“God does not play dice”, Einstein famously said. Einstein, like many of us, was unsettled by some corollaries of quantum mechanics (QM). In particular, QM says particles don’t have definite properties until the property is measured. That’s a heavy statement about the nature of reality and Einstein didn’t like it, “Do you really think the moon isn’t there if you aren’t looking at it?” Set on disproving this QM nonsense, Einstein, together with a few other brains, developed the EPR paradox.
The idea of the EPR paradox goes like this: We all agree nothing can travel faster than the speed of light. The EPR paradox shows that if “particles don’t have definite properties until the property is measured” then information must travel faster than the speed of light. Laws of physics contradiction. Therefore particles must have had definite properties even before the measurement.
Details? Here we go (if you want the real scoop with no hand-wavy inaccuracies, Google is full of hits from actual physicists). QM describes a process called quantum entanglement. Quantum entanglement allows two particles to behave as one. I mean “one” in the Schrödinger equation one-wave-equation sense. Begin with two particles called A and B. These two particles don’t have definite properties until that property is measured. Entangle A with B then separate them by miles, millions if you will. Now measure a property of A (say its’ phase). Because A and B are entangled, you’ve not only measured the phase of A, but you now know the phase of B. But A and B are millions of miles away. Information must have travelled instantaneously from A to B so A could let B know the result of the measurement. We know nothing can travel faster than the speed of light. Therefore QM is wrong and B had that phase all along, even before the measurement. “In your face QM!” Einstein smugly said (Ok, he didn’t say that, but I bet he thought it).
The opposition responded to the EPR paradox. They argued since A and B is one wave equation, no information needs to travel between A and B and there is no violation of the speed of light. Einstein disagrees. For Einstein, the paradox remains. All information must be determined locally and this ‘spooky action at a distance’ is baloney. Those in Einstein’s camp are called “Local Realists”, and those that embrace the crazy (nearly all modern physicists) are not. So the question remains, do particles have definite properties even when we are not looking? If only there was an experiment that could prove which resolution of the paradox is correct.
But there is! Enter Bell and his inequality. Bell’s inequality is a simple mathematical description (baby probability simple) of a simple experiment that could prove or disprove Local Realism. The catch, Bell’s inequality experiment is simple only on paper. In reality, it is technically very difficult to implement. Since Bell published the idea in 1964, physicists have been working hard to test Local Realism in the lab. All tests (as far as I know) refuted Local Realism (thus most modern physicists reject Local Realism), yet, because the experiment is so difficult, loopholes remain. See here, here, here, and hereor a neat 2001 summary here, for a few examples.
Will Local Realism ever be truly disproven? According to an article recently posted and submitted to the Physical Review Letters, yes. The authors, researchers at the United States National Institute of Standards Technology, have designed a loop-hole free test of Bell’s inequality. Don’t trust U.S. federal researchers? The Dutch did it too – with help from U.S federal researchers (for the scoop and an image of a scientist with cool techy shades see here). The findings from the Dutch team were recently published in Nature. That means the publication has been peer reviewed, so it must be true. According to both teams, Local Realism is disproven, for good this time (not like all those other times). If this experiment holds the test of time and onslaught from hardcore local realists (are there any?), it means particles really really for realz don’t have definite properties until we measure them. It means Local Realism is dead and it means Einstein was wrong. God does play dice…but only when we aren’t looking.