The Everettian Doomsday Argument

If the most familiar overlapping version of Everettian quantum mechanics (EQM) is correct, then we are constantly branching into multiple people. The corresponding reconvergence is rendered effectively impossible by decoherence. This asymmetry between splitting and reconvergence generates a disparity in numbers between young and old temporal parts of people. To every newborn infant, there correspond a multitude of teenagers, and to every teenager, there corresponds a multitude of geriatrics. Thus the vast majority of temporal parts of people, spread out over the entire Everettian multiverse, are closer to the ends of the lives of the people of which they are a part of than they are to the beginnings of the lives of those people. This consequence gives rise to the Everettian Doomsday Argument.

The original Doomsday argument aims to show that the end of the human race may be sooner than we would initially have thought. Roughly, it assumes that we are a member of the human race typical with respect to birth rank – an ordering from the first human to be born to the last – and moves to the conclusion that we should expect the human race to end within a couple of hundred years, since otherwise our birth rank would be surprisingly low. The Everettian doomsday argument aims to show that death may come sooner than we would initially have thought. Roughly, it assumes that we are a member of the set of temporal parts of a branching person typical with respect to birth distance­ – an ordering from the temporal part closest to birth to the furthest – and moves to the conclusion that we should expect that we are much closer to death than to birth, since otherwise our birth distance would be surprisingly low.

I won’t assess the argument here. Those who think there is a problem with the original Doomsday argument may well find that the objection it carries over to the Everettian Doomsday argument. And some may baulk at applying the randomness assumption to the selection of observer-moments, rather than just to the selection of observers. What I think is striking is that the diverging version of EQM, which I’ve defended in print, completely undermines the Everettian Doomsday argument. If diverging EQM is correct, then there are just as many early temporal parts of a given person as there are late temporal parts. Is this is a reason to prefer divergence to overlap, or just another reason to reject the Doomsday argument?

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The Everettian Doomsday Argument