Assumptions: 1] I'm an observer at inertial rest 2] Light is going c 3] Gravitational interaction can't exceed c I'm going to use "photon" and "graviton" as shorthand to pose the questions initially. Maybe correcting the form or assumptions of the questions will provide the answer... The photon is moving past me at c, and if another photon was to be following the first one, I would not observe it catching up to the first one. If the photon could send a graviton backwards to a mass behind it, that mass might receive it, but my observation must be that the mass couldn't send a graviton that could catch up to the photon. Likewise, the photon might intercept a graviton from a mass in front of it, but my observation must be that the photon could never send a graviton forward to a mass in front of it. This makes the photon appear to my observation to have the inability to complete an exchange with masses either behind or in front of it, even though the photon is both sending and receiving gravitons, there is not a "proper" exchange. The photon is receiving from the mass in front and sending to the mass behind, and neither of the masses is enjoying a complete exchange. Questions: 1] Does an exchange interaction require sending and receipt completion for both parties to the exchange? 2] How do photons exchange gravitons with masses directly behind and in front if both photons and gravitons go c? 3] Do light cones apply here? Light cones have their causal regions in their interiors, but what I'm imagining is something like a light cone that describes the photon's interaction with gravitons... the interiors would be defining the regions where the gravitons from masses could (front cone) and could not (back cone) ever intercept the photon...? Thanks for any assistance on this.