Entropy
A quantity measuring how many ways a thing can be spread around. In a system that exchanges nothing with its surroundings, entropy can increase but never decrease.
In 1865 the German physicist Rudolf Clausius, working on the theory of heat engines, focused on the heat exchanged divided by the temperature at which it is exchanged, summed up, and named it entropy after the Greek word for transformation[1]. In a system that exchanges nothing with its surroundings this quantity can grow but never shrink. That is the second law of thermodynamics.
Ludwig Boltzmann showed that entropy is set by the number of ways the atoms can be arranged to make a given state. For the same energy, the more freedom there is in how it can be spread, the larger the entropy.
In 1905 Einstein calculated how the entropy of light changes with the volume of its container, found the same form as for a gas of independent particles, and argued that light behaves as though it were a collection of lumps of energy[2].
Related terms
- Boltzmann constantThe fundamental constant, written k, that links temperature to the energy per atom.
- Light quantumThe particle-like packet of energy that light behaves as. Now called the photon.
- Equipartition theoremThe classical law that in thermal balance, every way a system can move receives, on average, the same share of energy, proportional to temperature.