Question: Early history of QM

[patrickd]

Many popularized accounts of the development of quantum theory generally go like this:

• Maxwell shows that all electromagnetic radiation is a variant of one phenomenon.
• Experimental results measuring black body radiation are inconsistent with the radiation theory as understood.
• Planck shows that the black body results can be understood by supposing the radiation to be quantized in packets whose energy = constant x frequency
• Einstein shows that results of photoelectric effect experiments can be explained by supposing that radiation is absorbed, and an electron ejected, only if the radiation comes in packets each containing energy above a threshold amount.
• So by 1905 we have experiments and a theory postulating that energy is emitted and absorbed in discrete packets, quanta.
• Over the next five years or so, it is shown by Bohr and others that extending this concept to atomic emission and absorption explains how a planetary atomic model can exist without “spiraling down” due to emitted radiation, and gives an excellent fit to the known spectrum for hydrogen.

This brings us up to around 1910. We then seem to enter a time machine, and jump forward to the late twenties, where we find that the next generation of theorists are coming up with the uncertainty principle, wave mechanics, and the whole controversy surrounding the Copenhagen interpretation.

So, my question is, what happened in the experimental realm between 1915 and 1925 that necessitated this (to me) huge increase in the sophistication of the explanatory apparatus?

Pat Dennis

 

[al onestone]

The most relevant other development you're missing is the Thompson realization of De Brolie's hypothesis (that matter particles also have a wavelength related to their momentum). Thompson JR proved this with the double slit expt. with electrons.

Remember, theoretical insights like you've mentioned as "coming up with the uncertainty principle, wave mechanics, and the whole controversy surrounding the Copenhagen interpretation" aren't just overnight flings for the Gottingen and Copenhagen schools. Heisenberg and Schrodinger required insights from other physicists, they required conferences (like the Solvay conferences) to refine the interpretation.

You speak of quantum theory as though it indeed wasn't the most important theoretical insight in human history. Quantum mechanics is no incidental, trivial result of a few experiments. The experiments you've mentioned are necessary in the development of QM but not sufficient. There is much more to it.

 

[kith]

So, my question is, what happened in the experimental realm between 1915 and 1925 that necessitated this (to me) huge increase in the sophistication of the explanatory apparatus?

Not much. It's just that the old quantum theory is full of ad hoc assumptions and therefore not very appealing. So it's only natural to do further research. And when Heisenberg thought about the mechanism of how the quantum transitions occur and decided to use only observable quantities, the modern version of QM emerged.

Such paradigm shifts always require some time. In this case, the large time gap has probably also to do with the First World War.

 

[jtbell]

• Over the next five years or so, it is shown by Bohr and others that extending this concept to atomic emission and absorption explains how a planetary atomic model can exist without “spiraling down” due to emitted radiation, and gives an excellent fit to the known spectrum for hydrogen.

This brings us up to around 1910. We then seem to enter a time machine, and jump forward to the late twenties, where we find that the next generation of theorists are coming up with the uncertainty principle, wave mechanics, and the whole controversy surrounding the Copenhagen interpretation.

So, my question is, what happened in the experimental realm between 1915 and 1925 that necessitated this (to me) huge increase in the sophistication of the explanatory apparatus?

The simplified version that you find in most second-year university-level "intro modern physics" textbooks (at least the ones that I've used) goes something like this:

1. Bohr justified the discrete energy levels of hydrogen by assuming that orbital angular momentum must be quantized.

2. de Broglie proposed that one could justify Bohr's assumption by assuming that the electron has a wavelike character, therefore the circumference of the orbit must equal an integer number of wavelengths, otherwise there would be destructive interference.

3. Davisson and Germer observed diffraction effects in a beam of electrons impinging on a metal surface which agreed with de Broglie's formula for wavelength in terms of momentum.

4. Schrödinger came up with a differential wave equation for de Broglie's "matter waves", in which the wavelike character was now distributed in three dimensions around the atom instead of a circular ring. He thought at first that the wave function was associated with the density of electric charge of a "smeared-out" electron.

5. Max Born came up with the probabilistic interpretation of Schrödinger's wave function. Then arguments started about how to interpret this probability. The arguments continue to the present day, and there is still no generally-accepted resolution.