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John Wheeler & QM

  1. Feb 1, 2015 #1
    Does anyone know how Wheeler understood QM? Here are some well-known sayings of his (slightly paraphrased):
    "the wave function does not represent the electron- it represents what we know about the electron; the joint product of all the evidence that is available to those who communicate"
    "it is meaningless to talk about the position of a particle until a position measurement has been performed. No question- no answer!"
    He also often quoted Bohr: "no phenomenon is a phenomenon until it has been made macroscopic by an irreversible act of amplification"
    He seems to be treading a very confusing middle ground between realism and operationalism. On the one hand, he clearly does not take the wf as the "actual reality", as MWI, BM, collapse models, and even the Von Neumann/Wigner consciousness idea would have it. On the other hand, he does see observations, or "acts of observer/participancy" as playing a special- and very real- role; so much so that he went on to suggest that our observing the universe is necessary for its existence("it from bit")! I'm not asking about that particular idea; I just want to know whether his interpretation of QM is seen today as a self-consistent possibility, and if so, how to make sense of it.
     
    Last edited: Feb 1, 2015
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  3. Feb 1, 2015 #2

    atyy

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    Of course one should not take random sound bites from Wheeler, which were meant to be provocative, as indicating whether he had a coherent philosophy or not. Anyway, "it from bit" was the subject of an essay competition. I like Leifer's entry.

    http://fqxi.org/data/essay-contest-files/Leifer_FQXi2013.pdf
    "It from bit" and the quantum probability rule
    Matthew Saul Leifer
     
  4. Feb 1, 2015 #3
    I took the sound bites from "At home in the universe", a collection of Wheeler's speeches and popular essays. I did read through these seriously and spend some time thinking about them, but I didn't make much headway beyond being able to repeat the quotes. He seems to be saying that after a position measurement, the particle retrocausally had a position- but not a momentum- the whole time. Can anyone make sense of this, or am i totally misreading him?
     
  5. Feb 1, 2015 #4

    atyy

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    I'm not a big fan of Wheeler's retrocausation and delayed choice, which can be treated by standard forward in time quantum mechanics. However, retrocausation is a loophole for local causality in Bell tests, so it seems worth exploring. I think the transactional approach to interpretation uses retrocausation.
     
  6. Feb 1, 2015 #5
    I read the link you sent... it doesn't have much to do with Wheeler, other than the title (for contest purposes). He just uses "it from bit" as a way of saying "no hidden variables"- i.e. anything but BM or similar.
    Anyhow, I was hoping somebody could explain Wheeler's point of view to me. One particular difficulty that I can put my finger on is the assertion that the measured observable "did exist from now on" but other noncommuting observables didn't- specifically position & momentum. If we have a meaningful history of position as a function of time, that clearly describes momentum as well- just take the derivative. As far as I know, the uncertainty principle Is only meaningful as a description of possible quantum states, not retrocausally assigned variable values. Any comments?
     
  7. Feb 1, 2015 #6
    Okay, my last post wasn't so fair. Let me clarify. Wheeler made his claim about causing there to have been a position in the context of a particular setup: his version (the original) of the delayed- choice experiment. A single-particle beam passes through a double slit and propagates to the measuring point. At the last moment, a choice is made whether to measure position with a screen, giving the standard interference probabilities, or to measure momentum in the direction perpendicular to the slits, giving ww information. His claim is that if and only if we measure momentum, the particle originally passed through only one slit. In this context retrocausally defining a position just before measurement does not give a history, because there are two paths to each point. My problem is that he seems to be making sweeping statements about complementarity and noncommuting measurements. This would seem to apply just as well to a particle moving from an emitter to a screen through empty space-and there, if we take measurement as telling us a real variable value from the past, any position gives us the whole history.
     
  8. Feb 1, 2015 #7

    atyy

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  9. Feb 1, 2015 #8
    sure, I followed that thread very eagerly. it's a fascinaning experiment even though predicted by standard qm. but i don't think the two-state/transaction idea is what wheeler had in mind, though it's also retrocausal. two-state has a wf actually propagating backward from the measurement.
     
  10. Feb 1, 2015 #9

    atyy

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    My honest answer is that Wheeler knew about the measurement problem and was just trying to provoke others into exploring solutions. I don't believe he had an answer. Indeed till this day, all major approaches to the measurement problem have open questions. It's not any more specific than his idea of spacetime foam.
     
  11. Feb 2, 2015 #10

    bhobba

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    Wheeler was a Copenhagenist in the tradition of Bohr. But he used to explain it in a unique way like he did relativity which can be seen in his book - Space-Time Physics.

    Check out:
    http://www.applet-magic.com/CIwheeler.htm

    He, as you would suspect since Feynman was his student when he came up with the idea, was greatly influenced by Feynmans sum over history approach:
    http://www.eftaylor.com/download.html#quantum

    Thanks
    Bill
     
  12. Feb 2, 2015 #11

    bhobba

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    He is saying, as per the standard Copenhagen view, quantum objects do not have properties until they are measured to have those properties. After measurement they again don't have them until next measurement. He just had his own picturesque way of explaining it.

    IMHO its better to study the math of QM in conjunction with the writings of people like Wheeler.

    I think it can safely be said Wheeler understood observation in QM had nothing to do with linguistic 'shenanigans' associated with the word observer and equating that to concious observer. Usually in QM observer and observational apparatus are the same thing - its just historical baggage that a word like observer is used in QM when its meaning is different to everyday use.

    These days Copenhagen has had a few, admittedly minor, modifications that don't affect its consistency - simply addressing the issue of how a theory about observations that appear in an assumed classical world explains that world:
    http://motls.blogspot.com.au/2011/05/copenhagen-interpretation-of-quantum.html

    Thanks
    Bill
     
    Last edited: Feb 2, 2015
  13. Feb 2, 2015 #12

    atyy

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    Oh, I think that is far too correct for Wheeler's poetic view :) I think that because he was also a general relativist, a subject about the whole universe, he was very interested in the measurement problem and how to remove the observer, or have the universe observe itself, leading to this famous picture:

    http://lmgtfy.com/?q=wheeler+universe+observing+itself+images
     
  14. Feb 2, 2015 #13

    vanhees71

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    Well, again Einstein's old saying is valid: "Don't listen to their words, look at their deeds". This "backward-traveling in time" thing often envoked to make popular-science books selling a bit better, because it sounds (and is fact is) more like science fiction than science also in the context of the Feynman-Stückelberg trick in quantum-field theory, is highly misleading. The very point of the Feynman-Stückelberg trick is to interpret the negative-frequency modes of the decomposition of the free quantum fields in terms of creation operators instead of annihilation operators, flipping the momentum (not time!) for better convenience by writing everything covariant and at the same time make all particle-like excitations (in the sense of the Fock space) to represent particles and antiparticles propagating according to causal time order.

    Then there was, of course, the famous Wheeler-Feynman work on the problem of a consistent theory of motion of point particles in classical electromagnetism, leading to the famous "absorber theory", where one works with half advanced half retarded propagators for parts (!) of the electromagnetic field. Great care has been taken in the original paper to make clear that this theory is causal, and nothing is running backwards in time. Of course, at the end, it neither completely solved the problems of radiation reaction for pointlike classical particles in classical electrodynamics (there's no such solution until today, but only the conclusion that classical particles only make sense when considered as extended but small objects) nor lead it to a successful relativistic quantum theory.

    It's a great story about the seminar Feynman gave in Princeton, when all of a sudden he had Einstein and Pauli in the audience, the latter predicting correctly that it won't lead to any useful QFT.
     
  15. Feb 2, 2015 #14
    the issue here is "what is a measurement?". one thing Ensemble, MWI, and collapse models have in common is that a measurement result isn't really "measuring" anything-the value is created at the moment of observation. wheeler, on the other hand, seems to say that a measurement is a "question" about something, and generates a meaningful "answer". that he was serious about this is demonstrated by his later suggestion that the universe itself is generated in this way.
    also, he made very clear that "observer/participants" play a special role. he specified that this does not depend on consciousness per se but rather "those who communicate", including computers. this also puzzles me: without conscious users, why would the electronic signals exchanged by computers "represent" anything in particular? but that's a more philosophical issue. i'm just trying to get the various interpretations of qm straight.

    maybe a more general question will help, without specific reference to wheeler: if the quantum state is taken to represent our knowledge, can someone explain- our knowledge about what questions? is this any different from saying that the state doesn't represent anything but is a tool for calculating results? if i beam particles into a black hole, so they will never be measured, is it meaningful to talk about their quantum state? in Ensemble, i would think not. in psi-ontic models, definitely yes. where does the "our knowledge" idea fall on this?
     
  16. Feb 2, 2015 #15

    bhobba

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    Ensemble is compatible with BM which measures an actual property.

    I think you had better give some further info, including context, to support the idea he considered concious observers to be involved in some way. The quotes you gave doesn't support that. The environment is observing quantum objects all the time eg a few photons from the CMBR is enough to decohere a dust particle and give it a definite position.

    The theory is clear about what a state tells us. Given a state P and an observable O the expected outcome of the observation O is the observable of is trace(PO) - that's the full statement of the Born Rule. Like I said, with regard to such things, its best to discuss it along with the math.

    The Born Rule tells us, in the theory, what a state is and its purpose. In fact Gleason's Theorem tells us it can be reduced to non-contextuality - but that is another story. How you interpret it is another matter - it can be real, a state of knowledge, synonymous with a preparation procedure - all sorts of things. The theory is silent about it.

    The state tells us about if you were to observe it - regardless of if you actually do or not. However since the environment is observing things all the time that's highly unlikely.

    Thanks
    Bill
     
  17. Feb 2, 2015 #16
    I know the theory is silent about interpretation; that's why this forum is a great place to discuss it.:)
    my (last) question was about the "state of knowledge" interpretation(s), first propounded (I think) by Heisenberg, and followed by Wheeler. State of knowledge about what questions?
    The reason I mentioned black holes is to show it's possible for a state never to decohere at all. But I know black hole information loss is a controversial topic, so maybe I should have avoided that. My point is that if we take a purely operationalist view, the state only "comes up for discussion" in the context of making some prediction. If there is nothing to predict, any description is out of bounds. So I want to know, does "state of knowledge" offer any description beyond that?
    As far as Wheeler & "observership"- I already returned the book to the library, but the point came up in a great many of his essays & speeches. Here is one dramatic quote I found at todayinsci.com:

    Is the very mechanism for the universe to come into being meaningless or unworkable or both unless the universe is guaranteed to produce life, consciousness and observership somewhere and for some little time in its history-to-be?
    — John Wheeler
    Quoted in P.C.W. Davies, God and the New Physics (1984), 39, from J.A. Wheeler, 'Genesis and observership', Foundational Problems in the Special Science (1977), 39.
     
    Last edited: Feb 2, 2015
  18. Feb 2, 2015 #17

    bhobba

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    Don't know that one - and I know quite a few.

    That goes way beyond QM.

    Thanks
    Bill
     
  19. Feb 2, 2015 #18
    I called it state of knowledge borrowing your own wording from the previous post!:D I'm referring to the idea that the state represents our knowledge about the system.

    Yeah, I know the quote is a bit off topic, but it does show he considered observers important. There was an article on the Bohr-Einstein debate that expressed his position relatively clearly, but i can't find it online.
     
  20. Feb 2, 2015 #19

    atyy

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    It's the same as saying the state is a tool for calculating results - the quantum state represents our knowledge of the results of future experiments. If particles fall into a black hole and they will never be measured, it seems that it is not meaningful or at least not necessary to talk about their quantum state - but of course one don't know in advance whether one is going to fall into a black hole to measure them. As I understand it, the black hole information paradox is the loss of unitarity for the outside observer.
     
    Last edited: Feb 2, 2015
  21. Feb 2, 2015 #20
    Thanks atyy, that's what I wanted to know. But I'm still pretty sure Wheeler's idea was different, even before the question of the Universe.
     
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