Do other branches of modern science have interpretations like Quantum Mechanics does?

Question

As a layperson, when one learns about Quantum Mechanics, one also learns about the various interpretations of QM-- Many Worlds, Collapse, Pilot Wave theory, etc.

However, as far as I can tell (from google searches), other modern domains of science-- condensed matter physics, fluid mechanics, high energy physics-- don't have interpretations, in the same way that there are for QM.

Is this true? Why does QM garner interpretations of its theory, when other branches don't?

Edit I feel my initial wording was perhaps too parsimonious. I want to flesh out more what I am asking.

Throughout the history of science, various hypotheses and theories are proposed, and experiments are run, specifically designed to disprove a theory. Over time, generally, one theory wins out over the others, and comes to be the commonly accepted theory.

A scientific theory has certain properties, such as that it's falsifiable-- that there is, in theory, some experiment that can be designed such that, if a certain result was measured, it would show that the theory was not true. That a theory is not disproved by such experiments is taken as evidence of its accuracy.

However, I've noticed something peculiar about Quantum Mechanics. In addition to the theory, which has passed many different experiments over the decades, and is generally well-accepted, QM also has "interpretations" or "explanations". Some of these, such as Pilot Wave theory, are falsifiable-- if no Pilot Wave is measured in an experiment, then the theory can be taken to be false, or, not an explanation of the phenomenon.

However, there is an interpretation called Many Worlds, which makes no testable predictions, and no observations can be made of the proposed Worlds. But, this shouldn't be a problem, because it's not a theory, but rather an "interpretation" or "explanation". While the concept of a scientific theory is more strict, in that there are some criteria that a scientific must meet, I have found no similar definition of an interpretation or explanation.

Furthermore, after looking, I haven't even found interpretations at all of other scientific theories! It seems to me that QM is unique in that respect.

I wonder, is it true that only QM has garnered interpretations? Why might this be so, when no other theory has interpretations?

Answer 1

Every theory requires an interpretation to be more than a linguistic or mathematical exercise. At the very minimum, it has to outline procedures connecting its constructs to something empirically observed, so-called operational or minimal interpretation. "Shut up and calculate" is a parody of such interpretation, as it also needs to provide instructions on matching what is calculated to what is measured. Incorporating prevailing intuitions about reasoning in and applying a theory usually requires more than that. This can get complicated considering that most physical theories have entities that are not observed directly, and make general claims that can only be tested in roundabout ways. They have to rely on some pragmatic extra assumptions about observational and experimental setups (such as lack of unknown confounding factors or of superdeterminism that forces experimenters to measure what they "choose" to measure, for example).

For example, determinism was traditionally a big part of how classical mechanics was interpreted in the 19th century (and even today), but it is not forced by its mathematical formalism alone. There was a lively discussion of determinism back then, with some prominent figures (like Maxwell) backing alternative, indeterministic, interpretations, see History of the study of indeterminism in classical mechanics. Modern discussions of the Norton dome are a distant echo of those debates. But the incentive for pondering interpretations of classical mechanics is much reduced now. We already know it is a non-fundamental theory that holds only approximately. Whatever interpretation is adopted, its answers to "big questions" about the "nature of reality" have diminished value. The "burning" questions from 18th century interpretations: are live or dead forces (energy or momenta) more fundamental, see What was the vis viva controversy; does nature follow "mechanical" causes or "strives" to minimize action, etc., seem almost comical today.

Special relativity also had two competing interpretations in its hey day of the early 20th century. The idea of absolute space, and especially time, is quite ingrained in common perceptions, a new physical theory upsetting it met much resistance. The original version of relativistic mathematical formalism, developed by Lorentz and Poincare, came with an interpretation that did not require such a leap. It postulated an absolute frame of ether, but with dynamical effects (length contractions and time dilations) that forever veiled its presence from our eye. Eventually, most of the public, like Einstein originally, got over parting ways with ether. But not all. Those attuned to the subjective perception of time duration and dissatisfied with "mechanization" of time still invoke Lorentz's theory of ether as an alternative interpretation of special relativity to reconcile their intuitions with physics, see presentism.

But again, special relativity is no longer a fundamental theory, general relativity is. It challenges common intuitions still further, suggesting that spacetime itself, even relativistic, is merely a construct. Alternatives with a fixed background, in the spirit of Lorentz's interpretation, popped up, see e.g. Logunov's relativistic theory of gravitation. It is not quite an interpretation of GR, as it makes distinguishable in principle predictions, but it could be modified to be mathematically equivalent to GR, if one so wished. There are already both substantivalist (spacetime is real) and non-substantivalist (spacetime is just a bookeeping device) interpretations proper of GR. On the strength of the much discussed hole argument, Einstein was leaning towards the latter. But the psychological importance of this is undermined by the almost universally accepted fact that GR will have to be replaced by quantum gravity, and it is from there that the "final" answers about spacetime should come.

And quantum gravity, whatever it turns out to be, will inherit the quantum part. Ether and absolute (space)time were one thing, suggesting that the reality itself is observer-dependent, that its "elements" only come to be when looked at, as in the original Copenhagen interpretation, was too much even for Einstein. Strictly speaking, quantum mechanics is not a fundamental theory anymore, it has been supplanted by quantum field theory. But the interpretational issues of QFT (indeterminism, observer dependence, nature of collapse, etc.) are quite similar to those of QM, and the latter are much easier to explain and illustrate to the general public. Hence the proliferation of its interpretations and focus on them in the popular culture.