It is one of the most important laws of nature that we know: The famous second law of thermodynamics says that the world gets more and more disordered when random chance is at play. Or, to put it more precisely: that entropy must increase in every closed system.

Ordered structures lose their order, regular ice crystals turn into water, porcelain vases are broken up into shards. At first glance, however, quantum physics does not really seem to adhere to this rule: Mathematically speaking, entropy in quantum systems always remains the same.

A research team at TU Wien has now taken a closer look at this apparent contradiction and has been able to show that it depends on what kind of entropy you look at. If you define the concept of entropy in a way that is compatible with the basic ideas of quantum physics, then there is no longer any contradiction between quantum physics and thermodynamics.