| > One typically uses the Schrodinger equation to do so In my opinion knowing how to use the Schrodinger equation to get the "spectrum of the hydrogen atom" is essentially a matter of historical interest but really not relevant to understanding things. Its quite cool you can do these tricks to derive a nice analytical form for the spectrum, but this approach emphatically does not generalise to more complicated systems (any non-trivial molecule) and even for the hydrogen atom the spectrum you get will be wrong anyway because of relativistic corrections and QFT-corrections. > But you conveniently forgot how to "derive" the part of QM that actually gives you the value of the commutator sitting on the right-hand side. I'm not sure what you're arguing is missing here? Once you've derived Robertson-Schrödinger you've just got a commutator there, for whatever observables you want to apply it to you just plug in the value. >No teacher of QM should introduce POVMs before talking about positions and momenta. I'm not talking about teaching here but thinking. You are probably right that most physics undergrads would not cope well with learning about POVMs. On the other hand I am tempted to argue for not teaching about the position operator and position in Schrödinger-style QM at all, or at least leaving it until quite late on. The way people teach QM has this weird thing where its pretty obviously wrong, because every physics undergrad knows we have special relativity, so there should be some nice symmetry between space and time which is completely missing in the Schrödinger equation. Time in the Schrödinger equation is a coordinate, and space (position) is a self-adjoint operator, which is just manifestly weird. Once you get to quantum field theory this gets fixed and position isn't an operator/observable anymore, it gets demoted back to a coordinate exactly the same as time. |
FWIW, as someone who is interested in science pedagogy, and specifically as someone who actively engages with anti-science propaganda like young-earth creationism, I want to contribute this:
> In my opinion knowing how to use the Schrodinger equation to get the "spectrum of the hydrogen atom" is essentially a matter of historical interest but really not relevant to understanding things.
IMHO this is more than historical interest. It's a dramatic illustration of how science actually works, and specifically, that it does not rely on any appeal to authority, despite the superficial appearance of occasionally hearing people say things like, "Einstein teaches us that X" with the implication that X is therefore unquestionable gospel because Einstein said it. Here is an example of a calculation that anyone can do (with enough effort) and compare to the results of experiments that they can likewise do themselves (with enough effort). Of course, most people won't bother to put in this effort, but just knowing that they could if they wanted to is very powerful because it provides an actual reason why other people's results are generally trustworthy: even if you don't do the experiment, someone else might, and if the result turns out to be wrong then it will eventually be called out.
Also...
> this approach emphatically does not generalise to more complicated systems
This is spot on. Speaking from first-hand experience of my own intellectual journey into QM, focusing on single-particle systems and slogans like "any attempt to measure the position of the particle destroys the interference in the two-slit experiment" is extremely misleading. It leads to conceptual dead-ends that make it much harder to wrap your brain around entanglement than it should be. IMHO, QM pedagogy should start with entanglement and decoherence. In this respect, I think Aaronson gets it right.
But mainly I just wanted to thank you both for the privilege of being a fly on the wall while you discuss these things. It has generated a long reading list for me.