[sam-2727]

Perhaps I'm misunderstanding some of the philosophical aspects here, but the Copenhagen interpretation seems to just a label for the (now) undisputed basic facts of quantum mechanics, so the "interpretation" part of it seems to be a bit historical now. The Wikipedia page gives the following "principles" of the interpretation:

> Quantum mechanics is intrinsically indeterministic.

> The correspondence principle: in the appropriate limit, quantum theory comes to resemble classical physics and reproduces the classical predictions. The Born rule: the wave function of a system yields probabilities for the outcomes of measurements upon that system.

> Complementarity: certain properties cannot be jointly defined for the same system at the same time. In order to talk about a specific property of a system, that system must be considered within the context of a specific laboratory arrangement. Observable quantities corresponding to mutually exclusive laboratory arrangements cannot be predicted together, but considering multiple such mutually exclusive experiments is necessary to characterize a system.

These are universally accepted facts now (it would be silly for any physical theory to contradict the second one). Even a kind of "out there" theory like many worlds theory (https://en.wikipedia.org/wiki/Many-worlds_interpretation) would not dispute these facts (it just reinterprets the indeterminism of quantum mechanics as taking different "branches" in a multiverse). The "interpretation" part of it is a bit historical, as no "interpretation" should contradict these facts (a la Bell's theorem).

Edit: As a philosophy, any "interpretation" built off of these facts is cool, but until you calculate anything, it's essentially useless as a physical theory.

[DebtDeflation]

The collapse of the wave function upon measurement is the interpretation part that is disputed by other interpretations.

[sam-2727]

Interesting, although I struggle to see how this could be. The collapse (from our point of view, at least) of the wave function is pretty necessary in the math of QM. You can only observe the eigenstates of a wavefunction, which is what collapse is. To me, it seems that other theories are just disputing what the "collapse" fundamentally means. For example, Many Worlds theory (from my admittedly limited understanding) says that the wave function's eigenstates each become the new reality in different universes. Please correct me if I'm wrong here though.

[ravi-delia]

Decoherence is the normally proposed solution. Unlike collapse, it wouldn't be the only non-linear, non-unitary, discontinuous operator in all of quantum mechanics. It doesn't give Born's rule, but as long as we're allowed to derive Born's rule by saying "and the wavefunction obeys Born's rule" that's not a mark against it.

[lmm]

> The collapse (from our point of view, at least) of the wave function is pretty necessary in the math of QM.

The measurement results are mathematically necessary. The Copenhagen interpretation, that this means there is a physical collapse in the wavefunction, is not.

> To me, it seems that other theories are just disputing what the "collapse" fundamentally means.

If you substitute "collapse" for "measurement" this is pretty much true. What else could "different interpretations of a theory" mean?

[scotty79]

I think that QM is actually covering two very distinct things.

One is about what actually happens. This contains Shrodinger equation and similar things.

The other is about what results will we get if we poke the particles with macroscopic objects disturbing them beyond recognition. That's the all math where the word "measurement" is used.

Somehow we think the science of what happens to a frog when you poke it with a knife is a part of zoology. It's important, it might be even more important than zoology, but that's a different domain of science.

[ravi-delia]

That's all well and good in zoology, but since both the experiment and experimenter are quantum systems, QM has to explain any and all interactions between the two. Measurement can't be some extraneous thing to what's happening, since it too is happening. I think decoherence gets some points on that mark, I'd be shocked if anyone really thought collapse wasn't the product of some yet undiscovered (or at least unconsidered) aspect of quantum physics.

[jbay808]

> For example, Many Worlds theory (from my admittedly limited understanding) says that the wave function's eigenstates each become the new reality in different universes

That's perhaps the Narcissistic Many Worlds interpretation. Another way of thinking about Many Worlds is that there's only one universe, in which: after measuring a particle in a superposition of two states, you are now in a superposition of two states. Each of the superimposed you states thinks your instrument measured a single clear result.

The wavefunction of the universe goes on propagating as usual, including both of the superimposed yous, but within each of the superimposed states, that you thinks that it has witnessed wavefunction collapse.

[ravi-delia]

It annoys me that you really have to dig into Many Worlds for a while before someone says outright what you're saying here. I guess it's because to someone already familiar with QM, decoherence is more about explaining why the newly different portions of the wavefunction don't interact than that there's a superposition at all, but to the layman the takeaway should really be exactly what you wrote.

[akvadrako]

The main problem with the Copenhagen interpretation is it's too vague. Some say it up involves collapse as a physical process, some say it doesn't. Whatever collapse is in Copenhagen, it is fundamental.

In many worlds, collapse is a subjective and approximate phenomena, not something fundamental.

[jk20]

Once I was present at a discussion between a seasoned quantum optics expert and a group of aspiring physicists, and he asked us which interpretation of quantum mechanics do we prefer. Some chose the standard Copenhagen interpretation, others chose the Many Worlds interpretation, or an interpretation with hidden variables, and so on. But just over half of those present agreed on the standard interpretation. And the expert agreed - why? Because, as he put it, everything that had ever been done in the field of quantum physics had been done using the Copenhagen interpretation.

The way I see it, other interpretations have no purpose except to reconcile what we observe on small scales with incorrect statements such as "electron is a tiny ball", or "electron is a wave in the three-dimensional space", or "an observer which are themselves composed of quantum mechanical particles is independent of the system, and exerts free will to choose measurements while the observed system is purely deterministic".

[robbedpeter]

The Heisenberg uncertainty principle is the root of the Copenhagen interpretation.

The uncertainty principle says you can never know both the location and the momentum of a particle. This stems from the fact that because spacetime is the way it is, knowing either one of those things requires having made a measurement, which by its nature, prevents the measurement of the other thing.

The act of measurement is independent of there being an "observer" in the sense of some sort of intelligence or consciousness. The use of the word observer in early communications led to all sorts of woowoo garbage later on.

More recent interpretations suggest that instead of the requirement that particles be in a singular state, perhaps their fundamental nature is probabilistic. The wave function is the thing, the particle at a singular place and time is an illusion. There's no need for wave functions to collapse, and that view seems to be an imposition of human scale expectations on the quantum universe. An electron exists as a point cloud - what we observe is an artifact of the observation, not a fundamental property of the particle.

I am fond of the "universe is made of math" view suggested by Max Tegmark.

https://en.m.wikipedia.org/wiki/Mathematical_universe_hypoth...

We did not evolve to perceive the universe as it is. We have limitations built into the mechanics of our existence that have to be overcome and understood at every level of abstraction that our tools of reason and technology provide. The more degrees of separation between our evolved tools of perception and extrapolated and abstracted concepts about how the universe works, the more it will diverge from human experience and seem to be "weird." When we try to make weird things make sense to our monkey brains, we introduce a bias that can lead us astray.

https://www.quantamagazine.org/where-quantum-probability-com...

The Copenhagen interpretation is an attempt to make monkey sense of something that has no direct input into any of our tools of perception.

The Everett interpretation recognizes that any observation has to include the quantum states of the mechanism doing the measuring, the environment in which the mechanism resides, and the entities in proximity to the environment, and planet, solar system, galaxy, and universe - that by existing inside the universe, you are subject to the influence of everything else that exists within the universe, and that quantum states are one of the proxies we have for predicting the results of interactions between the states.

Stephen Hawking was an Everettian, and discounted the Copenhagen interpretation.

https://en.wikipedia.org/wiki/Many-worlds_interpretation

Tldr; there's no such consensus on Copenhagen, and the many worlds interpretation is gaining precedence, because it's got the most rigorous basis in mathematics.