In quantum mechanics we have the the state of a system represented entirely by a wave function, and the equation that dictates how it will evolve, i.e. the Schrodinger equation.

The Copenhagen interpretation requires, in addition to the above, the claim that the wave function "collapses" due to something called a "measurement", and the Born rule, which tells us that the probability distribution of collapse is given by the square norm of the wave function, and the basis in which the measurement is occurring. This is a claim about an additional mechanic to the Schrodinger equation, in describing how the wave function evolves.

The spontaneous collapse theory requires additional mechanics to determine the time-dependent probability distribution of spontaneous collapses (which favors the spatial basis), in addition to the Born rule. Like the Copenhagen interpretation, this is another additional mechanic to the Schrodinger equation in describing the wave function's evolution.

The De-Broglie Bohm theory requires the existence of a point particle in addition to a wave function. And, as with spontaneous collapse, it elevates the position basis as the "most important" one.

It seems like the Everettian interpretation is the only one that adds nothing to the theory, and simply states that the wave function and the Schrodinger time evolution is all there is. It doesn't require the Born rule. It doesn't favor a specific basis of states for the wave function, like spontaneous collapse or pilot wave does. And it doesn't add any new mechanics to the evolution of the wave function beyond the Schrodinger equation. It seems like the set of assumptions in any other interpretation of quantum mechanics is a superset of the set of assumptions in the Everettian interpretation.

Am I missing something? Does the Everettian interpretation impose something new to quantum theory that the others don't? Or is the Everettian interpretation the interpretation with the fewest assumptions, and thus the correct application of Occam's Razor to the measurement problem?

Note: I am not asking if the Everettian interpretation is correct, or "obvious", or the one that everybody should be adopting and textbooks should be teaching. Nothing like that. I'm asking if the Everettian interpretation of quantum mechanics is the one with the fewest assumptions. I can already foresee the objections that this question is ultimately subjective and opinion-based, which it would be if I was asking the former questions. But I think the question I'm asking is actually objective. It's perfectly compatible to believe, for example, that Everettian is the natural choice when applying Occam's Razor, but still not believe that it's the correct interpretation.

  • $\begingroup$ might make Ben Crowell's or Kakaz's answers look out of context if that is the case, and it does appear to be so, you should not edit it, since the edit invalidates the existing answers. Please roll back the changes at your earliest convenience. $\endgroup$
    – Kyle Kanos
    Oct 24, 2019 at 16:55

2 Answers 2


Occam's razor favors simplicity, but simplicity is subjective. I've heard people use Occam's razor for exactly the opposite purpose, to argue that it's extravagant to talk about many worlds, so we should favor the Copenhagen interpretation (CI) over the many-worlds interpretation (MWI).

I think wrangling over CI versus MWI is pretty pointless, because we have a better framework these days for discussing these things, which is decoherence.

Any explanation of what is going on in quantum mechanics needs to explain why we don't see interference between pointer states (or cat states). Decoherence explains that without any additional postulates, and it also explains why it's an approximation that we can't see interference between pointer states. This is something that CI and MWI don't address; they merely assert the result of the approximation, without explaining why it's an approximation or how to tell how good an approximation it is.

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    $\begingroup$ Simplicity isn't subjective when you measure it by number of assumptions made. $\endgroup$ Sep 26, 2019 at 15:42
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    $\begingroup$ Can you elaborate on how decoherence resolves the measurement problem? Even if we assume that, when a system interacts with its environment, all the interference terms in the basis of the measured quantity vanish entirely, we still have the problem that the system is in a superposition of multiple states, which is inconsistent with what we observe. As far as I can tell, we're still stuck at the position that we need an interpretation to explain this. If this is just a misconception, can you help alleviate it? $\endgroup$ Sep 26, 2019 at 15:47
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    $\begingroup$ @Bridgeburners: we still have the problem that the system is in a superposition of multiple states, which is inconsistent with what we observe. The observer is then in a superposition of states. Decoherence and linearity explain why each state of the observer can't tell that the other states also exist. $\endgroup$
    – user4552
    Sep 26, 2019 at 20:25
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    $\begingroup$ @BenCrowell I believe that would be the claim of the Everettian interpretation $\endgroup$ Sep 26, 2019 at 20:48
  • $\begingroup$ @Bridgeburners: The most austere versions of MWI are simply a statement of the postulates of quantum mechanics. In more baroque versions, you get claims about branching of universes and so on; those versions then require an explanation of why pointer states don't interfere. Unitary evolution is a feature of all versions of quantum mechanics, and unitarity guarantees that Copenhagen-style collapse can't be an actual physical process, only an approximation. $\endgroup$
    – user4552
    Sep 27, 2019 at 21:50

In quantum mechanics in fact there is no isolated systems. Interactions with environment are impossible to avoid partially because of real influence of small perturbations, partially because of quantum entanglement.

Saying that it is clear that measurements process, based on interaction between quantum system and macroscopic measurement aparatus is complicated, and may be very hard to describe.

Hard, doesn't mean impossible.

It reminds somehow thermodynamic versus classical mechanics. It is possible to compute ( via molecular dynamics) all interesting thermodynamical characteristics of the gas. But why would you like to do it as computing macroscopic characteristics like temperature or pressure is just enough.

The same situation is with quantum systems.

You are completely missing the point. Collapse/measurement etc is shortcut for interactions with environment.

Take for example simple particle in potential well. Ifyou want, you may start with Hamilton for all universe, and starting like that, you probably have to put in it Hamiltonian for Sygitarius black hole, and Hamiltonian for moon an much more. Then you may meditate on this monstrous expression and notice that probably nearly all you wrote have no real influence on the problem you are trying to solve. Then you have to drop anything which is not important. Sygitarius as well. But they're two things you cannot ignore: particle in the well and measurement aparatus.

And you have to solve such system. It is still way too hard in most situations, because of macroscopic characteristics of measurement process.

You cannot assume your measurement do not disturb your particle So next clever idea is to say, this disturbance is in a shape, you can move forward. It may be asumption about collapse. Of course it is a lie! Everyone knows that. But it's kind of cheating you may afford, because results are quite good correct!

Everett interpretation is exactly about this. It is just reminder, the whole system is irreducible. And if measurement aparatus is less influential, more subtle and more quantum one, the more important is this idea. But you have to remember: irreducible in platonic meaning, don't have to be irreducible in practice. Everything is connected to everything, but most of this connections may be neglected, because they are just weak, random without coherence so cancelling, and because of many other circumstances.

And maybe you could be disappointed, but three is probably no parallel universe where you agree with that answer. There's no parallel universe at all...

  • $\begingroup$ I'm not quite clear on what you're saying. But it sounds like you're offering a purely pragmatic argument, in that the Copenhagen interpretation is merely a convenient tool for describing what's going on during a measurement, so we may as well treat it as the correct ontological description even if it isn't. Because what's really going on with the wave function is really convoluted, but unnecessary for our purposes. Is that what you're saying? $\endgroup$ Sep 26, 2019 at 16:05
  • $\begingroup$ I want to explain that interpretations of Everett models where somebody argues that every quantum interaction grows really physical, separate universe, are not related to science, and base on completely misunderstanding what the whole thing is about. $\endgroup$
    – kakaz
    Sep 26, 2019 at 16:16
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    $\begingroup$ I don't think Everett ever actually said that we grow a physically separate universe. All he said was "let's take out the Born rule and the concept of 'measurement' from wave function evolution". "Multiple worlds" is just a way of looking at it. We're not (according to the interpretation) adding any dimensions to the Hilbert space by making a measurement. $\endgroup$ Sep 26, 2019 at 16:20
  • $\begingroup$ Exactly. The part which Everett wants to be still present is exactly "the rest" which is neglected where somebody wants to use paradigm were wavefunction collapse. Collapse is just a model of interaction with environment, and is element of quantum theory. If you want more precisely description you have to use quantum field theory, and calculate real interactions with environment $\endgroup$
    – kakaz
    Sep 26, 2019 at 16:23

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