The CMB becomes deadly well before these speeds (the rear facing light burns you; the forward facing light nukes you). That's not even accounting for objects in

The CMB becomes deadly well before these speeds (the rear facing light burns you; the forward facing light nukes you). That's not even accounting for objects in space, or the light from stars, which will be amplified according to the Lorentz factor. EDIT: here's a small discussion: https://physics.stackexchange.com/a/196171 https://physics.stackexchange.com/a/196171 There's a whole PBS SpaceTime episode about this, but I can't find it, rn.