The Unraveling of Classical Physics: Planck and Einstein

Abstract

It is not a particularly unusual observation that hot bodies emit radiation. This can be thermal radiation, which can be measured and felt, or it can be visible light, which can be measured and seen. The type and amount of radiation emitted depend on the temperature and the kind of material. One can measure both the total amount of energy emitted and the amount of energy of a given frequency (color). It is more unusual that when matter in the form of hollow balls is heated and observed through a small hole, all materials exhibit the same thermal spectrum; this is not the case for solid spheres. Although perhaps unexpected, this particular behavior has been observed and can be understood on the basis of the laws of thermodynamics, a well respected division of classical physics. Early in 1900, the first really accurate measurements of that thermal spectrum became available. Surprisingly, there was no theoretical formula that agreed with these observations. There was a theoretical expression derived on the sound basis of classical physics, but it agreed with the data only for low frequencies. For higher frequencies the theory and experiment did not agree at all. On the basis of a rather different semi-theoretical picture, it was possible to obtain an expression for the high-frequency part of the spectrum, but then the low-frequency part did not fit at all. There was no formula of any kind that fit the complete experiment.