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Gravitational quantum detection

  1. Sep 12, 2003 #1

    drag

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    Greetings !

    I remember posting a partially(I intend to add a different
    more profound example, if I get some feedback on this enitial
    message) similar thread here before called "gravitational
    double slit", but for some reason I couldn't find it.

    Anyway, basicly I was asking wheather physics can answer
    the following question today (or at least what is the majority
    opinion on this issue) :
    What would happen if gravitational waves were used instead
    of electromagnetic waves to detect the quantum in the
    double slit experiment ?

    Would there be a "detection" ? Would it collapse the WF ?
    Would the HUP be violated if the first answer is yes and
    the second is no ?
    (At least make your best guess. :wink:)

    Live long and prosper.
     
  2. jcsd
  3. Sep 12, 2003 #2

    selfAdjoint

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    Gravitational waves ought to diffract, and self-diffract. You would have to use a BIG slit though.

    Note: Gravitational waves are not the wave nature of the graviton.
     
  4. Sep 12, 2003 #3

    drag

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    Greetings !
    I think we've had a slight misunderstanding. I'm not talking
    about any gravitons or hypothetical theories of
    "GR + QM". I'm talking about a "normal" double slit experiment
    with photons/electrons/whatever where the detector/s at the
    slit/s uses gravity waves. (Abviously this is by far
    outside of the range our current technological capabilities
    but that does not invalidate the question :wink:).

    Thanks ! :smile:

    Live long and prosper.
     
  5. Sep 12, 2003 #4

    jcsd

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    gravitaional wwaves pass through pretty much everything so your not going to be able to diffract one. A gravitational wave is the result of a dynamic gravitational system.
     
  6. Sep 12, 2003 #5

    marcus

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    tho with all possible good will, nevertheless no one is understanding drag's question

    feynmann discussed a thought experiment where one tries to
    "fool" the double slit experiment by putting a detector by
    one slit so one can catch the electron in the act of going thru the slit

    putting a detector destroys the interference pattern
    so nature forces you to take the detector away or else she
    refuses to do her thing

    NOW DRAG SAYS what if you made a detector of photons or electrons that works by feeling the gravitational field have
    a little spasm when the electron goes thru the slit
    and by the wonders of technology this detector is very small (!)
    and can be put beside just one slit (!!!!)

    then drag says, wouldnt this be a way of spying on nature and
    finding out which slit it went thru without disturbing the fringes
    or some such thing---i dont quite understand

    this really is one for selfAdjoint or for jcsd
    it is a feynmannesque thought experiment so cheers to you drag
    but the answer is likely that no you could not use it to tell which slit (or if you set it up so you could the pattern would go away)
     
  7. Sep 13, 2003 #6

    jeff

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    The double slit experiment demonstrates that particles have a wave aspect, and in particular, a debroglie wavelength, given by hbar/p where p is it's momentum. I don't see why detection gravitationally would produce a different result than detection by a geiger counter, or any other suitable detection device for that matter.
     
  8. Sep 13, 2003 #7

    drag

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    Greetings !

    Well, marcus finally (I actually thought that what I said
    was quite clear but I appologize if I were mistaken)
    pretty much understood me. Although I have to say he
    really exhagerated somewhat - it's just a hypothetical
    question (for now) I didn't say we can actually do it
    and I did not say I prefer some some result.

    I am not quite clear as to why marcus is so certain of
    the answer. A detector near one or both slits would collapse
    the WF if its electreomagnetic (like in the normal experiment)
    a detector using nuclear forces probably will too (even if
    such experiments haven't been conducted, these forces
    are parts of the extended quantum theories that have so far
    proved to be correct for them).

    But gravity is a different thing. In fact, if for example
    our detectors will "feel" 60% of the particle passing through
    one slit and 40% through the other and there will be no
    WF collapse (we'll the diffraction pattern on the screen)
    than not only will we know that GR and QM can not be connected
    in many currently theorized ways but QM is infact incomplete
    or inaccurate altogether. On the other hand we could discover
    that the WF will collapse which probably means that gravity
    can and should be quantified somehow. Alternativly, we could
    detect 100% of the paretcile's mass passing through a single
    slit but still see diffraction on the screen (no WF collapse),
    thus confirming some "hidden variables" theory.

    What I want to know, again, is the answer or at least the
    most likely (as it seems today) answer to that (which may
    be what marcus said alright).

    I could also rephrase the question more clearly perhaps:
    Does a partcile produce space-time curvature (as we normally
    expect it to as well) when it is not interacting with other particles ?
    If the answer is yes - doesn't it mean that the HUP
    can be violated "through" gravity ?

    Live long and prosper.
     
  9. Sep 13, 2003 #8

    jeff

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    It doesn't work that way: A detector spying on a slit either detects a particle to have passed entirely through it, or it doesn't detect it at all.

    I think I pointed out to you the second time you asked you're question that since gravity couples to everything - including quantum fields etc. - it would be hard to see how to build a consistent QGT without gravity being governed by the same quantum laws that everything else is.
     
  10. Sep 13, 2003 #9

    LURCH

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    This is a very interesting question. I would have to say (and this is only my guess) that I think the presence of a detector, even a GW-based detector, would still alter the pattern on the detection plate at the end of the experiment. And my reasoning is this;

    It is a predicted quality of gravity waves that they carry energy away from any phenomenon that generates them. At the quantum level, where wave/particle duality becomes significant, the difference between energy and matter gets hazy. So I think that any thing that carries a detectable amount of energy away from a particle must alter that particle. Which, I suppose, brings up another rephrasing of your question,
    I would say that if there is a gravity wave to be detected, then the dispersion pattern will be altered. And if there isn't, the interference pattern will be produced at the end of the experiment, but the detector in or near the slit will detect nothing.
     
  11. Sep 13, 2003 #10

    selfAdjoint

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    I agree with you Lurch. I think if there is an interaction with the particle - some exchange of virtual bosons - then the particle is "pinned down" in the sense that its location is established(up to uncertainty) as being at one slit rather than then other, and it can't any more exhibit waviness because localization like that is not consistent with a wave encountering both slits.
     
  12. Sep 13, 2003 #11

    drag

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    Greetings !
    Have you ever heard of a working gravitational particle
    detector of this scale ?
    I'd love to see the design. :wink:
    For the moment I'm not trying to do anything but try
    to receive the most likely answer to a question.
    Also, you'll agree with me that for the moment we see
    that gravity is NOT quantified in any way what so ever
    (as far as I've heard at least) and while I'm not trying
    to defy QM, it is, in my opinion, quite reasonable to assume
    that to find the connection between these two foundations
    of modern physics we should not just think of new mathematics
    but also of new revolutionary experiments.

    Peace and long life.
     
  13. Sep 13, 2003 #12

    jeff

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    Firstly,

    Let me just say that your line of questioning from my point of view is just about the most interesting in the forum at the moment.

    Star Trek the next generation, episode sixty nin......Oh, you probably mean for real, right? My feeling is there's at this point no theoretical reason to doubt that such a thing is possible in principle and that it would operate analogously to any other kind of detector.

    Empirically yes, but the theoretical picture makes a strong - irresistable in my view - case for quantizing gravity.

    Yes, we must try our best.

    Let me ask, are you thinking that perhaps gravity need not be quantized, or that gravity may undermine quantum mechanics vis a vis the uncertainty principles?
     
  14. Sep 17, 2003 #13

    drag

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    Greetings !
    Thanks. :smile:
    I was wondering why my previous thread about this
    was so "dull".
    I'm no expert, but I've heard of a recent attempt, for example,
    that failed. They tried to detect gravitational quantization
    by observing very distant objects like quasars and looking
    for distortions, but failed.

    As for wheather the HUP can be violated, personally I do
    not think so, 'cause it would seem to me that this is
    not just a limmit but rather a fundumental border. Of course,
    I'm really totally not qualified to judge on the matter. :wink:

    As for gravity being quantized, again my personal insignificant
    opinion is that it won't happen, at least, not in the
    standard manner in which the other forces were quantized.
    (If I weren't an atheist, I'd say that God loves riddles. )

    Live long and prosper.
     
  15. Sep 18, 2003 #14

    drag

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    Greetings !
    Well, I suppose I could try to argue that a space-time
    wave may be completely different in its effect, but
    I won't. Besides, considering quantum limmitations on
    sensitivity I suppose there wouldn't be a way for us
    as observers (if not the experimental particle) to
    escape the HUP during the entire experimental process.

    HOWEVER, there is something else I believe I can see here.
    GR is not a quantum theory, it is a theory of geometry
    (that Einstein preffered and liked) hence, theoreticly,
    we may not even need any machine creating waves in space-time
    through very intense gravitational interactions. Basicly, we
    could just put something there and see if it gets attracted.
    Now, if I'm not mistaken, the HUP should have no effect here
    because it's not even a quantized interaction - rather a
    "continuos" geometry change the scale of the observation of
    which we can then increase(not enough though :frown:). For example, suppose that I send the same particle through two very long tunnel
    like slits - the gravitational attraction should become
    more apparent - wheather the WF will collapse or not and
    if there will at all be some attraction.

    In fact, the more I come to think of it as I write - let's
    consider wheather this IS a technologicly possible experiment
    TODAY:
    We run the double slit experiment, the slits are turned
    into long narrow (according to the wavelenght) tunnels
    one above the other and the particles (say electrons) are
    injected at very relativly low speeds. Now, we allow the
    tunnel slits to be capable of detecting an electron impact.
    Then we run the experiment for a great deal of them and
    see wheather amongst the electrons that did not emerge
    from the tunnels and were detected there, a greater, by some
    degree, amount impacted the lower walls of the tunnels
    slit and a lesser amount impacted the upper walls.
    Thus we might just be able to see wheather the uncollapsed
    WF "flows" in accordance with space-time or just the other
    3 forces we've already quantized.
    So, does the above make any sense ?

    Further more, if I am nor mistaken this could even have
    ramifications for the Equivalence Principle, since we know
    that if the same occured in a "normally" accelerating frame
    the WF will feel the acceleration, right?
    But, aren't you assigning quantum propeties and behaviour
    to gravity ? If I'm not mistaken, it has so far refused
    to work according to such models, or am I indeed mistaken ?

    Well anyway, I'd really appreciate your responses and I'd
    like to remind you, again, that I'm not claiming anything
    or am specificly in favour of some option, I'm just trying
    to examine them in general and make some general sense of
    the question, and the rest is a matter for you the experts to
    decide upon. (Though if it's possible, I'd appreciate you telling
    me at which stage I should either start copyrighting or go do something more useful like water some plants or somethin'... )

    Live long and prosper.
     
  16. Sep 23, 2003 #15

    drag

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    Oops... :frown: Sorry for that one, I thought about it the
    next day, but I couldn't get online since then. The above
    experiment, of course, would not test what I wanted
    to find out. It would test wheather the WF follows space-time
    curvature (geodesic lines) which, I believe, is almost
    certainly known as the case, rather than test wheather the
    WF itself would create space-time curvature - which is
    the object of my inquiry.

    Anybody care to make suggestions as to technologicly possible
    experiments or some comments on the matter ?

    (One abvious possibility is to trap an intense laser beam
    between two mirrors and see wheather the medium in between
    produces graviatational pull - but, abviously, that would require
    "slightly" more accurate instruments than the ones availible to
    us today. :wink:)

    Live long and prosper.
     
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