The full, weird story of the quantum world is much too large for a single article, but the period from 1905, when Einstein first published his solution to the photoelectric puzzle, to the 1960’s, when a complete, well-tested, rigorous, and insanely complicated quantum theory of the subatomic world finally emerged, is quite the story.
This quantum theory would come to provide, in its own way, its own complete and total revision of our understanding of light. In the quantum picture of the subatomic world, what we call the electromagnetic force is really the product of countless microscopic interactions, the work of indivisible photons, who interact in mysterious ways. As in, literally mysterious. The quantum framework provides no picture as to how subatomic interactions actually proceed. Rather, it merely gives us a mathematical toolset for calculating predictions. And so while we can only answer the question of how photons actually work with a beleaguered shrug, we are at least equipped with some predictive power, which helps assuage the pain of quantum incomprehensibility.
Doing the business of physics – that is, using mathematical models to make predictions to validate against experiment – is rather hard in quantum mechanics. And that’s because of the simple fact that quantum rules are not normal rules, and that in the subatomic realm all bets are off.
Interactions and processes at the subatomic level are not ruled by the predictability and reliability of macroscopic processes. In the macroscopic world, everything makes sense (largely because we’ve evolved to make sense of the world we live in). I can toss a ball enough times to a child that their brain can quickly pick up on the reliable pattern: the ball leaves my hand, the ball follows an arcing path, the ball moves forward and eventually falls to the ground. Sure, there are variations based on speed and angle and wind, but the basic gist of a tossed ball is the same, every single time.
Not so in the quantum world, where perfect prediction is impossible and reliable statements are lacking. At subatomic scales, probabilities rule the day – it’s impossible to say exactly what any given particle will do at any given moment. And this absence of predictability and reliability at first troubled, and then disgusted, Einstein, who would eventually leave the quantum world behind with nothing more than a regretful shake of his head at the misguided work of his colleagues. And so he continued his labors, attempting to find a unified approach to joining the two known forces of nature, electromagnetism and gravity, with an emphatically not quantum framework.
When two new forces were first proposed in the 1930’s to explain the deep workings of atomic nuclei – the strong and weak nuclear forces, respectively – this did not deter Einstein. Once electromagnetism and gravity were successfully united, it would not take much additional effort to work in new forces of nature. Meanwhile, his quantum-leaning contemporaries took to the new forces with gusto, eventually folding them into the quantum worldview and framework.
By the end of Einstein’s life, quantum mechanics could describe three forces of nature, while gravity stood alone, his general theory of relativity a monument to his intellect and creativity.
The article defies the title, there is nothing in the article that supports the title.
Perhaps some clarification is useful. Quantum mechanics is usually the label for non-relativistic quantum physics in a semi-classical setting. (I.e. assuming a classical low energy gravity background, of newtonian gravity or general relativity.) While quantum field theory is the label for a relativistic quantum physics, where particles are subsumed to be field excitations and method infinities are replaced by the need for observations to pin down parameters. Sometimes the label “quantum mechanics” is taken for the whole, but one should be careful – quantum field theory *can* describe gravity in a low gravity setting!
I quote myself again: “And the effective quantum gravity field theory is rather mundane in a flat space universe where effective theories like it and general relativity has to come to an accommodation: “The perturbative treatment of quantum General Relativity behaves as an effective field theory, and well defined quantum corrections can be calculated.” [Donohue, 2017] Both theories would break down at the Planck scale, implying that is perhaps not what happen in reality.”
So there is today nothing “alone” with gravity, which is an effective non-linear first order derivative approximation of systems that linear, also effective, quantum field may need many orders of derivatives to describe. It is easier to describe large scale behavior in general relativity, while it is easier to describe small scale behavior in quantum gravity field theory.
Here is something to think about (IMO of course): while quantum field theory and gravity is perfectly compatible, there is also a sense where the former stands out as a consequence of relativity. Not only because special relativity goes into the field description but also because it can be used to model absence of non-probabilistic predictability. By allowing observations of quantum spin obeying a relativistic principle of no preference for reference frames, the quantum mechanic model of “wavefunction collapse” appears as a relativistic aspect of nature on the same footing as time dilation and length contraction. [“Answering Mermin’s challenge with conservation per no preferred reference frame”, Stuckey et al, Nature Scientific Reports, 2020.]