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. 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.

  2. 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

  3. 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.

  4. Avattoir says:

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

    Also, fictional.

  5. 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.

  6. 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.

  7. 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:

  8. 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.

  9. Bob Zannelli says:

    In my opinion the most difficult issue , or arguably the only unsolved problem (other than what process selects a particular basis for measurement. ) for Everett’s Universally valid Quantum Mechanics model , is its implementation of the Born rule. Tony Aguirre and Max Tegmark offer a possible solution to this problem that is very much at odds with Sean’s views on this question. Basically they examine the Everett model in the context of cosmology, invoking eternal’s inflation prediction of a Multiverse. Based on this they argue that the Born rule is really about an observer’s inability to self locate in any given O region. If this makes sense, the problem that Everett’s model has with unequal probability predictions disappear. It’s very unfortunate , in my opinion that Tegmark and/or Aguirre were not in attendance at this event and that instead time was wasted discussing Qbism and Bohm’s wave mechanics. However, I am glad that Sean was there to represent Everett.

    Born in an Infinite Universe: a Cosmological Interpretation of Quantum Mechanics
    Anthony Aguirre, Max Tegmark
    (Submitted on 5 Aug 2010 (v1), last revised 12 Jun 2012 (this version, v2))


  10. Bob Zannelli says:

    But, as I pointed out on the other thread, Re: Everett Universally Valid Quantum Mechanics, what you’ve presented as the Everett formulation is very far from being parsimonious. For example, when the alleged branching occurs, how do you transition from a superposition of states that is entangled with its environment, to an untangled superposition? This is more or less the same problem as the collapse model has; a sudden, inexplicable change in the form of the WF without a process to explain it. Further, why are all possible outcomes eventually measured? Where is the necessity for this occurring within the Everett model? Nothing you have presented even touches on this issue, which is one of the key claims of the Everett model. What part of a world is realized in which a possible outcome is measured that isn’t measured in our world? — a full lab with geek within, part of a world, no world but nature measuring spontaneously? And what process makes these different choices — and from what material basis — or is it all imagination?


    I reply

    You don’t get. It’s called Decoherence and it’s an actual experimentally supported process that explains all the things you claim there is no explanation for. You described all the problems with Copenhagen and laid on Everett’s model. As to your question how things get entangled again, they never do get untangled, it’s the presence of all the information not available to any possible observer that reveals ( Mathematically) the continued existence of the wave function, or if you prefer the physical reality described by the wave function. It all comes done to whether you think human language is a more objective description of reality than mathematics. I don’t. I suggest you read Carrroll’s “Quantum Mechanics Made Easy” The truth is , there are only two real possibilities for the measurement process, some kind of modification of the equations of quantum evolution, this would mean that time symmetry IS NOT operative in the laws of physics or Everett’s universally valid quantum mechanics, which mathematically is just the pure application of quantum theory without any magical collapse process, for example like looking at something makes the wave function collapse, which is the Copenhagen answer , or even worst if that is possible, there is no objective reality , just disembodied thoughts as proposed by Qbism. Up until Bohr physicist hadn’t given up on there being an actual physical reality we could understand by building predictive models. Bohr changed that for far too many physicists with his William James inspired radical subjectivism. Bohr argued there is nothing to know so we mustn’t try. This stopped , to a large extent , most progress in understanding the measurement problem for 40 to 50 years, since so many drank the spiked cool aid he served up. Real progress began in the 70’s with the discovery of decoherence , making the measurement process something that could actually be investigated empirically, you know like scientists do.

  11. 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.

  12. 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.

  13. Zygmunt T. Raszkowski says:

    My suppouse: There is no mass in any material existens, quantum included.
    There is only plus or minus spin of electromagnetic waves.
    The mass can be found only when observed, in other words only when you want to be

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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

  20. 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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  23. 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.

  24. Ray Gedaly says:

    or d. I need to adjust my meds.

  25. 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!