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 for entropy that were previously thought to be equivalent don’t always match for quantum objects.

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 exchange quantities like particles and heat. For almost 200 years, physicists have used the same set…