I have read this question:
Every time a photon changes direction, it requires something else to gain momentum in the opposite direction, whether a solar sail or a star bending light by gravity.
Now I sincerely do believe that this answer is only for diffraction, and not gravitational lensing. In the case of diffraction, there is an actual interaction between the particles of the slit and the photons, that changes the photons' angle. In this case, the photon is able to exchange momentum with the particles of the slit.
Though, in the case of gravitational lensing, the photon is simply traveling along a null geodesic, a path in curved space. There is no need nor possibility to exchange momentum with the star that caused the lensing. But the reason I am in doubt is, that although, it is not possible for the photon to interact with the atoms of the star directly, it is possible for the photon to interact with the gravitational field of the star (to maybe exchange momentum).
So there are a few things that come to mind:
in the case of gravitational lensing, the photon moves along a null geodesic, and no need and no possibility to exchange momentum with the star directly
the photon is interacting with the star's gravitational field, and could exchange momenutum
So basically, I am asking whether the star moves (even if a tiny bit) towards the photon that passes by because of the (exchange of the) photon's momentum in the case of gravitational lensing.
- Momentum conservation in gravitational lensing?