Cosmologic philosophy: Weinberg on quantum theory (Introduction)

by David Turell , Saturday, January 07, 2017, 01:46 (2878 days ago) @ David Turell

And Steven Weinberg adds his confusion about quantum theory and non-locality:

http://www.nybooks.com/articles/2017/01/19/trouble-with-quantum-mechanics/

"Probability enters Newtonian physics only when our knowledge is imperfect, as for example when we do not have precise knowledge of how a pair of dice is thrown. But with the new quantum mechanics, the moment-to-moment determinism of the laws of physics themselves seemed to be lost.

"All very strange. In a 1926 letter to Born, Einstein complained:
Quantum mechanics is very impressive. But an inner voice tells me that it is not yet the real thing. The theory produces a good deal but hardly brings us closer to the secret of the Old One. I am at all events convinced that He does not play dice.

"As late as 1964, in his Messenger lectures at Cornell, Richard Feynman lamented, “I think I can safely say that no one understands quantum mechanics.”3 With quantum mechanics, the break with the past was so sharp that all earlier physical theories became known as “classical.”

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Today there are two widely followed approaches to quantum mechanics, the “realist” and “instrumentalist” approaches, which view the origin of probability in measurement in two very different ways.9 For reasons I will explain, neither approach seems to me quite satisfactory.

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"The realist approach to quantum mechanics had already run into a different sort of trouble long before Everett wrote about multiple histories. It was emphasized in a 1935 paper by Einstein with his coworkers Boris Podolsky and Nathan Rosen, and arises in connection with the phenomenon of “entanglement.”

"We naturally tend to think that reality can be described locally. I can say what is happening in my laboratory, and you can say what is happening in yours, but we don’t have to talk about both at the same time. But in quantum mechanics it is possible for a system to be in an entangled state that involves correlations between parts of the system that are arbitrarily far apart, like the two ends of a very long rigid stick.

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"Strange as it is, the entanglement entailed by quantum mechanics is actually observed experimentally. But how can something so nonlocal represent reality?

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"What then must be done about the shortcomings of quantum mechanics? One reasonable response is contained in the legendary advice to inquiring students: “Shut up and calculate!” There is no argument about how to use quantum mechanics, only how to describe what it means, so perhaps the problem is merely one of words.

"On the other hand, the problems of understanding measurement in the present form of quantum mechanics may be warning us that the theory needs modification. Quantum mechanics works so well for atoms that any new theory would have to be nearly indistinguishable from quantum mechanics when applied to such small things. But a new theory might be designed so that the superpositions of states of large things like physicists and their apparatus even in isolation suffer an actual rapid spontaneous collapse, in which probabilities evolve to give the results expected in quantum mechanics.

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"One difficulty in developing such a new theory is that we get no direction from experiment—all data so far agree with ordinary quantum mechanics. We do get some help, however, from some general principles, which turn out to provide surprisingly strict constraints on any new theory."

Comment: Weinberg is unhappy as we all are about the confusion that is quantum theory but we are stuck with the knowledge that our universe works because of quantum mechanics. Is it God's trick to make us confused? Or perhaps we are not bright enough to understand the answers.