# Light and Mass

1. Jun 1, 2006

### mdj

I have a question that has been bugging me since my Special relativity course... (uhm that would be two years now...)
We all know that matter produces a gravitywell that bends light. That has been shown lots of times, in fact astrophysics would be a bit dull if that didn't happen... But how about the other way around? Can light curve space-time? and create a gravity well?
If so has it been detected? how much would all those cute little photons "in-transit" between stars contribute to the space-time curvature? I guess that has all long been thought of (could be ironic though if dark matter turned out to be light )

Anyway, would someone please enlighten me?

Thanks :)

2. Jun 1, 2006

### mikeu

Yes, the energy in a photon does curve spacetime, although really not very much :). The stress energy tensor on the right-hand side of Einstein's equation $G_{\mu\nu}=8\pi T_{\mu\nu}$ describes how the existence and flow of both matter and energy curve spacetime. I have no idea if this has ever been measured, although I would have to guess not due to how little a photon would actually bend spacetime and how short a time it would stay in any one place. Perhaps a laser would have enough of an effect in a really sensitive lab setup...

3. Jun 1, 2006

### pervect

Staff Emeritus
There's no question that light contributes to the stress-energy tensor, and hence gravitation.

Quantifying how much is a bit tricky. You might take a look at

https://www.physicsforums.com/showthread.php?t=116769

This gives some results for what happens if you have a "box" of light, which is a simple static case.

The detailed results for this case are not particularly intuitve, because pressure causes gravity.

You might also want to check out the "radiation dominated" era of cosmology. In the early history of the universe, cosmology was dominated by radiation - most of the gravity in the universe was due to "light".

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