Max Planck On Entropy and Ludwig Boltzmann

Entropy, together with energy, is probably the most vital concept in nature. Energy remains constant. Entropy instead increases continuously. It can never decrease. This is exactly the essence on which the second law of thermodynamics is based. The entropy of a system of bodies may only increase. When brought to its maximum levels, it remains constant. If it increases, then the process is irreversible. If it remains constant, it is reversible and may even be inverted. This is the major difference between the laws of nature. Thermodynamical equilibrium occurs when entropy has reached its maximum level. If it does not increase, then no change can occur either. I was greatly impressed by Ludwig Boltzmann's ideas, by which I was guided when formulating the laws I discovered during my experiments. He described entropy as the logarithm of the probability of the state of gases, starting from the atomic theory approach. Successfully applying the adequate method to radiation fields could only be possible by considering radiation as also being made up of atoms, though still considering it as made up of special quanta. This theory inevitably created the assumption that they were atom-beams or well-defined dimensions of quanta, which are recognizable through measurements. At first I was reluctant in formulating this hypothesis because it contrasted every classical atomic representation. Yet no other solution was possible. The study proved, thanks to the cooperation of many colleagues, that the theory was indeed true.