Originally Posted by trinitree88

jlh. In a quantum sense, the loss of energy by the photon must be accompanied by the gain elsewhere. When the General relativists get involved they point out that conservation of energy....inviolate in a particle physics lab...can be considered not conserved locally, but is eventually conserved globally (somewhere else...unspecified). It addresses the problem that a free photon traveling in space that is "empty", needs a victim to deliver it's momentum and energy to....hence voiding many arguments that the light is merely tired from it's lengthy journey.
I have pointed out that the representation of a vacuum as "empty" and free of potential "victims" for the photon is itself a false model, based on grade school representations of Bell jars and ringing doorbells. Space is not empty, and while bosons may pass freely through each other, fermions will interact. Nuff said. pete

As Antoniseb said there's "lots of hand waving out there"...I prefer a collision with particles involved, conservation laws upheld, and all species accounted for with numerical algorithms....that's how the LHC is set up.