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Can a computer be an observer?

  1. May 10, 2010 #1
    Hello all, I am writing an article on the definition of "observer" in a quantum mechanics context.

    From what I know about QM, most consider that an inanimate apparatus and even individual particles can function as the "observer" in quantum measurements. I also know that the 1991 experiment by Mandel, Wang, and Zou pretty much established that the experimental setup is what determines whether quanta behave as particles or waves, not what the experimenter actually determines or chooses to learn.

    However, I came across a 1996 talk by the late computer scientist Seymour Cray, who described a then-recent experiment which would seem to contradict this. A computer performed wave/particle duality experiments and stored the results in memory. This data then appeared to remain in macroscopic superposition until a human experimenter actually checked the files, thus suggesting (incredibly) that a machine cannot function as a quantum observer in the capacity that a human observer can.

    I have spent literally hours searching Google Scholar (and regular Google) for this experiment, turning up absolutely nothing. I am starting to think that Cray was mistaken, that this experiment never happened.

    http://americanhistory.si.edu/collections/comphist/montic/cray.htm" [Broken] of Cray's talk -- he discusses the experiment in the sections "Wave/Particle Duality and Computers" and "Giving Meaning to Binary Data."

    Whether you know anything about this experiment or not, any insight is greatly appreciated!
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 10, 2010 #2
    If anything, it sounds like your speaker may have misunderstood (and further mangled) something he had only heard indirectly, it's mentioned more as an aside than as something he had any personal expertise on. (Science journalism tends to be oversimplified and excessively sensationalised, and it's difficult for typical reporters themselves to get a subtle understanding of cutting edge work.)
  4. May 10, 2010 #3


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    I'm not familiar with the experiment you're talking about, but it sounds like this Cray guy has misunderstood something. It's certainly plausible that the memory was in a superposition for a while, but it certainly didn't have to interact with a human for the coherence of the superposition to be delocalized into the environment.

    As for the title question, I'll just quote myself:
  5. May 10, 2010 #4
    Depends on the interpretation.
  6. May 10, 2010 #5
    According to Wikipedia ( http://en.wikipedia.org/wiki/Observer_(quantum_physics) ) "In quantum mechanics, the observer and the system being observed became mysteriously linked so that the results of any observation seemed to be determined in part by actual choices made by the observer."

    I am not sure if that is right or wrong, but supposing it is true, your question "Can a computer be an observer?" would be translated into "Can a computer make a choice?".

    If not (and if Wikipedia is correct... and if my logic is also correct...), then Seymour Cray would be right.
  7. May 10, 2010 #6


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    That's a pretty poorly written Wikipedia article.

    By the "choices of the observer", all they mean is which property the 'observer' chose to measure (if any). Which ultimately means:
    - "What kind of interaction [between the "observing" system and "observed" system] occured, if any"?

    Now if you're a human conducting an experiment where you, say, change polarization filters, then you could say
    "your choice affects the observed system". But that's because you change the nature of the interaction.
    Nothing to do with the choosing in itself, nothing to do whether or not you look at the results. Etc
  8. May 10, 2010 #7
    That's being overly generous. In context, he's suggesting that megabytes of data stored in memory and disk (not to mention all the circuit activity) is in a macroscopic superposition (of completely different classical states) for an arbitrary number of hours, never decohering with the room-temperature environment, until the moment that the computer is directed to execute (using a chosen basis) an analysis routine on this archived data. His sense is not of some extreme minority interpretation (human consciousness mediated collapse), nor in the sense of something possible in principle and just astronomically low probability, but in the usual QM sense of repeatably preparing a system in a superposition. It's flat wrong.
  9. May 10, 2010 #8


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    OK. I didn't click the link in #1, so I assumed that we were talking about a few bits, and not necessarily a typical computer memory.

    Are you saying that there is an interpretation in which the "observer" needs to be human?
    Last edited: May 10, 2010
  10. May 10, 2010 #9

  11. May 10, 2010 #10


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    I know about that, but it would be hard to take that idea seriously even if we didn't know anything about decoherence, and now that we do I think it's impossible.
  12. May 10, 2010 #11

    How does decoherence explain the transition from a mixed state to single outcomes? There still appears to be a MP, unless you believed in the MWI.
    Last edited: May 10, 2010
  13. May 10, 2010 #12
    In MWI no explanation is needed
    In BM, hidden particles tag the outcome which is 'real'. Dont ask me why it becomes real :)
  14. May 10, 2010 #13
    Thank you for all of the commentary. It is very helpful.

    But this is something I find confusing about QM. On the one hand there is the mainstream decoherence camp, who insist that the phenomenon that Cray describes could not be possible. Then there is the (smaller) Von Neumann/Wigner camp, who I imagine would say those experimental results are not only possible, but probable, if computers are not "conscious." So, you would think that one of them would have tried this experiment, if in fact it has never been done the way Cray describes.

    In other words it would seem that the described experiment would falsify or support Von Neumann/Wigner, and/or suggest that decoherence theory is incomplete. So I would be surprised if no one actually has tried it, to settle this significant dispute and make some headway in QM interpretation.
  15. May 10, 2010 #14
    What difference would a human find between a superposition and a definite state of the computer data?

    How can you know the state of data before a human measure?

    Here, the question is "Can a computer be an observer?". In Schrödinger's cat, "Can a cat be a quantum observer?"

    Where can I find a reliable definition of "quantum observer"?
  16. May 10, 2010 #15


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    This is not at all true. (I'll elaborate a little below).

    I don't know what you mean by "MP". Decoherence explains the appearence of collapse in the framework of the MWI. The interaction between the system and its environment selects a preferred basis for the Hilbert space, and that basis also identifies the "worlds" in which stable records of the state of the system can exist. A memory in your brain is such a stable record, and there can be no consciousness without memories, so decoherence identifies the worlds which can contain conscious observers and they are the same as the worlds where the measurement had a well-defined result.

    In the ensemble interpretation, there's no need, and no possibility, to describe the measurement process itself. State preparation and measurement are primitives, i.e. they're not defined in other terms, and they aren't (and can't) be described as dynamical processes.

    This is the main reason why MWI intepretations are worth taking seriously. They can explain some things that the ensemble interpretation can't.
  17. May 10, 2010 #16
    Frederik, measurement problem has 2 components:
    1. Why we dont observe macroscopic objects in superposition
    2. What outcome becomes real, or is cat dead or alive?

    By saying "no explanation is needed" I meant N2, not N1
    N1 (decoherence) is in fact an important thing to study
  18. May 10, 2010 #17

    The word "real" has been undefinied for at least a century. What does it even mean anymore? Even in a MWI setting, what does it mean for a system to be real? Had we been able to seal it off from the environement and keep it in superposition and avoid entanglement, would the Moon be still there? No. Is the Moon real when it's not decohered? No. Does the MWI look like a relational interpretation. I think it does as it sets a limit to what can be observed as soon as a entanglement with the environment takes place.
    Last edited: May 10, 2010
  19. May 10, 2010 #18

    The measurement problem.

    Yes, i agree with that. MWI is the interpretation that explains the most. And given the number of realities, it's nearly as powerful as "God did it".
  20. May 10, 2010 #19
    What does it mean to "observe macroscopic objects in superposition"?

    For me, "to observe" means to form a memory of the outcome of the interaction. Whether I observe electrons or macroscopic objects, my memories are definite-valued. Even if there is a superposition of states of "me" formed at the moment of observation, I have no way of knowing that and I can devise no experiment to determine that.
  21. May 10, 2010 #20


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    I don't think that's a correct representation of Wigner & Co's view; they were imagining consciousness emerging from quantum properties, not vice-versa.

    But more importantly, Wigner abandoned his ideas when the process of decoherence began to clarify (in the 1970's IIRC). Since, "quantum consciousness" ideas have increasingly become the domain of crackpots and new age charlatans. I don't know of a single big-name physicist (or perhaps even reputable physicist) today who believes in quantum-consciousness ideas. (Roger Penrose is a gifted mathematician, but he is not a physicist) I do know several who've openly criticized it (Penrose's friend Stephen Hawking being one)

    It's a dead idea. It was never more than speculation to begin with, and now it's debunked speculation.
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