Temperatures Below Absolute Zero Are Hotter Than Infinity

Absolute zero, −273.15 °C, is usually treated as the bottom of the temperature scale. But in January 2013, physicists at the Ludwig Maximilian University of Munich and the Max Planck Institute of Quantum Optics, led by Ulrich Schneider and Immanuel Bloch, pushed a gas of about 100,000 potassium atoms past it into negative absolute temperature.

Temperature reflects how energy is shared among particles. Normally, low-energy states are crowded and high-energy ones are sparse. Using laser traps and magnetic fields, the team inverted this, packing most atoms into the highest available energies — an “upside-down” distribution that, by the strict thermodynamic definition, carries a negative Kelvin value.

The twist is that such a gas is not cold at all. It is hotter than any positive temperature.

The scale does not simply end at infinity; it jumps to negative values, which sit above it.

A negative-temperature system would dump heat into anything with a positive temperature it touched, making these the hottest systems ever created in this precise thermodynamic sense.