The first law of thermodynamics states that energy can neither be created nor destroyed but can be converted into different forms. It has been used to describe many things — including how refrigerators and car engines work. It’s one of the pillars of physics.

The first law of thermodynamics, created in the 1850s, is only valid for systems in equilibrium or a state where a temperature can be precisely determined. For instance, a cup of cold water and a cup of hot water will gradually achieve a warm temperature when combined. This warm temperature is the equilibrium. The water is out of equilibrium when the hot and cold water has not yet reached that endpoint, though.

Likewise, systems need equilibrium in many branches of modern research. The first law has been expanded for common materials out of equilibrium for over a century. However, such ideas only hold when the system is approximately there, such as when the hot and cold water are almost mixed. For instance, in space plasmas, which are far from equilibrium, the ideas do not apply.

West Virginia University physicists have made a breakthrough on an age-old limitation of the first law of thermodynamics. The study is expected to revamp scientists’ understanding of how plasmas in space and laboratories get heated up and may have a wide variety of further applications across physics and other sciences.