Heat always spontaneously flows from a hotter place to a colder one 鈥� so says the second law of thermodynamics. But within an extremely sparse gas, the opposite may be possible, with heat flowing from cold to hot. This finding could reveal cracks in a foundational law of physics.
鈥淕iven the second law [of thermodynamics] has been known for 150 years, you would imagine that if a counterexample exists, then it must be in some exotic system, like in string theory,鈥� says at the University of California, San Diego. To his surprise, he found one by modelling the behaviour of a few particles zipping around in a box.
Qiao and his colleagues simulated a box filled with a very dilute gas 鈥� it is so thin that the particles will never interact with each other, but they can bounce off the sides of the box. The top and bottom of the box absorb heat when any particles touch them. Typically, a box filled with gas like this would show all the hallmarks of the second law of thermodynamics 鈥� if any part of it started off hotter than the rest, heat would flow to colder parts of the box until the whole thing evened out at the same temperature. This equilibrium state would also be the one with most entropy, or the least order.
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Qiao says he expected the gas would never reach equilibrium because the particles could not collide with each other, but something more unusual seemed to happen: even when the temperature at the bottom of the box was colder than the top, particles would still move upwards and carry heat from the colder part of the box to the warmer one. Qiao says this is because warmer particles always move more quickly than colder ones, so they are more likely to overcome gravity鈥檚 pull and make it to the top of the box and lose their heat there, while the already slow and cold ones will, in the absence of collisions, stay at the bottom.
It may be that instead of breaking a tenet of the second law of thermodynamics, this odd system reveals that under some circumstances, the law鈥檚 two parts 鈥� the direction of heat transfer and the increase of entropy 鈥� may not always go hand in hand.
鈥淚t鈥檚 just under these very special conditions without particle-particle interaction, [entropy] is maximised in such a way that allows cold to hot heat transfer,鈥� says Qiao.
at Northern Illinois University says that claims of breaking the second law of thermodynamics are common and often lead to fanciful proposals like perpetual motion machines, but in more than 150 years no one has managed to provide unambiguous experimental evidence for having broken the law.
鈥淭his is a nice effect of fundamental interest, but perhaps not of much practical relevance,鈥� says at the University of Stuttgart in Germany.
Still, Qiao is confident in his theory, and has experimental tests in mind. In fact, he says that working on this mathematical investigation made him realise that one of his own experiments from 15 years ago may fit the bill: in that set-up, small amounts of liquid were pushed into tiny cavities within a porous solid and consequently could not touch each other. He says that some of the data from that old experiment had struck him as odd but unexplainable at the time, but now he is seeing it with new eyes.
Ultimately, he wants to design a useful device based on his new theory, such as one that could generate electricity based on the energetically cheap cold-to-hot heat transfer. 鈥淏ut because the second law is so fundamental, this [new work] is something that I would also happily retire with,鈥� he says.
Journal reference:
Physical Review E,
