In some ways, Einstein invented the basics of quantum theory multiple times. Special Relativity and the light-quantum in 1905, Specific Heats in 1906, electromagnetic emission and a generalized coordinate invariant version of the Bohr-Sommerfeld electron in 1917 and his insistence (along with Schroedinger, Debye and Compton) on the light-quantum in the mid-1920s during his first controversy with Bohr. As usual, his biographer Pais is driven to remark that some of Einstein’s insights are so advanced that they don’t fit into the ordinary chronology of the logical development of particle physics. That’s especially true of the whole batch of quantum papers from 1917. Pais says he notes them where he does in his narrative for purely chronological reasons since they don’t fit in logically – the insights are so far ahead of where everyone else was at that time.
But in the logical chronology of the theory of Specific Heats, Einstein, for once pretty much fits right in. On page 394 of Subtle is the Lord, Pais writes “Until 1906, Planck’s quantum had played a role only in the rather isolated problem of blackbody radiation. Einstein’s work on specific heats is above all important because it made clear for the first time that quantum concepts have a far more general applicability.” However, even though it seems Einstein was right in the chronological flow in terms of the development of theories, Einstein’s approach was not picked up right away by anyone else until around 1911 when, Nernst, Peter Debye and eventually Max Born and Theodore von Karman, worked out a more complete theoretical description.
But, as long as we are pretty much following Abraham Pais’ narrative of Einstein’s odd, but very bold, inventive, and fruitful work on quantum theory, just how twisted was the journey of the quantum in and out of Einstein’s work? First, apparently people had a lot of trouble nailing down just which phenomena they wanted to associate with quanta. For example, Einstein pointed out the quantum nature of the photoelectric effect in 1905, but that description of the phenomena was not widely accepted until the early 1920s. And for another example, X-rays were shown to have light-like qualities in 1912 by von Laue, but even Compton’s work showing that X-rays carried momentum like the postulated light-quantum was not conclusive evidence of the characteristics of light-quanta for Niels Bohr in 1924.
Finally, as Pais observes on page 197 of Subtle is the Lord, “it was Einstein’s style forever to avoid the quantum theory if he could help it.” Possibly because he knew it was not widely accepted as he started to work on General Relativity in 1907, and perhaps partly to have the pleasure of demonstrating its effectiveness in many new ways. As Pais continues his exposition, while Einstein started working out the gravitational red shift, Einstein skipped the easy method of showing that energy could be written as Planck’s constant times the light frequency, and “starts all over again” to derive the frequency shift.
But – to back up and go over all of this all over again and “start all over again” ourselves – what does the chronology of the theory of the light quantum look like?
1900 – Black body (heat) radiation is quantized (Planck’s constant)
1905 – Photo-electric effect is shown to be quantized (Einstein)
1906—Specific Heats are shown to be quantized (Einstein)
1907 – Gravitational Redshift related to Planck’s constant (Einstein)
1913-1915 Bohr-Sommerfeld atom uses quantized values and works for describing the X-ray spectrum
1917 – quantized emission, absorption and stimulated emission all quantized with directional momentum (Einstein – the first step to quantum mechanics)
1922 – Schroedinger’s quantized molecular emission Doppler effect description
1923 – Compton Effect shows X-ray quanta with directional momenta
1925 – Bothe-Geiger and Compton-Simon experiments confirm quanta with directional momenta
1925 – Facing the BKS proposal of Bohr and company (Einstein and others save the quantum
{I’ll explain this more in the next post})
1926 – Lewis names the light-quantum the “Photon” and the name catches on very fast
So…next post, what was up with the BKS proposal?
