rationalskepticism.org

andrewk wrote:I find every-world hypotheses, whether in the quantum-mechanics based form or any other, somewhat morally disheartening. If whenever faced with a moral decision I believe that, regardless of what choice I make, there will always be just as many worlds in which I make the immoral decision, it seems to make it pointless to weigh up ethical choices. In short, there is nothing I can do to make the totality of all worlds any better.

But it doesn't actually say that - some things would occur in more worlds than others. This is because any "high level" outcome like this is going to be the result of a long sequence of quantum events. Some kinds of world would result from a big proportion of sequences of quantum events and would be "common". Others would only result from a small proportion of quantum events. Are there any worlds in which you turn into a serial killer? Well, it is hard to see how we can rule it out. We can imagine a small, random change to your brain which nudges it just a bit towards serial killing, and another one, and still another. However, how many such nudges do we need, and if we are allowed to have a lot of such nudges, and then compare the number of resulting worlds with the ones in which the same number of events occurs and you stay sane, it is probably a very small proportion. In MWI, it remains the case that some outcomes are more likely than others. The difference is that even the unlikely outcomes still happen - they just happen in fewer worlds. A good way of looking at this is asking yourself what the chance is, in the absence of MWI, that you will do immoral things. If you suddenly found out that MWI is true you should merely convert that to "a proportion of future worlds in which you do immoral things". Another point - this moral issue, if it is one, is still there if MWI is not true! It just has a purely probabilistic nature without the "many worlds" issue.

Teuton wrote:

Paul Almond wrote:To put it simply, it proposes that the basis for "reality as we know it" is the quantum wave function, and that this has real existence.

Functions are abstract mathematical entities, so the quantum wave function cannot be the basis of physical reality, which must itself be a concrete physical entity (a quantum field?). It can at most represent that basis mathematically.

That sounds like a semantics issue to me, really - about whether we can use the word "function" or whether we are being lazy with semantics and ontology: what is being claimed by MWI is that "something" exists and that the "something" is described by the quantum wave function, and that that "something" can explain both quantum mechanics and the appearance of separate events to us. The term "quantum wave function" is used by both advocates and detractors of MWI without too much issue - and I think they all understand what they mean.