Why were the founders of quantum mechanics idealists?

Question

Max Planck, Schrodinger, Bohr, and other founders of quantum mechanics have all expressed an idealist or subjective idealist position: in other words, they believed reality is created by or dependent on the mind. I see why quantum mechanics lends itself to scientific antirealism, but if you were to accept it as just instrumental I don't see why you would then be committed to idealism. So what was their reasoning?

Answer 1

I suppose one of the reasons for their idealism¹ is a particular interpretation of quantum mechanics, which also happens to be the commonly accepted one. Basically, it suggests that the state of the Universe is undefined until someone looks. Each particle appears to be everywhere, filling up the entire Universe² -- until, again, someone tries to actually observe it. Once they do, however, they always find the particle at some specific location, with the probability determined by the so-called wave function.³

The particle's transition from being everywhere to being found at a specific point in space is referred to as wave function collapse. And again, the naive interpretation is that the collapse is triggered by the act of observation, thus suggesting that the sentient(?) observer creates (or shapes, or forms) the reality.

Some prominent physicists -- most notably Albert Einstein -- never bought into this idea. It should be also noted that no one has ever seen a particle in its undefined state. Indeed, eventually we came up with other interpretations of QM that do not require an observer for the Universe to take its shape (most notably, the many worlds interpretation, which, ironically, does not require many worlds -- the multiverse -- to actually exist either).

However, in the early days, the naive interpretation was only one of the two available (the other being "shut up and calculate"). Finally, there were other serious, if personal, reasons why many found (and still find) this interpretation appealing.

¹ Alleged -- and I think Bohr was less guilty of it. His position was closer to "shut up and calculate".

² This crazy idea comes from experiments, in particular the so-called double-slit experiment. In it, a single particle has to pass through a solid plate with two slits in it. One would think that it can only pass through one slit or the other. In practice, however, the particle acts as if it knew whether only one or both slits are open -- as if it went through both slits on its way through the plate.* In fact, it appears as if the particle took every possible path -- also going around the Moon, or around the most distant galaxy, etc. -- the speed of light limit be damned.

* And this we know because the probability of the particle being observed at specific location changes depending on whether one or both slits are open.

³ Every particle and every chunk of matter has its wave function. The universal wave function determines the probable position of every particle in the Universe.

Answer 2

The development of modern physics shattered the old dichotomy between perceptual idealism, and materialist realism.

The classical refutation of idealism was by Samuel Johnson kicking a stone. This was not decisive, https://www.essentiafoundation.org/idealist-stones-hurt-too/reading/#:~:text=Johnson%20is%20implying%20that%20it,stones%20does%20not%20refute%20idealism. but it does illustrate the options that were under debate.

Under modern physics, consisting of QM and relativity, BOTH our perception and our "real" world of the matter around us, are mistaken. This view is called scientific realism, and it holds that what is "real" are the fundamental objects and equations of physics -- and that "rock" is just a transitory artifact, with ill-defined boundaries, and "really" consisting of mostly empty space, that the principles of Electro Magnetism prevented Johnson's foot from taking advantage of and passing through the "rock". Note under scientific realism, "matter" is no longer fundamental -- it too is a transient and ill-defined manifestation of the more fundamental principles of energy, entropy, momentum, probability fields, etc. Note all of these "real" things in scientific realism fall under the general class of "abstractions". Scientific realism basically holds that matter reduces to math, and is a form of platonism. "Shut up and calculate" is a conclusion of a belief in the fundamental "reality" of the math behind physics.

Counter proposals to scientific realism include instrumentalism, which holds that energy, momentum, entropy, and probability fields are just useful ciphers in our minds to deal with the "real world". This has been seized upon by materialists to challenge "scientific realism", but this tactic is poorly thought thru. What is "real" in instrumentalism are the experiences we make our inferences from, and the mind that holds them as useful instrumental models. Matter, and the world around us, is as much an "instrumental speculation" as the complexities of modern physics are. Instrumentalism is actually a classical phenomenological idealism.

One of the other objections to scientific realism was the criticality of an observer in the tests that established QM. QM today is generally treated as having wave function collapse when there are "interactions" rather than "observations" (which require an observer, putting US directly as agents in the "reality of the world). But this is incorrect. Einstein forced this difference into physics attention with one of his later papers. https://plato.stanford.edu/entries/qt-epr/

EPR led to Bell demonstrating his inequality. Per Bell's Inequality, QM cannot be both "local" and "real", where local is defined as satisfying the light cone interaction boundaries of relativity, and "real" is defined as a mind-independent reality. Bell's Inequality has been tested, and confirmed. This confirmation is widely interpreted as a refutation of the "locality" of light cone boundaries of relativity, but it could alternatively be treated as a refutation of "realism".

Another poster proposed MWI as another alternative, but this is a misunderstanding of MWI. MWI is fully subject to Bells Inequality, and does not rescue classical realism from the challenges it faces due to modern physics. Note also that MWI is just another variant on the platonism of scientific realism, where "math is what is real" is taken to an even greater extreme, and an infinity of universes are proposed because the math is cleaner if they split rather than collapsing.