It seems my comment was a bit too subtle for you. Infinite uncountable infinities of universes is not parsimonious.

Pretend that you're talking to someone who doesn't see many-worlds as an obvious corollary of the current QM theories. What experiments do you propose to indicate the simultaneous existence of e.g. a universe in which the photon chose Slit A and another in which the photon chose Slit B?

Your appeal to BitII fails because that concerns e.g. conservation laws that are regularly observed to be true, and which therefore can be assumed true when a particular situation makes observation impossible. Many-worlds has never been shown by experiment, so it is not eligible for such treatment.

First, a hint about where I'm coming from: http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ (I recommend you read the whole thing, it's interesting, insightful, and even fun.)

We already agree that there is no experimental difference between collapse/Copehagen interpretation, and the Many Worlds interpretation. In the name of what are you demanding experimental evidence for one interpretation, and not the other?

> Infinite uncountable infinities of universes is not parsimonious.

You need to remember that those universes are in no way postulated. They are derived —from the equations. So you can't use that impressive infinity to claim that the Many World interpretation would somehow have a higher Kolmogorov complexity than Copenhagen.

Many Worlds just takes the equations and run with them. It's Copenhagen that makes additional assumptions by manipulating the results of the equations: They're not real, or there's a collapse… Either way, that's an additional hypothesis on top of those equations (which by the way have massive amounts of experimental evidence behind them, and are accepted by everyone as the current best guess). That additional hypothesis is not very parsimonious, don't you think?

(You have to remember how Occam's razor really works. From the Wikipedia, "The principle states that among competing hypotheses that predict equally well, the one with the fewest assumptions should be selected." As a simple matter of fact, Many Worlds makes strictly fewer assumptions than Copenhagen. Parsimony is not measured by the size of the universe predicted by the theory. Parsimony is measured by the number of core assumptions. Not the same thing at all.)

To get a feel of how utterly ridiculous collapse postulates are (in 20/20 hindsight, I don't want to make fun of physicists), I must quote Eliezer Yudkowsky: http://lesswrong.com/lw/q6/collapse_postulates/

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.

WHAT DOES THE GOD-DAMNED COLLAPSE POSTULATE HAVE TO DO FOR PHYSICISTS TO REJECT IT? KILL A GOD-DAMNED PUPPY?

I'll read that eventually, but I won't be surprised if it turns out to be like much of the rest of LW: a sophisticated, entertaining, ultimately unconvincing apology for EY's particular brand of mysticism [EDIT:], layered over many perfectly cromulent observations about probability[/EDIT]. I don't need the truth so badly that I would swallow the best truth-substitute I can find. I'm perfectly content to categorize something as a "known unknown".

If I were to stipulate that the Copenhagen interpretation(s) is (are) silly philosophical daydreaming, could you do the same for many-worlds?

> EY's particular brand of mysticism

Funny how different people perceive his writings. To me, most of what he writes feels obvious —at least in retrospect. "Obvious" doesn't feel very mystic to me. I do understand however how many people would be distrustful of his casual writing style. He tends to sound like a lowly blogger, not like a respectable academic. (Personally, I don't care for those status signals.)

> I'll read that eventually, but I won't be surprised if it turns out to be like much of the rest of LW

Hmm, if you're already familiar with this material, then don't bother just yet. Start with the first few chapters of E.T. Jayne's Probability Theory: the Logic of Science. That's more basic, less crazy sounding, and more generally applicable.

Now I don't exactly know how physicists deal with quantum mechanics. I asked one, and he didn't even bother with any interpretation, sticking with the observable consequences of the equations (a more prudent attitude than either Copenhagen or Many-World).

Something however bothers me deeply: insisting on calling amplitudes "probabilities", while they're anything but. That only makes teaching harder. Seriously, I was tempted to mass delete every occurrences of "probability" from the "Probability Amplitude" article in the Wikipedia.

> If I were to stipulate that the Copenhagen interpretation(s) is (are) silly philosophical daydreaming, could you do the same for many-worlds?

Err, it doesn't work like that. If I were to convince you, I would have learned nothing, and believe then what I believe now. I can only give you my current best guess.

Which is, many-worlds is by no means certain. At the very least we don't have a Theory of Everything, and we could miss something. Collapse postulates however are just crazy. Rejecting the existence of the amplitude you didn't observe is just as insane (no less, no more) as rejecting the existence of a photon which just passed the limits of our observable universe.

When we do find a theory of everything, I bet the equations will predict the existence of things that can't be observed —not even in theory. I just hope people won't see that as a licence to not believe in those things at all.

Thanks for the cordial discussion, but I think the physicist you mention has it right. "Prediction" of phenomena that can never be observed seems like an oxymoron.

Yeah, but there is one practical application, if we ever conquer the stars: if we send colonists so far away that they eventually cross the boundary of our observable universe, we care a great deal about their continued survival, even though we will never observe it directly.

Other unobservable stuff may or may not be incredibly important. (Though at a first glance, QM interpretation isn't.)