Phillip Ball has a long aggrieved essay about the Many-Worlds Interpretation, which is, as Sean Carroll notes, pretty bad. Ball declares that Many-Worlds is “incoherent, both philosophically and logically,” but in fact, he’s got this exactly backwards: Many-Worlds is, in fact, a marvel of logical and philosophical coherence, while Ball’s objections are incoherent and illogical.
The fundamental problem with Many-Worlds is that every argument about it devolves very quickly into stoner dorm-room bull session nonsense about parallel worlds and identity and morality. But none of that is physics. It’s all science-fiction window dressing touted to sell books. Human identity and consciousness are not subjects that physics can say anything sensible about– cognitive scientists can barely say anything sensible about identity and consciousness, and that’s what they do for a living.
Physics is about fundamental rules and interactions, and those are the only things you can treat rigorously. And Many-Worlds is, in fact, a rigorous and logical treatment of these issues. It’s most clearly explicable in thinking about a single particle with two possible states. Could be an electron that’s either spin-up or spin-down, or a photon that goes through either the left slit or right slit of a Young’s double slit experiment. Doesn’t really matter.
The sticking point is that quantum theory predicts that single particles can exist in multiple states at the same time– the electron can be both spin-up and spin-down, or the photon can go both left and right. This is weird, but experimentally confirmed, because we can do interference experiments that unambiguously show the contribution of two distinct states– a photon going through a single slit makes a single stripe, but open a second slit, and you see an interference pattern characteristic of waves passing through both slits.
Of course, while quantum mechanics excels at predicting the probability distribution for the final states, in everyday life, we only see a single outcome of a given measurement. This is where the various interpretations of quantum physics come in. Collapse-type theories, of the sort evidently favored by Ball, hold that something happens in the detection process that physically forces the particle to choose one final state, while Many-Worlds and its variants hold that the particle continues to occupy multiple states at the same time, and the superposition simply expands to incorporate the detector– so now you have an electron that’s both spin-up and spin-down and a detector that’s showing both spin-up and spin-down, with those states entangled together in the appropriate manner.
If everything in the universe exists in multiple states at once, though, why don’t we see that? This is where the solid philosophical grounding of Many-Worlds becomes apparent, because an essential part of the whole business is that you can’t say what the state of a thing “really” is without specifying how you measure that. This is an idea that’s central to a lot of the best physics we have– I’ve been teaching from Peter Galison’s Einstein’s Clocks, Poincaré’s Maps again this term, and a point I hammer on to the students is that relativity comes out of the realization that you can’t talk meaningfully about simultaneity without explaining how you establish the timing of events. Poincaré goes on at length about this in The Measure of Time, and Einstein’s 1905 paper on Special Relativity makes the same point. Once you realize that simultaneity needs to be defined via a concrete mechanism, relativity is inevitable.
And this is what (my preferred understanding of) Many-Worlds does with the measurement problem. The question “Why don’t we see superposition states?” is meaningless without specifying how you would detect that a particular object is in more than one state. But we already said that: You measure some sort of interference effect that arises from two separate components of the wavefunction.
How do you measure an interference effect? Well, you look for some oscillation in the probability distribution. But that’s not a task you can accomplish with a single measurement of a single system– you can only measure probability from repeated measurements of identically prepared systems.
If you’re talking about a simple system, like a single electron or a single photon in a carefully controlled apparatus, this is easy. Everything will behave the same way from one experiment to the next, and with a bit of care, you can pick out the interference pattern. As your system gets bigger, though, “repeated measurements of identically prepared systems” become much harder to achieve. If you’re talking about a big molecule, there are lots more states it could start in, and lots more ways for it to interact with the rest of the universe. And those extra states and interactions mess up the interference effects you need to see to detect the presence of a superposition state. At some point, you can no longer confidently say that the particle of interest is in both states at once; instead, it looks like it was in a single state the whole time.
And that’s it. You appear to have picked out a single possibility at the point where your system becomes too big for you to reliably detect the fact that it’s really in a superposition. Ball’s complaint that Many-Worlds needs an explicit criterion for “when the universe splits” is the incoherent and illogical demand, because it’s not properly defined. There’s no magical difference between combined and split universes, there are just superpositions that are small enough to measure and superpositions too big to be measurable. How big is that? Well, there’s not a sharp line because that’s not a meaningful question in the abstract– there’s only the question of what you can measure, and how you can measure it. Without a clear and rigorous definition of those things, you’re in the same situation as electrodynamics before relativity, floundering around because the idea of absolute universal simultaneity is fundamentally incoherent.
What about the parallel worlds and the differences in identity and morality? Who cares? All that stuff is just a collection of foggily defined emergent phenomena that arising from vast numbers of simple quantum systems. Absent a concrete definition, and most importantly a solid idea of how you would measure any of these things, any argument about theories of mind and selfhood and all that stuff is inescapably incoherent.
(I should note that this goes for Many-Worlds popularizers like Max Tegmark, as well– Ball is correct that most of the pop versions of Many-Worlds don’t make a great deal of sense. But challenging pop versions of Many-Worlds on the basis of pop philosophy of mind just throws incoherence against incoherence like a battle scene in a comic-book movie. It generates a lot of noise, but doesn’t really amount to much. If you want to claim that Many-Worlds is philosophically incoherent as physics, then you need to talk about physics, and that demands a level of rigor that’s missing from Ball’s piece.)
(I’m also shorting the other interpretations a bit here, mostly for reasons of space. As Sean points out in his post, there are, in fact, versions of collapse theories that make sensible and rigorous statements about how things work, and hint at ways we might be able to test them. There are also consistent theories in the Bohmian vein that approach the whole business from a very different angle, but again ground their work properly in discussions of what you can measure and how you can measure it.)
(Finally, the above probably sounds more strongly in favor of Many-Worlds than my actual position, which shades toward agnosticism. But nothing makes me incline more toward believing in Many-Worlds than the gibberish that people write when they try to oppose it.)