Here's an excerpt from a letter that 22 year-old Albert Einstein wrote on April 10, 1901 to his girlfriend or maybe by then fiancée, Mileva Maric:
Planck assumes that a completely definite kind of resonator (fixed period and damping) causes the conversion of energy to radiation, an assumption I cannot really warm up to.
Einstein was making a play on words at the end of the sentence, since Planck's work involved the theory of "heat radiation," the radiation emitted by an object due solely to its temperature. See the full English translation of the letter, plus much more, at the Einstein Papers Project website.
(The reference in Einstein's letter to "Dude" should be to "Drude" instead, one of the great physicists of the early 20th century who committed suicide. Boltzmann and Ehrenfest are the other two.)
(The reference in Einstein's letter to "Dude" should be to "Drude" instead, one of the great physicists of the early 20th century who committed suicide. Boltzmann and Ehrenfest are the other two.)
For about 15 more years, Einstein continued to be dissatisfied with the general lack of a physical description of "the mechanism of energy transfer" for the radiation absorption and emission of a charged harmonic oscillator ("resonator"). One of the relevant questions was, how can an oscillator emit or absorb light in discrete packets when it's oscillating continuously?
In fact, a physically meaningful description still doesn't exist for either the quantum harmonic oscillator or the quantum emission and absorption of light by atoms. As a substitute explanation, we currently have only the highly developed perturbation theory of quantum mechanics, which works spectacularly but is nevertheless a mathematical invention, not a physical explanation. This state of affairs is one that Einstein himself helped to set into motion in 1916-17, when he re-derived the Planck black-body radiation spectrum equation using what are now called the Einstein A and B coefficients, which are based on probability considerations.
