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Physics

Heat can flow backwards in a gas so thin its particles never touch

A surprising reversal of our usual understanding of the second law of thermodynamics shows that it may be possible for heat to move in the “wrong” direction, flowing from a cold area to a warm one

By Karmela Padavic-Callaghan

4 November 2024

Diffuse gas particles can behave in a way that contradicts a fundamental law of physics

Shutterstock / North10

Heat always spontaneously flows from a hotter place to a colder one – 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.

“Given the second law [of thermodynamics] has been known for 150 years, you would imagine that if a counterexample exists, then…

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