At the end of the nineteenth century, theoretical physics was in disarray. Competing theories regarding very small- and very large-scale phenomena were incomplete and conflicted to such an extent that it seemed there could be no resolution. The Michelson-Morley experiment’s failure to detect an immovable luminiferous aether led to absurd fabrications such as the Lorentz–FitzGerald contraction. At the micro level, a world far too small to see with available instrumentation seemed unknowable.
After 1900, theoretical and experimental advances came rapidly, and in the twentieth century we have moved toward a detailed knowledge of the very small and the very large, though many issues remain unresolved.
Niels Bohr’s life work, which spanned the first half of the twentieth century and a few years into the 1960’s, focused on the atom. He began with Ernest Rutherford’s view that the atom consisted of a small nucleus surrounded by still smaller electrons. The problem with Rutherford’s model, however, was that the electrons would be expected to lose energy over a period of time – very brief in this submicroscopic domain – so they would crash into the nucleus, perhaps fusing with it, much as the earth will one day plunge into the sun.
The fact that electrons don’t crash into their nucleus became the starting point for Bohr’s investigations. Much like Albert Einstein’s thought experiments, Bohr’s methodology had a distinctly philosophical character. At his time and dealing with such a remotely scaled domain, it would have been difficult to be empirical. What Bohr proposed would seem preposterous to some individuals, but it is now widely accepted, as amended by Werner Heisenberg and others.
Bohr described an atom composed of a nucleus surrounded by electrons that orbited in concentric shells. His great innovation was in describing how electrons, situated in concentric shells, have distinct energy levels and how they jump from one to another in spontaneous events.
The great innovation was that light and even matter can correctly be described as either a particle or a wave. When described as a wave, it also (believe it or not) exhibits the property known as bandwidth. This notion, applied to a nominally solid body, though perhaps hard to believe, has been abundantly demonstrated experimentally. Philosophically, the idea that particles have a dual nature is known as complementarity, which means that seemingly mutually exclusive properties may be seen to co-exist, particularly in the realm of very small physical bodies.
Bohr’s thinking dovetailed with that of another great twentieth century thinker, Werner Heisenberg, whose uncertainty principle extended classical physics into decidedly new territory. The two great theoreticians differed sharply in regard to Germany’s plunge into darkness in the 1930’s. Bohr escaped the fate of others, traveling to Sweden, England and the United States before returning to liberated Copenhagen, where he was honored for his life work.
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