I found an article on the "Reference Frame" titled Simple QM proof implies many worlds don't exist. I tried to read it, but being a complete layman, I did not understand a thing. Could somebody tell me if this proof is valid and the many worlds interpretation is no longer considered as an option? Or does the proof have a critical flaw?

Also, if possible, can somebody give a brief summary of the argument in layman's terms?

Thank you.

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    $\begingroup$ When scientists make ground-breaking claims which they can back up with sound argument or evidence, they publish in a peer-reviewed journal. When they are slagging off their opponents, like everyone else they write a blog. You found this "proof" in a blog... $\endgroup$ Jan 27, 2017 at 6:33
  • $\begingroup$ @sammygerbil Yes you're right; but as I explained in a comment above, the sad truth is that even if this proof were valid and published in a journal, I might not have known about it; the reason being that "Interpretation X has been shown to be inconsistent" would probably not make the news. It would probably come up on pop science websites, but I still might have missed it as I don't read these things every day. I'm sure that if Lobus actually published it in a journal the article would say so itself, but I thought that maybe this was canon knowledge discovered some time ago and Lobus... $\endgroup$
    – Ovi
    Jan 27, 2017 at 6:47
  • $\begingroup$ just chose to explain it when he did. This possibility, along with my knowledge that Lobus' work is highly regarded at least in his specific field, prompted me to ask this question. $\endgroup$
    – Ovi
    Jan 27, 2017 at 6:47
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    $\begingroup$ In the parallel world in which Lubos' proof is valid you would not have missed it. You would still read his blog, but in that world it would contain numerous references to his peer-reviewed journal article and the glowing reviews in The New York Times. This would still be the case even if the proof was old news. Sadly, the blog you read did not contain such references, therefore at the time of writing the blog no such journal article - and no peer-reviewed proof - existed. You are in the wrong world. $\endgroup$ Jan 27, 2017 at 7:07
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    $\begingroup$ Downvoted, because I believe this is not Skeptics.SE where we'd analyze everything we find on internet blogs. $\endgroup$ Jan 27, 2017 at 11:21

2 Answers 2


Lubos Motl's argument isn't right; it's shooting down a strawman of the MWI, not the MWI itself. To recap, the argument goes like this:

  1. Many worlds claims that after a spin measurement, there are separate 'worlds' with different measurement results. For example, there could be one world where the electron is spin up, and one where the electron is spin down.
  2. We can identify whether a quantum state is spin up or spin down, and there are no quantum states that are both at once.
  3. Therefore, the electron can't be both spin up and spin down, so many worlds is false.

The trick is that in step (3), Lubos has assumed that the state of the electron in the MWI is a standard quantum state (and that the "worlds where the electron is spin up/down" are simply a quantum superposition).

However, this isn't what the MWI says at all! Instead, it says that after measurement, the electron is entangled with the measuring apparatus, so their joint quantum state is something like $$|\text{screen says +1, electron spin up}\rangle + |\text{screen says -1, electron spin down} \rangle$$ where I'm neglecting coefficients and phases. Because the electron is entangled with something else, it doesn't have a quantum state of its own, so step (3) doesn't work.

The simplest thing we can do to extract a "state" for the electron is to ignore ('trace out') the state of the apparatus. When we do this, we find that the electron is actually described by a mixed state, i.e. something like $$\text{50% chance of } |\text{spin up} \rangle + \text{50% chance of } |\text{spin down} \rangle.$$ This is a probabilistic, not quantum, mixture of states, and the $+$ sign is not quantum superposition. In accordance with step (2) above, there are no quantum states here that are both spin up and spin down at once -- just a mixture of two that are spin up and down separately. These two possibilities are what MWI people would call the two 'worlds'.

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    $\begingroup$ +1 I know what I hate in ( often endless and pointles discussions /arguments) about interpretations and I don't hate your answer. Thanks very much for posting a straightforward reply that taught me a lot. And did not require aspirin. $\endgroup$
    – user140606
    Jan 27, 2017 at 5:08

I doubt it, because the notion that any interpretation of quantum mechanics is provably true or false by experiment seems to be fundamentally wrong. A pretty mathematical "proof that something is impossible" doesn't count, even if the math is correct.

To quote from the blog itself,

Stupid monkeys are obsessed by questions whether MWI and other things are "not even wrong", "politically correct", "obeying Occam's razor", "pretty", and all such irrational adjectives, but no one seems to care about the question whether it is scientifically false or true.

IMO what he should have written is "scientists don't waste their time arguing about whether different QM interpretations are "true" or "false" because (1) there is no way of doing any experiment to answer the question, and (2) it makes no difference to any practical use of QM (and it certainly does have a practical use - for example almost every modern electronic device uses components which can only be designed and understood using quantum mechanics.)

The blog author quotes Bohr, but doesn't seem to understand what the quote means:

As Niels Bohr used to say, Physics is not a tool to describe how the reality is. Physics is a tool to say right things about what we can see.

I think the majority of working physicists don't spend much time on "interpretations" at all - or in other words, they prefer the slogan "shut up and calculate" to "shut up and contemplate". But of course the pop-science books love this sort of thing, because it sounds cool and doesn't need any equations to describe it.

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    $\begingroup$ You say 'the notion that interpretation of quantum mechanics is provably true or false by experiment seems to be fundamentally wrong'. However, as an example, there exist ideas for experimentally distinguishing between the pilot wave 'interpretation' and other 'interpretations' (at least for certain initial conditions for the universe), as in e.g. this paper. In this case, this 'interpretation' agrees with canonical quantum theory in 'equilibrium' situations (in some precise sense), but can be novel outside of 'equilibrium'. $\endgroup$
    – diracula
    Feb 12, 2017 at 15:35

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