EPR paradox?

Question

What is the EPR Paradox and what is Einstein's 'hidden variables idea'?

Answer 1

Unbeknownst to most, Einstein was a major opponent of quantum physics (that's right, he hated it). Ironically, Einstein was also one of the foundng fathers of QM (along with Planck, Heisenberg, Bohr and Schrodinger), and he won the Nobel prize in 1921, not for Theory of Relativity, but for the Photoelectric Effect (http://en.wikipedia.org/wiki/Photoelectric_effect), which illustrated the quantum nature of light and the wave-particle duality.

For over 30 years, Einstein attacked quantum physics and its devleopment. He nearly drove Neil Bohr insane, and estranged many of the young brilliant minds coming into the field at the time. They simply didn't know what to do with a living legend which is one of the greatest intellects ever lived, and yet he was not contributing to their efforts to understand nature. Quite the opposite, they have to often defend their efforts.

In 1935, Einstein, Podolsky, and Rosen published a paper that many physicists at the time thought casted a great shadow over the legitimacy of quantum mechanics. And the only reason that they didn't abandon QM was that they didn't have another theory capable of replacing it. But the EPR Paradox kept many physicists wondering if QM was on the wrong track.

So here is the main ideas behind EPR's thought experiment:

In physics, there is a notion of "principle of locality", which states that distant objects cannot have direct influence on one another: an object is influenced directly only by its immediate surroundings. This also follows after Einstein publishes Special Relativity, making C, the speed of light, the absolute speed limit on what objects can or cannot influence another object.

So Einstein devised a thought experiment that would demonstrate the incompleteness of quantum mechanics, by violating the "principle of locality". There is a very famous quote by Einstein where he was quoted saying that quantum mechanics is "spooky action at a distance".

In any case, the EPR paradox draws on a quatum phenomenon predicted by quantum mechanics, known as quantum entanglement, where the quantum states of two or more objects have to be described with reference to each other, even though the individual objects may be spatially separated by some arbitrarily large distance.

The actual thought experiment involves the meaurement of the spins of a pair of entangled-particles (which we really don't need to get into here). And the main point of Einstein's argument is that, either:

(1) The result of a measurement performed on one part A of a quantum system has a "non-local" effect on the physical reality of another distant part B, in the sense that quantum mechanics violates "locality", or

(2) Quantum mechanics is incomplete in the sense that some element of physical reality corresponding to B cannot be accounted for by quantum mechanics (that is, some extra hidden variable is needed to account for it.)

At the time of the publication of the EPR Paradox, no one understood enough about quantum entanglement to refute it, and no one was able to perform actual experiements to verify the phenomenon. So Einstein's proposal of "hidden variables" stuck as an explanation for the nature of QM.

Answer 2

Wikipedia, help!