robryk @robryk@qoto.org

@_thegeoff

IIUC you interfere two frequencies that are closeby and thus you can read out the value by looking for a non-zero-frequency envelope in the output. I haven't read enough to know how that works: do they use nonlinear material to literally create a MHzish signal, or do they sample the output light at a MHzish frequency.

(This gives you better tolerance wrt stray light, but I don't know if this is the only reason.)

@johncarlosbaez

Is instantenous speed a central example here? I'm asking because I'm surprised to find it's not an intuitive notion: I don't remember fellow classmates struggling with it (and I do remember struggles with abstractions such as a function that happens to be linear. I will probably ask my 6~8yr old "nephews" in the coming days; suggestions on concrete questions that show the difficulty are very welcome.)

@bert_hubert @johncarlosbaez

Solid state physics had lots of such, ttbomk basically at any time. (I'm not sure whether y'all would call it fundamental physics though; it's in large part about creating models for situations where we already think we have good, albeit intractable, models.)

@_thegeoff btw I've realized that heterodynic(sp?) interferometry is a thing, but haven't managed to dig up how it works exactly (in particular, what's the nonlinear mixer that can accept light and can emit RF, or how does it work without that).

@sophieschmieg

That would be surprisingly nonxenophobic.

@munin What does shot day mean?

@_thegeoff @drskyskull

Yes, sum-up amplitudes and take argument of that. (And it might be brighter/dimmer in different directions. I haven't figured out what will happen if I e.g. take a bunch of sources with positions taken from a Gaussian and with polarisations picked uniformly from a sphere.)

The other thing I wanted to point out is that this is not the thermodynamic case, where the information on which particle went where is there, but unretrievable without investing lots of entropy. In this case the information about source of the particular photon you captured doesn't exist.

@_thegeoff @drskyskull

Not exactly average phase, but something along these lines.

The reason I say along these lines instead of exactly that is quibbles around the difference between "there's literally no way to distinguish" and "our description considers these equivalent".

BTW. If the source is not monochromatic (as it never is), the notion of in phase becomes weird and complicated.

@_thegeoff @drskyskull

I think I figured it out in the meantime.

Assume a star is a collection of ideally monochromatic point sources. Then you can clearly see how they will interfere with each other at infinity in a particular direction. The net phase you observe there will be a function of phases of these sources, which avoids the "where does the timeshift asymmetry come from" problem. (Also, amusingly, the "you can't emit polarisationful wave in all directions" seems to still be the case, so there will be a line along which nothing gets emitted. If we allow the sources some bandwidth, this restriction disappears because it doesn't apply across different frequencies.)

Re "which photons we actually see": the question is ill-formed, because they interfere with each other.

@_thegeoff

I don't think this is what matters for visibility of beats. I would rather expect that what matters is relationship between 1/deltafrequency and time period you are integrating over. I'm not sure what's the effective time period eyes-as-EM-receiving-apparatus are integrating over, but it can't be more than ~1/60s. (If we had more resolving power in wavelength I could also estimate based on that, but alas.)

If I'm right and my naive estimate is roughly correct, sadly seeing beats with one's own eyes would require narrower spectra than any laser I know of (IIUC tens of kHz is already extremely narrow).

BTW an easier way of adjusting the frequency (if we had something with a narrow enough emission spectrum to start with) would be to put it in a magnetic field to split the excited state (see Quantum Light Dimmer in https://www.iypt.org/problems/problems-iypt-2024/).

@_thegeoff If you want to think in terms of photons, then I guess uncertainty principle on transverse directions might be relevant (as it's the source of diffraction limits).

@_thegeoff

Ah, this surely also relies on the size of the receiver, because it seems to be a consequence of the diffraction limit.

@_thegeoff

Yeah, I'm labouring under the same confusion. (It started from wondering what it would take to get visible beats: that would require absurdly narrow peaks (~single Hz wide) at a distance of less than ~40Hz. But then, if we somehow had that, what would determine the time offset of beats, assuming the star is large and emitting noncoherently across its surface?)

Perhaps thinking of a classical planar wave (and what can be observed about it in finite time) will be helpful here.

@_thegeoff

> Sorry for not being clearer: I was thinking more of the spatial coherence than spectral.

Or, actually, to be more precise, I was wondering how similar the situation is to a multitude of emitters emitting at different frequencies, esp. as far as phase coherence between them would manifest. (For non-spatially coherent sources the obvious answer is that you have all the possible phases coming from different directions.)

@_thegeoff

> The spectral lines are all single wavelengths when emitted

That's certainly wrong: the excited states would be stable if they were energy eigenstates. Are you saying that rotational Doppler smear is much larger than this?

> Further complicated by interstellar hydrogen clouds absorbing different wavelengths depending on their relative velocity, causing the "Lyman-alpha forest", which is a great name for a prog-rock band.

TIL. Thank you very much. Do I UC that this basically applies a filter (so can shift peaks only insofar they are not ideal Dirac deltas)?

Sorry for not being clearer: I was thinking more of the spatial coherence than spectral.

@mcc

You might wish to look up time delays involved in using melatonin first: I don't remember what they are but I do remember that they sounded unintuitive to me.

@_thegeoff

Unless I get distracted, I will probably try to make loudnesses of different frequencies to scale.

Now that I think of it: do stars produce coherent light from our POV?

@cstross

It sounds useful as a source of poison and a component of a trap.

@_thegeoff

I'm surprised to hear beats, given that I don't think I've seen destructive interference of nearby peaks in spectrum ever. Where does this difference come from?