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General relativity predicts that electromagnetic fields contribute to the stress-energy tensor, and that they therefore have gravitational fields. Kreuzer (1968) did laboratory experiments that were interpreted by Will (1976) as confirmation of this prediction in the case of the static electric fields of nuclei. The precision of the test was later improved by orders of magnitude by Bartlett and Van Buren (1986) based on lunar laser ranging.

But these are all tests of static electric fields in nuclei. Is there any direct empirical test of GR's prediction in the case of electromagnetic radiation? A test of the static case does not trivially imply a test of the radiation case, since the distinction between radiation and non-radiation fields is generally covariant.

The early universe was radiation-dominated. Do CMB observations allow us to probe early enough times so that if GR was wrong about this, discrepancies would have shown up? Radiation-dominated cosmologies are one of the standard closed-form solutions of the FRW equations, but I'm not clear on whether any of the features of these models can actually be tested empirically. The CMB comes from the surface of last scattering, which was at about 400,000 yr. The switch-over from radiation-dominated to matter-dominated happened at about 2,000 yr. Based on those figures, which differ by a factor of about 100, I'd kind of crudely expect that gravitation by radiation would perturb observable features of the CMB by no more than 1% or something...?

How about nucleosynthesis, which probes times as early as 100 s? Is our knowledge of nucleosynthesis too crude to test this prediction of GR?

-Ben

Kreuzer, Phys. Rev. 169 (1968) 1007. I've written a description here: http://www.lightandmatter.com/html_books/genrel/ch08/ch08.html [Broken]

Will, “Active mass in relativistic gravity: Theoretical interpretation of the Kreuzer experiment,” Ap. J. 204 (1976) 234, available online at adsabs.harvard.edu. A broader review of experimental tests of general relativity is given in Will, “The Confrontation between General Relativity and Experiment,” relativity.livingreviews.org/Articles/lrr-2006-3/.

Bartlett and Van Buren, Phys. Rev. Lett. 57 (1986) 21. The result is summarized in section 3.7.3 of the review by Will.

But these are all tests of static electric fields in nuclei. Is there any direct empirical test of GR's prediction in the case of electromagnetic radiation? A test of the static case does not trivially imply a test of the radiation case, since the distinction between radiation and non-radiation fields is generally covariant.

The early universe was radiation-dominated. Do CMB observations allow us to probe early enough times so that if GR was wrong about this, discrepancies would have shown up? Radiation-dominated cosmologies are one of the standard closed-form solutions of the FRW equations, but I'm not clear on whether any of the features of these models can actually be tested empirically. The CMB comes from the surface of last scattering, which was at about 400,000 yr. The switch-over from radiation-dominated to matter-dominated happened at about 2,000 yr. Based on those figures, which differ by a factor of about 100, I'd kind of crudely expect that gravitation by radiation would perturb observable features of the CMB by no more than 1% or something...?

How about nucleosynthesis, which probes times as early as 100 s? Is our knowledge of nucleosynthesis too crude to test this prediction of GR?

-Ben

Kreuzer, Phys. Rev. 169 (1968) 1007. I've written a description here: http://www.lightandmatter.com/html_books/genrel/ch08/ch08.html [Broken]

Will, “Active mass in relativistic gravity: Theoretical interpretation of the Kreuzer experiment,” Ap. J. 204 (1976) 234, available online at adsabs.harvard.edu. A broader review of experimental tests of general relativity is given in Will, “The Confrontation between General Relativity and Experiment,” relativity.livingreviews.org/Articles/lrr-2006-3/.

Bartlett and Van Buren, Phys. Rev. Lett. 57 (1986) 21. The result is summarized in section 3.7.3 of the review by Will.

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