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A question regarding the orbit of material in a galaxy such as ours

  1. Mar 12, 2008 #1
    This is a question I recently posted on Lubos Motl's blog. I am hoping that maybe someone here will also have some insight into this...


    I have been reading about gravitational waves in hopes that they will prepare me for my inevitable look into the gravitons that I asked you about in a previous comment section.

    I've come to understand (from Cooperstock & Tieu's papers, take them or leave them) that the self-interaction between the stars of a galaxy occurs mostly due to gravitational effects. A "pressureless" fluid, so to speak, when ignoring the interiors of the stars.

    My own extrapolation from this is that when an individual star is accelerated due to the gravitational pull of its neighbours, this causes the star to emit energy as a gravitational wave, which in turn causes a disturbance in any neighbours that lay upon the trajectory of this wave, which causes that [neighbouring] star to emit a gravitational wave, and on and so forth. It would seem to me then that since this collection of stars is not distributed isotropically, that this back-and-forth stimulation of gravitational waves would tend to occur more along the galactic plane (ex: xz axis) than in the up-down direction (ex: y axis).

    Wouldn't it then be unreasonable to expect the stars to orbit around the galactic centre following standard Newtonian approximation, since that would require gravitational energy (including these gravitational waves) to be distributed isotropically?

    I guess the closest analogy that I have in mind would be a laser, where the emission of photons is focused along a somewhat unified trajectory, rather than in all directions (more or less) equally.

    I appreciate any information that you or your readers might have on this topic.

    - Shawn
  2. jcsd
  3. Mar 12, 2008 #2


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    I think the energy lost by gravitational waves is so small as to have no effect on the orbit of stars or structure of a galxy.
  4. Mar 12, 2008 #3
    Fair enough, thank you for this answer.

    I understand that gravitons are effectively braking radiation, and that gravitational waves are taken to be a group of gravitons possessing the same velocity (direction and speed). Since the braking of the individual atoms/etc would also occur anisotropically, wouldn't this affect the gravitational field by making it non-spherically symmetric? This is assuming that the spherical symmetry of the Schwarzschild solution springs from the fact that the braking experienced by the Sun's constituent particles causes the emission of gravitons in all directions (roughly) equally simply because self-interaction also occurs (roughly) isotropically.

    The simplest example that I can come up with is a single non-composite particle (ex: electron) made to oscillate along a single axis. If it were to emit gravitons only along one axis, wouldn't the resulting gravitational "field" be completely anisotropic, and not spherically symmetric, or isotropic, like Newtonian approximation implies? I am taking this example from how I know the electron to emit photons. I guess the analogy would be a gravitational "laser", where the beam divergence is effectively 0.
    Last edited: Mar 12, 2008
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