There were very compelling reasons why most scientists believed this and these reasons have never been explained by other ideas. The most compelling reason was the phenomenon of relativity. H. Ziegler pointed out in a 1909 discussion with Einstein, Planck, and Stark that relativity would be a natural result if all of the most basic components of mass moved at the constant speed of light.
If these scientists were right, Hofstadter's shells would be made of photons, and each shell would exist in its most simple state as just one sine-wave cycle of electromagnetic energy. Since such photon shells must complete their loops at the speed of light in one wavelength, and we know the mass of each shell, and their wavelength is determined by their energy content in accord with Einstein's mass-energy equation, we can calculate the diameter that each shell would be if it were a circle."
That's the way the Current Loop-Disk theory see's it too. :-)
