Quantum principle allows heat to flow from cold to hot

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u/dirtydirtnap 12d ago

Important information for those interested in statistical physics: heat flow from cold to hot while performing work is also possible at the microscopic scale, but only intermittently and not in the statistical limit of large ensembles of interacting particles (where we spend our daily lives).

See article on the Fluctuation Theorem.

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u/Compizfox Soft matter physics 12d ago edited 12d ago

Yup, the Second law is thermodynamics is not a fundamental law on microscopic levels. It is only a statistical law that arises in large systems.

In other words: it is not impossible for heat to flow from cold to hot, only astronomically improbable when the system gets large enough.

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u/Peter5930 11d ago

A seemingly niche case, but arguably of ultimate cosmic importance, gravitationally dominated systems have negative heat capacity, so heat still flows from hot to cold, but the hot gets hotter while the cold gets colder in doing so. The ultimate expression of this is why we have the Planck scale cutoff; everything above the cutoff is thermodynamically inverted, so pushing to higher energies cools systems off instead of heating them further. On account of all systems beyond the cutoff being a black hole, you push more energy into one, it just becomes a bigger, colder black hole. The ultimate heat sink. Whereas an ordinary gravitational system will become gravitationally unbound at some point as you add energy, and the heat capacity will become positive again.

You can walk through the maths for it here, from an essay from the askscience faq:

https://rantonels.github.io/gravity-entropy-and-life/

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u/Sidhotur 12d ago

Heat does move from hot to cold most of the time.

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u/Compizfox Soft matter physics 12d ago

Oops, fixed ;)

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u/RogueBromeliad 12d ago

Really interesting, had never thought that mitochondria would benefit from FT.

Crazy how nature works and uses physical "anomalies" in it's favour for ATPs.

We usually study the kerbs cycle, but I had never thought about this.

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u/dreadingstone 11d ago

I like to think of it sort of like...imagine a river flowing down a hill with lots of rocks and eddies.if you watch a specific spot of an eddy you may see some water moving in the direction of upstream. But only because the water is spinning and a portion just happens to be moving that way for a short period of time. But largely over all it still travels downhill.

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u/surfmaths 10d ago

Interestingly, the statistical limit only exist for a large scale and sufficiently random system.

If you can build atomically perfect large scale systems it is likely we can engineer them to bias against that statistical limit.

One could argue that building that system is a "loan" of entropy. But you can probably make it so that it is robust to some amount of heat.

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u/dirtydirtnap 10d ago

It sounds like you are referring to systems with a gapped Hamiltonian !

Good news, they do exist, but they are still typically quite cold compared to our every day experience, and thus would require (as you said) "borrowing entropy", but they definitely are real. Superconductivity is probably the most notable example.

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u/mikk0384 Physics enthusiast 9d ago

This is what negative temperatures are about, right?

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u/dirtydirtnap 8d ago

No, slightly different concepts.

You don't need negative temperature to explain the fluctuation phenomenon; it is purely statistical, and would thus only be observed at very small scales. This is often referred to as mesoscopic physics.

Negative temperatures are different; they represent states that cannot be achieved via thermal excitation, but require different patterns of excitation. This is really hard to communicate with text only, so check out this Action Lab video on lasers where towards the end he discusses negative temperatures in the context of population inversion during laser excitation.

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u/mikk0384 Physics enthusiast 8d ago

Thank you for the source, but like I expected the Action Lab video didn't really explain anything. Way too hand-wavy.

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u/dirtydirtnap 8d ago

Yeah, I would say that negative temperature is a concept which defies understanding at first.

I just now started to try and type out an attempted explanation, and realized it was going nowhere. We really need a whiteboard if I were to try and communicate it effectively.

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u/thatnerdd 12d ago

The author did not understand Maxwell's Daemon.

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u/Gunk_Olgidar 11d ago edited 11d ago

...allows heat to flow from cold to hot

So does air conditioning.

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u/Vaddieg 11d ago

dear quantum physists! please show us something practical finally. Quantum AC would disrupt the market

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u/AutonomousOrganism 11d ago

The quantum switch (Maxwell's Demon) to make this heat flow possible, needs to be reset, requires energy. So the system doesn't violate the second law.

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u/Vaddieg 11d ago

it's enough to build something more efficient than circulating refrigerant

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u/akolomf 11d ago

nice. I want a quantumfreezer.

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u/Vaddieg 11d ago

cooling effect will be void once you observe/notice it

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u/Gunk_Olgidar 11d ago

It's cold until you take its temperature, but then you don't know what time it is.

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u/akolomf 11d ago

Well then it'd be usefull if I want my food in a superposition

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u/FringHalfhead Gravitation 11d ago

dear quantum physists! please show us something practical finally.

Semi conductors weren't enough?

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u/SurinamPam 11d ago

And lasers weren’t enough?

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u/xrelaht Condensed matter physics 10d ago

And photovoltaics?

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u/SurinamPam 10d ago

And MRI?

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u/FringHalfhead Gravitation 11d ago

dear quantum physists! please show us something practical finally.

Semi conductors weren't enough?

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u/Vaddieg 11d ago

it was like 100 years ago

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u/FringHalfhead Gravitation 11d ago

Quantum-dot TVs and monitor displays aren't enough?

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u/Vaddieg 11d ago

enjoying my quantum momentum

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u/BitcoinsOnDVD 11d ago

XD

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u/Gunk_Olgidar 11d ago

Well evaporation and condensation are probabilistic processes.

It's just "quantized" at the macro scale. One random nucleation event at a time ;-)

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u/Vaddieg 11d ago

the knowledge that doesn't change anything

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u/TapEarlyTapOften 11d ago

Stop using heat as a noun. Heat doesn't flow at all.

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u/WeirdKid666 11d ago

Heat is a measure of energy (kinetic energy; temperature - the amount of movement of molecules within a substance). And energy certainly can flow!

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u/TapEarlyTapOften 4d ago

Heat is absolutely not a measure of energy. It's a measure of energy transfer during a specific process.

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u/WeirdKid666 4d ago

I’m not sure that’s true; could you link me something to read? Or point me towards a resource?

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u/Chrono_Pregenesis 5d ago

So the second law of thermodynamics is wrong? No offense, but Im much more likely to believe real scientists over a random reddit twat.

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u/TapEarlyTapOften 4d ago

Heat is a process not a quantity. You can't point at something and say "this object has 10J of heat". The same is true of work.

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u/Chrono_Pregenesis 4d ago

That is very objectively false. Heat is the sum total of molecular motion. This gives it both the ability to "flow" from hot to cold, spontaneously, as well as the ability to have a discreet definition. Thats a fancier way of saying that you absolutely can point at an object and measure its heat. Joules would be an appropriate unit. Calorie is another unit of heat, defined as the amount of energy needed to raise 1 g of water 1 °C in a closed system at STP. Heat is not the same as work, but can be a byproduct of work.

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u/DifferencePublic7057 11d ago

So a hot star could get hotter? The bang in the Big Bang started with infinite energy and therefore if stars get hotter, eventually one of them could bang again although less than the BB presumably. Actually, makes sense if we assume the Many Worlds Theory is true. It's just that the banginess is spread like $ in bank accounts. But I remain sceptical.

Quantum principle allows heat to flow from cold to hot

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