Quantum theory is challenging long-standing ideas about entropy

Measuring disorder in the quantum realm is creating a bit of a mess. A mathematical study found that three different definitions of entropy that were previously thought to be equivalent don’t always match for quantum objects. This could have far-reaching implications, as it could force a rewrite of well-established models for thermodynamics.

When two objects are connected – say a container of hot gas is linked to a colder gas reservoir – the entropy of the colder one can increase because the two will…