Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Gravitational waves and gravitation

  1. Dec 22, 2009 #1
    Gravitational waves are generated when the mass quadrupole moment changes in time.
    We also know motion of mass contributes to its gravitation. Does the producing process of gravitational waves, which involves mass in accelerated motion, produce gravitation as well? If so, is it of less, equal or more magnitude than the gravitational waves being generated?
     
  2. jcsd
  3. Dec 22, 2009 #2

    Jonathan Scott

    User Avatar
    Gold Member

    Firstly, this question is a bit mixed up, as "gravitation" is normally considered to mean a gravitational field, but strength of gravitational waves is related to the rate of change of the gravitational field, so these quantities cannot be compared.

    Secondly, you can't move or accelerate a mass in isolation (the center of mass of any complete system moves with constant velocity), so for example you can't cause the gravitational field of a system to change abruptly by pushing it sideways, because the gravitational field of the pushing mechanism will cancel out the change to first order.

    The best you can do is to change the distribution of mass, creating tidal effects, plus gravitational waves if you do it fast enough.
     
  4. Dec 22, 2009 #3
    Ok, let me put it in a quantum mechanical way. Gravity waves are real gravitons. Gravity is virtual gravitons. Does any process that produces real gravitons also produce virtual gravitons?
     
  5. Dec 22, 2009 #4

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    For concreteness, let's imagine a system consisting of two objects of mass m, in circular orbits of radius r about their common center of mass. The power radiated in gravitational waves scales like [itex](m/r)^5[/itex]. The kinetic energy and binding energy of the system scale like [itex]m^2/r[/itex]. Although this is an apples-and-oranges comparison (power versus energy), I think it should be pretty clear that the two effects scale differently, so there is no particular reason to expect them to be on the same order of magnitude. To do more of an apples-to-apples comparison, you'd probably want to consider some measure of tidal forces (e.g., the Weyl tensor) at some distance from the system. In realistic observations, we wouldn't even use the same apparatus to try to detect both effects. For a system whose gravitational waves we might hope to be able to detect, we would be looking for an AC component of the tidal forces, whereas any DC component of such a system would be masked by the presence of all the other masses in our galaxy.
     
  6. Dec 23, 2009 #5
    So what is meaning of "AC" and "DC" components of tidal force?

    Also, can you answer the gravity waves question quantum mechanically: Gravity waves are real gravitons. Gravity is virtual gravitons. Does the process that produces real gravitons also produce virtual gravitons?
     
  7. Dec 23, 2009 #6

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The same as for an electric current:the oscillating and non-oscillating terms in the Fourier spectrum.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook