Can quantum particles really communicate over vast distances in an instant, or are we missing something?
Neither Max Planck nor Albert Einstein, who between them brought photons into the world, was ever completely happy about what they had done. Planck came up with the idea of little packets of light, but was reluctant to believe they were real objects rather than just mathematical devices that made life easier for physicists.
Einstein had a different problem. He had no difficulty picturing light as a rain of tiny bullets that could literally knock electrons loose from their atoms. He used that idea, in fact, as the basis for a simple explanation of the photoelectric effect, in which light shining on certain metals generates an electric current. Later, he even argued that it made good sense to think of light as a "gas" of photons not unlike a conventional gas of physical atoms.
What really bothered Einstein was that, if photons were real objects, an undesirable element of probability crept into physics. And Einstein, revolutionary though he was, remained a strict classicist in one respect: he believed unswervingly in cause and effect. If you know all the properties and characteristics of any object, in other words, you ought to be able to predict exactly what it will do in any situation.
Quantum theory, by contrast, can only ever give the probability that something might happen. When a stream of photons falls on the lens of Polaroid sunglasses, some will go through and some will not. But there is no way you can predict exactly what any individual photon will do. You can only know the chances of one result or another.
Quantum theory upset Einstein because it gave him nothing better to grapple with than frustrating probabilities. In 1936, he got together with Boris Podolsky and Nathan Rosen to create the "EPR paradox". It's ironic that the spooky EPR connection has now been used in the lab to teleport photons, because the original reason for inventing the EPR paradox was to show that one of the implications of quantum theory was so unacceptable that it must be wrong or incomplete in some respect. What the EPR trio couldn't accept was the idea that measuring a photon in one place could have an instantaneous physical consequence somewhere else -- all because quantum measurements are about probabilities.
The original EPR argument has been recast in many different forms, but let's stick with photons for now. Suppose you entangled a pair of photons polarised at 90 degrees to each other. You can't know what the polarisations are until you measure them; they could be vertical, horizontal or any angle in between. All you do know for sure is that they are perpendicular to each other. You send these photons off in different directions. At some point as they shoot off into the distance the photons will run into polarising filters you've cunningly put in their path.
Suppose one photon passes straight through a vertically aligned filter. It must be vertically polarised, so its partner must be horizontally polarised. The second photon would therefore pass through any horizontal filter in its way, but not through a vertical filter. So far so good. One photon is vertically polarised, the other is horizontally polarised, so they are at right angles as they should be, and all's well with the world.
Not quite. Until the first photon hits the filter, you have no idea whether it will go through or not. And for that matter, the photon doesn't know, what sort of filter it is going to hit until it gets there. Since you know nothing about either photon's individual polarisation until you make a measurement, you only know that the odds of it going through are fifty-fifty, no matter what angle the filter is set at. So the second photon can't know what the first photon will do until it actually does it. Yet the actions of the first photon determine the actions of the second. The second photon has to get some sort of tip-off from the first, even though they are physically a long way from each other.
What's more, this tip-off has to be instantaneous, because it has to work even if the two photons hit their filters at exactly the same time. It's impossible to predict what either photon will do, and yet the two of them must act in concert so that their polarisations have the correct relationship to each other. This is the crux of the spookiness that Einstein, Podolsky, and Rosen took such exception to. It arises precisely because the results of quantum measurements are uncertain or indeterminate until they are actually made.
The story would be different if the polarisations of the two photons were somehow fixed at the outset, even though we don't know what they are. The results of any polarisation measurements would still be fifty-fifty either way, because you have no prior knowledge of what the photons will do when they get to the filters. But from the photons' perspective, everything is predetermined: each photon is in a definite state, so the fact that the two measurements come out as they do is the result of prearrangement, not of a spooky communication.
This is what the classically minded Einstein seized on. It's like saying that a billiard ball headed your way is either red or blue, but you don't know which until you actually look at it. That's an entirely different proposition from saying that the billiard ball is neither red nor blue until you look at it, and it only becomes one colour or the other at the moment you see it.
The argument, in other words, was that if photons' polarisations are truly not determined until they are measured, then the paired EPR photons have to conspire with each other in some instantaneous way in order to guarantee that simultaneous measurements on them come out right. That seemed absurd! Much more sensible, Einstein and his friends believed, was the notion that quantum theory is incomplete, and that each photon has some secret property that, if only you knew it, could tell you what the result of a polarisation measurement would be.
All well and good, except . . . how would you find out the photon's secret except by making the very measurement whose result it is supposed to help you predict? Which rather spoils the point. Most physicists agree that the EPR conundrum is indeed something of a puzzle. But does it mean that quantum theory is wrong, or just hard to understand? What would be the point of giving photons extra properties if there is no independent way of finding out what they are, especially when they don't seem to make jot of difference to the outcomes of experiments?
In the words of the physicist John Bell, who generally sympathised with Einstein's disquiet over quantum theory, the EPR paradox is one of those questions that most physicists feel they will fully understand if they can ever spare twenty minutes to think about it. But in the meantime, why worry?