 Imagine that the Andromeda galaxy, which is about two million light years or \(2\times 10^{19}\) kilometers from Earth, is at rest with respect to Earth. On Earth two friends walk past each other, Alice walking along the Earth-Andromeda line towards Andromeda, Bob walking along that line but away from Andromeda. Each is walking at a comfortable pace, say 4 km/hour. One can calculate that their planes (or spaces) of simultaneity at the instant at which they pass each other on Earth (Call the event of their meeting \(\mathbf{O})\) intersect the history or world line of Andromeda about 5 ¾ days apart. (Call these two events \(\mathbf{A}\) and \(\mathbf{B}\), respectively. We are idealizing Andromeda as a point, for the purpose of this example.) Imagine, finally, that during this 5 ¾ day period between \(\mathbf{B}\) and \(\mathbf{A}\) a momentous thing happens. The Andromedeans launch a space fleet aimed at invading Earth.