This is worded too strongly but I do not know why it is being down voted. For those who feel any interpretation is warranted, many worlds is entirely credible and cleaner in some respects than Copenhagen (no collapse).

"Many Worlds is a theory that's cheap on assumptions but expensive on universes."

Sure, but universes are cheap. There were those who said that the theory that "nebulae" were distant galaxies should be dismissed according to Occam's Razor, because it meant vastly multiplying the number of galaxies, stars etc. in the universe. But it was a simpler theory than the idea that nebulae were some new kind of object, and it turned out to be correct.

I'm with you. I think Occam's razor trumps a discomfort over the scale of the multiverse. The latter is just a failure of imagination.

It is clear to me that penalizing assumptions rather than "stuff" has worked out well in the past. It isn't clear to me that we shouldn't penalize stuff (or in this case, universes) at all. The fewest-assumptions version of MW seems to lead to such an explosion of "stuff" that there might well (in our optimal version of Occam's razor) exist some term that doesn't show up in evaluating nebulae, but dominates in considering MW. That said, applying a meta-Occam's-Razor for now should leave us with the focus on assumptions, and thus I think with MW.

As someone said elsewhere, the problem there is philosophical - I like the idea, it makes a lot of intuitive sense, but if I can't visit any of the many worlds then it's not really falsifiable. Also, if there was ever a reason to break out Occam's razor it's this one - conjuring entire new universes into being for every available variation at every step of the Planck time seems awfully hairy. I appreciate that it's axiomatically simpler but this seems like an awful lot of information overhead. It would be nice if there were a clearer description of the multiverse in which the many worlds exist in superposition; I'll try Deutsch's Fabric of Reality book to give it another shot.

Some people call it the many-worlds interpretation. I just call it quantum mechanics. An important part of this view is that the scientist running an experiment is not an external observer but he is subject to the same rules of physics. Just as an electron can be simultaneously in many states, so can the human observer.

The interpretation I subscribe to is that measurement is a situation of entanglement. In the electron slit experiment, when you observe the electron traveling through lets say the right slit, subsequent behavior of the electron and the "memory" in the person will be correlated.

I don't know how memory and the brain really work here. I think that is the real question - the mechanics of the brain and an observation.

Regarding Occam's razor, to me this seems like a much simpler explanation of observation than having the wavefunction of the entire universe collapsing when a random human is in the loop.

The trouble with many worlds, for me at any rate, is say you have a mirror where the photons go one way 94% of the time, the other 6% of the time. How does that work? 94 worlds of one type and 6 of the other? Everett did some hand wavy, we assign a probability P to each outcome stuff but you still wonder how on earth that works physically. If the universe branches with one version of you seeing it go one way and one the other is one of you more existy or what?

The only thing that's fundamental, that's physically "real", is the wavefunction. So you have your mirror and your photon and you, and all that's described according to some wavefunction |P(t)>, evolving according to the laws of quantum mechanics.

Now we observe that after the photon passes through the slit, we can decompose the function as |P(t)> = sqrt(0.94)|Q(t)> + sqrt(0.06)|R(t)> where Q and R are independent, each individually evolving according to the laws of quantum mechanics. This is already derived rather than fundamental, but the phenomenon is real, because the causality relation is real. Our neurons under Q (and of course a "neuron" isn't really fundamental, it's an interpretation of a particular group of particles behaving in a particular way, i.e. of particular aspects of the wavefunction) have no effect on our neurons in R and vice versa.

All that is physically real; the only remaining question is what we should expect to subjectively experience. Since our neurons in Q have no effect on our neurons in R and vice versa, it seems like we'd experience either being in Q or being in R. With what probability? Well, whatever it is it had better be conserved; it makes no sense to say that we'd experience R with 4% probability in 5 minutes and then 8% probability in 10 minutes. If we send another photon through in Q, splitting the wavefunction further into |S(t)> + |T(t)> + |R(t)> where Q = S + T, then our subjective probabilities should be such that the probability we find ourselves in S or T = the probability that we found ourselves in Q before sending the second photon.

What's the probability-like quantity that's conserved by quantum evolution of a system? Why, it's the norm, ||P>|^2. I guess that's what we'd expect to be the subjective probability then.

If you want a rigorously defined and clear version of many worlds, I recommend http://arxiv.org/pdf/0903.2211.pdf

It uses the (uncollapsed) wave function to have a mass density on physical space. There are many realities, if you like, implied, but they all move about in a self-consistent way. The analogy is that it is as if two tv channels were overlying each other. You can follow each separately if you watch the evolution, but at any one time it would look a bit like a mess.

The problem with just having the wavefunction and nothing else is that it is not at all clear what the probabilities would be about. You need something that the theory describes that makes contact with our 3 dimensional experience.

I thought there are 94 you (and universes) that seen one result and 6 that seen the other.

Then there are irrational probabilities :)