Quantum Mechanics Smackdown

Greetings from the Big Apple, where the World Science Festival got off to a swinging start with the announcement of the Kavli Prize winners. The local favorite will of course be the Astrophysics prize, which was awarded to Alan Guth, Andrei Linde, and Alexei Starobinsky for pioneering the theory of cosmic inflation. But we should also congratulate Nanoscience winners Thomas Ebbesen, Stefan Hell, and Sir John B. Pendry, as well as Neuroscience winners Brenda Milner, John O’Keefe, and Marcus E. Raichle.

I’m participating in several WSF events, and one of them tonight will be live-streamed in this very internet. The title is Measure for Measure: Quantum Physics and Reality, and we kick off at 8pm Eastern, 5pm Pacific. The live-stream is here, but I’ll also try to embed it and see how that goes:

The other participants are David Albert, Sheldon Goldstein, and Rüdiger Schack, with the conversation moderated by Brian Greene. The group is not merely a randomly-selected collection of people who know and love quantum mechanics; each participant was carefully chosen to defend a certain favorite version of this most mysterious of physical theories.

  • David Albert will propound the idea of dynamical collapse theories, such as the Ghirardi-Rimini-Weber (GRW) model. They posit that QM is truly stochastic, with wave functions really “collapsing” at unpredictable times, with a tiny rate that is negligible for individual particles but becomes rapid for macroscopic objects.
  • Shelly Goldstein will support some version of hidden-variable theories such as Bohmian mechanics. It’s sometimes thought that hidden variables have been ruled out by experimental tests of Bell’s inequalities, but that’s not right; only local hidden variables have been excluded. Non-local hidden variables are still very viable!
  • Rüdiger Schack will be telling us about a relatively new approach called Quantum Bayesianism, or QBism for short. (Don’t love the approach, but the nickname is awesome.) The idea here is that QM is really a theory about our ignorance of the world, similar to what Tom Banks defended here way back when.
  • My job, of course, will be to defend the honor of the Everett (many-worlds) formulation. I’ve done a lot less serious research on this issue than the other folks, but I will make up for that disadvantage by supporting the theory that is actually true. And coincidentally, by the time we’ve started debating I should have my first official paper on the foundations of QM appear on the arxiv: new work on deriving the Born Rule in Everett with Chip Sebens.

(For what it’s worth, I cannot resist quoting David Wallace in this context: when faced with the measurement problem in quantum mechanics, philosophers are eager to change the physics, while physicists are sure it’s just a matter of better philosophy.)

(Note also that both Steven Weinberg and Gerard ‘t Hooft have proposed new approaches to thinking about quantum mechanics. Neither of them were judged to be sufficiently distinguished to appear on our panel.)

It’s not accidental that I call these “formulations” rather than “interpretations” of quantum mechanics. I’d like to see people abandon the phrase “interpretation of quantum mechanics” entirely (though I often slip up and use it myself). The options listed above are not different interpretations of the same underlying structure — they are legitimately different physical theories, with potentially different experimental consequences (as our recent work on quantum fluctuations shows).

Relatedly, I discovered this morning that celebrated philosopher Hilary Putnam has joined the blogosphere, with the whimsically titled “Sardonic Comment.” His very first post shares an email conversation he had about the measurement problem in QM, including my co-panelists David and Shelly, and also Tim Maudlin and Roderich Tumulka (but not me). I therefore had the honor of leaving the very first comment on Hilary Putnam’s blog, encouraging him to bring more Everettians into the discussion!

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53 Responses to Quantum Mechanics Smackdown

  1. Bob Zannelli says:

    Over the last several days there has been a knockdown fight about the Universally Valid Quantum Mechanics Model AKA the Everett model on Vic Stenger’s discussion list. On the whole list only two of us buy the Everett interpretation or really more accurately the Everett model. We were being swarmed by a bunch of those instrumentalists , nice guys but very subjective. I look forward to Sean’s paper on this and whatever is available from this conference

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  2. Mike says:


    Look forward to the paper. Only available via Springer so far, but will take a look when it’s posted on arXiv.

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  3. david says:

    Can you recommend a good explanation of Everett (many-worlds) formulation? I have already read the Wikipedia article but I think I would like something more in depth.

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  4. Sean Carroll says:

    david– My explanation is here, although not in much quantitative detail, admittedly:


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  5. Steve Ruis says:

    Be careful. If you keep your tongue firmly in cheek (the Weinberg quip) you’ll end up biting it. Good luck; should be a good show.

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  6. James Gallagher says:

    Sean and Chip’s derivation of the Born Rule was published in this book, they call it “self-locating uncertainty”. (unfortunately the whole section isn’t viewable in google books, so wait for the arxiv paper)

    It has the usual problem of putting a square root in for arbitrary reasons – why not a fourth root or 100th root?

    The power 2 in the Born Rule is a crucial bit to explain – and so far we only have experimental proof of it: eg http://arxiv.org/abs/1007.4193

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  7. Sean Carroll says:

    James, if you think we “put a square root in for arbitrary reasons,” you clearly haven’t read the paper. We’re deriving the Born rule, not asserting it.

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  8. Another hidden-variables model approach is that of Huw Price and Richard Corry (Retrocausal Models for EPR).

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  9. James Gallagher says:

    Sean, some of the pages were missing in google books, but wouldn’t the argument work in the case where the ratio of the amplitudes is a fourth root of a rational number?

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  10. Sean Carroll says:

    The value of the ratio of the amplitudes is irrelevant — what matters is that the ratio of probabilities is given by the square of the ratios of the amplitudes.

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  11. James Gallagher says:

    Sean, ok I have misunderstood something basic in the argument, I should wait to read the full paper – it is interesting if you have derived this without making any non-trivial assumptions.

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  12. Craig Gidney says:

    Sounds interesting. I would go in person, but the talk is sold out.

    Science and Story is not sold out, apparently, but the WSF site doesn’t seem to allow for people buying tickets at the last minute. It says I should go to the box office instead… but doesn’t say where that is or give a phone number. I guess they don’t want the 40$, and it’s live stream or nothing.

    I’ll have to ask for your thoughts on the Kolmogorov complexity of the laws of physics some other day.

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  13. Dom Miketa says:

    david – I’d have a look at this article by David Wallace. That’s fairly in-depth, but not overly technical.

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  14. James Gallagher says:

    The dart animation is rubbish.

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  15. Mitchell Porter says:

    Sean’s promised derivation of the Born rule has appeared on the arxiv. Skimming through it, I see it is in the same spirit as the Deutsch-Wallace attempt to derive the Born rule, which I criticized on this blog back in February.

    I will repeat my basic objections: If you are a many-worlds theorist, and you want to explain e.g. why QM says event A is twice as probable as event B, the logical explanation is that event A is twice as common as event B, when all the parallel worlds are considered. But Deutsch, Wallace, and now Carroll and Sebens, all reject this approach.

    Carroll and Sebens explicitly recommend against trying to count parallel worlds / branches of the wavefunction. For example, on page 15 they speak of “the unrealistic assumption that the number of branches in which a certain outcome occurs is well-defined”.

    Instead, they endeavor to show that using Born-rule probabilities is the “rational” thing to do in a quantum universe. Deutsch and Wallace defined rationality in terms of game theory. It’s not yet clear to me what Carroll and Sebens are up to… But it might be a sleight of hand employing the conflict between QM and “local realism”.

    Bell’s theorem showed a problem for any attempt to reproduce QM using local causality. One response to this is an explicitly nonlocal theory like Bohmian mechanics; another is to say physics is local in an operational sense (I can’t make a change happen at a distance) and also to simply refrain from even trying to exhibit a causal mechanism responsible for Bell correlations.

    Carroll and Sebens say that they will obtain the Born rule from the principle that “Alice’s probabilities should be unaffected by changes in the state of her environment”. That sounds innocuous. But to me it sounds like a statement of locality in the operational sense, and the lesson of Bell’s theorem is that this is accompanied by some sort of antirealism. And indeed, C&S have an antirealist position, not regarding worlds, but regarding the number of worlds – see the earlier quote where they say that the “number of branches” where something happens is not “well-defined”.

    I can’t say for sure that this is what they are doing. It takes a while to decode these quasi-philosophical arguments in which one is told not to ask certain questions or not to think in certain ways. Indeed, good luck to the novice reader who might be wondering what I mean by “antirealism” or “locality in the operational sense”. Awful jargon exists because people don’t want to explicitly say things like, nothing exists between measurements; or, we shouldn’t care what exists between measurements; or, parallel worlds exist, but we can’t and shouldn’t try to count them.

    For now, all I can do is beg the reader to not believe that any problem has been solved in this paper.

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  17. Josh says:

    I think Mitchell made some interesting points, but I feel the the rationale on why one might interpret the number of worlds as undefined glossed over (and thereby whats acceptable in deriving the probabilities themselves). Unless of course there is no rationale to this but that’s nonsense.

    The number of worlds in an Everret multiverse is quite interesting then. Anyone care to elucidate?

    Mitchell your statements about the jargon involved are not lost on me though. Ive consistently found myself reclarifying much of QM concepts in more explicit terms. Its as if mathematical positivism has entirely won over the scientific culture, and that we no longer desire to articulate explicitly what our theories imply about the fundamental nature of reality IMHO.

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  18. arXiv says:

    Just adding some heat into the discussion, here’s part of the summary of the paper linked under the “Website” input for this comment:

    “[A]fter the lapse of fourscore years, the terminology is more precise today than it was during the founding years. Concepts get clearer in time, and one learns to avoid sloppy terms and misleading phrases. The foundations, however, have not been touched by these refinements.

    As way of summary, here are our answers to the questions asked at the beginning:
    – Yes, quantum theory well defined.
    – Yes, quantum theory has a clear interpretation.
    – Yes, quantum theory is a local theory.
    – No, quantum evolution is not reversible.
    – No, wave functions do not collapse; you reduce your state.
    – No, there is no instant action at a distance.
    – Heisenberg’s cut is where you put it.
    – No, Schrödinger’s cat is not half dead and half alive.
    – No, there is no “measurement problem.”

    Tersely: Quantum theory is a well-defined local theory with a clear interpretation. No “measurement problem” or any other foundational matters are waiting to be settled.

    What, then, about the steady stream of publications that offer solutions for alleged fundamental problems, each of them wrongly identified on the basis of one misunderstanding of quantum theory or another? Well, one could be annoyed by that and join van Kampen in calling it a scandal when a respectable journal prints yet another such article. No-one, however, is advocating censorship, even of the mildest kind, because the scientific debate cannot tolerate it. Yet, is it not saddening that so much of diligent effort is wasted on studying pseudo-problems?”

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  19. Avattoir says:

    I note from near the end of the panel that Professor Carroll has come up with a way now to make bets guaranteeing his death precedes before him being called on to settle up.

    That’s offered just in fun, much in the spirit of the panel session – which seemed even to infect Professor Schack, by the end at least; but, oddly, never quite Professor Greene, whose efforts to dumb down the presentation for the audience to me proved unnecessary.

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  20. Both Sean’s recent paper 1405.7577 and Wallace’s paper 0906.2718 explicitly assume continuity at some point. To me, this seems like the key assumption. If a world “appears” in the wave function with coefficient zero, that’s the same as it not appearing, so we assume that observing that world has probability zero. Then continuity tells you that worlds with small amplitude must have small probability.

    Once you’ve got that, I believe Everett already argued that worlds that don’t satisfy the Born rule have amplitudes that go to zero when experiments are repeated (the so-called “maverick” worlds), and so by continuity they should have small probability even after finitely many experiments, which means we should expect to observe the Born rule.

    I’m curious about what people think of this kind of argument. I have a gut feeling that it is at the core of other arguments given, but would love to understand this better.

    Incidentally, an alternative to assuming continuity is assuming some kind of discreteness of amplitudes, which would cause small amplitudes to become zero.

    Stephen Hsu has a nice summary of this issue in 1110.0549, with an accompanying blog post. His earlier paper 0606062 with Buniy and Zee describes how discreteness can solve this problem. This is discussed in this blog post at The Quantum Pontiff.

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  21. Josh says:

    arXiv, would you not agree though that such is a rather positivist perspective? I.e. in the sense that our current QM framework works and does not preclude or rely upon interpretive matters such as the question of reality regarding Schrodingers cat, but does not necessarily describe what is “reality” fundamentally. This would would seem much more accurate imho. From here, saying that QM doesn’t have an issue with such foundational matters seems a bit disingenuous. It would seem better to simply state that QM, in it’s pure form, doesn’t (yet) affirm anything about those interpretive, foundational matters.

    After all, if all such were already settled within our current QM framework, there would be no need for Sean’s debate.

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  22. Max says:

    A question about the Many-Worlds formulation:
    When the universe splits, and in one version of the universe a copy of me observes the particle in one place and in another version of the universe a different copy of me observes the particle in a different place, then how come I don’t experience all these outcomes? What determines which version of me I will be in? If there truly are multiple copies of me being created, with my brain and therefore my consciousness copied perfectly, then surely I should experience all the outcomes?
    I don’t really know how to express this well but my question is about why, when consciousness is replicated, I still only experience one version of the events and what determines which version I experience.

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  23. James Gallagher says:

    Carroll and Sebens paper is a lot longer than the article in the book I linked to above – but to be honest it seems like a heck of a lot of obfuscation to get the Born rule – I mean if it’s that much trouble it maybe indicates that MWI is wrong.

    In the standard interpretation it is easy to deduce that the simplest candidate for probability conserved by the Schrödinger evolution is |psi|^2 ( = psi.psi*), and then since we have never noticed a single deviation from this in experiments we have no need to think a more complicated expression involving quartic or higher even powers is required.

    God is subtle but he is not malicious.

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  24. CB says:

    Awesome, the first comment on Mr. Putnam’s blog is Sean suggesting they include Everettians in the discussion of QM interpretations^Wformulations… and the second comment is an internet kook telling all those people they don’t understand GR, QM, or electromagnetism.

    Oh well, such is the physics blogosphere.

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  25. Mike says:

    Max said:

    “When the universe splits, and in one version of the universe a copy of me observes the particle in one place and in another version of the universe a different copy of me observes the particle in a different place, then how come I don’t experience all these outcomes?”

    According to our present knowledge of physics whilst it is possible to detect the presence of other nearby worlds, through the existence of interference effects, it is impossible travel to or communicate with them. Mathematically this corresponds to an empirically verified property of all quantum theories called linearity. Linearity implies that the worlds can interfere with each other with respect to a external, unsplit, observer or system but the interfering worlds can’t influence each other in the sense that an experimenter in one of the worlds can arrange to communicate with their own, already split-off, quantum copies in other worlds.

    Specifically, the wave equation is linear, with respect to the wavefunction or state vector, which means that given any two solutions of the wavefunction, with identical boundary conditions, then any linear combination of the solutions is another solution. Since each component of a linear solution evolves with complete indifference as to the presence or absence of the other terms/solutions then we can conclude that no experiment in one world can have any effect on another experiment in another world. Hence no communication is possible between quantum worlds. (This type of linearity mustn’t be confused with the evident non-linearity of the equations with respect to the fields.)

    Non communication between the splitting Everett-worlds also explains why we are not aware of any splitting process, since such awareness needs communication between worlds. To be aware of the world splitting you would have to be receiving sensory information from, and thereby effect by the reverse process, more than one world. This would enable communication between worlds, which is forbidden by linearity. Ergo, we are not aware of any splitting precisely because we are split into non-interfering copies along with the rest of the world.

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  26. Ignacio says:


    The entire video discussion is quite unhelpful in that it portrays a sense of division in the science that simply isn’t there.

    Even Weinberg surprisingly falls into the trap of making assertions that are clearly unjustified in his latest paper. He states that QM or the interpretation of QM implies that there is a microscopic and macroscopic separation and that QM only applies in one regime and not the other.

    There is no such division of realms. All of nature is governed by QM. The description of reality is at its core probabilistic and this is not based on our limited abilities. The basic unwillingness to accept this accounts for the majority of the noise the human mind produces on this subject.

    If you are still troubled by the introduction of observers then you should just look at decoherent histories.

    There is no point in pretending that one is saying something more profound other than for entertainment purposes, in which case one should honestly state as much.

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  27. paul kramarchyk says:

    I am 66 years old. Retired engineer (commercial nuclear power). And the fire that continues to burn in this old body is my love of science and the wonderful mix of people who dare do it. When others say prayers, I say this:
    “The effort to understand the universe is one of the very few things that lifts human life a little above the level of farce, and gives it some of the grace of tragedy.” — Steven Weinberg

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  28. Avattoir says:

    Some time back, I made a value judgment to the effect that, when confronted with a choice between the closely-reasoned views of a Nobel Prize winning HEP physicist as responsible as any living human for the assembly of the most successful prediction of physical behavior in recorded history, and the fumes of shortened interpretation and abbreviated dismissal of him by an anonymous blogger with the Italianate version of one cartoon cat and the Socratic technique of another, default to the former.

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  29. Avattoir says:

    James Gallagher – “subtle, but … not malicious”.

    Also, fictional.

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  30. Avattoir says:

    Mike – On top of the communication barriers, there’s also that while you can remember what happened to the you that’s here with us now, you can’t remember what happened to all the other yous that are off with all the other us-es behind the barriers.

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  31. James Gallagher says:


    Yes of course, but if he (god) had been involved he surely would have chosen |psi|^2 for the probability, anything else would give a really weird probability law that would be too silly, and as we all know, God is certainly not silly.

    I certainly would have chosen |psi|^2 from the possible choices of conserved quantities and I don’t have even have close to godlike intelligence.

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  32. James Gallagher says:

    Note that the simplest quantities conserved by the Schrödinger evolution are expressions made up of terms involving psi.psi* (= |psi|^2) and their powers , you can’t have psi, or psi* (and their powers) in a term on its own since it will not be conserved by the evolution.

    So why is the Born rule not |psi| or |psi|^4 (for example)?

    Well that’s the point I’m trying to make when I say if “god” had to choose he would not choose such expressions since that would give a really weird probability law (where mutally exclusive outcomes would not have their probabilities just added together), and as we know “he is not malicious” :-)

    Maybe it could be an anthropic argument, so that universes where the Born rule is different don’t enable intelligent life, I have read some arguments like this, not so convincing for me.

    There is also the possibility that |psi|^2 is selected dynamically from the evolution as some Bohmians have argued, or maybe even is a thermodynamic property of a fundamentally probabilistic evolution.

    The Born rule can’t be fully derived from the other postulates of Quantum Mechanics, but an appeal to simplicity is very compelling, and in the end, that is enough, for me.

    btw, it is quite amusing that Born’s original paper got the Born rule wrong even after an edited footnote – see A Pais’ article in SCIENCE, VOL. 218, 17 DECEMBER 1982 “Max Born’s Statistical Interpretation of Quantum Mechanics” (he originally says psi is the probability, then in the edited footnote changes this to psi^2 – when he should have said |psi|^2 )

    Also, some of the founding fathers were (temporarily) confused as to whether |psi| was the probability.

    early draft for those who don’t have access:

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  33. Bob Zannelli says:

    Ignacio says:
    May 30, 2014 at 2:34 pm

    The entire video discussion is quite unhelpful in that it portrays a sense of division in the science that simply isn’t there.

    Even Weinberg surprisingly falls into the trap of making assertions that are clearly unjustified in his latest paper. He states that QM or the interpretation of QM implies that there is a microscopic and macroscopic separation and that QM only applies in one regime and not the other.

    There is no such division of realms. All of nature is governed by QM. The description of reality is at its core probabilistic and this is not based on our limited abilities. The basic unwillingness to accept this accounts for the majority of the noise the human mind produces on this subject.

    If you are still troubled by the introduction of observers then you should just look at decoherent histories.

    There is no point in pretending that one is saying something more profound other than for entertainment purposes, in which case one should honestly state as much.


    This is an excellent comment, I couldn’t agree more. Tegmark hit the nail on the head when he made the argument that in Quantum Interpretation it all comes down to which is more objective, everyday language or Mathematics.

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  34. Bob Zannelli says:

    Hidden due to low comment rating. Click here to see.

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  35. Bob Zannelli says:

    Hidden due to low comment rating. Click here to see.

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  36. Josh says:

    Sean, I would like to add some optimism to what otherwise seems a bit of a reprisal in terms of discussion from this debate.

    To me, I thought it was fantastic to get to see this. We must all remember that science is unfortunately not entirely an objective matter but also a social one. Even if one theory is objectively true does not mean that such theory is what will be accepted and applied by all in practice. It needs to be shown, demonstrated, talked about, and to garnish consensus. Open discussion is crucial then.

    And I did not particularly feel the air of the event divisive. Certainly the panel’s opinion’s were different, and I wasn’t the biggest fan of the chart for/against graphic, but disagreement does not mean discord. And the disagreement does exist. There’s a fair bit of differing views, and even some confusion by the authorities, out there on whether or not QM needs an interpretation and what that should be. Consensus does not yet exist. So if we want to move forward that dialogue on disagreements should happen.

    Finally, I’m immensely pleased that the discussion was purveyed to the public sphere. If we want interpretations of QM to stop being abused to mysticism by the public or otherwise, then in the least, we could begin by helping educate. I also think by showing what is currently a bit of an open question, it takes away a sentiment many sometimes feel that scientists are becoming the “high priests” of a necessarily materialistic worldview in that only they can discuss and apply reason to it. The concepts of science should be accessible and communicated to all. And I think this dialogue was a great example of attempting that.

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  37. Craig McGillivary says:

    It would be nice if you could blog a description of what the Born rule is and why Everettians need to derive it. The Many-Worlds interpretation seems obviously right until you think about the Born rule. Then it becomes less obvious.

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  38. Zygmunt T. Raszkowski says:

    Hidden due to low comment rating. Click here to see.

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  39. kashyap vasavada says:

    “If we want interpretations of QM to stop being abused to mysticism by the public or otherwise, then in the least, we could begin by helping educate.”
    Such remarks are amusing. You know scientists who believe in MWI, say with a straight face that one copy of you in one universe observes one result and a second copy in an alternate universe observes another result. Isn’t this fantastic? If this is not mysterious , then I do not know your definition of mysticism.

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  40. Josh says:


    In short, I was throwing a bone.
    Whereas I certainly find some interpretations of QM fantastic, there are certainly others that are simply unfounded (one may disagree on the extent to which this or that theory is as so… for instance, I for one, would like a bit more falsifiability out of Many Worlds). Perhaps I come across over-focused on this. I’m not, but I know there is often frustration in the scientific community in seeing QM used as a carte blanche explanation for whatever purposed belief or fantasy. So I put those words there for those who might have that perspective.

    Either way, I’m particularly interested in the dialogue simply occurring so that, hopefully, a consensus can be reached, and ideally we can all be educated on that.

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  41. hoyboyt says:

    1.) Shelly Goldstein’s response to the Einstein question is great. I think that’s where everyone has trouble understanding quantum mechanics when they start out. That’s the first big philosophical hurdle to be overcome, and it’s easy to forget just how eye opening it can be. It’s such a big revelation that it tripped up Einstein. His reaction was like PTSD for physicists.

    2.) I think any interpretation which treats matter like a particle (not part of the wave function other than just riding it) is less precise than one which doesn’t. Of all possible interpretations of quantum mechanics, you could easily split those which include particle treatments into one group (which should be called approximate quantum mechanics) and all those which treat it as strictly waves into the other, which would be the full interpretation of quantum mechanics. Any time wave-particle duality is mentioned as something other than an artifact of increasing complexity, it’s a red flag.

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  42. John Barrett says:

    It doesn’t seem like the MWI would be able to use the same equation of the Schrodinger equation. Like it was explained in the video, the Schrodinger Equation just tells the story of the results of the experiment. Then in order to get the same results in the MWI, it would seem like that the universe would have to split to each result at the same rate the Schrodinger equation says that each result should be obtained. Then the Schrodinger equation in the MWI would actually be describing how often the universe splits to different universes that have a certain result. That would be the only way each universe could sustain the same principles in physics, like the Schrodinger equation.

    This brings me to what I will just call the “Schrodinger Monkey Paradox”. Say you had a monkey flipping a light switch that controlled the beam of light being tested to prove the Schrodinger equation. The universe would only be able to split due to the uncertainty of the light of the beam when the light is on. Then the monkey would flick the switch completely at random. Then since the universe only splits when the light is on and it wasn’t continually on the entire time for a continual pattern of the universe being able to split, then the results of the experiments should get different results other than the Schrodinger equation if the MWI is true. The universe would split at a different rate, simply because the means for it to do so was not always there. Then the Schrodinger equation would be accurate no matter how often you allowed the opportunity for decoherence of the beam.

    Then it would be kind of funny, because it would mean that for someone that runs test on monkeys that the creator of their universe or God could just be one of the monkeys flipping a light switch, lol.

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  43. James Gallagher says:

    Craig McGillivary

    Sean briefly discusses the issue in the introduction to his recent arxiv paper.

    The Born rule is a postulate of standard quantum mechanics which tells us how to assign probabilities to outcomes of measurements (a non-unitary “collapse” of the wavefunction to a specific eigenvalue).

    Because MWI is a purely deterministic theory it has no postulate to explain the probabilities of the measurement outcomes and instead must explain them via purely deterministic Schrödinger (unitary) evolution and MW branching.

    Other deterministic theories have a similar problem to explain the Born rule, eg in Bohmian mechanics you have to assume the initial cosmological state of the wavefunction of the universe was already distributed according to the |psi|^2 density, or else show that an arbitrary initial probability density would quickly converge to |psi|^2 (as done in this paper)

    With standard QM we do not have such difficulties, as we already have assumed a fundamentally probabilistic interpretation, and in which case we just need to find a suitable positive definite conserved quantity (since probability is positive and conserved).

    Now, there are highfaluting ( :-) ) arguments such as Gleason’s Theorem which precisely derive why |psi|^2 is the only possible candidate which gives a coherent probability fuction, but in the early days, the creators of quantum mechanics just noticed it was the simplest positive definite quadratic form conserved by the evolution – and that quartic forms or higher were ruled out by experimental results (eg see General Principles of Quantum Mechanics – W Pauli, sec 3, p 15) Thus the Born Rule was introduced as a separate postulate in the standard (copenhagen) model of Quantum Mechanics right from the beginning.

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  44. Bob Zannelli says:

    Worth sharing I think.

    Well it depends on information. So I guess in some sense you need an
    > information storing entity as part of the story. But mind has
    > no causative relationship in creating a mixed state.


    So what does the job, then? It seems from what they have said recently,
    that both Brent and Lawrence tend towards the idea that consciousness
    does play a central role in the emergence of the classical. I must say
    that, for myself, this is even more distasteful than MWI.


    What I find distasteful in the notion that somehow mind has some magical role in wave function collapse. I find this distasteful because it’s certainly not true. But based on Everett’s Universally Valid Quantum mechanics, the classical world is emergent based on the limitation of local access to information. You have to think of brain processes in terms of quantum mechanics, that is , any information storage entity must be included in the global wave function having a presence in each branch. This does not mean that the brain isn’t fully classical in the sense that anything is classical in Everett’s model, the Penrose assertions about brains are unlikely to be true because of Decoherence.

    Bob Zannelli

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  45. Daniel Shawen says:

    A pair of quantum entangled socks might be genuinely useful for interstellar communication, particularly if you have a wormhole for a clothes dryer.

    Sorry, that’s about all I get out of it. I think I’m with the Qbist.

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  48. Ray Gedaly says:

    Maybe a little off topic but …
    Given that an electron has a non-zero probability of appearing anywhere; does this mean it could appear anywhere in the greater, possibly infinite universe (or Type 1 multiverse) beyond the observable universe? Assuming so, then an electron beyond the observable universe could appear here inside a measuring device and its properties measured. In fact, given an infinite universe, isn’t it a certainty that such electrons would be common. Now if physical laws and constants vary across the larger universe, then this electron could have a different mass. But since we haven’t come across an electron with a different mass, does this imply that:
    a. The physical laws and constants do not vary across the greater universe (or Type 1 multiverse).
    b. The universe is not infinite.
    c. Infinities cancel out.

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  49. Ray Gedaly says:

    or d. I need to adjust my meds.

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  50. Ray Gedaly says:

    While I’m here, something way off topic …
    A massive star collapses into a black hole. But to an outside observer, time slows to a stop at the event horizon for all the matter falling into the black hole. So to us, in our universe, there is nothing inside the event horizon of a black hole, including no singularity. Or the singularity would only form infinitely far into the future. When a black hole evaporates, matter/energy is simply radiating from the event horizon (nothing new) but there can be no naked singularity. Also, probably no wormholes, but that’s a different argument. Of course I’m mainly arguing from classical physics, quantum effects could change all this. Nevermind!

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  51. Ray Gedaly says:

    Fermi Paradox implies either that faster-than-light communications (or travel) is impossible, or that the universe is not infinite (Type 1 multiverse). Were it possible to communicate ftl, then a civilization beyond the observable universe could communicate with us. Were the universe infinite, then there is a certainty that there would be an infinite number of civilizations with ftl technology who would want to communicate with us. Thus we don’t live in a universe (or Type 1 multiverse) where both of these are true.

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  52. Quantum Mechanics Smack-down, a fun name :) based upon the contentions and rivalry of the vastly different factions of Quantum Theory. As can be seen by the likes and dislikes of comments on this thread, there may be no mainstream topic in physics more divisive than Quantum Theory.

    “….only local hidden variables have been excluded. Non-local hidden variables are still very viable!”

    Regarding the quote above, although nearly all theorists would agree with this statement above there are still those that theorize that even local hidden variables have not been ruled out, and some that non-local hidden variables are highly improbable (somewhat the opposite of the quote above). Such local hidden variable theories usually involve a type of aether. Today’s aether-like theories for such explanations might include dark matter, dark energy, Higgs particles as a Higgs field, gravitons, quantum foam, and many other theorized possibilities. .

    Although Einstein and others of his time disagreed with the premises of QM, most did not invoke aether to explain observations in the quantum world. But for those that today propose an aether in one form or another for this or other reasons, examples as mentioned above, some theorize that there may be nothing mysterious about the quantum world, including wave-particle duality, quantum entanglement etc., that cannot be readily explained by an aether of some kind – proposing that the universe is a far simpler place than most versions of Quantum Theory otherwise contend.

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