This came up in a discussion in another forum - the context was the gravity of a photon... for which we need quantum gravity. But imagining a pulse of monochromatic light, it was asserted that it's gravity could be well approximated by replacing one of the mass terms in Newton's Law by E/c^2.(adsbygoogle = window.adsbygoogle || []).push({});

I am of the impression that this is dodgy ... ie that means the gravity is spherically symmetrical in all reference frames. But it was asserted that the result can be derived as an approximation from the stress energy tensor derived here. @pervect et al.

I confess I've forgotten how to get from the stress energy tensor to a gravitational field for some observer.

So my question is: can it?

For the sake of an example: if I put an observer mass m at the origin, and a pulse of light energy E passes on trajectory: ##\vec r = (b,0,ct)^T## then the replacement suggests: $$F =\frac{GEm}{(b^2+c^2t^2)c^2}$$

I do recall that the approximation works for a container of light travelling at speed v<<c though.

qv. https://www.quora.com/Does-photons-...energy-generates-gravity/answer/Rick-McGeer-1

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# A Relation of gravity of a pulse of light to E/c^2

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