Two Objections in 1935 — Entanglement and the Cat

In 1935 Einstein argued that the strange parts of quantum mechanics are like this. It is not that values are absent before measurement; the values were there all along and the theory fails to record them.

The argument looked airtight. When it was finally tested, the gloves turned out not to be gloves.

Entanglement

Make two particles in a single event and their properties are tied together — if one spins up the other must spin down. Which one is up is not settled; only the relation between them is.

This is entanglement, and no distance loosens it.

The first objection — EPR

In May 1935 Einstein, Podolsky and Rosen published a short paper, known ever since by their initials.

1. Separate two entangled particles by a great distance. Measure here and the value there is fixed at once.
2. What is done here cannot instantly affect there; no influence outruns light.
3. Then the value there must have been fixed all along, before any measurement.
4. But quantum mechanics says there is no value before measurement. So it is not wrong — it is incomplete.

If quantum mechanics were right, distant things would be linked instantly, which cannot be. So there must be hidden variables we do not yet know, fixing the values in advance. That was Einstein's picture — the picture of gloves.

Spooky action at a distance.Einstein's phrase for the situation

The second objection — the cat

That autumn Schrödinger wrote an objection from another angle.

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

His aim was accurate. An electron in superposition can be tolerated; a cat cannot. So where does the quantum end and the ordinary begin? Where exactly is that line drawn?

It is written nowhere in the theory. This is the measurement problem, and it is still open.

A little further in

After the two objections, nothing happened for nearly thirty years.

Bohr's reply was long and vague. Most physicists set the matter aside as philosophy. The arithmetic worked and there was much to build with it — and, decisively, there seemed to be no way to tell the two positions apart.

Done as a hobby

In 1964 John Bell was an accelerator physicist at CERN. This was not his job but a question that had nagged at him for years. A short paper written on sabbatical was the result.

His insight was this: if Einstein's picture is right, then there is a ceiling the experimental results cannot cross.

Three questions

The inequality itself has nothing to do with quantum mechanics. It is arithmetic. Put in terms of people it runs like this.

Take a group of people with three properties: tall or not, wearing glasses or not, left-handed or not. For each person all three answers are already fixed.

Now run that count with entangled particles: instead of three properties, three detector angles, measuring the two particles at different ones. If the answers were fixed in advance — if they were gloves — the inequality cannot be broken.

Quantum mechanics predicts results that break it.

And when it was measured

John Clauser ran the first such experiment in 1972. He hoped Einstein would be vindicated and expected his own results to favour hidden variables. They did the opposite.

Early experiments left a loophole: with the detector angles set in advance, the particles might somehow be tuned to them. In 1982 Alain Aspect closed it by switching the angles while the particles were in flight.

1. 1935EPR and the cat — two objections
2. 1964Bell — the argument can be settled by experiment
3. 1972Clauser — the first test, against his own hopes
4. 1982Aspect — switching the angles in mid-flight
5. 2015Delft — the remaining loopholes closed
6. 2022The Nobel Prize — Clauser, Aspect, Zeilinger

The results all pointed one way. Not gloves. The values had not been fixed in advance.

But it does not carry messages

One misreading must be cut off here: that entanglement allows communication faster than light.

Talk of teleportation by entanglement, or of entangled minds, sells here. Entanglement is a correlation between two outcomes, not a sending of anything from one place to the other.

Its uses lie elsewhere. Because eavesdropping on entangled particles always leaves a trace, it allows encryption whose interception can be detected in principle. Quantum computers use entanglement too.

Why we never see the cat

Back to Schrödinger's question. Why an electron and not a cat?

Half the modern answer is decoherence. Anything cat-sized is struck constantly by air molecules, photons and its own atoms, and each collision leaks which-path information outwards (article three). It happens so fast that superposition has no room to persist.

The most productive wrong objection

Einstein lost. His picture was refuted by experiment.

And the EPR paper became the most cited thing he ever wrote — more than relativity. The whole field of quantum information grew out of the objection, and with it the idea that entanglement is a resource.

After five articles

Trying to calculate the colour of a stove, we found that energy comes in lumps (one); the lumps fixed the seats an electron may take (two); writing those seats as a wave left us unable to say what is waving (three); there turned out to be no state holding position and momentum together (four); and distant things are not separate in the way we assumed (five).

For a hundred years this theory has never made a wrong prediction. For a hundred years what it says has not been settled.

The question that remainsIf a calculation that has never failed is still not understood, is understanding something other than the calculation coming out right? Or is what we call understanding simply the point at which we grow used to it?