I have been thinking about the relevance of the many-worlds interpretation of quantum mechanics to ethics.

Brian Tomasik seems to have the idea that it is still rational for you to believe that there will be some connections between the intentions you form and how things go in the different branches of the multiverse which you can affect by your decision, similarly to the situation with versions of determinism with just one universe. But I have been wondering whether this is really the case. Because it seems to me that in some branches of the multiverse there might be quantum fluctuations which might prevent your intentions from being efficacious, so that really you cannot rationally plan to make things as good as possible in the branches of the multiverse that you can affect, but only in the branch of the multiverse which "your consciousness" will follow (whatever that might mean).

This is a rather vaguely-expressed worry. I am just concerned that the whole situation with the nature of rationality might be different in the many-worlds interpretation than on other versions of determinism.

Let me know whether this makes any sense.

I thought the Many-Worlds Interpretation explicitly states that the worlds do not interact. Thus, there's nothing you can do, even counterfactually, to reduce suffering in the other worlds. It's like trying to reduce suffering in the past.

David Pearce's essay, 'Quantum Ethics', discusses some of the moral implications of the Many-Worlds interpretation of quantum mechanics. The particular worry you have is not explicitly addressed there, though.

You could acausally change other worlds from an EDT point of view by doing things that correlate with good things happening in other worlds. For example, in Parfit's hitchhiker, you might decide to pay the guy on the basis that it means that he will have picked you up in a higher portion of universes.