Consider instead of time traveling from time T’ to T, that you were given a choice at time T which of the universes you would prefer: A or B. If B was better you would clearly pick it. Now consider someone gave you the choice instead between B and “B plus A until time T’ when it gets destroyed”. If A is by itself a better universe than nothing, surely having A around for a short while is better than not having A around at all. So “B plus A until time T’ when it gets destroyed” is better than B which in turn is better than A. So if you want your preferences to be transitive you should prefer the scenario where you destroy A at time T’ by time traveling to B.
There are two weaknesses in the above: perhaps A is better than oblivion, but A between the times T and T’ is really horrible (ie it is better in long term but negative value in short term). Then you wouldn’t prefer having A around for a while over not having it at all. But this is a very exceptional scenario, not the world goes on as usual but you go back and change something to the better that we seem to be discussing.
Another way this can fail is if you don’t think that saying you have both universes B and A (for a while) around is meaningful. I agree that it is not obvious what this would actually mean, since existence of universes is not something that’s measurable inside said universes. You would need to invent some kind of meta-time and meta-universe, kind of like the simulation scenario EY was describing in the main article. But if you are uncomfortable with this you should be equally uncomfortable with saying that A used to exist but now doesn’t, since this is also a statement about universes which only makes sense if we posit some kind of meta-time outside of the universes.
Consider instead of time traveling from time T’ to T, that you were given a choice at time T which of the universes you would prefer: A or B. If B was better you would clearly pick it. Now consider someone gave you the choice instead between B and “B plus A until time T’ when it gets destroyed”. If A is by itself a better universe than nothing, surely having A around for a short while is better than not having A around at all. So “B plus A until time T’ when it gets destroyed” is better than B which in turn is better than A. So if you want your preferences to be transitive you should prefer the scenario where you destroy A at time T’ by time traveling to B.
There are two weaknesses in the above: perhaps A is better than oblivion, but A between the times T and T’ is really horrible (ie it is better in long term but negative value in short term). Then you wouldn’t prefer having A around for a while over not having it at all. But this is a very exceptional scenario, not the world goes on as usual but you go back and change something to the better that we seem to be discussing.
Another way this can fail is if you don’t think that saying you have both universes B and A (for a while) around is meaningful. I agree that it is not obvious what this would actually mean, since existence of universes is not something that’s measurable inside said universes. You would need to invent some kind of meta-time and meta-universe, kind of like the simulation scenario EY was describing in the main article. But if you are uncomfortable with this you should be equally uncomfortable with saying that A used to exist but now doesn’t, since this is also a statement about universes which only makes sense if we posit some kind of meta-time outside of the universes.