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Hulse-Taylor pulsar and GR what if orbital velocities were higher?

  1. Jan 3, 2014 #1
    The Hulse-Taylor (PSR B1913+16) binary pulsar 'observed vs. predicted' orbit decay is one of the great validations of GR. The orbit decay over the recording period of 30+ years is very close to GR prediction as per predicted energy loss through gravitational waves.

    Hulse-Taylor experiment is seen as a confirmation of the existence of gravitational waves, though none have yet been directly detected (LIGO has failed to do so, but the sensitivity of Advanced LIGO in 2014 may succeed, if the underlying theory and computations are correct. We're still waiting on that).

    However, what would have happened if the two pulsars involved happened to have a 'slightly higher' orbital velocity around their CG than they do (as possibly may be the case for some other binary pulsars)? Would they have then maintained their existing orbits without decay or slowly spiraled outwards, and been seen as a refutation of the existence of gravitational waves? Have any other binary pulsar experiments confirmed or refuted this? Or is such a situation impossible?

    [Bit of a naive question as I have very little understanding of the GR area that deals with gravitational waves, but hoping to get some inputs through this question.]
     
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  3. Jan 3, 2014 #2

    PAllen

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    All orbits decay, in principle, if GR is correct. An orbit that expanded (without some explanation like mass emission, or spin locking), would violate not only GR but Newtonian gravity. Not only have no violations been discovered, but several other binary neutron star systems have been discovered which match GR predictions precisely for different orbital and mass parameters.
     
  4. Jan 3, 2014 #3
    The question was not about the orbit decay predicted by GR. That can be assumed validated.

    Let me clarify - the question was - what would we observe if we detected a pair of binary pulsars which have orbital velocities somewhere between 'Newtonian orbital velocity' and 'escape velocity'? What would we observe over a small period of 30-40 years? A high enough orbital velocity would show the binary pulsars spiraling outwards from each other, wouldn't it? (Correct me please if I am wrong in this basic concept!)
     
  5. Jan 3, 2014 #4

    Jonathan Scott

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    Orbits aren't necessarily circular. Closed orbits are usually elliptical.

    If we observed a pair of massive objects which had an orbital velocity higher than that necessary for a circular orbit but less than that for escape, then by Newtonian theory we would expect to see them continue in elliptical orbits about their center of mass, where the current point in the orbit is closer together than the average for a whole orbit. By GR we would then expect to see that orbit modified in the long term by a slight decay (plus other effects such as perihelion precession).
     
  6. Jan 4, 2014 #5
    Thanks for the response. This makes sense.
     
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