I'm assuming that ben_w is referring to the blue shift in the CMB that you will see in front of you. And pair production is the process of turning sufficiently high energy light into matter.
But I've never given thought to the fact that for some observers the microwave background are low energy photons, and for others, they could be high enough in energy to produce electron-positron pairs. Seems like another one of those relativity related paradoxes. Only fast-moving observers (relative to the CMB) are seeing constant pair production from the CMB photons?
> Seems like another one of those relativity related paradoxes. Only fast-moving observers (relative to the CMB) are seeing constant pair production from the CMB photons?
Both observers agree about pair production.
Pair production obeys conservation of momentum and energy — although the equations allow both to be conserved with two photons interacting, the practical effect here is CMB photons interacting with the spaceship hull: the observer in the CMB-rest-frame will see very low energy photons doing this with a high-velocity hull, and the spaceship frame will see very high energy photons doing this with a stationary (to them) hull.
That said, I'm told that accelerating observers can, during the acceleration, disagree about what particles exist around them: https://en.wikipedia.org/wiki/Unruh_effect
(Hmm, and now I'm wondering if the Unruh effect is really as described — if different quantum fields contribute at different rates, or have cut-offs at different scales, that would automatically also create something that, if you squint, will look a bit like an approximation of MOND… sadly, this is beyond my competence at this time).
I was under the (probably mistaken) assumption that high enough energy photons could spontaneously turn into an electron and positron. But it is sounding like you need at least two photons to do that. And maybe those photons can't be traveling in essentially the same direction for that to occur? Like the cross products of the momentum of each photon needs to be above a threshold or something?