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Light and gravity

  1. Apr 9, 2014 #1
    Light can be affected by gravity and as I understand it light can be a source of gravitational effects on other objects. But have such effects been detected? References would be helpful.
    Thank you
     
  2. jcsd
  3. Apr 9, 2014 #2

    UltrafastPED

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    The Pound-Rebka experiment (1959) exploited the Mossbauer effect to verify the effect of gravity upon light:
    http://en.wikipedia.org/wiki/Pound–Rebka_experiment

    The effect of light upon gravity is very weak; light simply adds to the momentum flows and energy density. AFAIK there have been no proposed experiments to detect this.

    You can read about all of the standard tests in Clifford M. Will's
    "Was Einstein Right? Putting General Relativity To The Test".
     
  4. Apr 10, 2014 #3
    Thank you. The main thing I am interested in is whether photons can be detected remotely without the necessity of them ending up at a detector where they get destroyed. I have found some information on this but need to read up on it.
    As you said gravitational effects are very weak but I wonder if these effects can be detected in principle, if not in practise.
     
    Last edited: Apr 10, 2014
  5. Apr 10, 2014 #4

    pervect

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    If you look at Steve Carlip's "Kinetic Energy and the Equivalence Principle", http://arxiv.org/abs/gr-qc/9909014 , you'll see some references that discuss measurements that concern the effects of static electric and magnetic fields on gravity. For instance, Eotovos experiments on different elements have different amounts of electrostatic energy.

    From this, it's not much of a leap to conclude that non-static fields also have an effect. But it may not be direct as you desire.
     
  6. Apr 10, 2014 #5

    UltrafastPED

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    Steve Carlip begins his analysis with a brief summary of experiments which show that potential energy "has weight"; he ends his analysis with:

    "We can thus tell our students with confidence that kinetic energy has weight, not just as a theoretical expectation, but as an experimental fact."
     
  7. Apr 10, 2014 #6

    UltrafastPED

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    This seems most unlikely. If you have references, please provide them!
     
  8. Apr 10, 2014 #7

    Bill_K

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    Gravitational deflection. When a photon passes near a compact object, it will be slightly deflected. The change in momentum of the photon will result in a change in momentum of the object, serving to detect the passage of the photon without destroying it.
     
  9. Apr 10, 2014 #8
    I think my post may have been a bit misleading. I am interested in whether photons can be detected remotely by observing gravitational effects and also by any other methods. The information I found and referred to can be found by googling:

    "single photon detected but not destroyed".

    The method uses quantum effects.I think the nobel prize was awarded for a team who observed photons in the microwave region of the spectrum. I don't know much about any of this yet and need to read up on it.
     
  10. Apr 10, 2014 #9
    Thanks Bill-K. I wonder if anything can be made sensitive enough to detect such small effects.
     
  11. Apr 10, 2014 #10
    Thank you pervect. I will take a look
     
  12. Apr 10, 2014 #11

    UltrafastPED

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    One of several techniques for taking what is called "a weak measurement" - it provides limited quantum information about the object being "somewhat" observed.

    See http://en.wikipedia.org/wiki/Weak_measurement

    It helps if you have a good grasp of quantum mechanics.
     
  13. Apr 12, 2014 #12
    Thank you again Ultrafastped. Weak measurements has now been added to my growing list of things to look at. I rather like your description that the photon is "somewhat" observed.
     
  14. Apr 12, 2014 #13

    BruceW

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    EM radiation is a source of gravity. And it can be detected in principle. But if we're talking about a single photon, that's a whole different kettle of fish. I don't think physics currently has much to say about what effect a single photon has on spacetime curvature. I mean, that would pretty much be a quantum theory of gravity, right?

    More generally, about detecting a photon without destroying it: In principle, there's nothing wrong with doing that. But I don't know if it is so easy in practice. This website seems to report that it has been done:
    http://www.nature.com/news/photons-detected-without-being-destroyed-1.14179
     
  15. Apr 16, 2014 #14

    PAllen

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    I will try to find the reference, but a GR theory paper I recall, that analyzed the mutual attraction of oppositely directed light beams (parallel light beams, somewhat surprisingly, do not - even in theory - deflect each other per GR), calculated a figure for two high powered laser beams. My recollection is that this showed how 'in principle' an effect we're talking about e.g. 10**-29 newtons for two high power beams.
     
  16. Apr 16, 2014 #15

    UltrafastPED

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    Here is a 1931 paper by Tolman, Ehrenfest, and Podolsky:
    http://journals.aps.org/pr/abstract/10.1103/PhysRev.37.602

    And something more modern (1998): http://arxiv.org/pdf/gr-qc/9811052v1.pdf

    I only read the abstracts.
     
  17. Apr 16, 2014 #16

    PAllen

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    Thanks! Your arxiv reference is the one I remember reading. However, my memory of the figure is wrong. The 10**-29 acceleration is due to gravitational waves from the Virgo cluster, while for two laser beams 10 cm apart, the acceleration is only 10**-110 !!. Thus GW from virgo cluster have 10**81 time larger effect than a laser 10 cm away. That's really "in principle".
     
  18. Apr 18, 2014 #17

    PAllen

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    If my calculation is correct, for a laser beam to have a comparable gravitational influence on nearby objects as a 1 gram weight, it would need a power of 10^24 watts.
     
  19. Apr 18, 2014 #18

    UltrafastPED

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    My thesis advisor is working on such a system, though I don't think they plan to test it that way!
    More along the lines of "breaking the vacuum", looking for new physics, and a host of applications:

    http://www.technology.org/2013/08/1...elop-new-generation-of-petawatt-scale-lasers/

    http://www.eli-beams.eu/

    http://en.wikipedia.org/wiki/Gérard_Mourou


    Though I think he is about due to retire ... we're celebrating his 70th birthday later this year at the University of Michigan.
     
  20. Apr 18, 2014 #19

    PAllen

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    Well, that's still about a billion fold too weak, but impressive. Since the pulses are so short, the effect is reduced e.g. equivalent to measuring the gravitational influence of a bacteria zooming by fast (but not so fast that gamma is significant, say 1/10 speed of light).
     
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