# How does the photon interact with the changing spacetime (say when massive object appers nearby)? How does it know how to "bend"?

How does the photon interact with the curved spacetime?

Say, we have the photon travelling through the universe. Then it enters the region of gravitatonal lens (or a curved spacetime created by some moving massive object) The massive object moves in, and it "brings" with it the curved spacetime, right? And this "distortion" spreads immediately and influences the trajectories of all the photons nearby.

So when an object moves in, the photon starts to take a different path following the spacetime curvature.

But HOW does the photon know this?

If we assume 2 things my question will become more clear:

• say that a massive object like a black whole was born instantly out of some other photons collided in this region (its possible in theory)

• the gravitational distortion it brought with it, bended the spacetime immediately (faster then light) AFAIK the "gravitational waves" are instant, FTL

Will the photon travelling nearby start to follow the (newely emerged) curvature immediately?

Anyway how does it know how to react to spacetime change? There is NO physical interaction between the photon and the (newely born) massive object. They do not exchange (virtual) particles like in electromagnetic interactions.

It just works. But how?

• Photons curve space-time since the stress-energy tensor of the electromagnetic field is non-zero. Thus, space and time were deformed before the black hole you describe was born. In fact, this is why there's a black hole in the second instance. Commented Mar 1, 2021 at 9:17
• "And this "distortion" spreads immediately and influences the trajectories of all the photons nearby." No, definitely not, this is the point of relativity!! Commented Mar 1, 2021 at 10:09
• "AFAIK the gravitational waves are instant, FTL." This part isn't true though, that's the whole point of gravitational waves, like EM waves, they take time to travel from source to detection point. Commented Mar 1, 2021 at 11:56