 However, if formula_23 then formula_24 and Alice will receive the message back from Bob before she sends her message to him in the first place.As an example, imagine that Alice and Bob are aboard spaceships moving inertially with a relative speed of 0.8"c".At some point they pass right next to each other, and Alice defines the position and time of their passing to be at position "x" = 0, time "t" = 0 in her frame, while Bob defines it to be at position "x′" = 0 and time "t′" = 0 in his frame (note that this is different from the convention used in the previous section, where the origin of the coordinates was the event of Bob receiving a tachyon signal from Alice).