Dead as a Doornail? Zero-Point Energy and Low-Temperature Physics in Early Quantum Theory

According to quantum mechanics, an oscillator at zero absolute temperature will continue to vibrate with a zero-point energy. This non-classical phenomenon was predicted by Max Planck in 1911, but for a long time it was contested and remained hypothetical. Only in experiments with molecular spectra from 1924 was the zero-point energy confirmed experimentally, and the following year it emerged as a consequence of quantum mechanics. Although Walther Nernst claimed that also vacuum was filled with zero-point energy, specialists in quantum physics denied that this was the case. Nonetheless, with the discovery of the Casimir effect predicted in 1948 Nernst’s idea was vindicated. Later still, the zero-point energy of free space turned up in the vacuum energy or so-called dark energy associated with Einstein’s cosmological constant. The chapter offers a history of the concept of zero-point energy with a focus on the period from 1911 to about 1928.