Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Scientists Watch as Heat Moves at the Speed of Sound

scientificamerican.com

Post reply on HN
Print view View original

21–30 of 33 posts

Re: Scientists Watch as Heat Moves at the Speed of Sound

#21

Post by freethemullet »

rfrey wrote: post #2

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 see…

The experiment describes an unusual process observed in a system driven out of equilibrium (i.e., non-equilibrium), where, strictly speaking, the 2nd law does not apply.

Re: Scientists Watch as Heat Moves at the Speed of Sound

#23

Post by freethemullet »

mcguire wrote: post #9

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?

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.

Re: Scientists Watch as Heat Moves at the Speed of Sound

#24

Post by freethemullet »

renholder wrote: post #18

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, w…

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.

Re: Scientists Watch as Heat Moves at the Speed of Sound

#25

Post by freethemullet »

pps43 wrote: post #6

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.

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.

Re: Scientists Watch as Heat Moves at the Speed of Sound

#26

Post by phaedrus »

rubatuga wrote: post #17
pontifier wrote: post #11

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 cond…

It wouldn't be possible because you need a temperature gradient. Creating sound from plain heat is the wrong direction for entropy.

For the idea to work for cooling, he would need to expand the air in a different location than it was compressed in. (I wonder if standing waves exhibit this effect?) IF it works out that this is possible, I imagine it would be like standing in front of a CO2 fire extinguisher (gets cold as pressure is released).

Re: Scientists Watch as Heat Moves at the Speed of Sound

#27

Post by heavenlyblue »

trentlott wrote: post #16
Waterluvian wrote: post #14

Earlier quoted context omitted.

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

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 .

>> 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.

Re: Scientists Watch as Heat Moves at the Speed of Sound

#28

Post by smoyer »

heavenlyblue wrote: post #27
trentlott wrote: post #16

Earlier quoted context omitted.

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 .

>> 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.

Yes ... a sinusoidal sound wave has an RMS amplitude based on the peak amplitude regardless of the frequency.

Re: Scientists Watch as Heat Moves at the Speed of Sound

#29

Post by robocat »

heavenlyblue wrote: post #27
trentlott wrote: post #16

Earlier quoted context omitted.

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 .

>> 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.

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.

Re: Scientists Watch as Heat Moves at the Speed of Sound

#30

Post by xelxebar »

heavenlyblue wrote: post #27
trentlott wrote: post #16

Earlier quoted context omitted.

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 .

>> 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.

For a sine wave, the energy scales like

    E ∝α ω^2
where α is amplitude and ω is frequency. So lower frequencies need propotionally larger amplitudes to carry the same energy.
Post reply on HN