July 5, 2014


Quantum state may be a real thing : Physicists summon up their courage and go after the nature of reality. (Chris Lee - July 2 2014, Ars Technica)
At the very heart of quantum mechanics lies a monster waiting to consume unwary minds. This monster goes by the name The Nature of Reality™. The greatest of physicists have taken one look into its mouth, saw the size of its teeth, and were consumed. Niels Bohr denied the existence of the monster after he nonchalantly (and very quietly) exited the monster's lair muttering "shut up and calculate." Einstein caught a glimpse of the teeth and fainted. He was reportedly rescued by Erwin Schrödinger at great personal risk, but neither really recovered from their encounter with the beast.

The upshot is that we had a group of physicists and philosophers who didn't believe that quantum mechanics represents reality but that it was all we could see of some deeper, more fundamental theory. A subclass of these scientists believed that the randomness of quantum mechanics would eventually be explained by some non-random, deterministic property that we simply couldn't directly observe (otherwise known as a hidden variable). Another group ended up believing that quantum mechanics did represent reality, and that, yes, reality was non-local, and possibly not very real either. [...]

[S]ay I shoot a single photon at a single atom, which may or may not absorb the photon. According to quantum mechanics, the atom enters a superposition state where it's both in its ground state and its excited state. We describe this superposition state with a wave function. One view of quantum mechanics states that the wave function really represents the atom. But an alternative interpretation is that the wave function represents what I, the observer, know about the atom--reality may be something else entirely. [...]

Now, a group of researchers has extended previous work to show that, yes, under a wide range of conditions, these two points of view do differ. They show that the wave function must in some sense represent the observed system rather than what the observer knows about the system.

Their work essentially boils down to creating a measure of how much two probability distribution functions overlap. They use this latest research to argue that no matter what wave function is used and what it represents, the measurement results must remain the same. That is, over multiple measurements, we should obtain the same probability distribution function. So even if the wave functions are indistinguishable, do they all reproduce the measurement results?

The answer is not very simple. The proof that the answer is correct is only understandable to other theoretical physicists, and it only applies to wave functions with three or more dimensions (these are very common). Nevertheless, let me take a shot at explaining it anyway.

If you take the view that the wave function only produces a probability distribution and then take all the wave functions that produce the same probability distribution--in other words, the observer's possible choices of wave functions, based on his or her knowledge of the system--and try to reproduce measurement results, you'll fail. Consequently, there is a single wave function that must represent reality.

Two thousand years of Science just to get back to where we started : the homocentric universe.
Posted by at July 5, 2014 6:48 AM

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