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8:29 AM
When we're talking about a group central extension in QM, are we using that as shorthand for specifically a U(1) central extension?
 
9:23 AM
At least in the basic setup, I've never seen a discussion of whether one can study say global $\mathrm{SU}(2)$ or something
 
 
2 hours later…
11:12 AM
@Slereah sometimes it's also a $\mathbb{Z}_p$ extension (covering groups are such discrete extensions by the fundamental group of the covered group)
@bolbteppa Since $\mathrm{SU}(2)$ is not abelian, it cannot be a center
 
Right, but you can still have $\mathrm{SU}(2)$ 'phase factors' depending on your states, and this projective rep discussion looks like it could be adapted, but I'm not sure
 
 
1 hour later…
12:39 PM
Why would one expect that highly exicted harmonic oscillator eigenstates give the classical probability density of harmonic oscillator? Of course by correspondence principle, but the classical probability density is about finding a h.o. in a given position at a *random time*, while the statistical interpretation of QM has nothing to do with time

Sounds handwavy to me
 
yes, the correspondence principle is a handwave
 
This is the first time I find a situation where it barely makes sense
I mean, the "classical probability density" is a trivial consequence of its oscillatory motion. Quantum probability density is about the statistical collapse of a wavefunction :/
 
classical limits are hard to get right, and they are not unique
you can equally well (perhaps even better) argue that the correct notion of "classical state" for the QHO is not "large $n$" but a coherent state with large $\alpha$
 
Exactly what I was writing
high $\alpha$ coherent states describe a localized system oscillating (what a classical h.o. actually is). The thing I wrote above seems to compare two conceptually different things
 
1:35 PM
Just found a useful footnote on Griffiths. We can instead think of an ensemble of oscillators with different initial positions and we measure the ensemble positions all at the same time. This at leats makes it somehow closer to QM
 
 
2 hours later…
3:43 PM
Side note: does anyone else find it ironic that someone with the username feynman is critical of a hand waving argument
 
4:08 PM
I wasn't criticizing the handwaving per se. I was trying to make sense of the idea underlying
 
I understand, I was just making an observation :-)
 
5:06 PM
Regarding your observation, I sometimes feel "guilty" about this username :P
 
5:45 PM
Hi, everybody.
Why do so few figures in physics journals have grid lines?
 
6:01 PM
@DanielSank why would they include grid lines?
especially in greyscale I find graphs on grids more annoying to look at
 
 
1 hour later…
7:31 PM
It's a shame the stack exchange app was abandoned
Was cool having a widget for the HNQ and getting notifications about replies
 
yeah, I liked it much better than the mobile site
 
fqq
@DanielSank I agree with ACM that most figures are just better without grid lines. Anecdotally it seems to me that they used to be more common a few decades ago and then we collectively moved away from that. Not sure if there's an actual reason or just due to default settings in software
 

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