Where I last left off, Einstein (with colleagues Podolsky and Rosen, a trio sometimes known as EPR) had questioned the Heisenberg Uncertainty Principle's suggestion that a particle could not simultaneously have a definite position and a definite velocity, and had questioned quantum mechanics more generally. One of the arguments put forth by EPR leads into a discussion of the topic I'd like to cover in today's entry, quantum entanglement.
In his book The Fabric of the Cosmos, Brian Greene notes how there are analogies in particle physics to the example of two people standing back to back then each starting to walk in opposite directions at the same speed. However many miles east, let's say, of the starting point one person is, the other person would be the same number of miles west of it. Greene notes that:
...if an initial single particle should disintegrate into two particles of equal mass that fly off "back-to-back"..., something that is common in the realm of subatomic particle physics, the velocities of the two constituents will be equal and opposite (p. 100).
According to Greene, EPR's argument followed thusly:
...imagine you measure the position of the right-moving particle and in this way learn, indirectly, the position of the left-moving particle. EPR argued that since you have done nothing, absolutely nothing, to the left-moving particle, it must have had this position, and all you have done is determine it, albeit indirectly (p. 101).
The research that ultimately contradicted EPR and supported quantum mechanics and all its weirdness showed that measurement of one particle of a pair actually did appear to affect the properties of the other particle. This type of finding has now been demonstrated with the two particles being as far as 11 kilometers (over 6 miles) apart. Adds Greene:
On the microscopic scale of the photon's wavelengths, 11 kilometers is gargantuan. It might as well be 11 million kilometers -- or 11 billion light-years, for that matter. There is every reason to believe that the correlation between the photons would persist no matter how far apart the detectors are placed (p. 115).
When two particles of a pair had corresponding (correlated) properties, skeptics initially could argue that such correlation may have been preprogrammed into the pair from their common origin. Such an objection was ultimately overcome by a statistical type of argument (credited to John Bell) and experiments supportive of it. This is all summarized at great length in Greene's chapter on quantum entanglement.
On this whole phenomenon and whether special relativity's provision that nothing can travel faster than the speed of light is possibly violated, Greene offers some reflections:
At the end of the day... two widely separated particles... somehow stay sufficiently "in touch" so that whatever one does, the other instantly does too. And that seems to suggest that some kind of faster-than-light something is operating between them. Where do we stand? There is no ironclad, universally accepted answer... (pp. 117-118).
Quantum entanglement may also serve as a mechanism for teleportation, which is sort of like "faxing" a physical object from one place to another. Greene discusses this in another of his chapters.
For further reading, general-audience articles on quantum entanglement and its possible implications are available here and here.