I’ve been re-reading David Deutsch’s The Fabric of Reality, which is a polemic for the many-worlds interpretation of quantum mechanics (or, taking the maths seriously, as Dr. Deutsch would contend).
Consider past-directed time travel. The standard argument here is the Grandfather Paradox: if you could journey back in time, you could kill your own grandfather, thereby preventing your father being conceived and therefore yourself from existing. So then you couldn’t go back in time, could you?
Incidentally, it’s always puzzled me that the grandfather gets it. Why not the father? In both cases timing is everything as you have to get to the eponymous male before he commits the act of insemination. It’s no good killing your grandfather yesterday.
Anyway, the standard argument makes backwards time-travel a logical impossibility. But given the problematic nature of “time” itself, this seems a strong statement to make. Perhaps past-directed time travel is merely contingently impossible like travel faster than light, or maybe it’s just technically hard?
Deutsch’s great contribution is to show the logical consequences of admitting that past-directed time travel is a logical possibility. Given a propensity to consequentially alter one’s past, the past one travels to cannot be the past from which one has emerged.
No, the logical possibility of past-directed time travel implies of necessity the kind of multiverse which Deutsch is lobbying for. You end up in another past which looks much like your own, but which differs henceforth in the consequences of your arrival.
I like his slogan: “Time does not flow. Other times are just special cases of other universes (in the multiverse)” [p. 288].
Amazing how far you can get from pure logic alone.