
Scientists Watch as Heat Moves at the Speed of Sound - pseudolus
https://www.scientificamerican.com/article/scientists-watch-as-heat-moves-at-the-speed-of-sound/
======
pontifier
This is so strange. Just 2 days ago I realised that although excessive sound
production is usually a symptom of a machine that is not working properly, if
a machine is designed to produce sound to dissipate energy, that energy would
not be converted into heat within the machine. The energy is carried away at
the speed of sound.

I have been thinking about using a similar process to make a "middle of the
room" air conditioner that creates a localised cold spot, and sends the heat
absorbed out of the room through the walls and into the surrounding atmosphere
using low frequency pressure waves.

All of these processes are likely related, and macroscopic systems based on
these principles almost seem magical.

~~~
Waterluvian
I'm curious if the numbers work out to yield you anything within the galaxy of
useful.

~~~
trentlott
Yeah, that's the actual question and probably a reason why it's not an
established technique

Also, low-frequency waves naturally are lower energy. Why not bury the output
and increase the frequency?

It would have the added benefit of supposedly keeping away ground-dwelling
pests; _supposedly_.

~~~
heavenlyblue
>> low-frequency waves naturally are lower energy

Just to be clear here: are they really? Sound waves have an amplitude which
also defines the amount of energy they contain.

~~~
robocat
Sound waves also have a maximum amplitude (without distortion) because the
minimum of the pressure wave can't be below atmospheric pressure. Although
Wikipedia indicates that maximum is quite loud.

------
rfrey
As I understood the article, in one part of the experiment they set up
alternate regions of heat and cold within a sample of graphite using
interference patterns from two laser sources. When they turned off the lasers,
instead of the hot regions dispersing their energy until the troughs were the
same temperature, the hot regions "overshot", becoming cooler than the
(former) troughs, with wave-like behaviour.

This seems to me like energy is momentarily flowing from a cooler region (the
former "peaks") to a hotter area (the former "troughs").

Would a Real Physicist be kind enough to explain to me why this doesn't
violate the 2nd law? Probably the answer is "you misunderstood".

~~~
arithma
Not a "Real Physicist" but the information is not being lost, it's only being
kept static-ish. This is the same as that waves don't violate the 2nd law
(it's telling that they being called the phenomenon second sound.)

> The second law of thermodynamics states that the total entropy of an
> isolated system can never decrease over time.

So instead of entropy increasing, it's hanging around for a while. No
violation, still unusual though.

~~~
sp332
Heat often correlates with entropy, but they are not the same thing.
Increasing heat doesn't always mean increasing entropy.

~~~
EngineerBetter
I'd highly recommend Arieh Ben-Naim's books on entropy for anyone looking for
more on this point.

TLDR he makes the argument that entropy is a measure of the probability of
probability distributions.

------
pps43
So the effect was known for a long time, and the new development is that it
was now observed at higher temperatures. But 120 K is still pretty cold, so it
doesn't really matter much for practical applications.

~~~
freethemullet
Depending on the concentration of defects and the length scale of the system,
theory suggests that it may be possible to observe second sound at much higher
temperatures beyond 120 K.

------
freethemullet
Here is the arxiv version of the Science paper mentioned in the article:
[https://arxiv.org/abs/1901.09160](https://arxiv.org/abs/1901.09160).

------
hyeonwho4
This is the theory of phonons: pseudo particles which transfer heat in
materials.

------
renholder
Asking because it's an interesting phenomenon and I don't know enough to
understand the question I'm putting forth:

If conductive heat is radiated through quanta, phonons[0], why would it be
limited to the speed of sound? Is there a limitation to waveforms in matter
that limits the speed at which it travels or is it purely dependent on the
energy "type"? (Thinking of light travelling in waveform, when I posit this,
which is the reason the ridiculous ask.)

[0] -
[https://en.wikipedia.org/wiki/Phonon#Acoustic_and_optical_ph...](https://en.wikipedia.org/wiki/Phonon#Acoustic_and_optical_phonons)

~~~
freethemullet
Long wavelength phonons are the familiar sound waves. Smaller wavelength
phonons have different group velocities due to dispersion.

The speed of second sound is not the same speed of first sound. The speed of
second sound is essentially determined by a weighted contribution of the
different of speeds of the different phonon modes that collectively
participate to produce a temperature wave.

------
hannasanarion
The effect appears at 120 Kelvin, or -240°F, so the article's assertion that
this could revolutionize microelectronics seems like a stretch

~~~
turdnagel
Right, and the article says you wouldn't need to do it at "cryogenic"
temperatures. I thought this was funny: "Nobody ever thought that you would
actually be able to do this at such high temperatures."

~~~
ricardobeat
-150℃ is almost halfway up to ambient temperature though, so it halves the energy requirement.

------
mcguire
A quick first impression question: how important is the laser interference?
Could you do this with a multi-centimeter graphite rod, by heating one end?

~~~
freethemullet
Laser interference is not required. The original second sound measurements in
superfluid Helium used a geometry similar to the one you describe, where a
heat source (as a pulse or periodic in time) is positioned at one end and the
temperature is recorded at some point(s) along the sample.

