Welcome to DanielSank's weekly featured post chat room

Each week I'll feature a post from the main site, comment on why I find it interesting, and invite any and all discussion on that post and related topics.

I think it appropriate to start with my favorite Physics Stack Exchange post:

I like this post for many reasons.

Blackbody radiation is a very important topic in physics. It plays an important role in history in that deriving the blackbody spectrum from first principles is what initially lead Max Planck to invent the idea of quantized occupation of modes.

it is a rather interesting post

Blackbody radiation plays an important role in theory, in all things from the cosmic microwave background, to the voltage noise of a resistor!

i like the idea of this chatroom, by the way.

However, despite the incredibly far reaching and deep importance of blackbody radiation, we never really have a chance to experience it because of the limitations of our senses. Looking at a blackbody radiator, we most perceive a single color, or at least, I would say we fail to perceive the full spectral density of the emitted radiation.

After all, our eyes are only able to view a tiny range of the electromagnetic spectrum.

The featured post, however, gives us blackbody radiation in a form we can really experience!

By varying the temperature of the noise, the author gives us example sound recordings in which the bulk of the noise energy lies within the range of human hearing.

This, alone, makes the post absolutely delightful in my opinion.

@DanielSank Now you've got me wondering if you could apply this to, say, stars, assuming the Stefan-Boltzmann law.

This is an excellent room, by the way. I love the idea.

Beyond that, I find the question very well posed, and the answer very thorough, clear, and practical in that the author actually provides sound recordings!

On top of all of that, I simply find noise fascinating and generally enjoy any question/answer on PSE offering an opportunity to learn more about noise.

@heather @HDE226868 glad you like the idea.

Perhaps this week we could put together an easy-to-download-and-run set of code that other users can play around with (to extend beyond the canned recordings provided in the featured post).

That would be interesting.

@HDE226868 what do you mean?

@DanielSank Assume that a star is a blackbody. Determine its temperature, then feed that into 彩音M's equation to get the frequency-dependent expression for $E(f,T_*)$.

Fluctuations - in variable stars, for instance - would lead to sounds that vary over time as the star pulsates.

@HDE226868 Well yeah but for the temperature of a star the energy would be at frequencies waaaaay above what you can hear.

Note that the temperatures used in the post are in the nanoKelvin range!

@DanielSank . . . Right. I forgot nanoKelvins.

One thing that I'd find interesting is to listen to blackbody noise for 1, 2, and 3 dimensions and see how they sound different.

The fomulae for the spectrum depend on dimension. The featured post uses 3D, I believe.

The Johnson-Nyquist formula is for 1D, I think.

Are there any major applications of 2D noise? Wikipedia indicates that the 1D formula can be applied to charge carriers.

@HDE226868 Well, sure. A drum head would emit 2D blackbody radiation.

Probably same thing for a thin metal plate.

@DanielSank Huh, really? I had no idea the blackbody formula was that wide-ranging.

@HDE226868 It applies to anything with modes.

Well, bosonic modes, I suppose.

If you read the original paper on Nyquist noise for a resistor you'll see why blackbody radiation applies to pretty much everything.

I think this is the paper.

@HDE226868 isnt this already done, like they can create acoustic signatures for stars

The argument is essentially just application of the equipartition theorem and some simple reasoning.

also howdy

so now a question based off this Dan

@Skyler I think those are often based on extrapolation from light curves, though I'm not familiar with the details.

as experimenters we are now hearing what exactly?

@Skyler I'm not sure what you're asking.

sound traditionally is thought of as vibrating air, are we now instead just directly mapping photons onto sound bands audible to us

@Skyler Blackbody radiation has a certain temperature dependent spectral density.

The featured post simply generates sound with that spectral density.

In some systems, that spectral density would be manifest in electromagnetic radiation. In this case, it's phonons (i.e. air vibration).

Note that to get sound in the human hearing range the author used a very low temperature in the nanoKelvin range.

what i mean is that basically sound here is homomorphic to photons emitted in the original problem right?

As temperature goes up, the bulk of the energy in blackbody radiation goes to higher frequencies.

@Skyler What do you mean "original problem"?

Blackbody radiation is by no means intrinsically tied to photons, even if that's historically where it was first discovered.

Hi, @ACuriousMind.

@DanielSank hmm, i think i know what you mean but an example would be helpful

@Skyler As mentioned above, a famous and important example is the voltage noise of a resistor.

also, in this case a 100 Hz photon corresponds to a 100Hz note

@DanielSank Hey

ok

i see what you mean

Using the same thermodynamics considerations as was originally used for blackbody radition of photons, Harry Nyquist proved that the voltage noise of a resistor follows a blackbody law.

I think a more interesting question would be to ask what if we took a perfect black body, then subtracted out a real black body from it and listened to those results. HBU Dan

I don't think I'm gonna say much about this post, but generally you should consider pinning the featured post of the week to the star board, as a room owner you should be able to.

By the way, if one of the GR/QFT/thery folks would like to talk about Unruh effect a bit, I'd appreciate it.

Otherwise people have to scroll potentially rather far up to see what the topic is

@Skyler HBU?

@DanielSank how (a)bout (yo)u

@Skyler What's a "real blackbody"? A real system that's close to an ideal blackbody?

@DanielSank yea

@ACuriousMind Done, but the text in the star board is unenlightening. It would be better if it said something like "This week's featured post".

Unfortunately, I can't do sudo-edits because I'm not a mod.

This week: listening to blackbody radiation

How about that, @ACuriousMind?

@DanielSank It's basically a consequence of there being no vaxcuum state that all observers can agree on, so what one sees as vacuum, the other sees as a thermal mess of particles. What do you want to know about it?

@DanielSank Much better :)

@ACuriousMind What do I need to know to recover the effect on a sheet of paper in my kitchen?

And, why is it impossible to agree on a vacuum?

@DanielSank so basically an example is that with the right scaling you could hear the excitation modes of hydrogen in a star

@Skyler Well sure.

or maybe the excitation modes of all the major materials in something

aka composition analysis

@Skyler If something is acting like a blackbody, you can't do composition analysis.

i think i came across a product thats doing this analysis but without the sonication

A blackbody spectrum only contains one parameter: the temperature.

@DanielSank non-ideal blackbody*

If you want composition you need to look for emission lines.

yea

so we can hear the emission lines if we did FT the deviation from ideal

lets say we did that with a ball of hydrogen gas, I wonder if there is any major meaning to the sounds we hear, like beat frequencies

@DanielSank To derive it properly, you'd need to do basic QFT in curved spacetime, write down the vacua of the accelerating and non-accelerating observer, then express the non-accelerating vacuum in the Fock basis of the accelerating observer. Here's a short overview from a seminar I partook in.

@ACuriousMind Thanks!

@DanielSank Basically, because the observers disagree what the proper mode decomposition of the field is. The field is generally covariant, but Fourier transformations are not, so different observers get different coefficients/modes.

@ACuriousMind I will poke you after reading the derivation. I just skimmed it and I think there are no concepts that will be impenetrable for me.