Gerry Canavan

the smartest kid on earth

Realism and Quantum Mechanics

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Seed has a long article on recent experiments in quantum mechanics that seem to disprove realism, at least on the quantum level:

Leggett doesn’t believe quantum mechanics is correct, and there are few places for a person of such disbelief to now turn. But Leggett decided to find out what believing in quantum mechanics might require. He worked out what would happen if one took the idea of nonlocality in quantum mechanics seriously, by allowing for just about any possible outside influences on a detector set to register polarizations of light. Any unknown event might change what is measured. The only assumption Leggett made was that a natural form of realism hold true; photons should have measurable polarizations that exist before they are measured. With this he laboriously derived a new set of hidden variables theorems and inequalities as Bell once had. But whereas Bell’s work could not distinguish between realism and locality, Leggett’s did. The two could be tested.

When Aspelmeyer returned to Vienna, he grabbed the nearest theorist he could find, Tomasz Paterek, whom everyone calls “Tomek.” Tomek was at the IQOQI on fellowship from his native Poland and together, they enlisted Simon Gröblacher, Aspelmeyer’s student. With Leggett’s assistance, the three spent six months painfully checking his calculations. They even found a small error. Then they set about recasting the idea, with a few of the other resident theorists, into a form they could test. When they were done, they went to visit Anton Zeilinger. The experiment wouldn’t be too difficult, but understanding it would. It took them months to reach their tentative conclusion: If quantum mechanics described the data, then the lights’ polarizations didn’t exist before being measured. Realism in quantum mechanics would be untenable.

This is a very good, really thought-provoking article on a subject that fascinates me but which I simply do not have the training to properly understand. Whenever I read about quantum mechanics I feel as though I can almost, but not quite, grasp it.

Especially thought-provoking is this:

Last year Brukner and his student Johannes Kofler decided to figure out why we do not perceive the quantum phenomena around us. If quantum mechanics holds universally for atoms, why do we not see directly its effects in bulk?

Most physicists believe that quantum effects get washed out when there are a large number of particles around. The particles are in constant interaction and their environment serves to “decohere” the quantum world—eliminate superpositions—to create the classical one we observe. Quantum mechanics has within it its own demise, and the process is too rapid to ever see. Zeilinger’s group, which has tested decoherence, does not believe there is a fundamental limit on the size of an object to observe superposition. Superpositions should exist even for objects we see, similar to the infamous example of Schrödinger’s cat. In fact, Gröblacher now spends his nights testing larger-scale quantum mechanics in which a small mirror is humanely substituted for a cat.

Brukner and Kofler had a simple idea. They wanted to find out what would happen if they assumed that a reality similar to the one we experience is true–every large object has only one value for each measurable property that does not change. In other words, you know your couch is blue, and you don’t expect to be able to alter it just by looking. This form of realism, “macrorealism,” was first posited by Leggett in the 1980s.

Late last year Brukner and Kofler showed that it does not matter how many particles are around, or how large an object is, quantum mechanics always holds true. The reason we see our world as we do is because of what we use to observe it. The human body is a just barely adequate measuring device. Quantum mechanics does not always wash itself out, but to observe its effects for larger and larger objects we would need more and more accurate measurement devices. We just do not have the sensitivity to observe the quantum effects around us. In essence we do create the classical world we perceive, and as Brukner said, “There could be other classical worlds completely different from ours.”

Written by gerrycanavan

June 9, 2008 at 2:41 pm

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