shake a heavy ball on a rubber sheet and the whole sheet ripples, and the ripple spreads outward.

If gravity is curvature, as general relativity says, then violently moving masses should send that curvature out as waves. Einstein predicted them in 1916 and thought them undetectable.

Because they are so faint. When the wave from two colliding black holes reached Earth, a four-kilometre instrument changed length by one ten-thousandth the width of a proton. Catching that took until 2015 — a hundred years.

See alsoGeneral relativityWaveMass

Gravity 중력 #

Physics

It drops apples and holds the Moon, yet between individual atoms it does almost nothing.

The weakest of nature's four forces: between two protons, gravity is smaller than their electric repulsion beyond counting. We feel it first because it only attracts, and a mass as large as Earth pulls all one way.

The electric force rules atoms and molecules; gravity rules stars and galaxies.

See alsoElectromagnetic forceThe gravitational constant G

Group theory 군론 #

Mathematics

Turn a Rubik's cube: one face, then another, then undo one. However you turn it, it is still a cube. And two turns in a row amount to a single turn of some kind.

Collect the operations and the collection itself obeys rules: do two in a row and you are still inside it, undo one and you are still inside it, and doing nothing counts as a member. Such a collection is a group.

Galois built this at twenty, to explain why the fifth-degree equation has no formula. The idea went far beyond equations, because it made symmetry calculable instead of merely describable. Noether used the language to derive conservation laws; Gell-Mann used it to sort a hundred-odd particles.

See alsoSymmetry and Noether's theoremAlgebraAxiom

Gut microbiota 장내 미생물 #

Biology & Medicine

Beans, then wind. The colon neighbours are eating what you could not digest.

Tenants, but they pay rent: they eat the fibre we cannot digest, make vitamins, train immunity, and above all occupy the rooms so bad guests have nowhere to stay.

The mix is as personal as a fingerprint, largely set by age three. There are no good or bad germs by nature; the same germ is a neighbour or a pathogen depending on place and number. What grows them is not a probiotic capsule but their food — vegetable fibre.

See alsoLactic bacteriaBacteriaMicroorganisms

Half-life 반감기 #

Physics

When any given nucleus will decay is unknowable — a second from now, or a million years. Yet gather a million of them and the time for half to go is exact.

Each one unknowable, the whole exact — this is what the world of probability is like.

And that exactness gives us a ruler. Measure the carbon-14 left in wood or bone and you know when it died; measure element ratios in rock and you get the age of the Earth. That is how we know the Earth is 4.6 billion years old.

See alsoRadioactivityProbabilityDeep time

Hash 해시 #

Method & instruments

Blending fruit into juice is easy. Turning the juice back into the fruit is not.

A hash is such a calculation. It runs one way only.

It has two uses.

The first is checking whether something has changed. Alter a single character in a file and the hash changes completely, so one value tells you whether a download arrived intact or was swapped.

The second is keeping passwords. A well-built service stores not the password but its hash, so even a stolen store yields no passwords. At sign-in, what you type is hashed again and only the values are compared.

Short and common passwords still fall to pre-computed tables, which is why length is safety.

See alsoEncryptionAlgorithmBits and information

Heat 열 #

Physics

Leave a spoon in hot soup and before long the handle is hot too. The heat moved from soup to spoon. The reverse never happens — the spoon's coldness does not move into the soup.

Heat is not a substance; it is motion being handed along. Vigorously shaking soup molecules strike the spoon's molecules, which start shaking, and so on up to the handle.

And it has a direction: always from the lively side to the quiet side. Why never the other way is the story of the second law of thermodynamics.

See alsoMoleculeEnergyThe second law of thermodynamics

Heliocentrism 지동설 #

Astronomy

Copernicus put the sun at the centre not because of new data but because the pile of epicycles had become ugly.

Put the sun at the centre and the planets' backward loops explain themselves. No extra devices are needed.

Yet at first it was not more accurate. Copernicus kept circular orbits, so errors remained. Only after Kepler made the orbits elliptical did it clearly win.

And the decisive evidence, parallax, was not observed until 1838 — 295 years after Copernicus died. People had believed it long before the evidence arrived, which raises the question of what evidence is for.

See alsoGeocentrismEpicyclesParallax

Hidden variables 숨은 변수 #

Physics

A die looks random because we do not know the hand, the angle, the air. Know them all and you could compute the result. The die does not roll on probability.

Perhaps quantum probability is like that: something we have not yet seen, which would explain everything. Einstein took this side.

For decades it was treated as a matter of taste, since both positions predicted the same results. Then John Bell found a setting where they give different numbers — and measurement came out against hidden variables. A rare case of a philosophical dispute settled by experiment.

See alsoEntanglementLocalityProbability

Hilbert's programme 힐베르트 프로그램 #

Mathematics

Could there be a contradiction hiding in one plus one equals two? Yet paradoxes kept surfacing in set theory in the 1890s. Ground that looked solid was shifting.

Hilbert's plan: rebuild mathematics as a system in which symbols are moved by rule, with no appeal to meaning — then prove, inside that system, that it can never produce a contradiction. Mathematics would be safe for good.

In 1930 he closed a retirement address, broadcast on radio, with 'we must know, we shall know'. The day before, in the same city, a twenty-four-year-old named Gödel had presented the proof that it could not be done.

Something came out of the wreck. Pinning down what 'by rule alone' means produced the Turing machine, and that became the computer.

See alsoThe incompleteness theoremsAxiomThe Turing machine

Humidity 습도 #

Earth Science

In summer the laundry stays damp all day; in winter it dries fast. Not because winter air is warmer — it is colder.

Water drifts into the air without being boiled. Water gone invisible and gaseous is water vapour.\n\nAir can hold only so much of it, and the warmer it is the more it holds. So the same amount of vapour leaves summer air with room to spare and winter air already full.\n\nThe humidity given in a forecast is not an amount but a share: sixty percent means the air is carrying sixty percent of what it could carry now. Laundry stays wet because the air has no room to take more.\n\nPush past the limit and the water reappears — that is condensation.

See alsoCondensationAir pressureHeat

Humoral theory 체액설 #

Biology & Medicine

A cold brings mucus; an infected wound brings pus. That the body leaks fluids when it is ill is something anyone can see.

So the ancients took illness to be an imbalance among those fluids: if one is in excess, draw it off. That is why bloodletting persisted for two thousand years.

The theory was wrong. But it did one thing: it made illness a matter of the body rather than a punishment from the gods. Medicine began the moment causes were sought inside the patient. That the real cause was microbes took another two thousand years.

See alsoGerm theoryMicroorganismsUseful wrongs

Hypothesis 가설 #

Method & instruments

You have caught a cold. Why? Perhaps the rain yesterday, perhaps the underground, perhaps not sleeping enough.

All three sound plausible. But a plausible story is not a hypothesis. It becomes one only when there is a way to find out that it is wrong.

'Getting rained on causes colds' can be checked: compare people who were rained on with people who were not (a control group). 'Bad luck' cannot be checked, because it accounts for whatever happens.

An account that explains everything explains nothing. This leads to falsification.

See alsoFalsificationControl groupProof

Ice core 빙하 코어 #

Earth Science

Look closely at ice from the freezer and there are tiny bubbles trapped inside — air that could not escape as the water froze.

Antarctic ice is the same. As snow piles up and is pressed into ice, that year's air is trapped as bubbles. A layer forms every year, so the deeper you go the older the air.

An ice core is therefore the Earth's diary of air. Drill deeper and you go further back; the deepest so far holds air from eight hundred thousand years ago.

Break the bubbles and you can measure how much carbon dioxide the air held then; analyse the ice itself and you can tell the temperature. A record from a time with no thermometers and no people.

It is this column that shows how the greenhouse effect has actually behaved over time.

See alsoThe greenhouse effectDensityIsotope

Ignition 점화 #

Physics

Lighting a fire, you keep feeding paper and twigs. Then at some point the logs themselves catch, and you can stop.

Fusion works the same way. Reaching a hundred million degrees takes electricity spent on heating from outside.

But fusion makes helium, and helium carries charge, so the magnetic field holds it in. It stays and keeps heating the plasma — a stove burning inside the fire.

When that inner stove grows large enough that no outside heating is needed, the fire is said to have ignited. It is the moment the match can be taken away.

No magnetically confined machine has reached it yet. That is why the contest reported in the news is not about temperature but about how long the plasma can be held.

See alsoNuclear fusionPlasmaQuantum tunnelling

Inertia 관성 #

Physics

A bus pulls away and you lurch backwards; it brakes and you pitch forwards. Nobody pushed you.

Because the bus has started moving while your body is still trying to stay put. That tendency to carry on as before is inertia.

For two thousand years people believed the opposite: that motion needs continual pushing, since a cart stops when you stop pushing. But that is friction, a force, holding it back — not the nature of things. Without friction, one push lasts forever.

See alsoForceMassMomentum

Infinity 무한 #

Mathematics

Name the largest number. Whatever you say, add one and it is larger. So there is no largest number.

Infinity is not a very large number; it is the state of having no end. So it is never put into a calculation directly — you watch where things head as they go on without stopping (see limit).

Odd things follow. Infinitely many pieces can add to a finite total, as with the halved room. And in the nineteenth century it emerged that some infinities are larger than others — the man who proved it was fiercely attacked for it.

See alsoLimitAxiomProof

Infrared 적외선 #

Physics

Through a thermal camera, people glow brightly even in the dark.

Our bodies give off infrared all the time: invisible, but it is the warmth you feel when you bring your hand near your cheek. Remote-control signals and a heater's glow are infrared too.

Earth, warmed by sunlight, sends heat back to space as infrared, and carbon dioxide traps some of it, keeping the planet warm.

See alsoElectromagnetic waveThe greenhouse effect

Inheritance of acquired characteristics 획득형질 유전 #

Biology & Medicine

A giraffe stretches for high leaves, its neck lengthens, and the longer neck passes to its young. So Lamarck explained it.

It sounds plausible. It is not. Lifting weights all your life does not produce a muscular baby; losing a finger does not produce a child with nine. What happens to a body is not written into its genes.

Darwin's account runs the other way: some individuals happened to have longer necks, and those survived to leave more offspring. Not the fruit of effort but of selection. Lamarck was wrong, but he left something important — he was the first to say that living things change over time.

See alsoThe Weismann barrierEpigeneticsNatural selectionGeneUseful wrongs

Interference 간섭 #

Physics

Drop two stones into still water. Where the ripples cross, some places heave higher and others go perfectly flat.

Crest meeting crest grows; crest meeting trough cancels.

Two things overlapping and leaving nothing is the decisive evidence of a wave. Grains cannot manage it — two handfuls of sand poured on one spot give more sand, never less.

That is why electrons making stripes in the double slit was such a shock. Something taken for a grain did what no grain can do.

See alsoWaveThe double slitSuperposition

Interferometer 간섭계 #

Physics

How do you measure a difference of one part in a hundred million of a second? No clock will do it. But set light against light and you can.

A half-silvered mirror splits one beam in two. The halves travel down two arms at right angles, bounce back and are recombined. If the two paths took even slightly different times, crests and troughs slip out of step and interference fringes appear.

Instead of timing anything you count how far the fringes have shifted. Because the wavelength of light is so short, differences too small to picture are magnified into a pattern you can see.

Michelson built it to find the aether. The aether was not found, but the instrument remained: LIGO, which first detected gravitational waves in 2015, has exactly this design, with the arms grown to four kilometres.

See alsoInterferenceThe aetherGravitational wavesThe double slit

Ion 이온 #

Chemistry

Salt seems to vanish into water. Yet that water conducts electricity, while clean water hardly does.

The salt did not vanish; it split in two. One side hands over an electron and the other takes it. The giver is left positive, the taker negative. Charged like that, each is an ion.

An atom normally has equal positive and negative charge and so is electrically quiet. Let one electron come or go and that balance breaks.

Once something is an ion, it can be pushed and pulled by electricity. That is enormously useful in a laboratory: you cannot pick up a single particle with your fingers, but give it a charge and an electric field will push it and a magnet will bend it.

Weighing by mass spectrometry, and accelerators that drive nuclei close to the speed of light, both begin by making ions.

See alsoThe electronAtomElectrolysisMass spectrometry

Irrational number 무리수 #

Mathematics

Draw a square one metre on a side and measure the diagonal: about 1.41 m. With a finer ruler, 1.414. Finer still, 1.4142.

It never ends and never repeats — unlike a third, which repeats as 0.333… It simply cannot be written as a fraction.

The Pythagoreans held that everything is a ratio of whole numbers, and this number fell out of a theorem they had proved themselves. They had demolished their own belief.

See alsoProofLimitInfinity

Isotope 동위원소 #

Chemistry

Brothers who share a name but not a weight. Elements have those too.

A nucleus holds two kinds of thing: charged particles and neutrons. The count of the charged ones decides which element it is.

So a few neutrons more or fewer gives the same element at a different weight. That is an isotope.

Some are unstable and turn into something else after a short while, giving off radiation as they go. Making the short-lived ones that nature does not keep is what a heavy-ion accelerator is for.

See alsoThe neutronElementRadiation

Kepler's laws 케플러 법칙 #

Astronomy

Kepler spent years fitting Mars to a circle and nearly succeeded — the discrepancy was only eight arcminutes, a very small angle.

But he did not discard those eight minutes. He knew Tycho's observations were better than that. When the residual is larger than the limit of your instrument, it is not error but signal.

So he gave up the circle — a premise nobody had questioned for two thousand years. The answer was an ellipse.

The three laws: orbits are ellipses with the sun at one focus, not the centre; a planet moves faster when near the sun and slower when far; and the further out a planet is, the longer its year, by an exact relation. That third one later became the clue that yielded universal gravitation.

See alsoHeliocentrismUniversal gravitationError and uncertainty

Kinetic energy 운동에너지 #

Physics

Fall off a bicycle going twice as fast and you hit not twice but about four times as hard.

The faster a body moves, the more energy it carries — not in proportion to speed but to its square. Twice the speed, four times the energy; three times, nine.

This is why stopping distance grows so fast with speed: the energy the brakes must remove rises with the square. In the eighteenth century scholars argued long over whether this quantity followed speed or its square, and du Châtelet assembled the case for the square.

See alsoEnergy

Lactic bacteria 유산균 #

Biology & Medicine

Kimchi sours as it ripens. Stir a spoon of yogurt into milk, leave it a day, and it sets and sours. The sourness is what these bacteria leave behind.

They stand up well to salt and acid. So when cabbage is salted the other microbes retreat and only these remain to ripen the kimchi. The acid they make then keeps other bacteria out.

They live in the gut too. They are called "good bacteria", but no bacterium is good by nature. Where it is and what it eats decides whether the same bacterium helps or harms. The series "We Have Never Lived Alone" tells that story.

See alsoFermentationMicroorganisms

Light 빛 #

Physics

Put sunlight through a prism and a rainbow spreads out — meaning white light was several colours mixed together.

What separates the colours is frequency. And beyond both ends of the rainbow there is more light we cannot see: infrared past the red (what we feel as heat), ultraviolet past the violet.

What light is was argued over for centuries — particle or wave? The present answer is both, and that strangeness is where the quantum story begins.

See alsoWaveFrequencyQuantum

Light-year 광년 #

Astronomy

The word ends in year, so it sounds like time. A light-year is a distance — the same family as a kilometre.

Space is so wide that kilometres pile on zeros, so distance is counted by how long light was travelling.\n\nA bonus comes with it: to look at a star ten light-years away is to see light that left ten years ago. Measuring distance is automatically looking into the past.\n\nSo the further you look, the older what you see. Observing the distant universe is observing the young universe.

See alsoLightParallaxStandard candle

Limit 극한 #

Mathematics

Walk half way across a room. Then half of what remains. Then half again. Keep going — do you ever reach the wall?

You never arrive. Yet the total distance walked closes in on the width of the room. You do not reach it, but where you are heading is unmistakable.

That destination is the limit. You do not compute the value at arrival; you read where it is going. This is how we can speak of speed at an instant despite being unable to divide by zero.

See alsoInfinityFunctionIrrational number

Locality 국소성 #

Physics

Flip a switch in Seoul and a lamp in Busan does not light instantly. However fast, the signal takes time.

It seems obvious, but it is a principle physics must respect, since relativity forbids anything outrunning light.

This is why Einstein could not accept entanglement: if measuring here settles what is there, locality appears to break. He called it spooky action at a distance. The present answer: entanglement cannot send information faster than light, so relativity survives — but the idea that values were fixed in advance does not.

See alsoEntanglementSpecial relativityThe constancy of light speed

Magnetic field 자기장 #

Physics

Lay paper over a magnet, scatter iron filings, and a pattern of lines appears — though the magnet never touched them.

There seems to be nothing between the magnet and the filings. In fact that empty space has itself taken on a property, and the property is the magnetic field. The filings only make it visible.

The idea is that force does not leap between two bodies; the space between has already changed. Faraday, who had no mathematics, pictured it as lines (electromagnetic induction).

A charged particle winds around such a line like a spring and travels along it, unable to leave it. Fusion machines use exactly this. Close the lines into a doughnut and the plasma goes round for ever without reaching a wall.

See alsoElectromagnetic inductionPlasmaElectric charge

Magnetic resonance 자기공명 #

Physics

Push a swing at random moments and it stalls. Push in time with its return and it climbs.

Force only transfers when the timing matches. That is resonance.

Some nuclei have a spinning-top quality (spin). Placed in a very strong magnet, they wobble at a steady rate like a tilted top. Radio waves tuned to that rate get a response; anything off it gets none.

The use follows from one detail: the rate shifts very slightly depending on what sits around the nucleus. Read the returning signal and you can work out how the atoms in a molecule are joined — the arrangement, without ever seeing it.

Hospital MRI works this way, watching hydrogen nuclei in the water of the body. Its original name was nuclear magnetic resonance; the word nuclear sounded alarming and was dropped. No radiation is involved.

See alsoSpinThe atomic nucleusFrequency

Mass 질량 #

Physics

Push an empty trolley, then a full one. The full one is much harder to get going — and once it is going, harder to stop.

That difficulty is mass. It is easily confused with weight, but they differ. Weight is the force with which Earth pulls, so on the Moon it drops to a sixth — mass does not change.

Which is why objects floating on a space station still feel heavy to push. The weight is gone; the mass is not.

See alsoForceInertiaUniversal gravitation

Mass spectrometry 질량분석 #

Method & instruments

Push a ping-pong ball and a steel ball with the same force and the ping-pong ball flies much further. The push alone tells you which is heavier.

A mass spectrometer does exactly that.

First the substance is made into ions. Then electricity flings them along and a magnet bends their path. Light ones bend far, heavy ones bend little, and the amount of bending gives the mass.

Why does knowing the mass tell you what it is? Because every molecule weighs its own amount. Measured finely enough, the mass itself works like a fingerprint. Break the molecule into pieces, weigh the pieces too, and you can work out how they were joined.

Its strength is catching very small amounts: doping tests on athletes, pesticide left on fruit, microplastics in seawater, proteins in the body. Quantities too small to see or smell are pinned down as numbers.

See alsoIonMoleculeIsotope

Matter waves 물질파 #

Physics

Light was a wave and turned out to be a grain as well. Then should the electron, a grain, not also be a wave? De Broglie's argument was really no more than that.

Astonishingly, it was right. Fire electrons at a crystal and interference stripes appear — a pattern no grain could make.

Why then does a person not look like a wave? Because wavelength runs inversely to momentum. The heavier and faster something is, the shorter its wavelength; a person's is far smaller than an atom, beyond any means of measuring.

The rules do not change in the small world. The rules are the same; we are simply too large.

See alsoWaveA standing waveThe double slit

Maxwell's equations 맥스웰 방정식 #

Physics

Electricity and magnetism were long taken to be separate: one came from rubbing, the other from a stone that pulled iron.

But they are one thing. A changing electric field makes a magnetic one; a changing magnetic field makes an electric one. Maxwell set the relationship down in four equations.

Solving them produced something unforeseen: there must be a wave in which the two fields generate each other as they travel — and its speed matched the already-measured speed of light. That is where light was identified.

See alsoLightWaveElectromagnetic induction

Method of least squares 최소제곱법 #

Mathematics

You measure the same desk five times and get 100.2, 99.8, 100.1, 100.4 and 99.9 cm. What is its true length?

Pick a candidate value and see how far each of the five measurements is from it. Square each miss and add them up. The candidate with the smallest total wins; here it is simply the average.

Squaring does two jobs: misses above and below cannot cancel out, and big misses are punished more. Gauss used it to predict where the lost asteroid Ceres would reappear. Today it is everywhere, from drawing the best straight line through data to training machine-learning models.

See alsoProbability

Microorganisms 미생물 #

Biology & Medicine

Leeuwenhoek looked at a drop of rainwater through a lens he had ground himself. Something in it was swimming.

A drop of water, scrapings from his teeth, an infusion of pepper — they were everywhere. He called them little animals.

That the world is full of living things we cannot see changed everything after it. Why illness arises, why food spoils, how wine ferments: all the work of these small things. He was a draper with no university education, and never told anyone how he made his lenses.

See alsoThe cellGerm theorySpontaneous generation

Molecule 분자 #

Chemistry

Halve a cup of water. Halve it again. Keep going and you reach a point where it cannot be divided further — divide once more and it stops being water.

One grain at that point is a molecule. A water molecule is three atoms stuck together; pull them apart and you no longer have water but oxygen and hydrogen.

You cannot see them, but they are never still. They shake, collide and wander. Hot means that motion is vigorous; cold means it is quiet.

See alsoAtomElementHeat

Momentum 운동량 #

Physics

A billiard ball hits a stationary one head-on: the first stops and the second rolls away, as if the motion had been handed over whole.

The amount of motion handed over is momentum: how heavy (mass) times how fast.

What matters is that the total before and after a collision is the same. Which is why a truck meeting a bicycle has an obvious outcome — the truck's momentum dwarfs it. A rocket moves for the same reason: it goes forward by exactly as much as it throws backward.

See alsoMassForceConservation law

Mutation 돌연변이 #

Biology & Medicine

Every time a cell divides it copies the whole genetic text — some three billion characters. However careful, it occasionally slips.

That slip is a mutation. Easy to think of as damage, but in evolution it is the raw material.

Most do nothing or do harm. Just occasionally one is useful, and only that one is caught by selection. Note what this means: mutations do not happen with a purpose. They are not produced because they are needed; they simply occur, and the fitting ones remain. The making is blind; only the choosing sees.

See alsoGeneNatural selectionTransposable elements

Nanometre and the nanoscale 나노미터와 나노 크기 #

Method & instruments

Look at a single hair. It seems thin, yet it must be split a hundred thousand ways to reach one nanometre.

A metre divided by a billion is one nanometre. The size means little on its own, so compare it.

An atom is about 0.1 nanometres across, so a nanometre is a few atoms side by side. A strand of DNA is about two; a virus is around a hundred.

What makes this scale special is not smallness. It is that the properties of matter begin to change here.

Atoms that sat buried inside a lump end up almost entirely on the outside once the grain shrinks to nanometres (surface area), and atoms on the outside behave differently from atoms within. So the same gold is yellow as a lump and red as nanoparticles.

Nanotechnology is less the handling of small things than the changing of properties by changing size.

See alsoSurface areaAtomSemiconductor

Natural selection 자연선택 #

Biology & Medicine

An antibiotic kills most of the bacteria. But if a few happen to withstand it, those are the ones that breed. A few generations later the drug no longer works.

That is natural selection. Nobody intends it. What survives remains, and what remains multiplies.

Only three conditions are needed: individuals differ a little, those differences pass to offspring, and not all survive. Given those three, direction appears on its own. Darwin was wary of 'survival of the fittest' for good reason: what remains is not the strongest but whatever suited that environment — and when the environment changes, so does the answer.

See alsoMutationTraits, dominant and recessiveThe modern synthesis

Negentropy 네겐트로피 #

Physics

The inside of a refrigerator gets colder: disordered heat is carried out and the inside becomes more ordered. Entropy inside it falls.

It is not free. The back of the machine heats up and it draws electricity: more entropy is added outside than removed inside, so the total still rises.

Life does the same. Gathering scattered materials into a body lowers entropy, and the bill is paid by eating and breathing. Schrödinger put it in 1944 as life feeding on negative entropy.

One thing must be underlined. While entropy falls inside the refrigerator, time inside it still runs forward — the milk is a day older. Entropy decreasing and time running backwards are not the same sentence.

See alsoEntropyThe arrow of timeThe second law of thermodynamics

Nitrogen 질소 #

Chemistry

Most of the air we breathe is not oxygen but nitrogen.

Two nitrogen atoms grip each other with all three hands, so they rarely react. Nitrogen goes in with a breath and out unchanged, and stays in the air for a very long time.

Yet life needs it: proteins and DNA contain nitrogen. Freeing that tight nitrogen into a usable form is done by microbes in the soil, by lightning, and by fertiliser factories.

See alsoMoleculeValence

No-signalling 신호 불가 #

Physics

Share a pair of entangled particles and measuring one settles the other at once. Surely that is communication faster than light?

It is not. Each result is entirely random, and since no chosen value can be forced, nothing can be loaded onto it.

The far side sees only a string of random outcomes. That the two are tied shows up only when the records are compared afterwards, and comparing them takes a telephone or an email like anything else.

So teleportation by entanglement, or entangled minds, is not what the experiments showed. Entanglement is a correlation between two outcomes, not a sending of anything.

See alsoEntanglementLocalityThe constancy of light speed

Novae and supernovae 신성과 초신성 #

Astronomy

In 1572 a star appeared in Cassiopeia where none had been. It shone as brightly as Venus and was visible in daylight. Eighteen months later it was gone.

This mattered because the heavens were supposed to be perfect and unchanging. Change belonged below the moon, to the earthly world.

Tycho measured the new star's parallax: if it were nearby, its position should shift slightly as the Earth turns. However precisely he measured, it did not move — meaning it lay far beyond the moon.

The heavens do change. We now know this is a star exploding at the end of its life, and that the elements from such explosions make up our bodies.

See alsoParallaxError and uncertaintyElement

Nuclear fission 핵분열 #

Physics

Weigh the two fragments and they total less than the original nucleus. Where did the missing mass go?

Into energy. As special relativity says, mass is itself a form of energy. The missing mass is tiny, but the conversion rate is so large that the result is enormous.

The person who worked this out was Lise Meitner, in exile in Sweden, told of the experiment by letter. She named the process fission. The 1944 Nobel Prize went to her experimental colleague alone.

See alsoThe atomic nucleusMassEnergy

Nuclear fusion 핵융합 #

Physics

The Sun has burned for four and a half billion years with no wood and no oil. No fuel lasts like that.

The Sun is not burning. Its nuclei are colliding and merging, and heat comes out at the moment they merge.

It is the reverse of fission: heavy nuclei give out energy when split, light ones when joined, because the steadiest place is in between.

Doing it on the ground needs a hundred million degrees — and nothing can hold something that hot. That is the problem so far.

See alsoNuclear fissionThe atomic nucleusPlasma

Numerical forecasting 수치예보 #

Method & instruments

Cut the sky into squares like a game board. Write today's temperature, wind and moisture into each. Use physical law to work out the values ten minutes on — then ten minutes again, and again.

A forecast is not divination but arithmetic: cut the sky into cells, apply the laws air obeys, and push the whole thing forward.\n\nSmaller cells catch a single valley or ridge and give a better answer — but halving the cell size multiplies the work more than tenfold. Hence the need for a very large calculator.\n\nAnd because of chaos, one run cannot be trusted. So the sums are run dozens of times from slightly different starting values, and the share of runs that produced rain is the probability of precipitation.\n\nSixty percent does not mean rain over sixty percent of the area, nor for sixty percent of the time. It means that in ten situations like this one, six of them bring rain.

See alsoChaosSimulationProbability

Orbit 궤도 #

Astronomy

Throw a ball hard and it lands far off. Throw it harder and it lands further off.

The Earth is round. While the ball falls, the ground beneath curves away. Throw hard enough and the ground runs away as fast as the ball drops, and it never lands.

So being in orbit is not floating. It is falling continuously and missing continuously.

People drift inside a space station not because gravity is absent but because they and the station fall together, so neither presses on the other.

See alsoUniversal gravitationMass

Orbital 오비탈 #

Chemistry

Textbook atoms usually look like a solar system, electrons circling a nucleus. Further into a chemistry book, odd dumbbell and doughnut shapes appear instead.

The later pictures are the right ones. Electrons do not travel in orbits.

Solving the wave function gives the probability of finding the electron in each place, and an orbital is a drawing of where that probability is dense. Not a path travelled but a cloud of likely locations.

That these come in several shapes is where chemistry begins. Which way a dumbbell points sets the angle of a chemical bond, the angle sets the shape of a water molecule, and that shape sets what water is like.

See alsoThe wave functionAtomChemical bond

Oxidation 산화 #

Chemistry

Leave an iron nail near water and it rusts red and grows heavier. Wood burns and grows lighter. Two things that look opposite are the same process.

Both are oxygen from the air attaching itself. Nothing leaves; something joins.

Wood seems lighter only because more escapes as smoke and vapour than attaches. Seal it in a vessel and weigh the whole and nothing changes. That experiment ended phlogiston. What was decisive was putting it on a balance instead of merely looking.

And the way we breathe and turn food into energy is the same reaction, running slowly.

See alsoConservation of massPhlogistonElement

Parallax 시차 #

Astronomy

Hold up a finger and close one eye, then the other. It jumps sideways against the background. Stretch your arm out and the jump gets smaller.

The gap between your eyes is the baseline; the jump is the angle. Further away means a smaller angle, so measuring the angle gives the distance.

The same method serves for stars — except two eyes are nowhere near enough, so the baseline is the distance the Earth travels in half a year. Even then the angle is under one arcsecond.

Tycho failed to find it and so rejected heliocentrism. His logic was sound: no parallax means either the Earth does not move or the stars are unimaginably far. Bessel found it in 1838. Tycho had not been wrong — the stars really were that far.

See alsoHeliocentrismStandard candleError and uncertainty

Parity violation 패리티 깨짐 #

Physics

In the mirror world, physics should surely work the same: balls fall, water flows, magnets stick — only left and right are swapped.

Until 1956 everyone believed it. Then Wu actually measured: she aligned radioactive nuclei at very low temperature and counted which way the emitted electrons preferred to go.

One side won. Nature does have a handedness. It was an experiment that measured what nobody had thought worth measuring and overturned a certainty — and the following year's Nobel Prize went only to the two men who had proposed the theory.

See alsoRadioactivitySymmetry and Noether's theoremConservation law

Pauli exclusion principle 파울리 배타원리 #

Physics

On board 6 of the element tool, tap a full room and a third electron bounces off.

In one room, one orbital, an electron's shape, place and energy are all the same. Only the direction of spin can differ, and it has just two values. So a room seats two.

Wolfgang Pauli found it in 1925. Without it every electron would crowd into the lowest room, elements would not differ, and there would be no periodic table.

See alsoSpinOrbitalElectron shell

Peer review 동료평가 #

Method & instruments

Mary Anning found the first ichthyosaur and plesiosaur skeletons. Her name appears on none of the papers written from her fossils. As a woman she could not join the Geological Society, which recognised her neither as member nor as author.

Peer review is a good institution. It filters out wrong claims, thin data and overstated conclusions, because others see the holes you cannot.

But what gets filtered depends on who counts as a peer. Anning is one case; McClintock going unrecognised for thirty years is another; results that contradict received opinion are always accepted late for the same reason.

It is not trustworthy because it is perfect. It is one layer among several, and works properly only alongside reproducibility.

See alsoReproducibilityFalsificationUseful wrongs

Phlogiston 플로지스톤 #

Chemistry

Wood burns down to a little ash. Something has clearly left — flown off as smoke and flame.

That something was named phlogiston: burning is phlogiston leaving.

A plausible account built on what you can see. Except that burnt metal weighs more. Nothing that leaves can make a thing heavier — which led to the contrivance that phlogiston has negative weight. That is what a theory tends to look like just before it falls: a growing pile of assumptions added to cover exceptions.

See alsoOxidationConservation of massUseful wrongs

Photosynthesis 광합성 #

Biology & Medicine

A big tree seems to have grown by eating soil, yet most of its body came from the air.

Carbon dioxide comes in through tiny leaf pores, water up through the roots; sunlight's energy splits and rebuilds them into sugar, and the leftover oxygen goes out. With that sugar the plant builds itself.

Most of the oxygen in Earth's air was made this way. The first to do it were not plants but cyanobacteria in the sea.

See alsoCarbon dioxideCyanobacteriaPhytoplankton

Phytoplankton 식물성 플랑크톤 #

Biology & Medicine

Too small to see, they make about half of Earth's oxygen.

What forests are on land, these are at sea. In sunlit shallow water they take in carbon dioxide and give out oxygen. Small animals eat them, fish eat those, whales and people eat fish: the bottom of the ocean food chain.

When they die they sink with their carbon into the deep, burying air's carbon under the sea for a long time.

See alsoPhotosynthesisCyanobacteria

Place-value notation 위치기수법 #

Mathematics

In Roman numerals, 3888 is MMMDCCCLXXXVIII — fifteen characters. Now try multiplying with it. It is close to impossible.

Our way is different. The same 3 means three, or thirty, or three hundred, depending on where it stands. Position sets value.

So you can line the digits up in columns and work down. Multiplication and division become procedures anyone can follow. Roman numerals needed a calculating specialist; this needs a schoolchild.

Calculation went from a talent to a procedure. And none of it works without zero.

See alsoZeroAlgebraAlgorithm

Planck's constant 플랑크 상수 #

Physics

0.00000000000000000000000000000000066 — thirty-three zeros after the point. Why should such a number matter?

That this number is small is itself the answer to why the quantum world is invisible to us.

A playground swing is a staircase in principle too, but one rung is this small, so a single push carries it up trillions of rungs at once. Naturally it looks like a smooth ramp.

The laws do not change in the small world; the steps are simply too fine relative to our size. Were h large, a baseball would make stripes in a double slit.

See alsoQuantisationQuantumEnergy

Plasma 플라스마 #

Physics

Water becomes ice, water, steam. So what does steam become if you heat it further?

Heat it more and the atoms come apart: electrons leave, and what remains drifts separately. That state is plasma.

It is sometimes called the fourth state. It looks exotic, but most of the visible universe is plasma — stars are.

What matters is the charge. Anything charged can be pushed by magnets, so it can be held in mid-air away from any wall. Fusion machines rely on this.

See alsoNuclear fusionAtomSuperconductivity

Plate tectonics 판구조론 #

Earth Science

Earthquakes and volcanoes do not happen just anywhere. Plot them and they line up along narrow belts — the ring around the Pacific most clearly of all.

Those belts are plate boundaries. The Earth's outer shell is not one piece but several, and they push, part and slide past one another. They collide at the edges, and that is where earthquakes and volcanoes occur.

Here was the mechanism Wegener lacked. Continents do not slide over the bedrock; the bedrock itself moves as plates. New crust rises at mid-ocean ridges and spreads outward, while elsewhere one plate descends beneath another.

The decisive evidence was magnetic striping on the sea floor. The Earth's magnetic field reverses from time to time, and those reversals are recorded in the ocean rock as stripes symmetrical about the ridge — a record of the sea floor spreading apart.

See alsoContinental driftStrataDeep time

Polymer 고분자 #

Chemistry

One bead is a bead. Thread tens of thousands of them and you no longer have a necklace but a rope.

Join one molecule to itself thousands of times and the properties change entirely: soft becomes tough, flowing becomes solid.

Plastics, rubber and nylon are all this. So are the proteins and DNA in our bodies.

So polymer research is about what to join, how long to make the chain, and in what shape — including making plastics that break down.

See alsoMoleculeProteinCatalyst

Prime numbers 소수 #

Mathematics

Twelve sweets divide evenly many ways — by two, three, four, six. Thirteen will not divide at all: one person takes all, or thirteen take one each.

Those are the primes: 2, 3, 5, 7, 11, 13, 17. Being unsplittable makes them the parts numbers are built from, and every number is a product of primes in exactly one way.

The odd thing is that no pattern shows in where they fall — gaps of 1, 2, 2, 4, 2, 4. Yet they are not arbitrary either; Euclid proved they never run out.

Neither random nor plain repetition, which is why they were chosen as a first greeting to anyone out there: the rule is visible only to something that knows how to count.

See alsoProofInfinityAlgorithm

Probability 확률 #

Mathematics

You cannot say what a die will show before you roll it. Yet roll it six hundred times and each face turns up around a hundred times. Each throw is unknown; the whole is not.

Probability writes that rule of the whole as a number. One in six does not predict a single roll; it says that over many rolls the proportion closes in on that value.

Insurance premiums, chance-of-rain forecasts and drug trials all rest on this. And in the quantum world, the discovery that probability is a property of nature rather than a mark of our ignorance set off a long argument.

See alsoError and uncertaintyQuantumFunction

Probability amplitude 확률진폭 #

Physics

With dice, opening a second route raises your chance of arriving; it cannot lower it. In the double slit, opening a second slit creates places where electrons stop arriving.

Probabilities are never negative, so adding them can never produce that cancellation.

Hence a two-stage calculation: assign each route a quantity that can cancel when added, sum those, and only then square the total into a probability. That intermediate quantity is the amplitude.

The order is everything. Add then square and interference appears; square then add and it does not. The whole difference between quantum mechanics and ordinary probability sits in that swap.

See alsoProbabilityInterferenceThe wave function

Proof 증명 #

Mathematics

You measure a hundred right triangles and Pythagoras' theorem holds for every one. Can you now say it holds for all of them?

No. The hundred-and-first might differ, and you cannot measure them all.

A proof lets you say it without measuring. Derive it by logic from a few starting points (axioms) and you can speak with certainty about triangles nobody has drawn.

This is where mathematics parts from surveying. The Egyptians made right angles with 3-4-5 and never asked why. Between knowing how to use something and knowing why it works stands proof. And proof runs the other way too — showing that something cannot exist, as with irrational numbers.

See alsoAxiomThe incompleteness theoremsIrrational number

Protein 단백질 #

Biology & Medicine

Proteins both build the body and do its work. Muscle is protein; so is what digests your lunch.

How can one kind of thing do so much? Shape.

Amino acids are strung together and then fold, each in its own way, and the folded shape decides the job. Some end up like locks, some like channels.

A medicine working is a story about shape too: finding a key that fits one protein. What protein to make is written in a gene.

See alsoGeneThe cellPolymer

Proton 양성자 #

Physics

Hydrogen, number 1, has one proton; carbon, number 6, has six; element 118 has 118.

The number of protons sets an element's name; add one and it becomes a different element. Neutrons and electrons can change without changing the name, but protons cannot. So the periodic table is not a ranking but a count of protons.

Protons are all positive and repel one another, but in the nucleus they are held together with neutrons by the strong nuclear force.

See alsoThe neutronThe atomic nucleusAtom

Public-key cryptography 공개키 암호 #

Method & instruments

A pile of open padlocks is left at the post office for anyone to take. Anyone can snap one shut; the key that opens it stays in my pocket.

So a stranger can send me a secret without our having met or agreed anything in advance.

This happens all day on the internet. Connect to a bank and you are handed the bank's padlock, snap it shut, and send. Whoever is watching in between cannot open it.

How is such a lock built? From a calculation that is easy one way and hard the other.

Multiplying two large primes is easy. Recovering those two numbers from the product alone, if the numbers are large enough, takes a very long time even with every computer on Earth. Multiplying is the locking; recovering is the opening.

See alsoEncryptionPrime numbersAlgorithm

Pulsar 펄서 #

Astronomy

Bell Burnell spotted a signal in the chart recordings repeating exactly every 1.3 seconds. It was so regular that an artificial source was suspected; in the lab it was jokingly labelled LGM, for little green men.

A second one in another direction ended the alien hypothesis — two separate civilisations would not both be signalling us.

The explanation: after a large star explodes (supernova), the remaining core is compressed enormously, packing the mass of the sun into something the size of a city. It spins very fast, sweeping out radio beams like a lighthouse, and each time a beam crosses the Earth we register a pulse.

The 1974 Nobel Prize went to her supervisor.

See alsoNovae and supernovaeThe atomic nucleusPeer review

Quantisation 양자화 #

Physics

A tap can be opened by any amount. A vending machine cannot: if the smallest coin is 100 won, a drink priced at 150 cannot be sold.

Energy in the small world is a vending machine, not a tap: it moves only in fixed units. A quantity restricted this way is said to be quantised.

One consequence matters a great deal. If the price of a single unit cannot be met, the transaction does not happen at all — not faintly, but not at all. That is why a hot stove does not pour out ultraviolet.

The size of one unit is the frequency times Planck's constant.

See alsoQuantumPlanck's constantThe quantum jump

Quantum 양자 #

Physics

A tap lets you adjust the flow by any amount you like. A staircase does not: you cannot stand on half a step. One step, or two.

The very small world turned out to be a staircase, not a tap. Energy is not handed over in arbitrary amounts but in lumps of a fixed size. One such lump is a quantum.

Nobody knew why. Planck, who first assumed it, took it for a bookkeeping trick and spent years trying to undo it. It was how nature actually works.

See alsoEnergyLightSuperposition

Quantum tunnelling 터널 효과 #

Physics

To roll a ball over a hill you need as much push as the hill is high. Short of that it climbs and rolls back. A hundred tries, the same result.

But in the very small world that ball sometimes simply appears beyond the hill. It did not climb over and did not push through; it is just on the other side.

The reason is that a particle is not a speck at a definite spot but something smeared out, like a wave (its wavefunction). It is faintly present beyond the hill as well, and being faint, it is found there only rarely.

This is why the Sun burns. At the temperature inside it, nuclei come nowhere near enough speed to beat the push between them. By the arithmetic the Sun should not burn at all. But now and then one gets through, and the Sun is vast enough to hold an unimaginable number of chances.

The rarity is our good fortune. Were it easy, the Sun would have burned out long ago. The reason fusion is hard and the reason the Sun lasts are the same reason.

See alsoNuclear fusionThe wave functionThe uncertainty principle

Quark 쿼크 #

Physics

As accelerators grew in the 1950s, new particles poured out — past a hundred names. Fermi is said to have remarked that if he could remember them all he would have been a botanist.

A hundred fundamental particles are not fundamental. As Mendeleev had arranged elements in a table, Gell-Mann arranged these by property — and found the table falls out by itself if three smaller things combine. Those three are quarks.

Two oddities. A quark's charge is one third or two thirds. And nobody has ever pulled one out alone: pull harder and the effort itself creates a new pair.

See alsoThe atomic nucleusElectric chargeAtom

Radiation 방사선 #

Physics

You cannot see it or smell it. No sense of ours detects it at all.

Which is part of why it frightens us more than other hazards: we cannot notice it.

But what our senses miss, instruments catch very well. Radiation is among the easiest dangers in the world to measure, down to tiny amounts.

It is easily confused with radioactivity: radioactivity is the capacity to emit, radiation is what comes out. In candle terms, the candle and the light.

See alsoRadioactivityIsotopeNuclear fission

Radioactivity 방사능 #

Physics

A photographic plate left in a drawer came out blackened, with no sunlight anywhere near. A piece of uranium ore had been lying beside it.

The ore was giving something off by itself, untouched.

Some nuclei are unstable and break down of their own accord, throwing off fragments and radiation and turning into a different element. Alchemy failed because chemistry cannot change one element into another — yet nature was doing exactly that, at the level of the nucleus.

See alsoThe atomic nucleusHalf-lifeElement

Random number 난수 #

Method & instruments

Roll a die and nobody knows what comes next. But ask a computer for any number at all — a computer is a machine that only follows rules.

There is a difficulty here: a number produced by a rule is predictable to anyone who knows the rule.

So the numbers an ordinary computer produces are not random but random-looking. Good enough for a game, dangerous for cryptography. If the key can be guessed, the lock is worthless.

Several real breaches happened exactly here — not because the cipher was weak but because the way keys were drawn was predictable.

So unpredictable natural events are borrowed instead: electrical noise in a circuit, the moment a mouse moved, radioactive decay whose timing is fixed by nothing. Chance cannot be manufactured, so it is taken from nature.

See alsoProbabilityEncryptionSimulation

Redshift 적색편이 #

Astronomy

An approaching ambulance sounds high-pitched; once past, the same siren sounds lower.

An approaching sound has its waves bunched up in front (a higher frequency); a receding one has them stretched. Light is a wave too, so the same happens: light from a receding star is stretched towards the red.

The measurement is clever. The spectral lines in starlight sit at fixed positions for each element. If the whole pattern is displaced towards the red, the displacement gives the speed of recession.

Hubble measured this across many galaxies and found that the further away a galaxy is, the faster it is receding. The universe is expanding — and run backwards, that implies a beginning from a point.

See alsoSpectral linesThe cosmic microwave backgroundStandard candle

Reproducibility 재현성 #

Method & instruments

When Leeuwenhoek wrote that things were swimming in a drop of water, the Royal Society did not believe him — nobody had seen such a thing. So they sent someone to look.

One person's sighting is not enough. They might have misread it, the instrument might have been faulty, they might be lying.

So science publishes not only results but methods, so that others can follow them. If they follow and get something else, the claim is in trouble.

This is a live problem. When famous studies in psychology and medicine were repeated, only about half came out the same. Part of the cause is that only successful studies get published. The failures have to be published too for the picture to be right.

See alsoPeer reviewControl groupStandardisation

Resolution 해상도 #

Method & instruments

Some stars look single to the eye and double through a telescope. The star did not split; it was always two.

That is resolution. Not how large something is made to look, but whether two things close together can be seen as two.

The two are easily confused. Stretch a blurred photograph with your fingers and you get a large blurred photograph; nothing appears that was not there. Magnifying without resolving is called empty magnification.

Microscopes and telescopes are alike in this. Better lenses help up to a point, and beyond that point lies a wall no craftsmanship can cross. The wall is set by wavelength.

So those who want to see smaller things do not grind better lenses. They look with something else entirely — with electrons instead of light.

See alsoWavelengthThe telescopeError and uncertainty

Retrograde motion 역행운동 #

Astronomy

Track Mars at the same hour for months and it drifts slowly eastward. Then, roughly every two years, it halts, travels back westward for some weeks, halts again and resumes.

The planet traces a loop across the sky. It was the hardest problem ancient astronomy had.

If the Earth stands still while planets circle it in one direction, this cannot happen — hence the device of the epicycle.

Under heliocentrism no device is needed. The Earth, on the inner track, overtakes Mars, and Mars briefly appears to slip backwards — as a slower car in the next lane appears to move backwards when you pass it.

One account has to add machinery for this; the other gets it for free. The question of how to choose between theories of equal accuracy starts here.

See alsoEpicyclesGeocentrismHeliocentrismParallax

Riemannian geometry 리만 기하학 #

Mathematics

Two people set off north from the equator, side by side. They walk parallel, and meet at the pole. But parallels never meet, we were told.

On flat paper it is true; on a curved surface it is not. Bend the floor and the geometry changes.

Riemann did not treat one particular curved surface but built a general method for any curvature. Instead of one ruler for the whole space, give each point its own — and how much the rulers differ from place to place is how much the space curves.

This was 1854, and nobody knew what it was for. Sixty-one years later, when Einstein wanted to describe gravity as curvature, the language was already written.

See alsoGeneral relativityAxiomUniversal gravitation

Saltpetre 염초 #

Chemistry

Put a glass over a candle and it goes out — the oxygen runs out. Yet gunpowder fires underwater, and inside a sealed gun barrel.

Because it can burn without outside air: it carries its own oxygen. Saltpetre is the ingredient that supplies it.

Charcoal and sulphur are easy to come by; saltpetre was the problem, being no mineral you can dig. It gathers slowly in the soil under old floors and in animal pens, and must be collected, leached, boiled down and filtered into crystals. Which soil, how many leachings — all of it knack, which is why the method itself was a state secret.

See alsoOxidationElementMolecule

Scale and symbols 축척과 기호 #

Earth Science

For a map to be a tool rather than a picture, two things are needed: measuring it must give a distance, and a mark must tell you what a thing is.

The Daedongyeojido put a dot every ten li along the roads: no ruler needed, just count the dots. Walled towns, beacon posts, relay stations and granaries each had their own mark, so the map could be read without writing everything out.

One more thing: it was cut into woodblocks. Hand copies drift a little with every copying; printed sheets are identical however many you make. Standardisation applied to maps.

And it was cut into twenty-two strips that fold like screens — 6.7 metres tall unfolded, the size of a book folded. The map stopped being an office's possession and became a traveller's.

See alsoStandardisationStrataDeep time

Schrödinger's cat 슈뢰딩거의 고양이 #

Physics

A cat, a radioactive atom and a flask of poison that breaks if the atom decays, all in one box. After an hour the odds of decay are even.

Correct the common misreading first: Schrödinger did not use the cat to explain superposition but to mock it.

His point: by your rules the atom is a superposition of decayed and not decayed, and since the cat's life is tied to the atom, the cat is a superposition of alive and dead. Does that make sense?

The counter-example became the emblem of the theory instead. His aim was exact: where does the quantum end and the ordinary begin (the measurement problem)? That line is still written nowhere.

See alsoSuperpositionThe measurement problemEntanglement

Sea-floor spreading 해저확장 #

Earth Science

Surveys of the sea floor turned up something odd. Continents carry rocks four billion years old, yet nowhere is the ocean floor older than two hundred million years. Why is the sea bed so young?

Because it is continually being made. A vast range runs down the middle of the oceans, and magma rises through the rift at its crest, freezes and is pushed aside. At the far margin one plate sinks beneath another. Old floor is erased and new floor laid down.

This is the engine Wegener lacked. Continents do not plough through solid oceanic crust; the floor itself moves like a conveyor, and the continents ride on it.

Confirmation came from magnetic stripes. The Earth's field reverses every few hundred thousand years, and rock freezing at a ridge records the direction of the moment. If the picture is right there must be mirror-image stripes on either side — pairs frozen together and carried apart. Vine and Matthews found them in 1963: a prediction made first and confirmed after, as cleanly as such things ever go.

With this, plate tectonics was complete.

See alsoPlate tectonicsContinental driftDeep time

Seismic waves 지진파 #

Earth Science

You tap a watermelon and know whether it is ripe, without cutting it open.

Because the sound changes as it passes through. The Earth is examined the same way.\n\nAn earthquake sends waves right through the planet. Recording where they speed up, where they bend and where they cannot pass at all, from stations around the world, draws out the layers inside.\n\nThere are two kinds and they travel at different speeds: the fast one arrives first, the harder-shaking one later. That gap is what earthquake early warning is built on.

See alsoWavePlate tectonicsStrata

Semiconductor 반도체 #

Physics

Some things conduct electricity and some do not. What is the in-between good for?

The in-between is exactly the useful part: it can be made to conduct or to block — and by another electrical signal rather than a hand on a switch.

A switch operated by electricity is a transistor, and a computer is billions of them.

The material is usually silicon, from sand. It is purified to an extreme and then given a trace of another element, and that trace changes everything.

See alsoCurrent and the batteryElement

Serendipity 뜻밖의 발견 #

Method & instruments

A dish left over a holiday had grown mould. Normally it would be thrown out. But around the mould, no bacteria were growing.

Antibiotics arrived this way. They were not what anyone was looking for.

Such things fill the history of science. The microwave oven came out of work on radar; a glue that deliberately does not stick well came from an attempt at a strong one.

But chance is not the whole of it. Thousands could have seen that dish and nearly all would have discarded it. Recognising that something is odd takes an eye that has watched the field for a long time.

So serendipity is less luck than something visible only to the prepared.

It is also why basic research cannot be selected in advance. If you knew what would come out, it would not be a discovery.

See alsoBasic and applied researchHypothesisUseful wrongs

Simulation 시뮬레이션 #

Method & instruments

What happens when two galaxies collide? You cannot run the experiment: it takes a billion years, and you have no galaxies.

So it is computed instead: put in the rule for how things pull on each other, work out the result after a very short interval, take that as the new starting point, and step again. Repeat it enough times and a billion years go by.\n\nIt is the third method, after thinking (theory) and doing (experiment). It exists for what cannot be experimented on.\n\nBut wrong rules give wrong results. That a computation is exact is not the same as its answer being right.

See alsoAlgorithmProbability

Singularity 특이점 #

Astronomy

Calculate the centre of a black hole and the density comes out infinite. So does the curvature of spacetime.

An infinity usually means the theory has stopped applying there — as classical physics stopped at the ultraviolet catastrophe.

So a singularity is less a claim that an infinitely small point exists than a marker saying: from here we do not know.

Treating it properly needs general relativity and quantum theory together, and no theory yet joins them. The same wall stands in the way of calculating the beginning of the universe.

See alsoThe event horizonGeneral relativityQuantum

Special relativity 특수상대성이론 #

Physics

One twin travels in a very fast spacecraft and returns to find the other has aged more.

Not a joke — it happens. Precision clocks have been flown and compared with ones left on the ground, and they disagreed by exactly the predicted amount. GPS satellites must be corrected for this daily or positions drift by kilometres.

The starting point was one premise: light has the same speed for everyone. Accept it and the conclusion follows logically that time and length are not absolute — and that mass itself is a form of energy.

See alsoThe constancy of light speedMassGeneral relativity

Spectral lines 스펙트럼선 #

Astronomy

Sunlight through a prism spreads into a rainbow — and looked at closely, thin dark lines run through it. Fraunhofer counted 574 of them.

Why? Electrons in an atom move only between fixed rungs (quantum jumps). The gaps are fixed, so the colours exchanged are fixed. Each element has its own combination of colours.

As sunlight passes through the sun's outer layers, the elements there take their own colours out of it, and the missing colours show as dark lines.

So reading the positions tells you the composition without going there. In 1835 Comte is said to have offered the chemical composition of the stars as an example of the permanently unknowable. The answer had been in Fraunhofer's notebook for twenty years.

See alsoAtomThe quantum jumpRedshift

Spin 스핀 #

Physics

The name says spinning, like a top. But nothing is turning.

Why not? Because its values come in halves, which no rotating object could give. Yet it is not meaningless: it responds to magnets, it has direction, and two of them add by a rule.

It was inserted by hand to fit experiment — and then fell out on its own when Dirac combined relativity with the quantum. Only then did it find its place. Because of it, electrons cannot pile into the same state, and that is why matter takes up space.

See alsoQuantumThe quantum jumpThe periodic table

Spontaneous generation 자연발생설 #

Biology & Medicine

Leave meat out for a few days and maggots appear. Leave a sack of grain long enough and mice appear. Nobody put them there.

It was a reasonable belief — it was what you could see.

Pasteur ended it. He boiled broth in a flask with a long swan-neck: air could pass, but microbes settled in the bend and never reached the liquid. Months passed and nothing grew. Snap the neck off and it spoiled within a day.

Nothing had arisen by itself. Things had come in from outside.

See alsoMicroorganismsGerm theoryThe cell

Standard candle 표준촛불 #

Astronomy

A light in the distance at night: a torch nearby or a lighthouse far off? You cannot tell whether it looks faint because it is dim or because it is distant.

But if you know it is a 100-watt bulb, you can compute: a 100-watt bulb this faint is at such a distance.

No star has its wattage written on it, so astronomers needed stars that announce it. Cepheid variables do: the period of their blinking gives their true brightness.

Parallax runs out at the nearest stars; standard candles reach far beyond. Measure the near ones by parallax, calibrate the candles against them, then reach further. If the bottom rung is wrong, everything above it is wrong.

See alsoCepheid variablesParallaxRedshift

Standardisation 표준화 #

Method & instruments

How do you measure how much rain fell? The old way was to dig after the rain and see how deep the soil was wet. But sand and clay drink differently.

Then one county's figure cannot be set beside another's. Joseon distributed identical vessels nationwide and measured the depth collected. With the vessels the same, the numbers became comparable.

That is what standardisation does: not make a measurement accurate, but make measurements comparable. Only comparable data accumulate, and only accumulated data show a trend.

It still holds. To combine temperature records across countries, the thermometers, their heights and the hours of reading must match. Otherwise you cannot tell whether the climate changed or the measuring did.

See alsoError and uncertaintyReproducibilityScale and symbols

Strata 지층 #

Earth Science

A cliff face shows bands of different colour and thickness stacked one on another, like a layer cake seen from the side.

Each band is mud and sand that once settled on a floor. What settles later lies on top, so the lower layer is older than the one above. It seems obvious, but Steno was the one who set it down as a principle.

With that, the ground becomes a record you can read for time: bottom to top gives the order, and the fossils caught between the layers read in the same order.

And where the layers are bent or broken, something happened afterwards — pushed, split, or turned over entirely. The Earth writes its history on itself.

See alsoFossilsDeep timeUniformitarianism

Strong nuclear force 강한 핵력 #

Physics

Protons are all positive and repel each other. Yet nuclei do not fly apart.

It works only at very short range but is strong enough to beat electric repulsion, which is why nuclei are small and tight. Neutrons wedged between protons add to it, so neutrons are often called the glue of the nucleus.

The force that joins atom to atom is not this but the electric force.

See alsoThe atomic nucleusThe neutronProton

Superconductivity 초전도 #

Physics

Push a large current through a wire and it heats. Push harder and it fails.

But some materials, made cold enough, let current pass with no obstruction at all. Nothing heats, so the current can be as large as you like.

The price is the cold: around 269 degrees below zero.

Why bother? Because very strong magnets are needed — in fusion machines, in accelerators, and in the MRI scanner at the hospital.

See alsoCurrent and the batteryNuclear fusionPlasma

Superposition 중첩 #

Physics

A coin in your pocket is already heads or tails; you merely do not know which. Taking it out does not decide it — it informs you.

The very small world is not like that. Before measurement the state is not yet settled — not merely unknown to you.

That sounds like word-play, but experiments tell them apart: already-decided-but-unknown gives different statistics from not-yet-decided. The experiments that actually measured the difference won the 2022 Nobel Prize.

See alsoQuantumThe wave functionEntanglement

Surface area 표면적 #

Chemistry

Drop equal amounts of lump sugar and caster sugar into water and the powder dissolves far faster. It is the same sugar.

Dissolving happens only where water touches. In a lump the inside is shut away and must wait its turn; in powder nearly every grain is already in contact.

Divide something finely and the amount stays the same while the outside grows. Cut a cube in half each way into eight and the surface doubles. Keep cutting and it keeps growing.

A great deal follows from this alone. Flour hardly burns, but flour dust in the air explodes. It is why a catalyst is used as a powder, and why the inside of our lungs is so finely divided.

At the nanoscale the effect reaches its extreme: there is no inside left to hide in, and almost every atom stands on the surface.

See alsoNanometre and the nanoscaleCatalystMolecule

Symmetry and Noether's theorem 대칭과 뇌터 정리 #

Mathematics

Turn an equilateral triangle by 120 degrees and it looks unchanged. Mirror a snowman left-right and it looks the same. Doing something and nothing changing — that is symmetry.

What Noether showed is this: every symmetry in the laws of physics brings with it a quantity that never changes.

If the result is the same whether you run the experiment yesterday or today (the laws unchanged by a shift in time) → energy is conserved.
If it is the same in Seoul or Busan (unchanged by a shift in place) → momentum is conserved.
If it is the same whichever way you turn the apparatus → angular momentum is conserved.

The conservation laws everyone is made to memorise all come from symmetries. The question of why energy is conserved finally had an answer.

See alsoConservation lawGroup theoryEnergy

Synchrotron light 방사광 #

Physics

Spin a wet umbrella and drops fly off outward — the faster it spins, the harder they fly.

Drive electrons round fast enough and much the same happens. Each time magnets bend their path, they fling light outward. That is synchrotron light.

At first it was a nuisance — the reason accelerators kept leaking energy. Then it turned out that the leaking light was millions of times brighter than ordinary light and its wavelength could be chosen at will, and the nuisance became treasure.

With it you can see where individual atoms sit: how a protein folds, what happens inside a battery, whether a chip's circuit was drawn correctly.

Pohang has two — one that sends electrons round a ring, one that fires them straight to make lightning-like X-ray pulses.

See alsoThe electronWavelengthDiffractionX-ray crystallography

The aether 에테르 #

Physics

Sound needs air; ripples need water. So what does light travel through?

Surely there had to be something — an invisible substance filling all space, named the aether.

Michelson and Morley set out to detect it: if the Earth moves through the aether, light should travel at different speeds along and across that motion. However precisely they measured, there was no difference. A beautiful failure. Special relativity was built on that firmly established absence.

See alsoLightThe constancy of light speedSpecial relativityInterferometerFrame of reference

The arrow of time 시간의 화살 #

Physics

Run two billiard balls colliding in reverse and nothing looks wrong. Run a cup shattering in reverse and it is wrong at once. Both obey the same laws — why does only one look wrong?

Reverse time in Newton's laws and they still hold; the same for electromagnetism and relativity. The basic laws do not distinguish past from future.

Only the second law of thermodynamics carries a direction, and even that is a count rather than a fundamental law: there are few ways to be a cup and uncountably many to be scattered pieces (entropy).

So with few grains the direction blurs and with many it sharpens. Two billiard balls have no arrow; a hundred million fragments do.

See alsoEntropyThe second law of thermodynamicsNegentropy

The atomic nucleus 원자핵 #

Physics

Alpha particles were fired at thin gold foil. Almost all passed straight through — but very occasionally one bounced straight back.

Rutherford said it was as if a shell fired at tissue paper had come back at you.

Most passing through means an atom is nearly empty. The occasional bounce means something very small and very hard sits at the centre. That is the nucleus. Blow an atom up to the size of a stadium and the nucleus is a pea at the centre; the rest is empty.

See alsoAtomThe neutronRadioactivity

The Carnot limit 카르노 한계 #

Physics

However well a power station is built, more than half the fuel is thrown away as heat. Not for want of engineering.

Carnot worked the limit out at twenty-eight. The striking part is that no machinery appears in the answer. It does not matter what it is made of or what it burns: only the temperature gap between hot side and cold side sets the ceiling.

Work can be extracted only while heat is on its way from high to low. Once it has arrived and the temperatures match, nothing more can be done.

See alsoHeatThe second law of thermodynamicsEntropy

The cell 세포 #

Biology & Medicine

Under the microscope, a slice of cork was packed with tiny chambers like a honeycomb. Hooke borrowed the word for a monk's small room and called them cells.

What he saw were dead husks, but the name stuck. What later emerged was this: every living thing is made of cells, and cells come only from cells.

The second half matters most. If cells never appear from nothing, then the cells in your body came from your parents' cells, and those from theirs. Follow it back and the chain never breaks. This is where the claim that all life is connected begins.

See alsoMicroorganismsGeneSpontaneous generation

The constancy of light speed 광속 불변 #

Physics

Throw a ball forward on a moving train and someone on the ground sees it going faster by the train's speed. Switch on a torch, though, and the light is not faster at all.

It sounds impossible, and it is true. However fast the person measuring is moving, light's speed comes out the same.

Then something else has to give. Speed is distance over time, so if speed is fixed, distance and time must differ between observers. That is where the slowed clock of a fast-moving traveller comes from.

See alsoLightSpecial relativityThe aether

The Copenhagen interpretation 코펜하겐 해석 #

Physics

Twenty-nine physicists met in Brussels in 1927, and the position that settled there was later given this name. No document by that name exists.

Three points: the wave function is a ledger of predictions, not a real thing; before measurement there is no value; why is not asked.

The third was the trouble. The arithmetic is perfect and the question why has no answer. The phrase shut up and calculate was later coined to mock the stance.

And on that stance were built transistors, lasers and MRI scanners. The world ran well without the reason — which is both why the interpretation has lasted a century and why Einstein and Schrödinger never accepted it.

See alsoCollapse of the wave functionComplementarityHidden variables

The Coriolis effect 전향력 #

Earth Science

Throw a ball across a turning carousel and it seems to swerve. The ball went straight; the floor turned under it.

The Earth turns once a day. Standing on it and turning with it, we feel nothing. Something moving far and long is another matter.\n\nA wind blowing south goes straight while the ground rotates beneath it, so in the northern hemisphere it appears to bend right.\n\nThat is why air does not rush straight into a low-pressure centre but spirals in, why typhoons are coils, and why the ones that reach Korea all curve rightward as they come.\n\nOne common myth: the way a basin drains has nothing to do with this. Over something that small and brief the effect is far too weak.

See alsoAir pressureChaos

The cosmic microwave background 우주배경복사 #

Astronomy

In 1964 two researchers found a hiss in their radio antenna that would not go away. It was the same in every direction. They evicted the pigeons roosting inside and scrubbed out the droppings, and it remained.

It was not noise. It was light arriving from the whole sky.

The early universe was too hot for light to travel freely. After about 380,000 years it cooled enough and light was released — and that light is still crossing the universe. As space expanded it stretched too (redshift), and is now a very cold radio hiss.

This is the decisive evidence for the Big Bang. And the faint blotches in it were the seeds that became galaxies. A fraction of the static on an old analogue television was this light.

See alsoRedshiftLightDeep time

The diagonal argument 대각선 논법 #

Mathematics

The whole numbers are endless and the decimals are endless. Is one lot bigger? Can 'bigger' even mean anything when neither ends?

Cantor's method: suppose you have a complete list of all the decimals between 0 and 1 — number one, number two, number three, on without end.

Now build a new number. Make its first digit differ from the first digit of number one, its second differ from the second digit of number two, its third from the third of number three, and so on down the diagonal.

This number differs from every entry — from number one in the first place, from number two in the second. You said the list was complete, and here is something missing. So the assumption was wrong.

Some infinities are larger than others. Cantor was fiercely attacked for the result.

See alsoInfinityThe incompleteness theoremsThe halting problem

The double slit 이중슬릿 #

Physics

Drop two stones into still water and the ripples overlap — in some places adding up, in others cancelling to flatness.

Try it with light: through two narrow slits, it makes stripes on the wall, bright and dark alternating. If light were grains you would get two bright marks. It was a wave.

A century later something far stranger appeared: fire electrons one at a time, well separated, and the stripes still build up. There is only one — what did it overlap with? That question leads to superposition.

See alsoWaveLightSuperposition

The Drake equation 드레이크 방정식 #

Astronomy

Is anyone else out there? The question cannot be answered. In 1961 Frank Drake did something else instead: he broke the question into pieces.

The rate stars form, times the fraction with planets, times how many could bear life, times the chance life begins, times the chance it becomes intelligent, times the chance it signals, times how long a civilisation lasts.

The equation gives no answer, since the numbers are unknown. It matters anyway, because it broke a vague question into terms that can each be researched separately.

The early terms are filling in: since the first exoplanet was confirmed in 1995 we know thousands. The later ones, the last especially, stay blank — we do not know how long our own civilisation will last either.

See alsoExoplanetsThe Fermi paradoxHypothesis

The electron 전자 #

Physics

You have been stung by a door handle in winter. Something moved across.

What moved was an electron. It sits in the outer part of an atom and, unlike the nucleus within, comes away and travels fairly easily.\n\nElectricity flowing is electrons moving along in succession (current). Materials sticking to one another, and light being absorbed, are also the work of electrons.\n\nIf the nucleus decides what an atom is, the electrons decide what it can do. Chemistry is very largely a story about electrons.

See alsoAtomThe atomic nucleusCurrent and the battery

The equivalence principle 등가원리 #

Physics

Suppose the cable of a lift snaps. Your feet leave the floor and whatever you are holding floats beside you. From inside, it is exactly as if gravity had vanished.

It works the other way too. Accelerate a lift upwards in space and whoever is inside is pressed to the floor — indistinguishable from gravity.

Einstein called this the happiest thought of his life. Why so large a matter?

Because an answer worked out for acceleration carries over to gravity. Light bends in an accelerating room, so gravity must bend light too (gravitational lensing). Time dilation follows the same way. One thought produced a theory.

See alsoGeneral relativityUniversal gravitationTime dilation

The event horizon 사건지평선 #

Astronomy

A fish swims above a waterfall. While it can swim faster than the current, it can hold its ground. Nearer the fall the current quickens, and past some point no amount of effort keeps it from being carried over.

That point is the event horizon — with curved space in place of the current and light in place of the fish. A boundary beyond which even the fastest thing in the universe cannot climb back.

It is called a horizon for a reason: it is not a wall but a limit. There is nothing special at that place. But what happens inside can never be known outside.

In 2019 the Event Horizon Telescope produced the first image — not of the hole, which cannot be seen, but of the ring of hot matter circling it. It needed a telescope the size of the Earth, so eight radio observatories around the world were combined into one.

See alsoGeneral relativityLightBlinding

The Fermi paradox 페르미 역설 #

Astronomy

Over lunch, Enrico Fermi is said to have asked: where is everybody?

The arithmetic runs like this. Our galaxy alone holds hundreds of billions of stars, many formed billions of years before the sun. Even if civilisations arise around a tiny fraction, they should be billions of years ahead of us.

And the sky is quiet. Sixty years of listening has turned up nothing.

There are several answers and none confirmed: life may be extremely rare, the road to intelligence may be blocked, civilisations may not last, or we may be listening the wrong way. Any of them means some term in the Drake equation is very small.

See alsoThe Drake equationExoplanetsExtinction

The gravitational constant G 중력상수 G #

Physics

Newton's law says heavier means a stronger pull — but not how much stronger.

G is the number that fills in the how-much. Know it and a falling apple gives you the mass of the Earth.

Measuring it is brutally hard. Gravity is so weak that the pull between two lead balls in a laboratory is smaller than the weight of a speck of dust. G remains among the least precisely known constants in physics.

See alsoUniversal gravitationMassError and uncertainty

The greenhouse effect 온실효과 #

Earth Science

Leave a car in winter sun and the inside gets hot: the glass lets sunlight in but does not readily let the warmth back out.

The atmosphere works similarly. Sunlight passes through and warms the ground; the heat the ground gives off as infrared is caught by gases such as carbon dioxide. In passes, out is held — a one-way blanket.

Without the effect at all, the Earth's mean temperature would be around minus eighteen degrees. So the question is not whether but how thick.

Arrhenius calculated by hand in 1896 that doubling carbon dioxide raises temperature by five or six degrees. He took it for good news, and reckoned coal consumption would need three thousand years to do it. It took a little over one hundred.

See alsoHeatThe ozone layerStandardisation

The ground state 바닥상태 #

Physics

You are standing on the bottom step. However much you want to go down, there is no step below.

The same holds for an electron in an atom: inside the innermost seat there is no seat at all.

This matters because the older theory said the electron should spiral into the nucleus — in a hundred-billionth of a second, by calculation. No matter would be left in the world.

That a desk does not pass through your hand, that we are here at all, comes down to this bottom rung: there is nowhere further down.

See alsoThe quantum jumpAtomEnergy

The halting problem 정지 문제 #

Mathematics

A program has been running for thirty minutes with no answer. Is it just slow, or stuck in an endless loop? Wait, or kill it?

It would be handy to have a program that tells you in advance: feed it any program and it answers 'halts' or 'never halts'.

Turing proved no such checker can exist. Suppose it does. Then you can build a spiteful program that asks the checker about itself and does the opposite: if told it halts, it loops forever; if told it loops, it stops at once. Either way the checker is wrong.

The argument has the same shape as incompleteness — make a thing refer to itself and draw out a contradiction. There are questions no computer can answer.

See alsoThe Turing machineThe incompleteness theoremsAlgorithm

The immune system 면역 #

Biology & Medicine

Measles does not come twice. The body has memorised the face.

Immunity studies the outside of an intruder, builds a key shaped to grip it, and holds on. Against a stranger this takes days, and during those days we are ill.

Afterwards it files the shape away, and next time the key comes in hours. A vaccine is a way to get that memory without the illness. A sore arm after a jab is that response starting up.

See alsoVaccines and inoculationVirusGut microbiota

The incompleteness theorems 불완전성 정리 #

Mathematics

'This sentence is false.' If true, it is false; if false, it is true. Neither way works.

Gödel made that word-play precise inside mathematics: he constructed a statement about numbers that says, in effect, this statement cannot be proved.

Then: if it can be proved, the system contains a contradiction; if it cannot, then it is true and unprovable. Either way, the hope of proving every truth is over.

The second theorem cuts deeper — no such system can prove its own consistency. Hilbert's programme collapsed here. It does not mean mathematics is broken. It means its limits were located exactly.

See alsoAxiomHilbert's programmeThe halting problem

The laws of thermodynamics 열역학 법칙 #

Physics

The steam engine came first and the theory afterwards. The machines worked; nobody knew why.

It comes to two sentences.

First law — energy is neither created nor destroyed; it only changes form.
Second law — but the usable form of it keeps shrinking.

Between them, perpetual motion fails twice over: you cannot make energy from nothing, and you cannot recover all of what you have.

See alsoEnergyHeatThe second law of thermodynamics

The measurement problem 측정 문제 #

Physics

A detector is made of atoms, and so is the person reading it. Both should obey quantum mechanics. So why does the needle never point two ways at once?

Quantum mechanics has two rules: one by which the wave function evolves smoothly, and one by which measurement yields a single value.

The problem is where the boundary between them lies — when the detector touches, when a record is made, when a person looks. That line is written nowhere in the theory.

The Copenhagen interpretation said not to ask. Schrödinger produced a cat in a box to mock that stance, and his counter-example became the emblem of the problem instead.

See alsoCollapse of the wave functionThe Copenhagen interpretationSuperposition

The modern synthesis 현대적 종합 #

Biology & Medicine

After Darwin's death, evolutionary theory ran into trouble. Mendel's rediscovery made inheritance look like discrete grains, while Darwin spoke of change accumulating in tiny steps. The two seemed not to fit.

The answer was simple. If twenty genes set height rather than one, then even though each passes on whole, heights spread smoothly. Many small effects, stacked, look continuous from outside.

The two theories were not in conflict; they were looking at different scales. Fieldwork confirmed it: within one fly species the genetic make-up differed valley by valley and shifted with the seasons. Evolution was not a past fixed in fossils but something happening now.

See alsoNatural selectionMutationGene

The neutron 중성자 #

Physics

Weigh a nucleus and something is missing: the known contents add up to about half of what it actually weighs.

What is the other half? Something with mass but no charge, and therefore invisible to the usual probes. That is the neutron.

Being uncharged turned out to matter enormously: with no charge, it is not repelled by a nucleus and can go straight in. That is what made fission possible.

See alsoThe atomic nucleusAtomNuclear fission

The ozone layer 오존층 #

Earth Science

Gathered at ground level, this layer would be about three millimetres thick — thinner than a coin. Without it, life on land could not survive.

In 1974 Molina and Rowland did the arithmetic. CFCs, used in refrigerators and spray cans, were considered safe because they react with nothing — but reacting with nothing also means not breaking down. So they do not vanish at ground level; they rise.

High in the stratosphere there is ultraviolet strong enough to split them, releasing chlorine — and one chlorine atom destroys tens of thousands of ozone molecules.

It was confirmed over Antarctica in 1985. What is striking is that satellites had been recording the same values for years, but the processing software discarded implausibly low readings as error. The machine had been filing the fact away as an outlier.

See alsoThe greenhouse effectLightError and uncertainty

The periodic table 주기율표 #

Chemistry

Line the elements up from lightest to heaviest and elements with similar properties recur at regular intervals.

So break the line at each interval and write it below: elements of like character line up in columns. That is the periodic table — recurrence turned into a shape.

Mendeleev's achievement was not the table but the gaps. Where the next element by weight did not fit the pattern, he left the place empty and moved on — and predicted the weight and properties of the element that belonged there. When they were found as described, the table stopped being a list and became a theory.

See alsoElementAtomSpectral lines

The photoelectric effect 광전효과 #

Physics

Shine light on a metal plate and electrons fly off. Shine it brighter and they should fly off harder — but they do not.

Red light, however bright, ejects nothing. Blue light, however faint, ejects at once. What mattered was colour, not intensity.

A wave cannot explain that: faint light shone long enough should accumulate enough energy eventually. Einstein's answer was that light arrives in lumps, the size of a lump set by its colour — and if the lump is too small, no quantity of them will do. This became the first evidence that the quantum is real.

See alsoLightQuantumWave

The quantum jump 양자도약 #

Physics

Climbing stairs you take one step or two — never half. At least your foot passes through the space between.

For an electron there is no in between. Moving from an inner orbit to an outer one, it does not cross the gap — it is simply there.

Making the jump, it emits the difference in energy as light. Since the gaps between allowed rungs are fixed, so are the colours emitted. That is why each element has its own spectral lines.

See alsoQuantumLightSpectral lines

The second law of thermodynamics 열역학 제2법칙 #

Physics

Where did the heat of the cooled coffee go? It did not vanish — it moved into the room's air and is still there.

But you cannot gather it back to reheat the coffee.

That is the heart of it: energy never disappears, but the usable form of it keeps shrinking.

The first law says the total stays the same; the second says it steadily becomes unusable. So no machine can run forever, and even the best power station throws away roughly half its fuel as heat.

Entropy is the number that measures how far along that road you are.

See alsoEntropyEnergyHeat

The self-consistency principle 자기일관성 원리 #

Physics

What if you went back and killed your own grandfather? Then you are never born — and never go back.

One answer to the paradox is this principle.

Changing the past is not forbidden; rather, only self-consistent histories happen at all. Anyone going back to kill a grandfather finds the gun jams, or the road is closed, or meets the wrong man. Not thwarted by luck — a history in which they are not thwarted cannot be one that occurs.

On that view a traveller does not change history but goes to do what they already did. Whether the principle is right is undecided, since whether the past is reachable at all is unknown.

See alsoWormholeThe arrow of timeThought experiment

The telescope 망원경 #

Astronomy

We are taught a telescope magnifies. Yet astronomers hardly mention magnification. They talk about aperture.

In astronomy a telescope is less a magnifier than a bucket. As a wider vessel catches more rain, a wider surface catches more light, and fainter things appear.\n\nA star stays a point however much you magnify it. The problem is not size but brightness — so building a larger telescope means seeing further, and therefore earlier.\n\nNor is it only visible light. A telescope for radio waves looks like a dish, and sees through dust that stops the eye.

See alsoLightLight-yearSpectral lines

The Turing machine 튜링 기계 #

Mathematics

What does 'to compute' actually mean? Watch someone doing long division on paper very closely and the actions are simple: read the symbol in the square you are looking at, write something by rule, move to the next square.

Turing stripped it to that: an endless tape, a device that reads and writes one square, and a table of rules. Nothing else — the bare skeleton of a person with paper and pencil.

The remarkable part is that this simple thing can compute anything computable. The machine on your desk is the same in principle, only faster.

And because computing now had a precise definition, one could ask what is *not* computable. That question is the halting problem.

See alsoAlgorithmThe halting problemHilbert's programme

The uncertainty principle 불확정성 원리 #

Physics

To find a ball in a dark room you feel around — and the moment you touch it, it rolls away. You learn where it was and lose track of where it is going.

In the very small world this is not a technical problem; being gentler does not help. There simply is no state in which both position and momentum have precise values.

Pin one down more sharply and the other blurs. It is not that we cannot measure well enough; it is how nature is. So the principle is not about measurement but about existence.

See alsoMomentumQuantumSuperposition

The wave function 파동함수 #

Physics

A wave on a guitar string is the string actually moving up and down; a ripple is water rising and falling. So in an electron's wave, what is moving?

Nobody can answer that with confidence.

The equation exists and it works. Solve it and you get exact probabilities for where the electron will be found, and those predictions have never failed. What the wave *is*, however, has been argued over for a century. Schrödinger wanted it to be something genuinely spread out in space; the arithmetic only came right when read as probability.

See alsoProbabilitySuperpositionQuantum

The Weismann barrier 바이스만 장벽 #

Biology & Medicine

Weismann cut the tails from mice and bred them, over five generations and more than nine hundred animals. Not one was born with a shortened tail.

The cells that make the body and the cells that will be passed on separate early in development, so nothing that happens to the body during life is written into them. Information runs one way.

This is why the inheritance of acquired characteristics cannot work. A lifetime of training does not produce a muscular baby, and cutting tails for generations never produces a tailless mouse.

Whether the barrier is absolute is discussed again in light of epigenetics, since some marks have been reported crossing a generation. That is a hairline gap in the wall rather than an open door: such marks are usually erased within a few generations, and the gene sequence itself is unchanged.

See alsoInheritance of acquired characteristicsEpigeneticsGeneNatural selection

Thought experiment 사고실험 #

Method & instruments

If you rode alongside a beam of light, what would the light look like? Einstein put the question to himself at sixteen. There is no way to actually try it.

But logic can carry you through. Riding alongside, the light should look stationary — and stationary light does not exist in Maxwell's equations. Something is wrong. Ten years later that question became special relativity.

A thought experiment is not daydreaming. It is a tool for colliding existing theories to expose a contradiction. Galileo asked what happens if you tie two falling bodies together, and brought down the theory that heavier falls faster. Schrödinger's cat and the EPR paper worked the same way.

Its limit is clear: it can find contradictions but cannot decide which way nature actually goes. Only experiment does that.

See alsoFalsificationSpecial relativityHypothesis

Tidal force 조석력 #

Physics

The tide comes in and out twice a day. If the moon simply pulled the sea towards it, once would do. Why twice?

Gravity is stronger closer in, so the sea facing the moon is pulled harder than the Earth is, and the sea on the far side less.

Subtract the part that moves the whole Earth together and the near side bulges towards the moon while the far side lags behind. Both bulge, so the tide comes twice.

Tidal force is therefore not a pull but a stretch. On Earth it raises the sea by metres; beside a black hole it stands water up by kilometres, and closer in it draws objects out like noodles.

See alsoUniversal gravitationThe event horizonForce

Time dilation 시간 지연 #

Physics

The map on your phone knows where you are. The satellite's clock gains 38 microseconds a day on one at the surface; uncorrected, the position drifts about ten kilometres a day.

Slower does not mean the clock is faulty. Everything there is slow — the clock, the heartbeat, the ageing.

So anyone inside notices nothing. It shows only when you meet again and compare.

There are two causes: moving fast (special relativity) and sitting where gravity is strong (general relativity). A satellite loses time for its speed and gains it for its altitude, and altitude wins.

See alsoGeneral relativitySpecial relativityThe equivalence principle

Traits, dominant and recessive 형질과 우성·열성 #

Biology & Medicine

Both parents have a crease in the eyelid and the child does not. Where did that come from? Not the neighbours.

Each parent carries two copies and passes on only one. If a parent has one 'crease' and one 'no crease', the crease is what shows. The version that masks is dominant; the one masked is recessive.

But if both parents pass on 'no crease', that is what the child shows. The recessive version had not disappeared, only hidden. Dominant does not mean better — only that it is the one that shows.

See alsoGeneChromosomeMutation

Transformation 형질전환 #

Biology & Medicine

Killed bacteria alone: the mouse is fine. Living harmless bacteria alone: fine. Put both in together and the mouse died.

And from the dead mouse came living, deadly bacteria whose descendants stayed deadly.

Something inside the dead had crossed into the living and changed its kind. Griffith, not knowing what, simply called it a principle. Sixteen years later Avery erased components one by one with enzymes: remove protein and it still happened, remove RNA and it still happened, remove DNA and it stopped.

See alsoDNA and the double helixGeneGerm theory

Transposable elements 전이인자 #

Biology & Medicine

Kernels on one ear of maize differ in colour, and single kernels come out mottled. If genes were fixed in place, this could not happen.

McClintock concluded that genes move about, switching other genes on and off as they go, and that this produces the mottling.

In the 1940s nobody believed her — genes sat at fixed addresses on chromosomes, everyone knew that. She stopped publishing and carried on alone for thirty years. When the same behaviour turned up in other organisms she was vindicated, and received the Nobel Prize at eighty-one.

See alsoGeneDNA and the double helixMutation

Turbine 터빈 #

Physics

Run with a paper windmill and it spins. Set a waterwheel in a stream and it turns. So far, nothing surprising.

This, in the end, is what a power station does. Whatever it burns, the last step is to boil water and let the steam turn a very large windmill. That windmill is the turbine.

Only the scale differs. Hard metal blades sit in ranks along a shaft, and hot fast steam passing between them spins it.

At the end of the shaft is a generator, where a magnet and a coil pass each other and electricity appears (electromagnetic induction). A bicycle dynamo works the same way.

Coal, gas, nuclear and fusion are identical here. Only what boils the water differs. Solar, hydro and wind are the ones that step out of this line.

See alsoElectromagnetic inductionThe Carnot limitEnergy

Uniformitarianism 동일과정설 #

Earth Science

How did a canyon like the Grand Canyon form? Did a vast catastrophe split the ground in one blow, or has the river you can see simply been running a very long time?

Hutton and Lyell said the latter: no special event is needed. The erosion, deposition and uplift you can watch today, continued for an unimaginably long time, will do it.

The view matters because it demands a scale of time — not thousands of years but hundreds of millions. And only that much time lets natural selection work. It is why Darwin carried Lyell's book aboard ship.

It has since been amended. Rare violent events, such as asteroid impacts, did occur. Mostly slow, occasionally sudden, is the present picture.

See alsoDeep timeStrataExtinction

Unit 단위 #

Method & instruments

How many paces is this room? It depends who walks it. You cannot trade in paces.

So people fixed one size and gave it a name: metre, kilogram, second.

For a long time the fixed thing was an object — a platinum bar in a vault that was one metre. But objects change, and a standard cannot measure itself.

So standards now rest on nature. A metre is how far light goes in a set time; a second is a set number of tremblings of an atom. The same anywhere.

See alsoStandardisationThe measurement problemLight

Universal gravitation 만유인력 #

Physics

An apple falls, so why does the Moon not? Newton's answer was unexpected: the Moon is falling too.

Throw a ball hard and it lands far off. Harder, further. Throw it hard enough and it falls exactly as fast as the ground curves away, so it never lands and instead goes round. The Moon is falling like that, continuously.

The same force governs apple and Moon. That there was no separate law for the heavens was the shock of it.

See alsoForceMassGeneral relativity

Use and disuse 용불용설 #

Biology & Medicine

Do push-ups every day and your arms thicken; wear a cast for weeks and that leg grows thin.

Up to here it is true: the body changes with use.

Lamarck went further and said such changes pass to offspring. That second half is the inheritance of acquired characteristics, and it is the part that was wrong. What happens to the body is not written into the genes.

See alsoInheritance of acquired characteristicsNatural selectionGene

Useful wrongs 쓸모 있는 오류 #

Method & instruments

There is no phlogiston, no aether, and geocentrism is wrong. But filing them simply as errors misses something.

Geocentrism produced serviceable prediction tables for fourteen centuries. Phlogiston left behind the habit of putting combustion on a balance — and that habit destroyed it. The hunt for the aether found nothing, and that nothing became the footing for relativity. Lamarck was wrong, but he was the first to say that living things change over time.

A wrong theory usually leaves something for the next one to stand on. So the history of science is not a list of winners. Knowing which roads were dead ends is part of knowing where we are standing.

See alsoFalsificationPhlogistonPeer review

Vaccines and inoculation 백신과 종두법 #

Biology & Medicine

Smallpox killed roughly one in three. Yet milkmaids rarely caught it — though they had often had a much milder disease of cattle.

Jenner tested the rumour: he introduced material from the cattle disease into a person, later exposed them to smallpox, and they did not fall ill.

The mechanism came later. The body remembers an intruder it has met and responds far faster the next time. A vaccine is a rehearsal with a weakened version. He began without knowing what he was doing — and smallpox became the only human infectious disease ever eradicated from the Earth.

See alsoGerm theoryMicroorganismsAntibiotics

Vacuum 진공 #

Physics

What force can nothing exert? And yet a lid held by vacuum is very hard to lift.

The vacuum does not pull. The air outside pushes. We live at the bottom of the air and simply do not feel its weight.

A perfect vacuum cannot be made; there are only degrees of emptied.

The usual reason for emptying is to remove interference. Any air inside an accelerator scatters the particles you worked to speed up, so the whole path is emptied.

See alsoAtomMolecule

Valence 원자가 #

Chemistry

Board 4 of the element tool calls this an atom's hands: hydrogen has one, oxygen two, carbon four.

Atoms want a full top shell. With few gaps they pull electrons in, with few electrons they give them away, and in between they share. That count is the valence. For the first twenty elements it is nearly always the smaller of top-shell electrons and top-shell gaps.

Helium and neon, already full, have valence zero and bond with nothing.

See alsoCovalent bondIonElectron shell

Virus 바이러스 #

Biology & Medicine

You go to the doctor with a cold and get no antibiotics. Drugs for bacteria do nothing to a virus.

Not a house — a letter. It does not eat or grow and cannot multiply alone. Once inside a cell, the cell builds the virus for it from the blueprint. The cell is the factory; the virus is the order form.

A hundredth the size of a bacterium. Its existence was deduced when tobacco-leaf juice passed a bacteria-proof filter and still spread disease; its face was first seen with the electron microscope in the 1930s. Whether it is alive is still argued.

See alsoBacteriaMicroorganismsGene

Vitalism 생기론 #

Biology & Medicine

For a long time people believed that what living things make, only living things could make.

It began to waver in 1828 when the German chemist Wöhler made urea, a substance in urine, from ordinary materials in the laboratory. It did not fall at once but retreated as similar experiments piled up.

Biology now holds that the atoms of life are the same as those of rock and air. What is special is not the atoms but their order: DNA.

See alsoDNA and the double helixNegentropy

Wafer 웨이퍼 #

Physics

In factory photographs someone holds a round plate that shines like a mirror. What is that plate?

Silicon drawn from sand is grown into a very pure crystal and then sliced thin, like a radish.

The point is that chips are not made one at a time. Hundreds of identical circuits are printed onto one round plate at once and cut apart afterwards — as a photograph is printed, several to a sheet.

So a larger plate is better: the same effort yields more. That is why wafers have grown steadily larger.

A flaw in the crystal or a speck of dust costs only the chips at that spot. How many survive is this industry's report card.

See alsoSemiconductorCleanroomNanometre and the nanoscale

Wave 파동 #

Physics

A crowd does a stadium wave, standing and sitting in turn. The wave travels across the stands — yet nobody has moved sideways.

That is a wave: what travels is not the stuff but the disturbance.

Sound works the same way. Air molecules shake in place; the air from your mouth does not fly to the other person's ear. Ripples, earthquakes and light are all like this. How often it shakes is its frequency — pitch in sound, colour in light.

See alsoFrequencyMoleculeLight

Wavelength 파장 #

Physics

Watching the sea, there is a gap between one crest and the next — close together on a calm day, far apart when the big waves come.

That repeat length is the wavelength.

It matters for a particular reason: a wave can only pick out things larger than its own wavelength.

Suppose a thin post stands in the sea. A large swell passes it as if it were not there, leaving no trace in the wave's shape. Ripples, though, break around it and their pattern is disturbed — and from that disturbance you can tell something is there.

Light is a wave, so the same holds. Visible light has a wavelength of about a two-thousandth of a millimetre. Anything smaller is simply passed over. There is a size no lens, however finely ground, can reach.

Wavelength pairs with frequency: a shorter wave passes more often in the same time, so its frequency is higher.

See alsoWaveFrequencyLightResolution

Weather front 전선 #

Earth Science

Yesterday short sleeves, this morning a coat. The season did not change overnight; different air moved in.

Air travels in masses. Air down from Siberia is cold and dry; air up from the southern sea is warm and wet. Each carries the character of wherever it sat longest.\n\nWhen they meet they do not blend, they shove. The warm side rides up over the cold, being lighter. Rising, it cools; cooling, it makes cloud; and so rain follows the front.\n\nThat is the line drawn with triangles and half-circles on a weather map: triangles where cold air is advancing, half-circles where warm air is.\n\nThe summer rainy season is this boundary parking over the country. While the southern and northern air are evenly matched, neither gives way, and the rain keeps falling in that one place.

See alsoAir pressureCondensationConvection

Which-path information 경로 정보 #

Physics

Fit the double slit with apparatus that reveals which slit the electron used and the stripes vanish. This is usually explained as being because someone looked.

It is not. Whether anyone reads the record, whether anyone is present at all, the outcome is the same.

What matters is not who looks but whether a trace is left. If the route is recorded — in the apparatus, in an air molecule, in a single photon — interference goes. If not, it stays.

The word observation did the damage. Here it means becoming entangled with something else and leaving a trace, not looking. Human consciousness enters nowhere in this.

See alsoInterferenceSuperpositionEntanglement

Wormhole 웜홀 #

Astronomy

Mark two dots on a sheet of paper. Crawling across the sheet, they are far apart. Fold the paper so the dots touch and they are next to each other.

In general relativity, gravity is not a force but the shape of curved space, and if space can curve, the equations permit such a fold.

Permitted is not the same as existing. The hole snaps shut as soon as it forms, and holding it open would require something that pushes where ordinary matter pulls. Whether nature contains such a thing is unknown.

It has neither been found nor ruled out — the commonest state of affairs in science: judgement suspended.

See alsoGeneral relativityThe event horizonUniversal gravitation

X-ray crystallography X선 결정학 #

Chemistry

Look at a street lamp through a fine curtain and the light spreads into a cross. The spacing of the threads makes that pattern — and from the pattern you can calculate the spacing.

A crystal is atoms in a regular array, so the same thing happens: X-rays scatter into a pattern set by the spacing between atoms.

Work the pattern backwards and you get where the atoms are. You cannot see an atom, but you can find out its address. From salt crystals it went on to the shapes of chemical bonds, the folding of proteins, and the double helix of DNA.

The arithmetic was enormous; early on, one structure could take years.

See alsoChemical bondDNA and the double helixAtom

Yeast 효모 #

Biology & Medicine

Leave bread dough somewhere warm and it rises. Makgeolli keeps fizzing inside the bottle. The same worker is at work in both.

Yeast eats sugar and gives off two things: alcohol and gas. In bread the gas puffs the dough and the alcohol bakes away. In makgeolli the alcohol stays and the gas becomes the tingle.

Yeast cannot eat starch. So to make drink from rice, the mould in nuruk must first snip the starch into sugar. That is why makgeolli works twice in one jar.

See alsoFermentationMicroorganisms

Zero 영(0) #

Mathematics

Consider 205 and 25. Without a zero, how do you write them differently? You could leave a gap between the 2 and the 5 — but is that one gap or two?

Zero marks that a column exists but holds nothing. Place-value notation only works because of it.

Brahmagupta went further: he treated zero not as a mark but as a number that takes part in calculation. Subtract a quantity from itself and you get zero; add zero and nothing changes; multiply by zero and you get zero. He set out negative numbers too, as debts.

One thing he could not settle was division by zero — and it is still unsettled. What is six divided by zero? How many zeros must you add to reach six? No number of them will do.

See alsoPlace-value notationAlgebraInfinity

Zero-point energy 영점에너지 #

Physics

Cool something to absolute zero and all motion should stop. It does not — because complete rest would fix both position and momentum at once.

The uncertainty principle forbids it, so a tremor remains even in the coldest state. It is why helium stays liquid near absolute zero instead of freezing.

A caution belongs here. The claim sells widely that empty space is full of energy and limitless free power can be drawn from it.

Zero-point means lowest (ground state). Extracting energy means dropping something lower, and there is nothing lower. The name itself says it cannot be taken out.

See alsoThe uncertainty principleThe ground stateEnergy