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Hiding the observer with gravitational measurements?

  1. May 26, 2015 #1
    Hello again. In a double slit experiment with electrons, suppose that we have the instrumentation to measure how space-time is curved by these particles. Would it be possible to obtain the electron's position and momentum by measuring the change in the direction of photons that move nearby but without contact? I think that would completely hide the observer, since photons won't strike the electron changing it's momentum. And, photons have no mass and no charge that could alter the electron's behavior in any other way. Unless there is actually some sort of fundamental inter-particle observation, maybe due to forces unification (GUT)?
     
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  3. May 26, 2015 #2

    atyy

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    I'm not entirely sure, but I believe that in a theory in which such a measurement is possible, the position and momentum of an electron do not exist as conjugate observables (so I think the question is meaningless).
     
  4. May 26, 2015 #3

    bhobba

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    No.

    The theory is unambiguous - you cant measure, with arbitrary precision, both position and momentum at the same time.

    That said, we do not yet have a theory of gravity beyond the plank scale - which you would almost certainly need to know to figure out how to do what you suggest - if it can be done at all - the space time curvature of an electron would be infinitesimal - likely way beyond even future technology - still one never knows.

    Thanks
    Bill
     
  5. May 26, 2015 #4
    Then why the experiment in first place, if the theory already provided the solution?

    Well I've read the phenomenon still occurs with atoms, molecules and even buckyballs! So there should be a way to measure the particle's gravitational effect without plank-scale technology... still years way, but seems possible anyways. Now these larger objects, while neutral, will alter the electrons direction, but maybe the change can be compensated in the measurements.

    So, if we wanted to measure the position, maybe we could send large particles in perpendicular directions, and measure their final direction or momentum to find out how near they got to the electron (or another large particle). Then the momentum... maybe the electron will make the particle to vibrate, rotate, or something else?
     
    Last edited: May 26, 2015
  6. May 27, 2015 #5

    atyy

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    If we are thinking of the same type of experiment, those do not measure the gravitational effect of the atom/molecule/buckyball. They measure the effect of gravity on the atom/molecule/buckyball.

    Here's a description of the type of experiment I am thinking about: http://backreaction.blogspot.com/2007/06/bouncing-neutrons-in-gravitational.html.
     
  7. May 27, 2015 #6

    Nugatory

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    In no particular order:
    1) You seem to be suggesting that because photons have no rest mass, they will not produce any gravitational effects. That's not correct - they have energy, and energy contributes to gravitatational effects as well as mass. Although the effects are too small to measure using any currently imaginable technology, both quantum mechanics and GR say that if the electron affects the path of the photon, the photon will also affect the path of the electron.
    2) Although photons have no electrical charge, they still interact gloriously, strongly, and promiscuously with any and all charged particles in their general vicinity. A photon is not like a little tiny bullet that either hits an electron or misses it altogether; it's a quantized excitation of the electromagnetic field, and charged particles are affected by electromagnetic fields.
    3) You have to consider that position-momentum uncertainty applies to the photons as well, both in determining their initial trajectories before the interaction with the electrons and knowing their position and momentum after the interaction. Thus, there will still be uncertainty about how exactly their trajectories have changed, and hence about the position and momentum of the electron.
    4) You are asking about "hiding the observer", which seems to suggest that you're thinking that the uncertainty principle comes from the observation disturbing the observed system. It doesn't - that interpretation of the uncertainty principle was discarded just a few years after Heisenberg suggested it. Unfortunately, by then it had leaked into non-technical popularizations and taken hold in the public imagination, so it's repeated as fact to this day. The uncertainty principle actually says something more along the lines of: if the electron is in a state such that a measurement of its position would yield the value X (to an arbitrarily large number of decimal places) with 100% certainty, then it is in a state in which an an accurate measurement of its momentum (to an arbitrarily large number of decimal places) will yield one of a range of values; we cannot be 100% certain of getting a particular value Y out of the momentum measurement.
     
  8. May 27, 2015 #7

    bhobba

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    Your the one that suggested it. As Nugatory explained your reasons for thinking it would give a different answer do not hold up.

    The space time curvature of those objects is just as negligible and would require technology way beyond what we have, or in the foreseeable future, expect to have.

    Thanks
    Bill
     
    Last edited: May 27, 2015
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