# Photons traveling in space

1. Feb 13, 2007

### MeJennifer

In several posting in Special & General Relativity it is suggested that photons influence the curvature of space-time.

Now, if I understand QM correctly, a photon does not posses a definite momentum, unless it interacts. So it appears that between the times of interactions it evolves in state space but not in real space.

So then how could we possibly say that photons influence the curvature of space-time while traveling?

What am I missing?

Last edited: Feb 13, 2007
2. Feb 13, 2007

### anantchowdhary

A photon still has energy whether it is interacting or not.So it should affect the curvature of space-time anyways isnt it?Tell me if im wrong.xD

3. Feb 14, 2007

### masudr

The electromagnetic field has an associated stress-energy tensor, see this Wikipedia article. If we are solving the Einstein equations and there is an EM field present, we must plug in the above tensor as $T_{ab}$ in addition to whatever dust there may be.

While I do not know the exact effect a single photon will have (since an EM-field is made up of many photons), classically the above approach is used.