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According to Special Relativity, there is no preferred inertial reference frame. And shifting reference frames can cause blue shifting or red shifting of photons.

And according to General Relativity, photons are both affected by the shape of space time as well as have an effect on space time such that a Kugelblitz, a black hole formed from nothing but photons is possible.

But then, for every photon, shouldn't there be some inertial reference frame where the photon is sufficiently blue shifted that it has enough energy confined within a small enough space that it would be a black hole?

And since it appears to not be the case by all empirical evidence (my room isn't filled with black holes when I turn on the light), how does adding additional photons let one create a Kugelblitz?

Shufflepants
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1 Answers1

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As your question indicates, if gravity depended only on energy then it would be frame dependent, which would conflict with relativity (or at least produce very odd effects like the ones you describe). This was one of the motivations for Einstein to come up with his theory of gravity, general relativity.

In fact gravity (spacetime curvature) is produced not only by energy, but by the whole stress-energy tensor, which includes energy, momentum, pressure, and other terms, and which is frame independent. When changing frames, any change to the energy part of the stress-energy tensor is offset by changes to other parts of the tensor.

Newton's law of gravity is only an approximation, valid when the non-energy terms of the stress-energy tensor are tiny. For light this is not the case, and to properly deal with the gravitational effects of light one needs to use the Einstein field equations, not Newton's.

Eric Smith
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