In ordinary discourse, a theory is a guess or a surmise, as in “that’s only a theory.” In science, however, a theory is a well-substantiated explanation of some aspect of the natural world that is supported by confirmed facts and/or observations. Verification of a theory’s predictions ensures its eventual acceptance by the community of scientists working in the particular discipline.
“Acceptance by the community” means that a consensus has been reached. In other words, at least a large majority, if not almost all, of the scientists who work in the discipline have agreed that the particular theory is the best way to explain or understand the relevant phenomena. In contrast to the bogus claim of some global warming deniers, reaching consensus is an integral feature of successful scientific theories. Once reached, the culmination of consensus is the publication of monographs and textbooks, and the introduction of university/college courses on the subject.
How consensus may be achieved is beautifully illustrated by the development of quantum theory.
It began somewhat inauspiciously with the concept that light and other forms of electromagnetic radiation consists of particles now called photons, but initially referred to as radiation “quanta.”
Max Planck introduced the concept as a means of justifying the formula he created in 1900 that successfully accounted for the experimental data on blackbody radiation. Despite this, the quantum-of-radiation concept was ignored or rejected until Albert Einstein invoked it in 1905, when he successfully explained the photoelectric effect, wherein radiation incident on a metal can eject electrons from it. Even then, the concept was not widely accepted, and experiments were carried out later that attempted to refute Einstein’s explanation, but ended up confirming it.
Why was the radiation quanta concept difficult to accept? Because the overwhelming consensus at that time held that electromagnetic radiation was composed of waves. It was a consensus that hardly wavered even though the 1887 Michelson-Morley experiment had failed to find evidence for the ether, postulated as the medium through which the waves traveled.
Of course, such steadfastness did not hinder the eventual development of quantum theory. The next step in its development was the conclusion of Ernest Rutherford in 1911 that the structure of atoms was analogous to the solar system, with the positively-charged nucleus playing the role of the sun and the electrons being the analog of the planets. Two years later, Niels Bohr used Rutherford’s solar-system model as part of his quantized theory of the hydrogen atom, which successfully accounted for the experimental data.
Bohr’s theory, which involved quantization of radiation and also of energy and angular momentum, was a dead end, since it failed when applied to any atom other than hydrogen. Furthermore, Bohr’s theory suffered from another drawback: according to standard electromagnetic theory, the electron in hydrogen, which Bohr stated was orbiting the proton, would quickly spiral onto it, thereby collapsing the atom. Such collapse did not occur, and though Bohr could not explain why, he made a virtue of necessity by declaring the obvious, that somehow atoms did not collapse.
Other attempts were made that utilized the quantization concept, but none provided insight, and while it seemed important, it effectively remained an orphan.
In fact, it wasn’t until 1924 that the penultimate step was taken, when Louis de Broglie introduced a vital new concept into the mix: he hypothesized that matter, in particular particles like electrons, would gain a wavelength once they were in motion.
So, in 1924 the concepts of quantization and matter waves had been articulated, as was Wolfgang Pauli’s Exclusion Principle, which he had hypothesized to explain the structure of the periodic table of elements. Not yet articulated was a theory that incorporated these concepts and explained microscopic phenomena.
That final step came next, when not one but three equivalent versions of quantum theory were proposed.
Werner Heisenberg produced the first one in 1925. Known subsequently as “Matrix Mechanics,” it was quickly and successfully applied by Pauli to the hydrogen atom. The second one was the brain child of Paul Dirac, a 23-year old English graduate student. Known as the “Transformation Theory,” he applied it to the hydrogen atom in an article that appeared five days after Pauli’s.
The third version was that of Erwin Schrödinger, who was motivated by a remark made after a seminar he gave in Germany on de Broglie’s hypothesis. The gist of the remark was that if matter displayed waves, then what was needed was a “wave equation.” Schrödinger produced a theory, and in 1926, like Pauli and Dirac, he successfully applied it to the hydrogen atom, among many other results. Schrödinger’s eventually became the most used version.
Various conclusions and confirmations were quickly reached. The theory was used to calculate or explain the structure of atoms and of molecules, including why atoms did not collapse. Electrons in a scattering experiment were shown to have the appropriate de Broglie wavelength. Equally important, Max Born resolved a puzzle concerning Schrödinger’s equation, whose solutions involve quantities called “wave functions.” What a wave function meant was initially unknown. Born’s 1927 interpretation was that wave functions are related to probabilities, for instance that an electron would be at a particular position in atom. It was the final ingredient needed to achieve consensus concerning quantum theory.
By 1928, quantum theory had become the new paradigm.
Featured image credit: Varsha ys by Varsha Y S. CC BY-SA 4.0 via Wikimedia Commons.