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Are gravity waves modeled as being lossy?

  1. Oct 23, 2015 #1


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    Not much more to my question than the title - I imagine that if they do exist they are probably lossy, else with all the potential wave generators that have been in existence generating waves for billions of years there would be lots of waves bouncing all over the place and they would be readily detectable.

    By lossy I mean do they lose energy as they travel through whatever medium they travel through, I guess somehow increasing entropy as they lose energy.
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  3. Oct 23, 2015 #2


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    I don't know what you mean by "bouncing all over the place". What would they be bouncing off?
  4. Oct 23, 2015 #3


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    The "medium", to the extent there is one, is spacetime. Gravitational waves do not lose energy as they travel through spacetime, except in the obvious sense that they spread out with distance. They only lose energy if they travel through matter and deposit energy in it (for example, by inducing vibrations in the matter that heat it up).

    Note that this is not unique to gravitational waves; electromagnetic waves don't lose energy traveling through vacuum either. (But see below.)

    Over billions of years, you have to take into account the expansion of the universe, which does cause waves (electromagnetic and gravitational) to redshift. That's why the CMBR is at a temperature today of only 2.7 K, whereas it was at a temperature of several thousand degrees when it was first emitted. This can be thought of as a "loss of energy" due to the expansion of the universe, but you have to be careful how you use that heuristic.

    Also, there aren't anywhere near as many "potential wave generators" for gravitational waves as there are for electromagnetic waves. EM waves are dipole radiation, so just about any change in a charge distribution will radiate EM waves. Gravitational waves are quadrupole radiation, so only a much more limited set of changes in mass distribution will radiate gravitational waves, and the waves will in general be very weak unless the changes in mass distribution are very rapid and violent.

    For example: binary pulsar systems radiate both gravitational and EM waves. (Hulse and Taylor won a Nobel Prize for making the measurements that showed that one particular such system was radiating gravitational waves.) We can easily detect the EM waves from the pulsars; that's how pulsars were discovered in the first place, by their precisely timed EM radiation. But we can't directly detect the gravitational waves at all; the only way we can detect that they are being emitted is indirect, by measuring the slow changes in the orbital parameters of the pulsars due to the wave emission. (Hulse and Taylor's measurements were taken over several decades.) Binary pulsars are neutron stars orbiting each other fairly close together, so they are about as compact as objects can get without being black holes. Yet their gravitational radiation is too weak for us to detect directly.
    Last edited: Oct 23, 2015
  5. Oct 23, 2015 #4


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    Yes, poor choice of words.

    Peter - thanks for the background - very helpful in tuning my intuition. I was thinking that gravity wave emitters would be order-of-magnitude as common as EM emitters, looks like I was way off on that.
  6. Oct 24, 2015 #5


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    Advanced LIGO would like to talk to you! :P
  7. Oct 24, 2015 #6
    A point of pedantry I know, but the name "Gravity Waves" is already taken. For clarity, I would advise referring to "Gravitational Waves".
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