User:JimJast/Dynamical friction of photons
Dynamical friction of photons (a.k.a. gravitational drag) is an effect observed in astronomy when photons move along other slower moving objects (while faster moving objects don't exist) and interact with them gravitationally. To an observer it looks as if there was some kind of friction between photons and other objects, however the interaction between them is only gravitational, and thus the "dynamical friction". The last part of name comes of course from the fact that objects that suffer this apparent "friction" are photons.
If the movement is along heavy objects moving in the same direction the effect is called slingshot effect and it is used to accelerate space probes while the space probe is flying over a planet moving in the same direction as the probe. Dynamical friction in this case changes sign and becomes acceleration that pushes the probe moving it to a higher orbit, or even eventually pushing it beyond the solar system as it happened in case of Pioneers 10 and 11.
Since it happens in the real world the amount of it may be calculated with almost Newtonian math but decisively Einsteinian physics as it is done here. It turns out that dynamical friction of photons if calculated this way, produces the amount of redshift equal the Hubble redshift observed in our universe and blamed on recessional speed of galaxies in the "expanding universe" (which in this scenario is not expanding any more since its behavior gets explained totally without any assumed expansion).
This redshift is also equal to amount of negative acceleration suffered by cosmic probes Pioneer 10 and 11 flying in two opposite directions in relation to the Sun, while the acceleration of Pioneers 10 and 11 remains officially a mystery and is named officially "Pioneer effect" while it used to be named "Pioneer anomaly". The dynamical friction of photons was not yet recognised officially as showing up in the real world while it is required in any place where sling effect works being a regular Newtonian effect and the "frame dragging" by masses of the universe is an Einsteinian effect cinfirmed by Gravity probe B. Einsteinian effect that previously was considered non existant or at least doubtful.