Hal wrote: > Those are interesting speculations, but I don't think it > really makes sense to imagine travelling between the worlds > of the Tegmark multiverse. > There are no causal connections between them of the type that > would be necessary for an information packet to travel in the > way we normally think of it happening. > > I think David Deutsch had some ideas about time travel in the > MWI going between parallel worlds, but again I didn't think > that could work, physically. Once worlds have decohered, > there are no physical mechanisms for them to interact to any > measurable degree. > > However I do think there are connections between time travel > and the MWI, different from Deutsch's rather simplistic > picture of travel to parallel worlds. > > The big problem with time travel is not so much the > kill-your-father paradox, because as Ben writes this can be > easily dealt with by postulating that only consistent time > travel works. The bigger puzzle then is the apparent > necessity of the universe to be intelligent, for the natural > laws to engage in strategic reasoning at least as advanced > and sophisticated as the intelligent beings whose free will > it is thwarting. > > When a time traveller tries to do something, there has to be > the potential for a sort of back-reaction from the universe > which can interfere with his actions if they would lead to a > paradox. Let's suppose he goes to do something, make a > change in the past which it turns out will be inconsistent > with his memories in the future. Something's going to stop > him. But how does the universe know that this has to be stopped? > It seems that there has to be at least a potential or virtual > universe created in which his actions play out, their > consequences extend through time into the future where the > time traveller departed from, and the inconsistency with his > mental state is detected.
Nature abhors a paradox. The principle of least (or minimal) action appears to prevent inconsistencies; at least according to Novikov et al. http://xxx.lanl.gov/abs/gr-qc/9607063 It's fine for billiard balls going through wormholes, but gets (philosophically at least, if not physically) problematic when applied to objects which like to think they have free will, such as me killing my grandfather. I hate to think that my decisions are reductively determinined by the principle of minimal action (much though my wife might agree). Jonathan Colvin