User talk:Johnjbarton/sandbox/introduction to quantum mechanics

Entry: Bad data to start (unmeasurable initial conditions)

(Copied from the page by Johnjbarton (talk) 01:39, 19 July 2023 (UTC))

I especially like the last entry--there are so many descriptions of the randomness in the double-slit experiment that fail to mention that "coherent light" is quite noisy: each individual photon has its own path--direction and offset through the slits, and they are not well aligned with each other. No wonder physicists have been quick to say that QM itself is all random, when it is not. There is nothing random about the Schrödinger equation. David Spector (talk) 17:43, 18 July 2023 (UTC) Johnjbarton (talk) 01:39, 19 July 2023 (UTC)

@David spector I moved your comment to Talk.

Yes, one of the issues I think we need to take up is "probability". To me is quite interesting that many different kinds of probability apply to QM. In fact we should have an article just on that. For the Introduction probability is important and I think it would be ok to explore the issue qualitatively. Johnjbarton (talk) 01:44, 19 July 2023 (UTC)

Current thinking on a topic list.

I've started a non-traditional topic list. I started off in this direction because 1) the subject is notoriously difficult, so retracing existing topic lists seems unlikely to win, 2) many of the issues for intro level concerns unknown or to little preparation so why not start with jargon free content, 3) the core issues that call for an introduction to QM are also core issues in the subject of QM.

So rather than "10 wild wacky things about QM', I'm thinking that we start off with quite a lot of material on 'why is QM difficult'? I originally just wrote "why difficult" as a topic but presto I realized the answer to that question is itself most of the introduction to QM. By describing the difficulties we are more on the readers side.

I'm hoping that readers equipped with 'why is difficult' will then recognize the jargony content as a second reinforcing path to the concepts. Johnjbarton (talk) 02:12, 19 July 2023 (UTC)

I'm fine with "why is QM so difficult" as a topic or article, but I would vastly prefer in its place, "why is QM actually not so difficult as it first appears?" There is, in my opinion, plenty of material, both qualitative and quantitative, that is easy to understand about how the physics of the very tiny is different from the familiar physics of our own very large size, and even the physics of the enormous Universe of galaxy clusters (with its Lorentz/SR character that is also different from our own regime).

This is especially true for QM if we were to boldly introduce the Bohmian point of view that Bell so loved (and proved) right at the beginning. It eliminates the strange nondeterminism that is at the heart of the current standard interpretation of QM, and simplifies wave/particle duality. Bohm also pointed out that a simplified measurement device can be included inside the experiment, so that it is included in the experiment's wave function, eliminating any collapse or other measurement problem.

What is left is nonlocality, which is not at all difficult to understand; it's just different, that's all. And states and wave functions, which are completely linear. To cap it all off, QM scales up to produce classical physics statistically, just like temperature and pressure are scaled-up understandings. That's easy to understand, too. Just my opinion. David Spector (talk) 10:24, 19 July 2023 (UTC)

Hopefully the material will cause a reader to come to the conclusion that QM is not that difficult after all ;-).

Whether determinism or non-determinism is strange depends on one's expectations: it's a matter of philosophical mismatch. (For me, determinism is completely weird, physically exotic, and unjustified.) Causality and locality are questions of physics, though not at all straight forward.

Many great minds of the last 100 years have tried to produce verifiable additions to QM that would select among or create anew an interpretation of QM. They have not succeeded. But so much has been accomplished that we have plenty to report on. Johnjbarton (talk) 17:34, 19 July 2023 (UTC)

I wish you wouldn't assume that David Bohm failed in his interpretation. At least read what he proposed before you voice such a definite opinion. Let me know if you need the online reference.

So, determinism is weird? What is weird about a force causing an acceleration according to a=F/m ? Can you even propose one example of determinism that is demonstrably weird? Nondeterminism doesn't even exist in classical mechanics, which is why the Cat in the Box thought experiment is so striking at first. David Spector (talk) 19:13, 19 July 2023 (UTC)

? Bohm didn't fail; interpretations can't fail. That's why they are interpretations and not theories. The only opinion I have concerning Bohm here is that we can't succeed at an Introduction based on Bohm. It would not represent physics, it would be controversial among editors, and it doesn't provide significant advantages. (My opinion on Bohm is not strong but, more important, not relevant).

Determinism is philosophical stance not a physical one. Newton's equations make great predictions: that's physics. Concluding that all future and past in the universe can be predicted because Newton's equation work well is philosophy. Bohm's model is physics (that doesn't happen to make predictions different from other interpretations). Concluding that Bohm's model for probabilisitic results is correct because it fits with some non-physics criteria is philosophy. The physics parts could be in an Introduction and I'm not against eventually trying an interpretations section, but it would need to cover the main players to be physics.

Classical physics is alone among sciences in providing some basis for determinism. Otherwise probabilities rule. That is what I meant about weird etc. Johnjbarton (talk) 22:23, 19 July 2023 (UTC)

We will have to agree to disagree about both issues, as I am not persuaded by your arguments. Bohm did indeed make predictions different from other interpretations, one of which I've mentioned to you at least twice before: the prediction of deterministic particle trajectories through experiments like the double slit. His prediction was for a very specific family of trajectories that reproduce interference fringes. The only other interpretation that would be quite similar is Many Worlds, and, as you know, that interpretation cannot be tested due to its reliance on, well, many worlds; we only live in one Universe. And, in 2011 or so, an experiment using weak measurement supported Bohm's prediction, which is the way predictions and their theories are tested for validity. Let me know if you would like a reference for the experiment, or its replication a few years later. As to the other point, I'll just go out on a limb and say that you're wrong: determinism makes more sense than nondeterminism, anywhere other than in grammar and computation theories, where it makes sense due to the ability of an algorithm to take two paths at the same time, comparing them both during and after. In real life, nondeterminism is indistinguishable from magic, religion, and insanity. Yet physicists believe in it only because it was enshrined as an axiom around 1927. Believing in nondeterminism doesn't make any QM calculation work better, but it does impress others with your ability to tolerate mysticism in the name of science. David Spector (talk) 22:37, 19 July 2023 (UTC)

Yes I guess we shall have to disagree. Ironically, I have almost exactly your opinion towards nondeterminism but about determinism. Determinism ultimately means there is a "cause of all causes".

I would be interested in any references to good physics articles. I would be especially interested in models for radioactivity.

I think you are confusing the broad lack of enthusiasm for Bohm for a "belief" in some alternative. There is no conclusive evidence for any QM interpretation. Full stop. That is the broad consensus in physics. It's not about some alternative. It's not that Copenhagen is a religion. This is simply something that is not known. Pretending that it is known and that Copenhagen or Bohm is the answer is just pretending. We don't know. I think Copenhagen is widely used because it is the one with the least additional context. True Copenhagen makes no theory about trajectories or anything other than observations.

My personal opinion is that all these interpretations, including Copenhagen and Bohm, are quite similar to the theories of Aether back in the 19th century. Lots of arguments about something that in the end was not a thing. Johnjbarton (talk) 02:10, 20 July 2023 (UTC)

I disagree with every major statement of fact that you've made here, perhaps four or so. The only one I'm willing to argue further about (because it is clear you will never understand my point of view on the others) is determinism, because you've confused determinism in physics with determinism in human affairs (which is philosophy). They are two different kettles of fish, the confusion or conflation of which accounts for your opposite view of nondeterminism.

My philosophical stance is nonduality, in which there are two quite different ways to view reality: from the individual and limited approach (the result of ignorance of who we really are, and the cause of selfishness, suffering, and war), where there seem to be innumerable causes for most effects, and where physics tries to simplify and locate just one at a time, and from the universal and unbounded subjective approach, in which there are no causes because there are no events (also no space, time, energy, matter, Universe, or physics). David Spector (talk) 10:31, 20 July 2023 (UTC)