 To make the exposition easier, let us add to the story of the Andromedean invasion a fourth “observer”, Ted, who is at rest with respect to Earth (and so also Andromeda) at the spot where Alice and Bob meet. Ted too defines an inertial frame, and there is a point at Andromeda (We can call it \(\mathbf{D})\) that (in Ted’s frame) is simultaneous with the meeting of Alice and Bob and Ted. To make our exposition easier still, let us suppose that Alice and Bob and Ted set their clocks to read 0 at the instant at which they all  meet.[10]  Let us focus on \(\mathbf{D}\).