Forensic Science — Every Touch Leaves Something

Touch leaves, contact exchanges

A century ago a French investigator set down a principle: when two things touch, each takes something from the other. The hand leaves sweat on the glass; dust from the glass clings to the hand.

Forensic science stands on that one line. It is never that there is no trace — only that it has not been found yet.

Fingerprints — a stamp on the fingertip

The ridges on a fingertip form as skin folds in the womb, so even twins differ and they never change. This is the fingerprint.

DNA — how a drop of saliva becomes an identity card

Human DNA is 99.9 percent the same from person to person. How, then, is one person picked out? Not by reading all of it.

DNA has places where a short spelling repeats — ten times in one person, thirteen in another — and the count is inherited. At about twenty such places, only the counts are read. This is DNA profiling.

Here is the striking part. A match at one place is quite likely. A match at all twenty is rarer than one in all the people on Earth. So twenty numbers become an identity card.

A little further in

One fragment into millions

A single hair or the saliva on a rim is enough because of a copying trick: put in one fragment, cycle the temperature up and down, and it doubles each cycle. Thirty cycles make a billion copies.

That is the PCR used in coronavirus tests. Testing and investigation share the same machine.

What a trace says and what it does not

An honest point belongs here. A fingerprint says only that this person touched here, not when. DNA says only that this person was here, not what they did.

More than prints and DNA

The service's work is far wider.

1. Drugs and poisons — what was in the body, found by the mass spectrometry of the fifteenth piece. Drug testing happens here.
2. Autopsy — reading what the body says about why it died.
3. Firearms and fire — where a blaze began, which gun fired a round.
4. Digital — recovering deleted files and reading phone records, where it meets the cryptography of the seventeenth piece.
5. Road accidents — reconstructing speed and sequence from skid marks and crumpled metal.
6. Documents and voices — whose handwriting, whose recorded voice.

They share one thing: making objects say what people will not.

Why Wonju

Once in Seoul, the service moved to Wonju in Gangwon province in 2013. Five regional branches share the evidence sent in by police across the country — hundreds of thousands of requests a year.

What you would study to do this

On television one person does everything. In reality each field is a different person.

1. Someone who handles DNA and blood — life science, genetics.
2. Someone who finds drugs and poisons — chemistry, pharmacy, at a mass spectrometer.
3. Someone who performs autopsies — a doctor with separate training in forensic medicine; a few dozen in the country.
4. Someone who digs out digital evidence — computer science, information security.
5. Someone who reconstructs fires and crashes — mechanical and electrical engineering, working at the scene rather than the bench.
6. Someone who keeps and records the evidence — every hand it has passed through, without gaps. A break in that record voids the evidence.

The last line is the unexpected job. Least visible, and if it breaks, everything above it is worthless in court. Meticulousness is the whole skill.

The fifth line likewise: the person who reads a burnt house or a crumpled car is someone who knows machines and wiring.

The question that remainsObjects cannot lie, but they do not say everything — so with whom should the final judgement rest?