What does hotter or colder actually mean? Negative absolute temperatures (or negative Kelvin temperatures) are hotter than all positive temperatures - even hotter than infinite temperature. No! Nothing can be colder than absolute zero (0K)! Is your system really colder than zero Kelvin? In fact, the idea of negative absolute temperature is old, but was seldom discussed for mobile particles because everybody assumed that it was impossible to realize. Even the speed of light as the ultimate speed limit for particles does not pose a limit to their kinetic energy: Although the speed of the particles is limited by the speed of light, the kinetic energy can still be arbitrarily high. Atoms in a classical gas, for example, do not possess such an upper energy limit - their kinetic energy can be arbitrarily high. It is an internal limit - the particles cannot absorb more energy even if there is plenty available. This limit is not an external limit in the sense that there is just no more energy available. a maximal possible energy a particle of the system can have. In order to be able to reach negative temperatures, a system needs to possess an upper bound for the energy of its particles, i.e.

the air around us), are limited to positive absolute temperatures. No! Most systems, including a classical gas (e.g.

If your question is not answered yet, please send an email to Simon Braun or Ulrich Schneider and we will add it.Ībsolute temperature refers to temperature on the Kelvin scale, where 0K is the absolute zero point, where all motion in a classical gas would stop.Ĭan all systems achieve negative temperatures? Here we want to answer some frequently asked questions concerning negative absolute temperatures.

Entropy of ideal gas at absolute zero