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Violating uncertainity principle

  1. Sep 26, 2009 #1
    Suppose in a double slit experiment, we place a machine that traps the gravitons emitted by the electrons and hence gives us the exact trajectory of the electrons(this would have no effect on the experiment as gravitons are liberated in any double slit experiment, just that we are able to measure it this time. This won't disturbing the system).At the same time, we find the velocity using our normal measurement using photons. Won't this violate the uncertainty principle??
    The Godel's theorem suggests that we can never get a complete and consistent theory of all the force together. The above experiment(if it works!) suggests that gravity would not follow quantum mechanics. If Godel's theorem is correct for this, it means there has to be some discrepancy in the adding the four forces together, something like above.
  2. jcsd
  3. Sep 27, 2009 #2


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    Welcome to PhysicsForums!

    Keep in mind that the uncertainty principle (HUP) does not prevent you from measuring 2 non-commuting observables. This can be done on entangled particle pairs, for example. However, the idea that you know *simultaneously* these values would violate the HUP. Indeed experiments show that the HUP is NOT violated in these situations. Certain knowledge of 1 observable always leads to complete uncertainty in its canonical partner. This shows up in experiments when you attempt to confirm an earlier value and discover it is no longer valid.

    As to the issue of gravitons and QM: Your point in general is a good one. If gravity is a quantum force, the HUP should apply to gravitons.
  4. Sep 28, 2009 #3
    thank you Drchinese
    But from the experiment, we will get trajectory(position) and momentum, which are commutable.
    what i was trying to say is this:
    suppose in a double slit experiment,electron will emit some gravitons(mass will almost remain same at all point even when photon collides). When we will make a photon collide with electron, we can either find its position or momentum more accurately. Now we choose to find momentum accurately. Also at the time of collision the number of gravitons emitted by electron will not be affected at all(leaving relativity alone). So, using some kind of graviton detector we can find the position of electron at the time of collision.
    this can give the position and momentum more accurately than possible.

    Iam not aware of a few facts though;
    1. whether photon graviton(if it exists!) can make the detector give a wrong estimate
    2. if we affect graviton somehow, can it affect the electron back.
  5. Sep 28, 2009 #4


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    Just repeating: there is nothing about the HUP that forbids us from taking measurements of unlimited precision for non-commuting observables. You cannot, however, have SIMULTANEOUS values that are objectively real.

    Imagine you have 2 numbers in your hand representing P and Q. Do you think subsequent measurements will again yield the same numbers? The answer is NO, and when you put the results together you will have a statistical distribution that respects the HUP. In other words, the measurements of P and Q can NEVER be simultaneous. You can pretend they are simultaneous but they won't be.

    As to gravitons: there is no experimental evidence at this point which confirms or disproves their existence.
  6. Sep 29, 2009 #5
    thanks, i got the point now.
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