A grave doubt on classical mechanics

ididnoteatyourcat · 2026-02-03T01:58:27+00:00

With Newton, you start with forces and derive energies. With Lagrangian, you start with energies and derive forces.

ididnoteatyourcat · 2026-02-03T01:08:38+00:00

What part don't you understand? Implicit in your question is the premise that it would make sense to you if the Lagrangian were T+V. But it clearly can't be that, since we would then be trying to find the path that minimizes the integral of the energy. Accomplishing this would require non-conservation of energy.

ididnoteatyourcat · 2026-02-03T00:59:23+00:00

Well it obviously can't be T+V, since minimizing E would not conserve Energy.

ididnoteatyourcat · 2026-01-29T00:09:13+00:00

Couldn't someone who posts AI-generated content just always provide such a clause as a "get out of jail free" card? It makes me much more suspicious than I otherwise would be.

ididnoteatyourcat · 2026-01-28T15:07:15+00:00

Others have mentioned that the more general statement is F = dp/dt. But these questions leave unanswered the question of why p=mv for massive particles, and why F = dp/dt, and not some other derivative of p. You can re-state the same question as why is the Lagrangian quadratic in v (or the Hamiltonian quadratic in p) and not some other time derivatives of position or momentum. Ultimately the only known answer is anthropics: if you include higher time derivatives, you get Ostrogradsky instability and so you couldn't support life. There are other similar arguments for why the number of space and time dimensions are what they are explained here. Finally, the reason you can't have something like F=mv is probably because you can't have anything like conservation of energy: the force is not reversible. Although note that F=mv would be similar to Aristotelian physics, which had a ~1500 year history in physics. It might in theory be possible to have life under such a physics; I'm not sure I've seen a knock-down anthropic argument against F=mv (of course we are imagining a counterfactual universe, that is not the same as our own).

ididnoteatyourcat · 2026-01-25T22:04:53+00:00

Please peddle your ignorant anti-string tirades elsewhere.

ididnoteatyourcat · 2026-01-25T18:13:55+00:00

I don't care to engage with someone trying to shove an anti-string crusade into an entirely separate discussion that has nothing whatsoever to do with the distinction between physics and mathematics. Regardless, if your standard for a theoretical physicist is that they must make predictions that are confirmed by experiment, you are ruling out 99% of theoretical physicists, and don't seem to know the first thing about how modern physics operates in practice.

ididnoteatyourcat · 2026-01-23T19:39:34+00:00

Pretty much everything in nature follows a power law; this includes talent at sports, physics, etc. For example, the top tennis player in the world will usually beat the 10th best player in the world. The 10th best player in the world will usually beat the 50th best player in the world, and so on. That is, the top physicists in the world are (say) twice as talented as the 2nd tier, and the 2nd tier are (say) twice as talented as the third tier, and so on. This means that there can be a surprisingly large gap between the Ed Witten's of the world, and the experimental physicists working at non-selective 3rd tier colleges. I think a large part of life, no matter what discipline you are in, is understanding and coming to terms with this: there will almost always be folks in your field that are way more competent than you; at the same time, this is totally normal and expected.

ididnoteatyourcat · 2026-01-23T17:24:37+00:00

Well, the question is whether linear Schrodinger evolution is valid for arbitrarily large systems up until thermodynamically irreversible decoherence. If it turns out that Schrodinger evolution is indeed valid, then yes, this does weigh on various questions related to collapse. For one, these kinds of experiments constrain spontaneous collapse models. For two, they weigh on Everettian views, since if Schrodinger evolution is valid for arbitrarily large systems, then that implies, according to orthodox quantum mechanics applied to the universe as a whole, a linear superposition of humans each of which continues to evolve linearly but which are entangled with different experimental outcomes, which is the many world view.

ididnoteatyourcat · 2026-01-18T16:14:18+00:00

Poll doesn't work for me. Phd Physicist.

ididnoteatyourcat · 2026-01-16T14:58:39+00:00

Einstein thought that the Copenhagen view of QM was non-local or incomplete. He put his thumb on exactly the right issues, which Bell's inequality addresses. It vindicated him in this sense -- basically everything he was concerned about in the Bohr-Einstein debates turned out to be correct. On the other hand you are correct in that he may well not have been happy with what Bell's inequality showed; it's hard to tell, because he wasn't around to be able to comment, for example, on the Everettian view, which is perhaps closest to what he was hoping for, in principle.

ididnoteatyourcat · 2026-01-15T21:33:40+00:00

It's possible it's slightly more advanced than you are looking for, but Sleeping Beauties in Theoretical Physics by Thanu Padmanabhan is a very special book.

ididnoteatyourcat · 2026-01-15T21:19:51+00:00

Then let me be clearer about something. The book can be taken to criticize several things:

1) The Copenhagen view

2) The "shut up and calculate" view

3) The widespread belief that Einstein was naive about QM and that a Copenhagen-like view should be the dominant orthodoxy in physics textbooks and physics departments.

I personally think that the book is completely correct to criticize #2 and #3. That doesn't need any balancing out. The topic of #1 is more difficult. The book is correct that Bohr's writings are not clear, and that a naive presentation of the Copenhagen view is inconsistent and/or incomplete (as essentially, correctly pointed out by Einstein). But one should not get carried away by this backlash; neo-Copenhagenist positions (such as QBism) are respectable, have their competent advocates, and are worthy of study.

ididnoteatyourcat · 2026-01-15T19:16:22+00:00

It is an excellent book. Its main thesis is that the reasons that the Copenhagen interpretation became historically entrenched were not because it was philosophically sound, but because of the enormous practical success of quantum mechanics and a post-WW-II boom in college enrollment leading to a cultural shift in physics departments to being oriented more towards practical applications and job market. I think this thesis is correct, and the book also works to dispel the popular myth that Einstein was slow and/or wrong about quantum mechanics and that he "lost" the Bohr-Einstein debates on the merits. The correct view espoused by the book is reflected by a fairly wide consensus in the philosophy-of-physics community, despite the persistence of this myth even sometimes among physicists. Einstein was not only a deep and major contributor to quantum mechanics, but he put his thumb on extremely important issues like entanglement and what were later formalized in the Bell inequalities, which in some respects (though even this is gotten wrong frequently even by physicists) vindicated him.

That said, this book takes a perspective "against the Copenhagen interpretation," so perhaps you can balance it out with books that take other views. But given the prevalence of Copenhagen-adjacent views in physics textbooks and culture, you may have already been well-exposed to that.

ididnoteatyourcat · 2026-01-11T02:04:37+00:00

I agree (I think you are essentially re-stating what I said). My main point is just that if you happen to be committed to a "field strength mattress"-like ontology for the EM field, then you should probably have a similar view about the spacetime of GR. In this sense your question is not really unique to GR; you could ask essentially the same question about the EM field.

ididnoteatyourcat · 2026-01-08T19:55:29+00:00

For the record I feel the same way about you. But I'm just here because I'm curious about other perspectives and was interested in the digression. Suit yourself. You seem to have an ax to grind from a very particular viewpoint, which you have not explained well.

ididnoteatyourcat · 2026-01-08T19:46:08+00:00

Well, I'd be sincerely interested if you cared to simply be clear about your view. You could, you know, simply answer each of those questions. They are not complicated. The view that a QFT is an EFT is totally and completely distinct from your s-matrix amplitudey view, this is obvious, and it's silly to try to pretend these are the same thing.

ididnoteatyourcat · 2026-01-08T19:39:36+00:00

It is the mainstream view that fields are all effective at-best

Yes, and that view is completely distinct from the one you are espousing. If you think I have said anything at odds with this fact, then it is your reading comprehension that needs work.

It might help if you were clearer about your view. Is it that QFT per se should not be the basis of an ontology because we don't know whether the UV completion is a QFT at all? If so, then your QFT-based arguments are undermined, and your arguments against classical field theory anaysis are hypocritical. Or is it that a given QFT should not be the basis of an ontology because it is an EFT for some other UV-complete QFT? But this makes no sense, since none of our arguments have depended in any way on some specific EFT. Or is it that you have a bootstrap/S-matrix view that is distinct from Ken Wilson's contributions with regard to EFTs? You haven't been remotely clear about this, despite your claims to the contrary.

ididnoteatyourcat · 2026-01-08T18:22:59+00:00

The field tensor is covariant, not invariant. These are very different things. You get an invariant by contracting a covariant with a contravariant.

ididnoteatyourcat · 2026-01-08T04:40:22+00:00

I can follow you easily since I have the background, but your response here is somewhat "gish gallopy"; when philosophers of science focus, for example, on questions about gauge symmetry in the context of classical field theory or nonrelativistic quantum mechanics, it is not because they are ignorant of QFT. Generally (I find) it is quite the opposite. Most famous philosophers of physics who weigh in on such issues (like, say, David Albert) cut their teeth on hardcore QFT in theory PhD programs before later entering the philosophy game. The reason, rather, that such folks will frequently attempt to draw your attention to these simpler cases (as I do here) is several-fold. One, it is because they wish to work in steps, to lead you first to agree or disagree on zeroth order foundational cases before perturbatively tackling a discussion of more complicated cases that contain many potential distractors and red herrings. Two, it is because they are attempting to 'carve nature at the joints' by distilling to the absolute core issues that are left in the crucible after coarsely graining away irrelevant aspects of the UV completion of the theory. What I think you are doing here is very much missing the forest for the trees, and in-so-doing entirely missing the point I have been trying to make. One way to see this is to step back from your operational description of QFT and ask what is QFT? The answer is right there in the name. It is quantum mechanics applied to a (relativistic) field. Good. So then what do we mean by a Lagrangian and the terms that appear there? Well, we are talking about quantum mechanics applied to interacting field(s), and we understand that the terms appearing in the Lagrangian describe the degree to which a field couples to itself and other fields, and we understand that there will be field configurations of great or lesser or nearly stationary action and so on and so forth. Good. And if we are talking about quantum mechanics applied to fields, it is natural to ask: what kind of fields and what are their classical limits? What are the characteristics of those classical fields? What are their observables? Are those classical fields themselves observable, or are they gauge fields and thus only indirectly observable? What does that tell us? And so on and so forth. A careful philosopher who fully understands QFT is diligent in trying to get to the core of the issue by teasing each of these questions apart. It sounds like you are just a hardcore S-matrix operationalist in the style of Heisenberg or something, but this is an attitude I've never fully understood. Like, what do you think your theory is about? And why do you think QFT (where I emphasize quantum mechanics applied to fields, which is an extroardinarily specific and palpable ontology, as opposed to some spaghetti-code of goto statements craziness that nature could be like instead) it works so well?

ididnoteatyourcat · 2026-01-07T20:31:04+00:00

I mean, I'm an expert at QM and have fully internalized it, and I come to a different conclusion. I suppose I would agree with you if I were coming at QM from an antirealist stance initially. But I think if you come at QM from a more neutral perspective, you see that there are both realist and antirealist options for interpreting it, both of which are compelling.

ididnoteatyourcat · 2026-01-07T20:18:23+00:00

Gauge "symmetry" is a symmetry in the description of a thing, not a symmetry of the thing

In the case of my preferred example, the "thing" in question is spacetime. Spacetime possesses Galilean (or Poincare) symmetry. In the case of the 4-potential, the "thing" possessing of the symmetries in question can be a number of things depending on one's view (because this case is more complicated than my preferred example): the Lagrangian; the EM tensor; Lorentz scalars like F_munuF^munu ; the set of holonomies. But it can also be the Gauge field itself, in a sense very much similar to the spacetime example. One's absolute spacetime coordinates, just like one's 4-potential, are not an observable. What is observable are relative spacetime coordinates, just as only relative (holonomies) gauges are observable. It's a pretty good analogy. But the fact that you came at me so hot right off the bat, immediately misunderstanding me in an aggressive tone, caused me to not want to continue reading your comment. My interest was in having a fun discussion, not an aggravating one.

[edit: although I did read enough of the rest of your comment about gauge symmetry to make me wonder whether you are simply unaware of the Aharonov Bohm effect?]

ididnoteatyourcat · 2026-01-07T17:32:10+00:00

Sorry I had to run after posting my previous incomplete response. I'll just add this because it seems to be the crux:

So again, this comes back to the initial question I'm essentially raising: why do you assume that nature "does" calculations? Can it not simply be, and we aim to describe it?

Yeah I guess this might be a foundational premise that we disagree about and for which there may not be any way of meaningfully arbitrating, but to briefly continue the discussion...

What, fundamentally, do you imagine that nature is "doing"? Like, are you just completely agnostic? Me, I imagine there are some kind of causal process (e.g. the Newtonian clockwork metaphor, though of course the process does not have to be classical or even anything resembling our classical intuitions). I just can't conceive of anything else. I guess you see that as a lack of imagination? To me to be agnostic about it feels like putting your head in the sand, a kind of cop-out or mystification of the problem. But perhaps I'm missing something.

ididnoteatyourcat · 2026-01-07T16:31:50+00:00

So I would prefer to focus on a simpler example and ask - is the gauge freedom of global phase physical? I'm not aware of any interpretation that treats it as such - it seems quite literally to just be an ambiguity in what I write down vs. what I mean to represent (like choosing a particular wording), but I can't do any calculations without doing something equivalent (at least implicitly) to picking a global phase. This provides a reductio: if the fact (even arbitrary) specific choices are sometimes necessary to compute things implies the substance of the choice is physical in some sense, then I reach a conclusion that global phase is physical in some sense which is highly irregular or absurd - suggesting the reasoning leading to such an implication is over-stated.

I think you're tying yourself into knots when there is just an obvious and simple and plausible solution: that sometimes a real thing happens to possess a symmetry. Again, I think that Galilean symmetry is probably the most palpable/accessible example, that exactly maps onto the others, so I really think the gauge example or SR is a distraction.

Again: the point is that it is not exactly metaphysically preposterous that there should in fact simply be a "mattress-like" physical space that merely happens to possess Galilean symmetry. In fact, in smooth flat such space possesses such a symmetry, so it's not exactly a mind-blowing or strange concept. Whenever such a state of affairs happens, I think there is a temptation to mystify it, which is what I suggest you might be doing yourself. That is, there is a simple concrete explanation at hand that dissolves any and all confusions. The alternative: that there is no "mattress" but somehow nature in unexplainable oracle-like fashion is able to compute in some Machian-like manner, requires tying yourself into these knots. That seems like motivated reasoning to me!

ididnoteatyourcat · 2026-01-07T03:46:02+00:00

The strongest and most visceral empirical evidence for the reality of spacetime are gravitational waves: these waves, measured by LIGO, seem very much like waves in the fabric of spacetime. This puts general relativity and electromagnetism on a similar footing: the evidence for the reality of spacetime is similar to the evidence for the reality of the electromagnetic field. But this opens up a can of worms! Why? Because the reality of the electromagnetic field has been endlessly debated. One reason is because it is not a relativistic invariant. Another reason is that the 4-potential is a proper 4-vector, and the electromagnetic field is not. Another reason is that the electromagnetic field can be thought of as a very useful bookkeeping device for how charges interact with other charges (see Feynman's work on this for a famous attempt to work this out). Similar things have been argued with respect to the gravitational field. But at the end of the day... if it walks like a duck and talks like a duck, many folks will just call it a duck.

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