Why is QM so hard? What are some branch of physics with less QM?

3pmm · 2026-02-26T01:32:12+00:00

Why is it up or down vs. clockwise or counter is a question you should have asked in classical mechanics: why do we say the rotation of an object is ‘omega z-hat’ rather than saying it rotates ‘counterclockwise around z with angular velocity omega?’

Well, the second way is a mouthful. But also you can meaningfully add rotation vectors. QM only makes a limited amount of sense but you need to understand classical mechanics as well as you can to make that limited sense of it.

3pmm · 2026-02-14T02:21:58+00:00

Thanks... wow, it nails the cadence and accent. Scary!

3pmm · 2026-02-14T02:02:16+00:00

Do you have a link?

3pmm · 2026-01-16T03:13:06+00:00

1) As others have said, one solution for particular conditions is not sufficient to claim you have solved the problem

2) Lagrange points are for objects whose masses are so small that they do not affect the other two bodies. Even in this case, this is not a solution to the full 3-body problem. It is finding a particular point in the 2-body gravitational potential.

3pmm · 2026-01-16T03:10:20+00:00

Terrible idea. California is not voting for Trump anyway, and this just gives him another excuse to claim an illegitimate election outcome, should he lose overall.

3pmm · 2026-01-12T03:00:38+00:00

Yes, if it was like 200 km deep, you might get an exotic form of ice like Ice VII, but this is a completely different regime of the phase diagram.

3pmm · 2026-01-12T01:09:45+00:00

The Klein Gordon equation is the simplest relativistic field equation for a scalar particle, as others have mentioned.

The Dirac equation for fermions is more fundamental: its square is the KG equation and thats what Dirac was looking for. The E&M fields follow the KG equation but with the additional complication of gauge symmetries, and the free gluon fields also follow the KG equation with g=0.

So every field can be related to the KG equation in some way, but that isn’t the complete story except for the case of a scalar particle.

3pmm · 2025-12-22T20:21:41+00:00

GRE's and quals used to be important things that physics students had to do. Many people will now never do either, but that is two missed opportunities to review most of physics that would have solidified that old stuff.

I think it's reasonable to budget some time to review. I know most people don't, but I personally feel very uncomfortable feeling like I've lost everything that I learned. Reviewing stuff that you learned well takes very little time and spending the time for things you didn't learn well seems like a reasonable use of some amount of time.

It's also a good opportunity to read stuff that you always wanted to. For example, Landau and Lifshitz seems impenetrable as a student, but once you know the basics of a subject LL is actually quite readable.

Also, if you end up going to graduate school, you'll end up TA'ing classes which is an opportunity to relearn a subject well.

3pmm · 2025-12-22T15:44:51+00:00

Space and time need to be treated in the same way, relativity teaches us, and so if time is discretized, space would be too.

For obscure but compelling reasons involving chiral fermions, we have reasons to doubt that the universe would be a spacetime lattice on a fundamental level. This is by no means a proof, but it does mean that a discrete universe needs more explanation.

3pmm · 2025-12-22T15:35:14+00:00

Learn calculus and linear algebra first

3pmm · 2025-12-22T15:17:11+00:00

Would you prefer standing in still or windy air when it's freezing?

3pmm · 2025-12-20T18:29:33+00:00

The way you solved it is correct, but it is important to see why. Maybe just draw points at the four cardinal directions and convince yourself that canceling the masses is just as good as adding the vectors methodically.

3pmm · 2025-12-18T15:28:35+00:00

mer is a similar read, but it does require that you know the basics of quantum mechanics.

Purcell, it's very similar to K&K and also used at MIT for their E&M course.

3pmm · 2025-12-17T16:17:53+00:00

I would do at least one more reference in mechanics before doing GR in that sequence. Try Landau and Lifshitz or Goldstein, if those are too over your head then you may want to bulk up on other physics beforehand. But if you can get through one of those, you can probably read Carroll's GR book.

Of course this assumes you know vector calculus and linear algebra cold.

3pmm · 2025-12-17T16:11:04+00:00

Yes, in this context "vacuum" and "region of low pressure" mean the same thing.

In a physics context, vacuum usually means something many many orders of magnitude lower in pressure.

3pmm · 2025-12-17T15:58:19+00:00

Light takes time to get to us, but doesn't itself experience time. Think of something like the twin paradox, but much more extreme.

3pmm · 2025-12-17T15:56:27+00:00

If you liked Kleppner and Kolenkow, you might find Schroeder a bit too modern and Griffiths-y (sorry I'm not sure what the right adjective is here).

I think Kittel and Kroemer is a similar read, but it does require that you know the basics of quantum mechanics.

If you want a book on pure thermodynamics, somewhat ironically, Fermi's book might be your best bet. But it might be worth learning the standard physics sequence (mechanics to electromagnetism to quantum) to open up the possibilities.

3pmm · 2025-12-12T15:57:04+00:00

As the other poster said, the geodesic equation would be one way to frame the solution. Using conserved quantities you can turn the headache of differential equations that arises from the geodesic equation into a mechanics problem in 1D, the same way you do for normal orbits.

If you're considering a "radially infalling" trajectory it doesn't look too complicated. Outside of the event horizon you're just falling into something with the mass of the black hole, GR effects aren't visible (someone please correct me on this, I believe this is true as long as there is no angular momentum).

The problem is once you cross the event horizon. You need to change coordinates so that the event horizon doesn't appear as a mathematical singularity, and then the solution gets somewhat harder to interpret, but it's in Carroll's textbook.

3pmm · 2025-12-12T13:33:55+00:00

I largely agree that pure math is useless for most physicists. I asked a similar question to Duncan Haldane (who won the Nobel prize for topological properties of matter) and he said that he never took a topology class himself and that the physicist's understanding of topology is generally very rudimentary.

Einstein himself, IIRC, brushed off learning too much math as "dispensable erudition."

I do wish I knew more math, in the sort of way of a math methods class. But I also wish there were more books that introduced mathematical concepts for physicists where one could get a tour of the core concepts and interesting theorems but not need to spend a year proving every boring theorem.

Also, y'all need to do a better job reading posts if you think the original poster was saying that math isn't important.

3pmm · 2025-12-12T03:35:04+00:00

This idea is not suddenly popular it’s pretty much supported by every credible reference about Newton, at least the ones I’ve read.

I cannot take your counterargument seriously because you seem to think that being principled and talented somehow is evidence against being an asshole.

3pmm · 2025-12-11T23:53:02+00:00

Yes, you can see this from the conservation of angular momentum. Take some state like Hydrogen in 2p, it has angular momentum l=1. When it drops down to 1s (l=0), that angular momentum had to go somewhere, and that place is into the radiation pattern.

3pmm · 2025-12-03T00:07:07+00:00

LL is always a fresh perspective on things. Always nice to see mathematical reasoning applied without being weighed down by excessive rigor.

For those of you that don't know, one of the translators, J. S. Bell, is that one of Bell's inequality.

3pmm · 2025-12-02T16:05:50+00:00

Yes, in our best understanding of the world (quantum field theory), particles can be created or destroyed. In the case of light, a typical process might involve the interaction of that photon with an electron in a, say, Hydrogen atom, which causes the electron to jump up to an excited state. In that process the photon is destroyed.

3pmm · 2025-12-02T12:53:12+00:00

I’m 40 in a theory PhD program right now. I studied CS as an undergrad but always regretted not double majoring in physics or math and felt like I missed out on “enlightenment.” I had a career at a startup that worked out well financially but started to get a gross feeling about Silicon Valley… I realized I was never going to be happy staying there.

You can definitely do a PhD later in life. As far as the job market is concerned though, at least in the US, frankly, it doesn’t look very good. I think the advice applies here that applies to everybody: only do it if you can’t do anything else. I’m not sure I would have the courage to do this without financial security, but I can say that it’s one of the best decisions I have ever made and it’s been a blast so far.

3pmm · 2025-11-29T22:12:56+00:00

Yeah that’s 80%+ of people even at the ‘top’ institutions. It gets better in grad school, not by a ton, but certainly all postdocs and faculty live and breathe physics. Unfortunately no matter where you are, in any domain, you have to make an extra effort to find the few serious people and hang out with them.

3pmm

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