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

  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.

  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.

  4. Sean Carroll says:

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


  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.

  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

  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.

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

  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?

  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.

  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.

  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.

  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.

  14. James Gallagher says:

    The dart animation is rubbish.

  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.

  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?”

  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.

  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.

  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.

  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.

  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.

  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.

  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.