I'm interested in the difference this makes to utilitarianism. I'm also interested in the question of whether the choice between the Copenhagen interpretation and the many-worlds interpretation could be resolved experimentally. If it cannot and it makes a difference to our view about what we ought to do, what then?

I don't have a formal education in quantum physics, so what I say may or may not be correct. Take it with a grain of salt. If someone corrects me, listen to them.

The difference between the two interpretations involves a certain point at which interactions become "macro". If the Copenhagen interpretation is correct, you can prove it by showing that quantum entanglement stops at this point. You can show the many-worlds interpretation to an arbitrarily high certainty by checking with higher and higher "macro" values. The problem is that this gets exponentially harder as the mass increases. You might be able to show a neuron is small enough, but you won't even be able to approach a brain.

As for the difference: in the many-worlds interpretation, every brain state will happen. You'd have to assume the density of the waveform matters. At that point, it's reasonable to assume the size of the brain matters, even if you're just using bigger neurons.

This is really the sort of thing that you work out as a prior. Either the size of the brain matters, or it doesn't. If it doesn't matter, you dogmatically deny many worlds, no matter how much evidence you get. You should imagine getting extremely powerful evidence before you settle on those priors.

If you say there's a certain probability that brain size matters, you run into the problem of incomparable possibilities. If there's a 50% chance of one brain being happy, and a 50% chance of 1kg of brains being happy, what's the expected utility? You can just try and give a weird version of probability and say the expected utility of one brain is the same as a certain mass of brain, but, again, this is a prior. You have to do it without using what you know about brain size.

Well, it sure seems to make a difference what we do. And I guess we can take that probablistic approach?

Thinking about it some more, there are ways to mess with Many Worlds so that there are distinct states. It could be a waveform made of particles, for example. As much of an obvious rules patch as this is, it's still better than waveform collapse.

Eliezer Yudkowsky wrote:If collapse actually worked the way its adherents say it does, it would be:

1. The only non-linear evolution in all of quantum mechanics.
2. The only non-unitary evolution in all of quantum mechanics.
3. The only non-differentiable (in fact, discontinuous) phenomenon in all of quantum mechanics.
4. The only phenomenon in all of quantum mechanics that is non-local in the configuration space.
5. The only phenomenon in all of physics that violates CPT symmetry.
6. The only phenomenon in all of physics that violates Liouville's Theorem (has a many-to-one mapping from initial conditions to outcomes).
7. The only phenomenon in all of physics that is acausal / non-deterministic / inherently random.
8. The only phenomenon in all of physics that is non-local in spacetime and propagates an influence faster than light.

There was a nice discussion of this question on the old felicifia.com forum, but it has since disappeared....

I mention a few points in the bottom half of "Why Maximize Expected Value?" In general, I don't think MWI makes much of a practical difference if you're already committed to expected-value maximization (except to the extent that it highlights issues of infinity, but so do almost all physical theories).

I'm fairly sure Seth has saved the old Felicifia's content somewhere. I'm guessing he's just been too busy to put it back up, but if you email him, he might be able to send you the contents of any specific thread...