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A comment on Scientists Watch as Heat Moves at the Speed of Sound.

Re:Scientists Watch as Heat Moves at the Speed of Sound by rfrey on Sunday March 24, 2019 @09:49AM (#19476318)
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".

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Re:Scientists Watch as Heat Moves at the Speed of Sound by arithma on Sunday March 24, 2019 @09:56AM (#19476347)
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.

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Re:Scientists Watch as Heat Moves at the Speed of Sound by sp332 on Sunday March 24, 2019 @12:15PM (#19477088)
Heat often correlates with entropy, but they are not the same thing. Increasing heat doesn't always mean increasing entropy.
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Re:Scientists Watch as Heat Moves at the Speed of Sound by EngineerBetter on Sunday March 24, 2019 @01:06PM (#19477410)
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.

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Re:Scientists Watch as Heat Moves at the Speed of Sound by tgtweak on Sunday March 24, 2019 @10:54AM (#19476665)
Doesn't seem as if total entropy is increasing, simply rebalancing to the other side (which is still kind of interesting, like a wave in a pool).
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Re:Scientists Watch as Heat Moves at the Speed of Sound by hannasanarion on Sunday March 24, 2019 @10:59AM (#19476692)
Yep, and energy is lost in the rebalancing process, and it oscillates until equilibrium, just like a pendulum.
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Re:Scientists Watch as Heat Moves at the Speed of Sound by teh_infallible on Sunday March 24, 2019 @12:07PM (#19477046)
Sounds like this would explain the Mpemba effect:

https://en.m.wikipedia.org/wiki/Mpemba_effect

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Re:Scientists Watch as Heat Moves at the Speed of Sound by freethemullet on Sunday March 24, 2019 @01:48PM (#19477636)
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.
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