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Quantum and Consciousness

  1. Mar 6, 2009 #1
    Hi I'm a complete novice when it comes to Quantum physics, or any physics for that matter, so I hope this isn't a very silly question. I'm currently reading Brian Greens's excellent book "The fabric of the cosmos" and I'm up to chapter 7 Time and Quantum. On pages 195 and 196 and in Figure 7.5 Green describes an interesting beam splitter experiment in which in photons are fired at a screen via a beam splitter and then two down converters. The down converters produce so called idler photons. The idler photons are allowed to pass through a further series of beam spitters and finally arrive at one of 4 detectors. Basically if the idler photon arrives at sensors 1or 2 we are able to determine the path of the signal photo which will eventually arrive at the screen. However, if the idler photon arrives at detectors 3 or 4 we are unable to determine the path. It turns out if the idler arrives at 1 or 2 no interference pattern is formed, but if it arrives at detector 3 or 4 an interference pattern is formed on the screen for those signal photons. What I would like to know is what would be the result if we preformed the following experiment. The beam is fired at the screen via a beam splitter and two detectors are placed on either path after the splitter to let us know which path the photon took. The detectors are connected to a computer and the results are stored on a hard disk. Obviously if we look at the results and then look at the screen no interference pattern would be formed. But, what would happen if we deleted the results from the hard disk without looking at them and before looking at the screen , so that although the "which path" information was detected it was unknown to us. In this experiment what would we see if we looked at the screen? Would the interference pattern be there or not? Maybe a stupid question and I'm guessing the Interference pattern would not be there. I have to ask though as the experiment described by Greene has got me wondering whether it is the measurement of the quantum event or the knowing of it that causes the interference pattern to not form. That is to say is it the interaction of the measuring device or the interaction of the human consciousness which causes the interference pattern to disappear.
     
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  3. Mar 6, 2009 #2

    alxm

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    You get the same result whether or not a naked ape is looking at the outcome.
     
  4. Mar 6, 2009 #3
    I think it is a philosophical question that will get different answers depending on who you ask. I know for a fact that some famous physicists (eg John Wheeler) adhered to the "consciousness causes collapse" viewpoint.
     
  5. Mar 6, 2009 #4
    Even to the point where some physicist asserted that the moon isn't there when nobody's looking. Total crap. The Schrodinger cat was a parable showing the stupidity of this view.
     
  6. Mar 6, 2009 #5

    alxm

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    No, it is not. You must have misinterpreted Wheeler.

    The result of the double-slit experiment depends on whether or not you detect (or _can_ detect) which slit the particle passed through. Whether or not you pay attention to that data, or perform a 'measurement' in the layman's sense is irrelevant, and quite trivial to show experimentally. You will not see an interference pattern if a detection mechanism is in place.
     
  7. Mar 6, 2009 #6

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    I don't know it for a fact, but it wouldn't surprise me to learn Wheeler supported this view (after all, he was the thesis advisor of Everett, who came up with the many worlds interpretation, which isn't that far off conceptually). The issue is much more subtle than you seem to realize, and there's no currently known way to rule out an interpretation like "consciousness causes collapse." If you disagree, can you describe a specific experiment that would rule it out?
     
  8. Mar 6, 2009 #7
    Conceptually there seems to be a pretty big difference: Everett says that the same laws apply to everything without exception (the atom entangles with the next atom which entangles with the cat which entangles with the observers and the whole universe ends up in a superposition) whereas Wheeler's participatory anthropic principle gives one set of laws for us humans and another set of laws for everything else (it is an explicit variation of Copenhagen).

    Yes. In our universe, there is a glaring distinction between a) whether something would have been impossible to have known in principle, and b) whether something would have been impossible for any human to have known. Quantum mechanics clearly depends on *a* rather than *b* (example experiments include decoherence in a thermal bath and that, say, double-slit outcomes do not depend on whether the which-path detector is turned on) and that wouldn't make much sense if consciousness caused collapse.

    There's also a slippery slope problem in treating consciousness as special (can collapse also be caused by a child, by an animal, one "living" cell, or a cleverly designed computer chip?). A third criticism, as mentioned before, is that if we're going to treat consciousness as special despite not needing to do so in the first place, why not also declare friction is caused by invisible demons?
     
  9. Mar 7, 2009 #8

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    You're right, I worded that wrong. I just meant they're both very strange ideas for dealing with collapse, so if Wheeler worked on one, it's not hard to believe he worked on the other. Still, they're related in that there's a gap in Everett's hypothesis when it comes to which of these many worlds we experience, which the consciousness causes collapse (CCC) interpretation tries to solve.

    Why would it not make sense if CCC is true? There's still decoherence to cause "apparent" collapse, ie, cause probabilities to obey approximately classical statistics (without interference terms).

    I'm not trying to defend the view, I'm just saying that, like any other interpretation, it can't be ruled out by experiment. That being said, there's no reason to give adult human scientists a monopoly on consciousness, there are views where it is possessed in some small way by any information processing system.
     
  10. Mar 7, 2009 #9
    No, CCC/PAP/Wigner's interpretation does not answer that question (which essentially asks the outcome of a quantum dice). The gap that does exist in Everett's Many-Worlds interpretation (viz. the preferred basis problem) was quietly already present in other interpretations (the specialness of position).


    You're not? Really?

    We can construct an experiment in which a pattern indicates whether the which-path information is knowable. Next, can we not construct a which-path detector that provably does not make the information knowable to any human being? Say, by entangling the information directly into thermal motions in the centre of the earth, so it becomes impossible even in principle for any human to obtain the information only because no human knew the wave-function of every atom in the planet beforehand. CCC now predicts a result contrary to every other interpretation, no? Are you familiar with quantum erasers?

    I think science has simply progressed onwards and that CCC/PAP/Wigner's interpretation has been left behind, along with Bohmian mechanics and Intelligent Design for example. Some philosophers try to preserve classical viewpoints by arguing that these haven't technically been disproven, but these interpretations seem not to be the ones giving fruit to continuing scientific advances.
     
    Last edited: Mar 7, 2009
  11. Mar 8, 2009 #10

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    I don't know if we're talking about the same gap. I meant the problem of determining which of the many worlds a given conscious observer experiences. I don't see what this has to do with position.

    That doesn't mean it's impossible in principle, just in practice.
     
  12. Mar 8, 2009 #11
    Conscious observer experiences ALL worlds (sor it splits too)
     
  13. Mar 8, 2009 #12
    Exactly, collapse doesn't wait for a human to observe the system.
     
  14. Mar 8, 2009 #13
    Ok I found a source claiming that Wheeler believed the CCC interpretation but you might not regard it as a credible source. It's from a Paul Davies book, and he says:
    "In 1979, John Wheeler, speaking at a symposium in Princeton celebrating Einstein's centenary, drew a still more mind boggling conclusion from the two hole experiment. He pointed out that it is possible to delay the choice of measurement until after the photon has passed through the screen... The precise nature of reality, Wheeler claims, has to away the participation of a conscious observer. In this way, mind can be made responsible for the retroactive creation of reality"
     
  15. Mar 8, 2009 #14
    No, no this Davies book tell big lies to sell book. No magic brains - if wavefunction used not particle/waveys then no problems with delayed choice. NOT backwards time or brains - go to see doctor.
     
  16. Mar 8, 2009 #15
    I'm not necessarily advocating the CCC point of view I just think that it is important to remember that some famous physicists supported it. It is not easy (but probably not impossible) to define a measurement without invoking a (conscious) observer. CCC is problematic but without it you run into other problems regarding what constitutes a measurement.
     
  17. Mar 8, 2009 #16
    Well thanks for all the replies and the above answer seems to be the most straightforward reply to the original question. I have no idea how a photon detector works, but I presume an electrical signal of some sort is sent when a photon passes. If the above reply is correct the wave-function would collapse if we connected the detector to earth and never knew the which path information. Or what about if we connected it to a light which flashed, but placed the light in a sealed light proof box so it was impossible to know if it had flashed or not and consequently the which path information, even though it had been detected. Would the wave-function still collapse and no interference pattern be formed.
     
  18. Mar 8, 2009 #17

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    Here's how I would think about it. Consider a classical version of the schrodinger's cat experiment. Here, an ordinary coin is flipped, and based on the outcome, the cat lives or dies. Now what should someone outside the room, who doesn't know the outcome, say about the state of the cat? Is it in some kind of alive/dead superposition? As far as he's concerned, this is as good a way as any to describe the state of the cat. Once he opens the door, the "wavefunction collapses", and the state of the cat becomes definite. In this sense, the weirdness doesn't really come from quantum mechanics, but is already present in classical systems with incomplete knowledge.

    Now, one might say that this superposition business is just a particular way of keeping track of an experiment, but doesn't correspond to the real world in any meaningful way. But quantum mechanics tells us this is wrong: we really need to think of a state as a superposition of different possible outcomes. There are two reasons for this. First, the uncertainty principle tells us there is always incomoplete knowldege, so the best we can do is determine a probability distribution of outcomes.

    The second reason is that, for systems where quantum mechanics is important (like an electron, but not like a coin), the different states in the superposition can interfere with each other, and cause a different outcome than if there was no superposition. For example, if we take the spin of an electron, there is a certain superposition of spin up and spin down that is equivalent to the spin left state. Clearly no such thing can happen in classical probability.

    Actually, this special "quantum superposition" is present in any system, but in macroscopic systems, the interference between different states is immeasurably small, so it is a good approximation to treat things as obeying ordinary classical statistics. This is why you don't see an interference pattern when you put a "which-path" detector in place: now the superposition is not of the state of the electron, but of the combined state of the electron + detector, which is a macroscopic system whose interference is negligible. This doesn't mean collapse has occcurred, but it doesn't mean it hasn't. It's still an unresolved question.

    In principle, if one had an accurate enough experiment, they could measure these tiny interferene effects for macroscopic systems. If the wavefunction really collapsed, there should be none, but if not, there should be some very tiny trace of the superposition. I don't see this experiment being feasible in the near (or not so near) future, but in principle it is an experimental question.
     
  19. Mar 8, 2009 #18
    If someone asked me if the cat were dead or alive as a result of the above coin experiment, I would not say that it was in some kind of dead or alive superposition. I would say "I don't know". The cat is definitely either dead or alive and not in-between It's just that I don't know the answer, because I don't have that information. What I'm looking for is a yes or no answer to a yes or no question. That is if we connect the which path detector to earth so that the signal is unrecorded by any machine or biological being (unless you include bacteria in the earth) will the interference pattern form? (yes or no question)
     
  20. Mar 8, 2009 #19

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    My point was it's not as clear cut as you seem to think, and superpositions need to be taken seriously.

    If you read and understood what I wrote above, you'd see I said, for all intents and purposes, the answer is no, but as a question of principle, there's no universally accepted answer.
     
  21. Mar 13, 2009 #20
    Cat can dead or alive!
     
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