Many decades ago, scientists were perplexed as to how the atom — presumed to be the elementary particle of the Universe– could glow with such a complex spectral pattern, AND why different elements released different patterns.
As the electron had only been recently “discovered,” some smartie decided that each element’s particular pattern of light is caused by electrons shifting from one sized orbit around the atom’s nucleus to another, simultaneously expelling energy in the form of photons. Because every element is presumed to have a different electron-configuration, this would explain the different spectra produced by different atoms.
With the technology of the time, experimenters first only saw the spectral emissions of atoms in broad strokes. As technology advanced, however, more detailed patterns of spectral emissions were observed. These so-called “fine spectrum” results convinced certain physicists that the inner life of an atom was more complex than first thought. However, the notion that shifting electron orbits were responsible for spectral emissions had already taken hold. Therefore, the natural scientific– heck, the natural human— thing to do was NOT to throw out the electron orbit theory– but to tweak it until it fit the new data.
Therefore, in addition to electron orbit-size, scientists decided that different angles or planes of orbits mattered, the different orbital planes producing different patterns of spectral emissions. As the technical analysis of spectral patterns grew still more detailed, it was decided that orbit shapes also must affect spectral emission patterns.
These first three “quantum numbers” (orbit size, plane-angle, and shape) satisfied physicists for a time– a time which turned-out to be very crucial, psychologically speaking, for the physics community… For it was during this time– the early twentieth century– that the scientific paradigm was shifting from Classical theories to the theories of Quantum and Relativity. Once these new theories begin to be fleshed-out in detail, the notion of tossing them away so soon was beyond anyone’s imagination at the time– at least the imagination of the young trendsetters. They had become enamored of all things Quantum and Relativistic. Only the old-timers were dragging their heels. To be on the cutting edge of physics meant being a Quantumist and a Relativist. The transition from Classical to Quantum/Relativisitic thought was, historically speaking, as fast and as powerful as a lightning bolt.
I believe that if the full “fine spectra” of atoms had been measured from the beginning, there is a good chance we might have chosen an alternative explanation for them– or if the idea of electron orbits had not already captured the imaginations of the day’s physicists. As it was, the electron-orbit theory continued to lumber forward, accruing (like a dirty snowball) each new “fix” to address each new batch of incoming data.
Eventually, better technology resulted in an even more detailed picture of atomic spectra. Unfortunately, the idea that the spectral patterns were caused by electron orbits had been milked dry– it just barely covered the data from the old technology. The new data was telling physicists that the mechanics they had assigned to the innards of the atom were not complex enough to explain the variety of the spectral patterns which were now being recorded. But where would this additional complexity come from? Physicists had already tweaked electron orbits in every possible way… They had used orbit size, shape, and angle. What was left?
Coming up with zilch, physicists moved from electron revolutions to electron rotations. If electrons were assumed to rotate in one of two directions, then the latest spectral emissions data could be perfectly explained. This rotational element, or “Spin” of the electron, became the fourth quantum number.
This was a fateful step. For even as spectral analyzers hardened in their belief in Spin, the rest of the physics community was moving away from the idea of individualized electrons! The solar system model of the atom, in fact, was outdated almost as soon as it achieved widespread notice. Physicists in other fields were beginning to think of electrons as “waves” or as “clouds”– or even as “probabilities.” Furthermore, the physics-math of the electron-as-interpreted produced a speed for the surface of the electron which was faster than light– which is supposed to be impossible since, according to the recently adopted Relativity theory, nothing can move faster than the speed of light.
Physicists then backtracked and said that, well, “Spin” is just a term for something that is not fully understood. What is really important about Spin is that it declares that electrons can exist in one of two states, thus doubling the complexity supplied by the first three quantum numbers and fitting incoming data nicely.
But the old, rejected notion that an electron was actually, physically “spinning” never died. The idea of a truly rotating electron is often the conception used when physicists are trying to explain the phenomenon of magnetism. Theorists of magnetism also rely on the idea that something at least quasi-material is revolving around the nucleus, resulting in a phenomenon known (confusingly) as “atomic Spin.” When physicists talk of Spin, in fact, they might be talking about electron Spin, atomic Spin, or the combined outcome of all such Spins within an atom. Meanwhile, of course, other physicists have long ago moved-on from such a solar-system-sounding scenario for the atom, considering the old conception out-dated, quaint, and naive.
It’s a muddle.