I have read online that light can produce a weak gravitational field (for example antiparallel beams should, in principle, attract weakly).

This made me wonder if light can produce minute gravitational waves?

Even if the waves were extremely weak (no disregarding of those high order terms in the applicable equation, whatever equation that may be), could the gravitational waves dissipate energy (on the order that is expected for cosmological redshifts) when light travels across cosmological distances?

I was thinking about the debunked tired-light hypothesis regarding the cosmological redshift, and I wondered if anyone has considered a mechanism whereby gravitational waves dissipate energy.

  • $\begingroup$ Unless somebody does the experiment there is no way of knowing. One can, however, apply the known theory to it (be it right or wrong) and the result of that is that cosmological redshifts are not the result of gravitational interaction of light with itself, it would be way too weak for that. Ultimately, of course, the redshift is the result of the interplay between spacetime and gravitating mass and one can use the observed redshifts to estimate the total amount of gravitating mass-energy of the universe (both normal and dark matter included). $\endgroup$
    – CuriousOne
    Sep 20 '14 at 14:17
  • $\begingroup$ Related: physics.stackexchange.com/q/14064/2451 and links therein. $\endgroup$
    – Qmechanic
    Oct 20 '14 at 20:24
  • 3
    $\begingroup$ @CuriousOne: Unless somebody does the experiment there is no way of knowing. Not true. General relativity makes unequivocal predictions about this kind of thing, and it is a well tested theory. In particular, the gravitational fields made by light need not be weak, and we have direct evidence of this. The universe was radiation-dominated up until it was about 50,000 years old. This period includes the period of big-bang nucleosynthesis (BBN), so empirical data on BBN are a test of these cosmological models. Therefore we can confidently use GR to address this type of question. $\endgroup$
    – user4552
    Oct 20 '14 at 21:59
  • $\begingroup$ @Bencrowell: There is a real difference between assuming that a relatively trivial extrapolation of a theory into an untested parameter range is valid and actually doing the measurement to confirm that it's actually valid. I think it's a pretty good guess that GR gives the correct answer, but it's important to explain the difference between applying theory and confirming theory. $\endgroup$
    – CuriousOne
    Oct 20 '14 at 23:42

The class of spacetimes that model the gravitational effects of massless radation are called "pp-wave spacetimes". They have a lot of interesting properties that make them worth studying. However, they do not dissipate energy, they transport it in the same way that other waves transport energy.

However, if you would like to model the cosmic microwave background radiation then a null dust spacetime will probably be more reasonable than a pp-wave spacetime. These two classes of spacetimes are closely related since any pp-wave spacetime can be interpreted as a null dust spacetime.


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