# Photons traveling in space

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?

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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.