Probability was not unfamiliar to the physicists of the 1920s, but it had generally been thought to reflect an imperfect knowledge of whatever was under study, not an indeterminism in the underlying physical laws. Newton’s theories of motion and gravitation had set the standard of deterministic laws. When we have reasonably precise knowledge of the location and velocity of each body in the solar system at a given moment, Newton’s laws tell us with good accuracy where they will all be for a long time in the future.
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....
In quantum mechanics the state of a system is not described by giving the position and velocity of every particle and the values and rates of change of various fields, as in classical physics. Instead, the state of any system at any moment is described by a wave function, essentially a list of numbers, one number for every possible configuration of the system.6 If the system is a single particle, then there is a number for every possible position in space that the particle may occupy. This is something like the description of a sound wave in classical physics, except that for a sound wave a number for each position in space gives the pressure of the air at that point, while for a particle in quantum mechanics the wave function’s number for a given position reflects the probability that the particle is at that position. What is so terrible about that? ...
Even so, I’m not as sure as I once was about the future of quantum mechanics. It is a bad sign that those physicists today who are most comfortable with quantum mechanics do not agree with one another about what it all means. The dispute arises chiefly regarding the nature of measurement in quantum mechanics.…
The introduction of probability into the principles of physics was disturbing to past physicists, but the trouble with quantum mechanics is not that it involves probabilities. We can live with that. The trouble is that in quantum mechanics the way that wave functions change with time is governed by an equation, the Schrödinger equation, that does not involve probabilities. It is just as deterministic as Newton’s equations of motion and gravitation. That is, given the wave function at any moment, the Schrödinger equation will tell you precisely what the wave function will be at any future time. There is not even the possibility of chaos, the extreme sensitivity to initial conditions that is possible in Newtonian mechanics. So if we regard the whole process of measurement as being governed by the equations of quantum mechanics, and these equations are perfectly deterministic, how do probabilities get into quantum mechanics?…
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. For reasons I will explain, neither approach seems to me quite satisfactory.…
The New York Review of Books
The Trouble with Quantum Mechanics
Steven Weinberg | Jack S. Josey-Welch Foundation Chair in Science and Regental Professor; Director, Theory Research Group,, University of Texas, and Nobel Prize (Physics) recipient, 1979
The Trouble with Quantum Mechanics
Steven Weinberg | Jack S. Josey-Welch Foundation Chair in Science and Regental Professor; Director, Theory Research Group,, University of Texas, and Nobel Prize (Physics) recipient, 1979
5 comments:
"Even so, I’m not as sure as I once was about the future of quantum mechanics. It is a bad sign that those physicists today who are most comfortable with quantum mechanics do not agree with one another about what it all means. The dispute arises chiefly regarding the nature of measurement in quantum mechanics.… "
Welcome to the party, pal! For decades Weinberg, a leading theorist, has been scornful at those who have raised questions, and only now does he raise the slightest concern? Better late than never, I suppose.
It's amazing that Weinberg still gets Einstein wrong in thinking that his concern was that "God plays dice". That was NOT Einstein's concern. What is it that he does not understand? Why does he get this wrong time and again? It's perfectly simple! Einstein's concern was that quantum mechanics gives rise to "telepathic methods". That there would be instantaneous action at a distance, or what we now call nonlocality, which is what we do in fact observe. The nature of this nonlocality is not well known, and is being actively researched.
What Weinberg doesn't really hit on is that nature can have locality (no instantaneous action) or reality (things existing without being observed). Physicists being perverse by nature decided long ago that things don't in fact exist unless measured by physicists and their clever instruments going "click", thus giving themselves a status reserved for deities. They'd rather keep locality than objects having an objective real existence. Some of this goes back to the creation of quantum mechanics and how some of the founders, Bohr especially, were befuddled with existentialism and other esoteric philosophies and insisted that nature must surely behave in that fashion.
Well, experiments are a bitch and they have conclusively proven that nature is nonlocal, which means we can keep reality as a concept. That hasn't stopped the theorists still maintaining that there is no objective reality! Weinberg may have inched slowly further towards the realist camp, but he still probably is an anti-realist.
Nature is strange, but not as strange as professional physicists. Einstein said that God was subtle, not malicious. Unfortunately a lot of today's physics is malicious and anything but subtle.
This is like the Darwin people too....
Maybe if they just looked at it like Ignacio says that we use stochastics when we don't fully understand something then they wouldn't get carried away with it....
There is a rote scripture about our non-Israelite ancestors: "alleging themselves to be wise, they are made stupid" Rom 1:22
This is probably a good example of this tendency in mankind....
If they would always lead with "we don't really know so we are using stochastics...." they might not ever get carried away with themselves.... another good example to always pay attention to and point out the methodology one is using at all times... both as teacher and student....
Weinberg is brilliant and his clarity of thought comes through in his writing in a rare way similar to Feynman. Even when he is writing non-technically, he doesn't short change his reader with a misleadingly dumbed down version but carefully explains what he is getting at. It means he really deeply understands the meaning of his material conceptually.
Ryan, I've read nearly all all of Weinberg's books, and Feynman for that matter. Weinberg is as clever as they come, although he does write some stupid political stuff. Everybody who has read Feynman loves him. It's impossible not to love Feynman. Weinberg's different, though: smart but not as engaging. His pop science books are legendary for good reason. His textbooks on gravitation and cosmology are utterly brilliant, clearly the work of a very keen mind, yet from someone who is not a general relativist or a cosmologist, although clearly he's a better general relativist cosmologist than most who are professors in these areas. His book on quantum mechanics is excellent if you already know quantum mechanics!
He can be mentioned in the same breath as Feynman, Witten and Wheeler, all of whom it must be said saw no real problems with quantum mechanics. Feynman was unsure that there was a problem, but also unsure that there wasn't a problem. In his later years, Wheeler returned to quantum mechanics but only to show that he had *conceptual* not technical solutions to the problems.
What Weinberg has said for decades about quantum mechanics was irrelevant. He just didn't seem to understand what the sceptics were saying. Weinberg was doing what Krugman does when talking about liquidity preference or banks. Weinberg is one of the greatest minds of the twentieth century and he can't seem to understand a perfectly straightforward argument that Einstein made again and again. It's amazing! It's not like the man isn't a genius. Here's a genius who can't seem to understand what even pop science writers understand! I just can't understand how someone as brilliant as Weinberg can't understand what is being said! It's baffling. But then MMTers have the same problems with the economics profession.
Now, Weinberg seems to have moved a fraction of an inch towards what people from Einstein to Penrose have been saying for decades. For decades Weinberg and people like him have been dismissing the sceptics as bad philosophers who weren't interested in doing real science. Apparently, Einstein and Penrose aren't real scientists! For the sceptics, this was a scientific problem that had to be resolved otherwise real progress would be hindered in other areas where you want to apply or extend quantum mechanics: quantum field theories and quantum gravity are two examples. Weinberg should have understood that.
The Bank of England economist Andy Haldane is making the news saying that the "economics profession is in crisis". Weinberg isn't saying that the theoretical physics profession is in crisis, but he's inching closer to Feynman sense of anxious uncertainty. That's progress! Depending on how good your physics is, a superb explanation of the problems that Weinberg has been brushing aside for decades can be found in Roger Penrose's new book "Fashion, Faith and Fantasy in the New Physics". Weinberg vs Penrose is a good contest! Genius vs genius squared!
For some insight on these arguments: https://www.youtube.com/watch?v=3l9HtG-VZCU
It's a talk by John Bell, for whom there are no superlatives. There's very little video footage of him, which is a real shame. He's a relative unknown compared to other theorists who trip off the tongue of the barely scientifically literate (Einstein for loads of reasons, Schrodinger and his cat, Heisenberg and his uncertainty principle, Feynman and his diagrams, etc), but for me there aren't many greater minds than his.
His work should have been awarded any number of Nobel prizes, but he died relatively young and before his work could be fully appreciated. He was decades ahead of everybody. He was apparently so humble that when others got Nobel Prizes for work that he substantially contributed towards, he shrugged it off: the work was important, not the prizes. He was a brilliant accelerator physicist, for which he could have got a Nobel; a near genius in quantum field theory and particle physics, for which he absolutely should have got a Nobel; but the greatest mind in the foundations of quantum mechanics, for which he should have got any number of Nobel Prizes but the man was too far ahead of his time.
In the next few years Anton Zeilinger and Nicolas Gisin will almost certainly be awarded a Nobel Prize, and they're working in an area that was largely shaped singlehandedly by Bell. All the work now being done in quantum information, quantum computation and quantum cryptography are in some fashion a consequence of Bell's work: he showed it could be done, and so it is!
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