Diffuse gas particles can behave in a way that contradicts a fundamental law of physicsShutterstock / 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…