What might be the practical implications of post-Everett quantum mechanics for intelligent moral agents? One practical implication of the reality of other macroscopic branches might be to compel a systematic reassessment of our notions of "acceptable" risk. Recognition of the freakish unlikelihood of various desired outcomes does not stop most of us playing the National Lottery; but the converse doesn't hold. Thus we are accustomed to thinking that various nasty scenarios are of negligible likelihood, and even vanishingly small possibility; and then disregarding them altogether in the way we behave. Yet if a realistic interpretation of quantum mechanics is correct, then all these physically possible events actually happen, albeit only in low-density branches of the universal wavefunction. So one should always act "unnaturally" responsibly, driving one's car not just slowly and cautiously, for instance, but ultracautiously. This is because one should aim to minimise the number of branches in which one injures anyone, even if leaving a trail of mayhem is, strictly speaking, unavoidable. If a motorist doesn't leave a (low-density) trail of mayhem, then quantum mechanics is false. This systematic re-evaluation of ethically acceptable risk needs to be adopted world-wide. Post-Everett decision theory should be placed on a sound institutional, research and socio-economic footing, not just pursued by responsible Everettistas on an individual basis. The ramifications of the Everett interpretation of quantum mechanics are ethically too momentous for purely private initiative. Our moral intuitions fail because natural selection equipped us to deal with a classical world rather than a Multiverse. Human beings tend to discount "remote" risks by treating the probability of such events as zero. Ultimately, perhaps ethical decision-making should be performed by quantum supercomputers doing felicific calculus across world-branches; quantum ethics may be computationally too difficult even for enhanced post-human brains. For it's worth stressing that Everett's relative state interpretation of quantum mechanics doesn't propose "anything goes". The branching structure of the Multiverse precisely replicates the probabilities predicted by the Born rule. There are no branches supporting civilisations in the middle of the Sun. Nor are there any branches where, for instance, one of the world's religions is true (as distinct from believed to be true): Everett is not a theory of magic. But the universal wavefunction does encode hell-worlds beyond our worst nightmares, albeit at very low density.
I'm going to watch Predestination now ... oh, and I missed Doctor Who, and the spin-off Torchwood in my list of what I liked.
