# 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