Temperature seems like something frozen: a substance has a temperature, and that's that. But look closer, and thermal equilibrium resembles a waterfall that appears motionless from the side even though it consists of a continuous stream of water. In the same way, any body at finite energy constantly exchanges photons with its surroundings. A photon is a quantum of electromagnetic radiation, and spontaneous emission is the spontaneous release of a photon by an excited system. Without a compensating influx of energy, the body cools, and equilibrium collapses.

Temperature used to be described as a static characteristic of a system in equilibrium, without asking what physically sustains that equilibrium. Now it is clear: thermal equilibrium is a dynamically maintained state in which a constant temperature is ensured by a continuous exchange of photons with the surroundings. The author has gathered known facts (spontaneous emission, the Stefan-Boltzmann law, the H-theorem) and shown that together they yield a single mechanism for maintaining temperature. The Maxwell distribution (the distribution of molecular speeds in a gas) is shaped by mechanical collisions, but the energy scale is set precisely by the boundary exchange.

Understanding the dynamic nature of temperature matters when designing cryostats, analyzing the thermal balance of planets and stars, and correctly interpreting experiments with small systems. The waterfall we see as a motionless wall of water actually consists of myriads of droplets continuously falling down. Temperature is likewise not a property of rest but a continuous flow of energy that has to be maintained.