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I Gravitational waves and energy

  1. Nov 24, 2016 #1
    I understand that any source of gravitational waves loses energy, which is carried away by the waves. But since the waves are perturbations in spacetime rather than a physical field, they cannot carry energy the way photons do. I have read that this used to be a source of considerable controversy in the past. Is there any consensus these days and what would the answer be?
     
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  3. Nov 24, 2016 #2

    Ibix

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  4. Nov 24, 2016 #3
    No, not really. I am aware of the sticky bead thought experiment. I want to reconcile that in my mind with the fact that spacetime is not a physical field.
     
  5. Nov 25, 2016 #4

    PeterDonis

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    What do you mean by "a physical field"?
     
  6. Nov 26, 2016 #5

    haushofer

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    In GR, spacetime is a manifold plus a metric on it (not just the metric, which was the source of great confusion for Einstein, see 'the hole argument'). But the metric is as "physical" as e.g. the el.magn. gauge field A. At least, in GR it is.
     
  7. Nov 26, 2016 #6
    What I mean is that ##T_{\alpha\beta}=0## away from the source. If the wave contains energy then ##T_{\alpha\beta}\neq0## at that point, leading to a contradiction.
     
  8. Nov 26, 2016 #7
    About that ...

    Note to the Fifteenth Edition
    In this edition I have added, as a fifth appendix, a presentation of my views on the problem of space in general and the gradual modifications of our ideas on space resulting from the influence of the relativistic view-point. I wished to show that space-time is not necessarily something to which one can ascribe a separate existence, independently of the actual objects of physical reality. Physical objects are not in space, but these objects are spatially extended. In this way the concept "empty space" loses its meaning.
    June 9th, 1952, A. Einstein, Relativity - The Special and The General Theory

    and

    A complete field theory knows fields and not the concepts of particle and motion. For these must not exist independently of the field but are to be treated as part of it.
    July 1935, A.Einstein, N.Rosen - The Particle Problem in the General Theory of Relativity

     
  9. Nov 26, 2016 #8
    Well, it's still true that that ##T_{\alpha\beta}=0## is the starting point from which we derive the gravitational wave equation.
     
  10. Nov 26, 2016 #9

    PeterDonis

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    This is not correct. Please take some time to learn what GR actually says about gravitational waves. The fact that they do carry energy even though they can propagate through regions where ##T_{\alpha \beta} = 0## was established in the 1960s and has been part of standard GR ever since. All of the major GR textbooks written since then cover this (for example, MTW has a good discussion of it).
     
  11. Nov 26, 2016 #10

    PeterDonis

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    The OP's question has been answered. Thread closed.
     
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