You might sometimes hear, even from people with some scientific background, that Quantum Physics is not a well established field of science, that, it is “just a theory” (an expression you often hear about Evolution) with a lot of use of probabilities showing that this theory has some “gaps” and lacks elements that it fills using those probabilities. The favorite weapon those people will use to argue with you is the famous quote from Einstein “God does not play dice“. Those people are simply more than 50 years late in the debate, because this debate is over, with a flurry of experimental evidence, including no-go proofs. Evidence that Quantum Mechanics works (including the computer you are using to read this article) are all in the favour of Niels Bohr, who answered Einstein with the less famous: “Einstein, stop telling God what to do“. Actually and as my friend and former classmate Lê summed up: Quantum mechanics is the most accurate and tested scientific theory ! And today quantum science is not just a field for wandering blue-sky physicists, but also a billion-dollar business technology covering a broad spectrum of topics from information security to urban lighting.
In November 4th 1964, John Stewart Bell submitted his work on the Einstein-Podolsky-Rosen (EPR) paradox. About thirty years earlier, Einstein was arguing that Quantum Mechanics was an incomplete theory, proving in the EPR article that Quantum Mechanics predicts non locality: meaning that an object can be affected by an action on another distant object instantly, with no physical interaction between them (call it teleportation if you like). For the brilliant but classically-oriented mind that was Einstein this could not prove anything less than an incompleteness in the Quantum Theory.
Simplifying to the extreme, Einstein did not like the fact that Quantum Mechanics was built on a probabilistic formalism. And like any normal human being with or without a scientific background, he could not help but think that probabilistic formulation is just a way to overcome what he thought was a lack of access to some sort of ‘hidden’ information (variables): Think about finance, when you do not have access to some market values, you assume some probabilistic behaviour based on likelihood, but when you have access to data, you can evaluate things with precision. For Einstein, formulating Quantum Theory with probabilities was just an incomplete approach, while the future would show that quantum behaviour of physical objects that rules our world are intrinsically probabilistic. Bell would in some way help proving that Einstein was wrong.
About twenty years latter, Alain Aspect, with the help of Philippe Grangier and Jean Dalibard (from whom I had the privilege to receive my first Quantum Physics course), performed one of the most acknowledgeable experimental evidence for one of the most disturbing aspects of quantum theory: entanglement, and more generally the rejection of local realism: it appears that in our real world, an object S1 and another one S2, specially prepared but then physically separated, can still keep some sort of instant correlation even after physical separation.
Einstein used this as a proof of incompleteness for Quantum Mechanicsm, because for him, after physical separation, the situation of the system S2 is independant of what is done with the system S1, as we read bellow:
The physical property that makes elements that are physically separated independant from each other is called Entaglement, Einstein and others considered such behaviour to be impossible, and as Quantum Mechanics predicted it, this theory is then incomplete and the missing part is those ‘hidden’ variables we do not know. To Einstein Podolsky and Rosen, Bell gave his answer in the famous Bell’s theorem stating that:
No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.
Then: If we can reproduce all the predication of quantum mechanics, especially entanglement, Einstein’s argument for missing hidden variables to replace probabilistic aspect of Quantum Mechanics is destroyed. And the fact is that, all over the second half of the previous century, experiments were routinely reproducing entanglement, the most famous being the Aspect experiment, and just recently, the success of a team from the University of Geneva to “teleport” a particle for as far as 25 kilometers ! This video might help to get an idea about entanglement.
For advanced readers, here is an impressive visualisation from the Viena Center for Quantum Science and Technology:
Thanks to Prof John Preskill’s tweet for sparking this blog post.