Good point. I may be thinking about this wrong, but I think Deutsch self-consistent time travel would still vastly concentrate measure in universes where time travel isn’t invented, because unless the measures are exactly correct then the universe is inconsistent. Whereas Novikov self-consistent time travel makes all universes with paradoxes inconsistent, Deutsch self-consistent time travel merely makes the vast majority of them inconsistent. It’s a bit like quantum suicide: creating temporal paradoxes seems to work because it concentrates your measure in universes where it does work, but it also vastly reduces your total measure.
That’s why it’s not usually called “Deutsch self-consistency.” It’s not supposed to be a filter on legal universes, but a dynamic rule that each initial condition does lead to a consistent universe. The resolution of the grandfather paradox is a 50-50 superposition of the universe where you are born and leave and the universe where you appear, kill your grandfather, and are never born. You could say that it filters out the 80-20 superposition, but that’s like saying that Newton’s self-consistency principle filters out universes that don’t obey his laws. (Well, maybe that’s Lagrange’s self-consistency principle...)
I’m not sure that comparison works. I can pick any initial universe-configuration and time-evolve it under Newtonian gravitation; a solution will always exist. But if I time-evolve initial conditions under laws that allow backwards time travel, it’s not clear to me that there necessarily exist any solutions. Maybe the Deutsch law forces the superposition to be 50-50, but the other physical laws force it to be 80-20. It may be that the Deutsch law is just a logical consequence of the other physical laws, in which case I think you’d be right. (This all with the caveat that I don’t really know physics, so I’m likely completely wrong.)
I can’t really think about this without having some idea of how it’s chosen which universes are real.
Good point. I may be thinking about this wrong, but I think Deutsch self-consistent time travel would still vastly concentrate measure in universes where time travel isn’t invented, because unless the measures are exactly correct then the universe is inconsistent. Whereas Novikov self-consistent time travel makes all universes with paradoxes inconsistent, Deutsch self-consistent time travel merely makes the vast majority of them inconsistent. It’s a bit like quantum suicide: creating temporal paradoxes seems to work because it concentrates your measure in universes where it does work, but it also vastly reduces your total measure.
That’s why it’s not usually called “Deutsch self-consistency.” It’s not supposed to be a filter on legal universes, but a dynamic rule that each initial condition does lead to a consistent universe. The resolution of the grandfather paradox is a 50-50 superposition of the universe where you are born and leave and the universe where you appear, kill your grandfather, and are never born. You could say that it filters out the 80-20 superposition, but that’s like saying that Newton’s self-consistency principle filters out universes that don’t obey his laws. (Well, maybe that’s Lagrange’s self-consistency principle...)
I’m not sure that comparison works. I can pick any initial universe-configuration and time-evolve it under Newtonian gravitation; a solution will always exist. But if I time-evolve initial conditions under laws that allow backwards time travel, it’s not clear to me that there necessarily exist any solutions. Maybe the Deutsch law forces the superposition to be 50-50, but the other physical laws force it to be 80-20. It may be that the Deutsch law is just a logical consequence of the other physical laws, in which case I think you’d be right. (This all with the caveat that I don’t really know physics, so I’m likely completely wrong.)
I can’t really think about this without having some idea of how it’s chosen which universes are real.