But I don't really like the preachy "Many Worlds is just the way it is, and anyone who thinks otherwise is a fool" tone of this series of articles. They're actually pretty well written (I'd rate them above David Deutsch's Fabric of Reality, frinstance -- though I don't consider that very high praise), but I recommend taking them with a grain of salt.

Example: We have embarrassed our Earth long enough by failing to see the obvious. So for the honor of my Earth, I write as if the existence of many-worlds were an established fact, because it is. The only question now is how long it will take for the people of this world to update. --- Sheesh, that kind of thing just doesn't belong in any scientific debate.

The domain name bothers me as well. Is it supposed to imply that anybody who doesn't agree with everything written in overcomingbias has simply done a poor job in overcoming their biases?

For example, it's commonly asserted that Bell's theorem rules out any deterministic quantum mechanical description of reality with local hidden variables. But this was, to my knowledge, never shown explicitly by Bell -- he simply ruled out a particular class of local hidden variable theories. There exist theories, consistent with experiment, which are local and deterministic: for what it's worth, a toy example is just a classical computer simulating everything in our universe we've yet seen.

We don't know where the Born probabilities come from, but we also don't really understand the implications of many body entanglement. We don't even have a consistent definition of entanglement for three or more particles. We don't know, we haven't been able to calculate, but I have reason to suspect, that measurement consistent with Born probabilities could be entirely explained by the deterministic axioms already part of Quantum Mechanics. We haven't really gone as far as we could go with them. And already people are treating Many-Worlds as axiomatic. I don't really buy it.

I think the state of the classical computer qualifies as a nonlocal hidden variable.

To use the language of formal logic, the classical computer is a model for quantum theory, i.e. (classical computer) |- QM. Within the QM theory, the computer qualifies as a non-local hidden variable, even though within the classical theory the computer is embedded in, it is local.

Another toy example is Bohmian mechanics on configuration space: the theory is just a local PDE + local particle. But that's non-local in physical space.

>We don't even have a consistent definition of entanglement for three or more particles. We don't know, we haven't been able to calculate, but I have reason to suspect, that measurement consistent with Born probabilities could be entirely explained by the deterministic axioms already part of Quantum Mechanics.

Decoherence makes sense on the macroscale (1000+ particles), although it's true that 40 particles is iffy. Different classical states (i.e., experimental apparatus has light on vs light off) are separated by a distance sqrt(number of particles) in configuration space, and don't interact.

As for explaining measurement with Born probabilities, that's reasonable. My co-conspirators and I currently have a physical, macroscale model where we show this to be true (no citation yet, but I'd be happy to explain more via email). But you still need some ontology.

All deterministic QM can show is that the probabilities work out correctly; i.e., the born probability of (measurement 1 says spin up, measurement 2 says spin down) = 0.

You still need a way to actually pick a configuration based on that probability distribution. The universe as we (you, me, even pg) know it is a point in configuration space, not a wavefunction. I'm happy with both MW and Bohm for that purpose.

> Decoherence makes sense on the macroscale (1000+ particles), although it's true that 40 particles is iffy. Different classical states (i.e., experimental apparatus has light on vs light off) are separated by a distance sqrt(number of particles) in configuration space, and don't interact.

> As for explaining measurement with Born probabilities, that's reasonable. My co-conspirators and I currently have a physical, macroscale model where we show this to be true (no citation yet, but I'd be happy to explain more via email). But you still need some ontology.

Please do. I looked for a while at decoherence and others, and the mechanism behind the processes kept fading from view. It's was like thermodynamics, where we can say something about the equilibrium states eventually reached, but we're having a hard time explaining the processes by which it reaches one state or another, and by those processes, the reasoning in other parts of physics break down. Like microscopic <-> macroscopic reversibility.

But the people in nonequilbrium statistical mechanics have made a lot of progress in reconciling microscopic reversibility and macroscopic apparent irreversibility. Is such a thing possible for quantum measurement, or more generally, the quantum classical transition, as well? Might there be a reversible description -- Schrodinger's all the way down, so to speak?

Finally, I'm not so sure that MW, decoherence, Bohmian mechanics, etc. are truly equivalent. In other words, I expect that one might start getting different answers.

And there's reason to believe that they're incomplete descriptions. If you take one measurement, you'll notice it takes time. And the microphysics of QM says that it's time evolution should be unitary. So, halfway done, if we stop the clock, when we're doing a measurement, what do we find? Or rather, what would our laws tell us we'd find?

The more popular interpretation seem to tell me 'don't ask this question.' But it seems there's something important hidden here.

Basically, what I've got is a model of a particle interacting with a measurement apparatus (a BEC, to make the calculations simple). You can reduce the many body schrodinger equation to a mean field model on reduced configuration space: (particle coordinate X BEC coordinate). So yes, it is schrodinger (actually madelung) all the way down.

Measurements (of position) correspond to the particle making a splash in the BEC (1). Splashes at different locations correspond to different measured outcomes. Once the difference between splash sizes is macroscopic ( (number of particles) * splash profile =O(1) ), the measurement is complete.

By "complete', I mean that if you pick a random BEC configuration (N BEC particle locations), and you can determine with statistical significance (i.e. 99.9999% sure) where the splash is.

Before this occurs, you've just got two overlapping probability distributions in configuration space. Picking random BEC configurations won't tell you (statistically significantly) the particle location.

The process is continuous, but it doesn't look that way to us since it is also very fast, i.e. t = O(1/number of particles in observation apparatus).

I suspect that there are all sorts of examples like this, accessible in theory and models of measurement in experiment, where things basically match up with with Born probabilities. Though I expect in some degenerate case they won't, just as, as the fluctuation theorem in nonequilibrium shows us, sometimes the second law is inaccurate, because whatever we call 'entropy' decreases.

One of my lecturers used to say that the main problem with MWI is that it looked like all the maths was done on the back of a napkin. I'm inclined to agree. It's very easy to explain Many Worlds with the example of a single particle and a single observer, but once you start throwing in a large number of mutually interacting particles, several observers, and take note of the fact that the observer him/herself is made up of multiple interacting particles, it suddenly becomes horribly complicated. As far as I know, nobody has ever done a many-worlds treatment beyond the simplest possible examples.

I suddenly feel the need for much, much more RAM...

No. "Bias" is a term of art among statisticians and social scientists who study human decision-making. It's my understanding that Overcomingbias started out as a collaboration between Robin Hanson ("futurist" economist at George Mason) and Eliezer Yudkowsky (the just-plain futurist who wrote the quantum sequence at OB).

Eliezer's got a pretty good introductory paper, which deserves to be read strictly on the merits of its opening paragraph:

http://www.singinst.org/Biases.pdf

OK, so you're a physicist, and you know that, when the dust settles and the true theory whatever-it-may-be is found, it's going to have multiple worlds in it, rather than violating relativity.

At what point, in your opinion, should physicists go ahead and announce this fact to the public as the official verdict of Science?

Make no mistake, quantum gibberish has trickle-down negative effects on the rest of the planet - people watching "What the Bleep Do We Know?" and losing hope in Reason. It may not hurt them any less than disbelief in natural selection, on average.

At what point should physicists get together and say: "Oops! Werner Heisenberg was wrong about the special role of consciousness. We now know this for sure. Everyone stop reading 'The Secret'."?

It's easy to say "Don't be too emphatic about that truth there" when you're a physicist and you can see the arguments clear as sunlight. If we were all evolutionary biologists we wouldn't need emphatic defenses of evolution, either.

I'm not a physicist, but I had studied physics for five years and I from what I have seen most physicists certainly wouldn't agree with you that the many-worlds interpretation is "the official verdict of Science." For the most part, physicists don't really care, because the all the results that can be tested experimentally remain the same no matter whether you talk about many worlds, about the wave function collapse or about something else entirely. This said, the majority of physicists who care at all choose to think in terms of the Copenhagen interpretation, which, as much as you may not like it, is simple and lets you get straight to the point. Which by the way does not imply that they think relativity is violated or that consciousness has any special status. If you want to consider relativity then the regular quantum mechanics is probably not the right framework anyway, since it is non-relativistic. What you want is the quantum field theory.

(Sorry for my English, I'm not a native English speaker)

So one macroscopic state (righty) corresponds to a set of finite volume; the set of all configurations who's macroscopic state (i.e., ignore quantum details humans can't see) looks like righty.

So all one can reasonably ask is "what is the probability my universe lives in that volume?" The obvious answer is the integral of |\psi|^2 over that volume, but that is a postulate, not something you can derive from the schrodinger equation.

Unless by configuration, you mean that each world is a world-line that contains a set of definite outcomes for all quantum events, and so all the quantum outcomes are predetermined for each world, and we explain probabilities by recourse to the proportion of some worlds to others. That's not really many-worlds, that's modal realism (every possible world exists, and "the actual world" just means "my world"). Which is more David Lewis than Hugh Everett.

Under evolution, [0,1] splits into [-0.5,0] U (0.5,1], two macroscopic states but still infinitely many microscopic ones. Every state in [0,0.5] was turned into a state in [-0.5, 0] and similarly (0.5,1] -> [0.5,1]. (Each real number corresponds to a world configuration.)

(note: there are quite a few variants of many worlds, and not everyone realizes they are talking about different theories. I don't know if I'm describing the most common view. )

My (non-mainstream) mental picture of MW is bohmian mechanics, with each possible bohmian trajectory corresponding to a different world history.

I hadn't heard this version, but it doesn't make sense to me. So these microscopic worlds each change when a quantum event occurs? If they can change, then why not think that there's only one world, which changes when you measure something? It's a lot simpler and seems to handle everything that this theory does. Of course, if they don't change, then we're in a microscopic world with a precise configuration at any given moment, and every observable has a value, and Einstein was right, and we wouldn't observe Bell's inequalities. But we do, and anyway, the whole reason we postulate this stuff is because we think that things in our world (you know, the one I'm sitting in, microscopic or otherwise) actually evolve according to the wave function.

BTW, if your picture is Bohmian mechanics, then you only need one world, where every particle has a well-defined position all the time, and these evolve according to the wave function. These different possible histories are epistemically possible because we don't know what region of the wave function the particle currently inhabits. But that's not a continuous infinity of worlds, microscopic or otherwise, unless you're just using the word "world" that way.

There are 6^1000 possible conscious minds. We call Lefty to the one that sees the first, very probable event. We call Righty to the one that sees the second, very unprobable event. There is actually only one Righty, but there are many, many Lefties. So it's actually a lot more probable to be a Lefty than to be a Righty. I think this works too with any other kind of event you could imagine (at least in a discrete (meta?)world, I'm not sure how you could have continuous probabilities if there are only discrete amounts of worlds available, or if you can have continuous amounts of worlds instead).

How does what you say work in this example? Why would both events have the same probability?

I don't know about QM more than the very, very basics, so please excuse me if I'm not understanding something very fundamental here.

But that's not MW. MW says that something happens, and when it happens, you get one world for each outcome. There's a problem here because what these outcomes are depend on the basis you write the wave function in, but that's a different issue. It's certainly not saying that there are an infinite number of worlds, each with a determined series of events, and we have probabilities because some of those worlds are more numerous than others. That's modal realism, as I said in a comment above.

By the way, that doesn't just raise its head in the context of quantum mechanics. It only does because it gives us reason to believe that it's physically possible that things could've been different than they actually are. But it seems possible-period that things could've been different, even if physics was deterministic. Even though it might've been physically determined that I went to the grocery store yesterday, it's certainly possible tout court that the whole universe had gone differently, and I could've gone to Fenway to see the Sox game instead. But does that mean that there's an alternate universe where my otherworldly counterpart went to Fenway? Isn't there a simpler explanation for why that's possible? QM has alternatives, like the Bohmian and GRW theories, which are far more plausible as well.

Maybe someday after I leave startupland.