Quantum Diavlog

Remember when I asked for suggested topics for an upcoming Bloggingheads discussion with David Albert about quantum mechanics? The finished dialogue is up and available here:

I would estimate that we covered about, say, three percent of the suggested topics. Sorry about that. But perhaps it’s better to speak carefully about a small number of subject than to rush through a larger number.

And I think the dialogue came out pretty well, if I do say so myself. (And if not me, who?) We started out by laying out our respective definitions of what quantum mechanics “is,” in terms that should be accessible to non-experts. (One user-friendly answer to that question is here.) Happily, that didn’t take up the whole dialogue, and we had the chance to home in on the real sticky issue in the field: what really happens when we observe something? This is known as the “measurement problem” — it is unique to quantum mechanics, and there is no consensus as to what the right answer is.

In classical mechanics, there is no problem at all; you can observe anything you like, and if you are careful you can observe to any precision you wish. But in quantum mechanics there is no option of “being careful”; a physical system can exist in a state that you can never observe it to be in. The famous example is Schrodinger’s cat, trapped in a box with some quantum-mechanical killing device. (Someone must write a thesis on the ease with which scientists turn to bloodthirsty examples to illustrate their theories.) After a certain time has passed, the cat exists in a superposition of states: half alive, half dead. It’s not that we don’t know; it is really in a superposition of both possibilities at once. But when you open the box and take a look, you never see that superposition; you see the cat alive or dead. The wave function, we say, has collapsed.

This raises all sorts of questions, the most basic of which are: “What counts as `looking’ vs. `not looking’?” and “Do we really need a separate law of physics to describe the evolution of systems that are being looked at?”

In our dialogue, David does a good job at laying out the three major schools of thought. One, following Niels Bohr, says “Yes, you really do need a new law, the wave function really does collapse.” Another, following David Bohm, says “Actually, the wave function doesn’t tell the whole story; you need extra (`hidden’) variables.” And the final one, following Hugh Everett, says “You don’t need a new law, and in fact the wave function never really collapses; it just appears that way to you.” This last one is the “Many Worlds Interpretation.”

I want to actually talk about the pros and cons of the MWI, but reality intervenes, so hopefully some time soon. Enjoy the dialogue.

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133 Responses to Quantum Diavlog

  1. GP says:

    I never really bought MWI, but every time I try to think about why not I confuse myself. I tend to think that even if both possibilities exist, observing one doesn’t necessitate a world where the other occurred, but it doesn’t have to do with a wave function “collapsing” (what does that even mean?!). The cat is alive or dead in that box whether you look at it or not. It state of being both alive and dead is just in the description. It’s the best description we can have before making the measurement but that doesn’t mean that’s what it actually is. It’s more about statistics—with one hundred cat-filled killing machines, half will come out dead and half alive. I guess that’s leaning towards a hidden variables interpretation, eh?

  2. Mark says:

    What about Cramer’s “Transactional” interpretation of QM?

  3. @Mark: in my experience, if you use broad enough strokes everything falls into one of those three categories. E.g. the much-touted “relational” interpretation is more or less many worlds in disguise. As another example, mechanisms like GRW collapse or Penrose-esque collapse fall into the category of what Sean called Bohr’s school.

    With regards to the transactional interpretation specifically, Wikipedia leads me to believe it falls into the first category, providing as it does a mechanism for collapse. However, it’s worth noting that the quantum foundations guys I worked with last summer at the Perimeter Institute had a rather poor opinion of the transactional interpretation. At the risk of misquoting, I believe on of them said that it only worked with a single particle. It’s also notable that it is supported almost entirely by a single person.

  4. Chris W. says:

    GP (comment 1):

    You don’t have to lean towards a hidden-variables interpretation. You can just say we don’t know, and we can’t know; the world has a fundamentally statistical structure. The question then becomes how to make this absolutely compelling. Does quantum mechanics leave the door open for a deep determinism or hidden causality?

    John Stachel has expressed the view[1] that Einstein’s real objection to quantum mechanics was that it asserted a kind of compromise between determinism and sheer chaos, but didn’t make clear the terms of the bargain. If the evolution of the universe isn’t fully determined, then why should it have any structure at all?

    [Sean: Maybe you should try exploring this with Stachel sometime.]

    [1] This was in Stachel’s own contribution to Conceptual Problems of Quantum Gravity, 1991 (A. Ashtekar, J. Stachel, editors).

  5. Count Iblis says:

    GP, you have to admit that the superposition of the two states is different from either one of the other. E.g., as long as the contents of the box is in the superposition and it hasn’t decohered yet (i.e. the state of the rest of the universe is the same in both parts of the superposition), you still have access to the initial state (in principle).

    This is perhaps best illustrated by changing the thought experiment by replacing the cat by a person and administering a deadly poison that kills with 100% certainty within, say, 5 minutes. So, the wavefunction of the person would describe a dead person after 5 minutes and after opening the box, we would see a dead person.

    However, according to quantum mechanics, we can still (in principle) talk to the person after the 5 minutes have passed provided we make sure the wavefunction of the contents of the box doesn’t decohere (in practice this is an unrealistic assumption). This works as follows. The process of asking a question to a person and then recoding the answer is equivalent to applying a perturbation to the wavefunction of the person and then measuring some observable. But in this case we want to undo the effect of time evolution since the poison was adminstered. The wavefunction evolves in time according to:

    psi(t) = Exp(-i H/hbar t) psi(0)

    The effect of the perturbation on the wavefunction is some unitary operator P applied to the wavefunction. We want to apply this to psi(0) when given psi(t):

    P psi(0) = P Exp(i H/hbar t) psi(t)

    Suppose that the person was sure to give some definite answer to the question. Then P psi(0) is an eigenstate of the operator, A, that corresponds to measuring the answer (the eigenvalue lambda):

    A P Exp(i H/hbar t) psi(t) = lambda P Exp(i H/hbar t) psi(t)

    And it follows that:

    B psi(t) = lambda psi(t)


    B = Exp(-i H/hbar t)P^(-1) A P Exp(i H/hbar t)

    So, measuring the observable B when the person is dead gives the same result as asking the question and recording the answer to the person when he was alive! You can then, of course, ask another question, so you can talk to the person as if he is alive and well.

    Of course, the operator B is extremely complicated and involves measuring all the molecules in the system. So, it cannot be done in practice. But because it can be done in principle, this means that the person as he was when he was still alive continues to exist in the box long after the poison has killed him.

  6. slide2112 says:

    So, an infinite number of “me’s” being created by a continuous series of events is a perfectly viable interpretation of QM.

    Yet the most obvious solution, that Consciousness causes the collapse, is silly because it allows new age folks to say mind can change personal reality.

    The idea that Consciousness is a fundamental aspect of reality is as well supported as any interpretation suggested here.

  7. Neil B. ? says:

    One thing I don’t understand, and furthermore am suspicious of: some attempts at measurement in QM aren’t possible in ways that involve literal inconsistency in the sought knowledge, but others aren’t. Because of Fourier composition of waves, you really can’t localize a wave packet and have a precise momentum as well. But I can create a photon with linear polarization axis at 20

  8. Mark Harrison says:

    @ slide 2112 #6

    One problem is what you mean by “perfectly viable interpretation.” At the moment, all QM interpretations are empirically indistinguishable. The last ten minutes of the discussion about the 2^1000000 Seans was pure speculation due to a lack of evidence. Prof. Albert was trying to get Prof. Carroll to see why the Many Worlds interpretation seems to undermine our understanding of QM in the area of probabilistic predictions. That has no bearing on whether it’s right or wrong, just that it’s odd.

    When physicists run the equations and accurately predict the results of experiments, no interpretation is used. Schrodinger’s equation is experimentally verified; Profs. Carroll and Albert were discussing what story we tell ourselves about what’s “really happening” in the universe described by QM.

    The other problem is the definition of “consciousness.” My objection would be the same as Prof. Albert’s when he said he didn’t believe in free will. The term is practically undefined and therefore useless as a “fundamental aspect of reality.” If Consciousness could be defined in terms of a hermitian operator in Hilbert space, that would be exciting news.

  9. Sad says:

    So, which “school of thought” seems to be held by most physicists? Inquiring minds really need to know.

  10. mathematician says:

    @ Mark Harrison

    If a physical theory does not accommodate consciousness, then it has been disproved by experiment, and it should be discarded.

  11. Eric Habegger says:

    I find Bohm’s theory of hidden variables most compelling. If one accepts the premise of vacuum energy, however you describe it qualitatively or quantitatively, it could neatly fall into the suspect territory. I really don’t buy for a minute the idea that not knowing the quantum state of a particle until you measure it indicates it exists in every possible state previous to the measurement. Why would that be any different from the axiom that if a tree falls in the forrest and no one is there to hear it did it still fall? Of course it fell!!!

    I just don’t understand how otherwise bright individuals could fall for that logic. Both the copenhagen interpretation and the many worlds interpretation fall into what I would call an “egocentric view” of the world. This view holds that unless an intelligent observer is there to observe a quantum event it doesn’t really happen. I think this view came about because the act of observing involves the exchange, sometimes in a complicated fashion, of photons between the observer and the observed. So you really do pin down while at the same time alter the quantum system in which the observed particle exists. The smaller and less massive the particle is that is observed the more this photon exchange will impact the state of the particle. But energy is always exchanged in the act of observation. But if I look at the moon and receive its photons it probably won’t affect the moon too much. Why would I ever “not” think the exact same thing is happening when observing particles in the quantum realm except that there the particles are so light the photon exhange actually does affect their state observably.

    I would call the MWs believers even more egocentric than the Copenhagen interpretation believers. They actually believe that by pinning down a quantum state they are creating a new entire universe that they inhabit that is different from all other universes where other potential observations might have been made.

    Information exchange is what makes the world go round and the information is finite but ever on the move. Energy is always conserved in the universe when information is exchanged. The observers and the observed are just the players in the drama.

  12. slide2112 says:

    #8, #10

    I am not signing up to go as far as mathematician does on this but…

    The key word here is interpretation. When interpreting the results of this hugely successful theory (theory does not seem to do QM justice) dismissing interpretations that include consciousness seems dishonest.

    It is one thing to say consciousness is too ill-defined (not undefined) to allow progress in a physical theory. It is another to ignore experimental results, dismiss the role of consciousness as one would a religion, and then come up with multi-verse like ideas. Where is the evidence for that?

  13. Jason Dick says:

    I would call the MWs believers even more egocentric than the Copenhagen interpretation believers. They actually believe that by pinning down a quantum state they are creating a new entire universe that they inhabit that is different from all other universes where other potential observations might have been made.

    Incorrect. The MWI interpretation merely states that there are interactions occurring all the time that prevent different components of wave functions from interfering with one another. And this isn’t even a controversial statement: it’s a simple conclusion from observing the dynamics of interactions within quantum mechanics. Quantum decoherence is a real phenomenon that has been directly observed.

    It seems more absurd to me that you would accept a theory that is not only more complex than quantum mechanics, but also discards locality.

  14. Eric Habegger says:

    I don’t think you are representing the many worlds theory as it is currently formulated. The individuals you represent actually believe any possibility that is represented by a wave function must exist simultaneously. And their theory is that once that wave function is identified by measurement the other possibilities must exist simultaneously, presumably in another universe.

    You tell me: is it more complicated to believe in conservation of energy and information or that there is infinitely more universes than the one we exist in, universes that have no potential for experimental contact. I prefer to believe in conservation of energy through the transferral of information. You are incorrect in your view.

  15. adamk87 says:

    Hey all, I just finished watching the excellent diavlog between carrol and albert over at bloggingheads and clicked on the link to this blog, and after reading the comments section thought Id ask what will probably be a stupid question.

    The extent of my knowledge on quantum mechanics is pretty much the aforementioned carrol/albert diavlog (deemed trustworthy), scenes from what the bleep do we know (suspiciously crackpot), and reading a few wiki submissions on the more famous aspects of the field.

    The most famous or the place you usually start when talking about quantum mech seems to be the double slit experiment, which wasn’t directly mentioned in the diavlog, but it sort of lurked in the background.

    Anyways my question, which has probably been addressed or pondered by someone, is the following: When the second slit is open wouldn’t we expect the variability or probability to change simply because as photons go through the slit they could presumably make contact with the disk and thus move it ever so slightly, which would make the photons that do “bump” but go through change, from just a single slit.

    My assumption is that my understanding of the slit experiment is wrong, but from what I understand (thankfully in this field ignorance is praised as knowledge) it seems like a plausible explanation for the different outcomes of the two experiments.

    Anyways Im interested to hear how you respond. Laymen terms, metaphors, etc. are much appreciated.


  16. adamk87 says:

    edit: by “ignorance is praised as knowledge” I meant to say “humility is deemed wise”.

  17. Jason Dick says:

    Eric Habegger,

    I don

  18. Eric Habegger says:

    You are asking the central question of what reality is. I have my own views but they are obviously controversial and I certainly wouldn’t want to spoil the fun of letting someone just beginning from having their own journey. If you are approaching it from the beginning I can think of no better start than reading the appropriate section in Richard Feynman’s famous physics lecture trilogy. After reading it, then sleep on it for about five years. I’m only partly kidding – the concept is an enigma- and (just my own view) let it seep up from your subconscious over time. Don’t let any “experts” of whatever stripe overly influence you in the journey, yours truly included.

  19. Eric Habegger says:

    That last comment was (obviously) addressed to Adam. Haven’t thought about Jason’s reply yet.

  20. Jason Dick says:


    I’ll describe the 2-slit experiment from the perspective of the many worlds interpretation (because I’m very much convinced that it’s accurate). Here’s the basic idea:

    First, you have a wavefunction of the particle. This wavefunction first passes through the two slits. After it has passed through the two slits, the part of it that has passed through one of the slits interferes with the part of it that has passed through the other. This interference depends upon the two components of the wave function having a coherent phase, which they do since both parts were once one part, and they haven’t yet interacted with anything that would mess up this phase. So, this wavefunction travels forward, oscillating as it goes, until it hits the screen.

    Now something interesting happens: an interaction that mucks up the phase. Basically, the part of the wavefunction that hits the screen at X gets its phase screwed up by the interaction with the screen such that it can’t oscillate any longer with the part that hits the screen at Y. These two parts both exist, but because their phases have been mucked up, there simply is no way for the part that says, “I hit the screen at X,” to talk with, “I hit the screen at Y.” And when we observe the experiment, we similarly end up with different parts of our wave function that can’t see one another, so we only observe one singular result.

    If we now change the experiment, so that we set up an apparatus to observe which slit the particle went through, the loss of coherence happens there, and we only observe one outcome once again.

    Did that help?

  21. Jason Dick says:

    Oops, that talk I quoted from was last year, not last month.

  22. Eric Habegger says:

    I certainly prefer a “decoherence” within a single universe better than the creation of multiverses. But it seems odd that I have never heard of it interpreted that way – maybe my own fault. I’m not sure I find the idea of decoherence satisfying as it seems a bit too abstract for my visually oriented mind. How does one characterize this decoherence iwthin a description that isn’t wholly mathematical in nature?

    My own inclination is that if we can eventually view quantum mechanics in the correct way it will transcend a strictly mathematical interpretation and be somewhat intuitive – in a way GR is. While this view of the many worlds interpretation is new to me I’m not sure it improves, at least for me, on some of the lack of intuition talked about in Feymann’s lectures. Would the lack of a discover of the Higg’s affect your view? People seem to fall into two groups regarding the primordial field – Higgs and ZPF. I’m afraid I fall into the latter.

  23. Jason Dick says:


    Let me first say that I always thought that the Many Worlds Interpretation was nonsense until I learned of it in the decoherence context. I think proposing the MWI in the language of multiply diverging universes is entirely backwards: that’s a conclusion of what the wavefunction appears to do to observers within it. It’s not the foundation of the interpretation, which is simply that what we have is a wavefunction that just evolves. The claim, then, is that this can account for all observer effects, though I’ll admit that the work isn’t done yet to demonstrate that this is actually the case. Still, the work done so far seems quite promising.

    How does one characterize this decoherence iwthin a description that isn

  24. MWI adherent says:

    [various anti-MWI comments]


  25. paul valletta says:

    So is there a chance,(no pun int!), that observing something, or not observing something, provides one with a greater or lesser knowledge of measure?

    For instance, if I do not observe something, can this be classed as actually increasing my knowledge about it’s whereabouts, being that in QM if you observe something, you lose your measured based knowledge about its momentum or position, thus by not actually observing something, I should actually increase my known accuracy of one or the other?