Thursday, May 16, 2019

Astronomer writes book on Bell

A new book asserts:
The fastest route to the insight into the ultimate nature of reality revealed by quantum mechanics, Greenstein writes, is through Bell's Theorem, which concerns reality at the quantum level; and Bell's 1964 discovery drives Greenstein's quest. Greenstein recounts a scientific odyssey that begins with Einstein, continues with Bell, and culminates with today's push to develop an industry of quantum machines. Along the way, he discusses spin, entanglement, experimental metaphysics, and quantum teleportation, often with easy-to-grasp analogies. We have known for decades that the world of the quantum was strange, but, Greenstein says, not until John Bell came along did we know just how strange.
No. The essence of Bell's theorem is that quantum mechanics is not a classical theory. In particular, the quantum probabilities are not just hiding a lack of knowledge about classical physical variables.

Bell just confirmed what had been conventional wisdom for 40 years.

The author explains in SciAm:
For in truth I believe that it is the story of everyone who has encountered this bizarre world. The realm of the quantum is utterly unfamiliar, utterly strange and utterly incomprehensible. Nothing in it corresponds to everyday reality. And more than that: nothing in it can be comprehended in ordinary terms.

“How can an electron be in two places at once?” I had been asking for so many years. “How can something happen without a cause?” I have not answered these questions. But so what? At long last I have achieved what to me is a great victory. I have expressed to myself clearly what the mystery is.

And sometimes I wonder if it is a mystery. Perhaps it is just a fact. This is the way the world is. Do I like this new cosmos that we have stumbled into? Do I dislike it? Is it congenial to my thoughts, or utterly alien to them?

It makes no difference: this is the new world — get used to it. ...

George Greenstein is professor emeritus of astronomy at Amherst College. He is the author of the new book, Quantum Strangeness: Wrestling with Bell's Theorem and the Ultimate Nature of Reality (The MIT Press, 2019).
I haven't seen the book, but this does not sound helpful. Electrons are not really in two places at once.

The electron goes thru both slits of the double-slit experiment, but that is because the electron is a wave that is not localized to a point. To describe the electron as being localized to two different points in different locations is weird.

Denying causation is even weird. Nothing in quantum mechanics is inconsistent with causality. It may be that you can only give a probability for a radioactive decay, but something is still causing that decay, as far as we know.

Lubos Motl explains explains why he rejects reality in the quantum domain:
As an undergraduate freshman, I was already exposed to basic texts on string theory – a librarian generously xeroxed a textbook by Green, Schwarz, Witten for me (for free), she probably risked copyright infringements as well – and I knew something about T-duality soon afterwords (other dualities were only discovered in the mid 1990s and made the picture thicker). So even if I had some tendencies for "realism" at that time, I had to choose: T-duality or the realist prejudices? Clearly, I would have chosen T-dualities because they're beautiful. They are really local symmetries of string theory and a symmetry is always a great improvement of a theory that makes the theory more likely. There exists absolutely no reason why the symmetry should be fundamentally broken so it's almost certainly not broken. Arguments based on symmetries and other kinds of mathematical beauty always trump purely metaphysical prejudices – everyone who has at least some physical intuition agrees.
His reasoning is a little peculiar. He sounds like a medieval monk who bases his belief on a rare sacred text that was recently transcribed for him.

Realism, in this context, means believing in things and mechanisms that are not directly observed. It is okay to believe in the dark side of the Moon, because that is consistent with our best theories. But believing in local hidden variables controlling electrons is not.

1 comment:

  1. Total nonsense. Bell was about an OBSERVABLE, space-like separated non-local correlation. Quantum mechanics was basically discovered in the 17th and 18th centuries. Newton's theory of fits of easy reflection and transmission!