A Copenhagen Bug

  1. So, let's say that a star 50,000 light years distant does not exist until it is observed. One day I look up and see a bright star and it's waveform collapses into existence as we know it. Now suppose my pet kitty observes it, does the waveform collapse? What if a tiny bug saw the star? It has been observed, does it's waveform collapse? No. Because, there was no information created about the existence of the star. It's really the connection between information theory, and reality. For the waveform to collapse, the information must come into existence at the same moment as the observation is made, and the waveform of there probably being a star at that location is collapsed by the act of the information coming into existence. No information, no star.

    So, what if a hermit sees the star, but never communicates that observation to anyone in any fashion, so the information dies with the hermit? Information cannot die, it lives on, perhaps in the remains of the hermit, but once created, it exists forever. And since the information about the observation of the star lives on forever, in some form or another, then the star is forever crystallized from only a probability of existence, into what we call reality.

    This then places humans back at the center of existence. In a very real sense, it invalidates the Copernican principle that we are not in a special or favored place in the Universe. It seems we are very special indeed. The Universe would not exist without us.
     
  2. jcsd
  3. Matterwave

    Matterwave 3,860
    Science Advisor
    Gold Member

    When defining the Copenhagen interpretation of QM, one must be very careful in the word "measurement" or "observation". "Measurement" or "observation" collapses the wave-function. But it is not usually taken as necessary that these "measurements" or "observations" be conducted by a human.

    In fact, if you set up a lamp inside of a 2-slit experiment, you make the interference go away even if you don't have a human checking which slit the electron moves through. The wave-function collapses as soon as it is possible to make such a measurement.

    No such measurement has to be carried out, though (i.e. it doesn't matter that a human reads the output of the measuring device).
     
  4. Drakkith

    Staff: Mentor

    Of course it exists. Don't read too much into the philosophical stuff behind QM. It's all merely speculation and such. Stay with the facts and leave it at that I say.
     
  5. I see your point, but I'd look at it a little differently. If no human ever looks at the result of the experiment, then no information was ever created, and the waveform has not collapsed. The cat is neither alive nor dead.

    My point is, there must be information generated to collapse the wave-function. I'm not sure how the information is generated in your example, but if you collapse the wave function, I'm sure it is. I'll have to think that one over.
     
  6. If a tree falls in the woods, and no one was there to hear it, does it make a sound? QM says no. There was only the probability of a sound but never the reality of one. In fact, there was only a probability of a tree, or a fall. Those are the facts. Unless, you chose to believe there is an infinite number of Universes, some in which the tree falls, and some in which it doesn't. Your choice, the math works equally well either way, but I like to stay as far away from infinities as possible.
     
  7. Matterwave

    Matterwave 3,860
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    I think you're putting too much stress on humans in Quantum Mechanics. Nowhere in QM does it mandate that measurements and observations can only be made by humans, or sentient beings...
     
  8. I think the problem behind this stuff is the lack of a precise definition of "information" (in quantum mechanics) and "reality".
     
  9. Ah! That's where I started. We are back to the bug. It's bug eyes observe the photons from the distant star, but it's bug brain can't generate information about what it has observed. So the wave-function does not collapse. It is still just a probability of a star at that location.

    Well, I could be wrong. Perhaps a tiny bug could collapse the wave-function of a star instantly 50,000 light years away without generating any information about what it has observed. That would be even more interesting, in my own humble opinion.

    I'm just musing about the connection between the generation of information and the collapsing of the wave function. I didn't mean to disturb anyone. Carry on.
     
  10. Well, there are 2 'flavours', with realistic wavefunction and where 'wavefunction is just a knowledge of the system'. Negative experiments also partially collapse the wavefunction 'because we understand, that if particle havent hit this target, then...'

    So the existence of sentinent beings (able to posess the 'knowledge') is somehow assumed. The mixture of 2 flavours makes it even more confusing. As before, I believe that in modern era Copenhagen Int makes more harm than good.
     
  11. Am I the only one who considers this absurd for one blindingly obvious reason - how can there be starlight to be observed unless it has already (maybe tens of thousands of years ago) been generated by that star? Do you not see a clear logical contradiction? As an aside, I admired your acting role in 'The Man from U.N.C.L.E.' :rofl:
     
  12. If you accept that nature is fundamentally probabilistic then there is no confusion, there is no need for a physical wave function collapse, observing the star simply confirms its existence, which can not be known otherwise (ie you can't do a calculation to confirm it exists, you can only ever calculate probabilities).

    Probabilities evolve deterministically and unitarily in the universe, the act of observation doesn't need to have any effect on the evolution of probabilities apart from enabling us to know something about the past evolution of the universe (which we could not know without making observations). The universe evolves on and on in its merry deterministic fashion according to schrödinger evolution just as it did in the billions of years before we even existed.

    But what is evolving deterministically are probabilities. Many people don't like this, they want a purely deterministic "real" universe, but as I tell my 5-year old "want want want" doesn't "get get get".

    Nature ain't really real, deal with it. :smile:

    ps. if a tree falls unobserved then that's what happened, it fell unobserved, but no-one will ever know this, just like trees that fell before we evolved on the planet (unless they left some fossil record)
     
  13. Demystifier

    Demystifier 5,382
    Science Advisor

    A deterministic universe is one thing, a real universe is something completely different. One may be without the other.
     
  14. Yes, that's what I mean by purely deterministic. If the universe can be reduced to purely deterministic rules then it is "real" in the epr sense, although (to explain modern experiments) it must have some non-classical feature like non-locality or information loss (essentially a many-to-one mapping in the deterministic laws cf 't Hooft).

    I like the simple picture of deterministically evolving probability states, which seems to explain everything quite "naturally", the universe has deterministic and non-deterministic components.
     
  15. I agree - a different interpretation can't be right. :smile:
     
  16. Right on Harry. :smile: Another important factor here imho is that the light our observer receives is not from 'the star' as such but some infinitesimal fraction of ionized gas in it's surface layers. Hence even if you believe in some mystical backward causation possibility here (not arguing with Quantum Erazer etc), it could at most effect those few actual source emitters of light. And I wonder how conservation of energy/momentum would survive our ability to 'realize' an entire star by observation?! I gained a lot from a (very long) thread on Schrodinger's cat here: https://www.physicsforums.com/showthread.php?t=468101&page=6, in particular #96 helped to clear up some of the misconceptions that abound. Just substitute 'star' for 'cat'.
     
  17. All a bit complicated, easier to just accept the universe has evolved along a definite path, but that path is probabilistically defined so we can't know what it is without "looking".

    The past has already happened, it cannot be changed.
     
  18. DrChinese

    DrChinese 5,745
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    Gold Member

    After it is determined, it cannot be changed. However, it is not clear when that point is actually reached!
     
  19. Yes, it's undetermined as far as conscious thinkers who haven't observed it yet are concerned, but I doubt if we destroy ourselves in some silly war this will change what the rest of the universe has done for 13 billion years!

    Obviously I have no experimental proof of this, but it seems reasonable :smile: (note: I have left out the elephant in the room - human free-will, but that gets us into very speculative areas ;-) )
     
  20. Admittedly even that entry was somewhat long, but a passage towards the end is relevant:
    "To say a wave function is collapsed, you must have a wave function in the first place. A mixture is not a wave function, it is a mixture of wave functions. Classically, it is the mixture that matters, not the quantum mechanics of the wave functions-- the evolution of a mixture is a classical evolution, what the individual wave functions are doing gets lost (like a thermodynamic treatment of an ideal gas where we are not a whit for what any given particle is actually doing, only the generic possibilities for what they are allowed to do). When a cat is a super-complicated statistical average of a bunch of possible individual wavefunctions, then it is a classical object, not a quantum mechanical one."
    How much more a star! May be wrong, but likely the OP got his que from that famous exchange between Einstein and Bohr where Einstein is quoted "I like to think that the moon is there even if I am not looking at it." An hyperbole, but combine that with the fact Bohr's Copenhagen viewpoint has almost unanimously considered to have won against the EPR argument, and hey presto - the Moon depends for it's existence on being observed. Or not.
    There have been a few recent threads where 'backward causation' as apparently confirmed in delicate experiments was discussed, but I would agree it has no relevance to 'the world at large'.
     
  21. Yes - a good thread, although not completely resolved. I am still scratching my head about some of those points.

    The wave function is not a physical entity with totally objective existence. Its a mathematical tool. It's an encoding of what has been measured, and along with QM theory which allows us to predict probabilities of future observations. The wave function collapses when our knowledge (as the result of measurement) changes. 200 years ago there were no wave functions, because QM had not been developed. To a duck observing a star there is no wave function, because the duck does not understand things in terms of QM.

    All of the apparent paradoxes are resolved by this interpretation, which is, I believe, the Copenhagen interpretation. But after the above thread, its clear that things are not this simple.
     
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