OK, I get the point. Sean Carroll really doesn’t care about problems of the ontology of personhood in the Many World Interpretation. I figured that, as a physicist, these would not be at the forefront of his mind, which is fair enough. But philosophically they are valid questions – which is why David Lewis thought a fair bit about them in his Model Realism theory. It seems to me that a supposedly scientific theory that walks up and says “Sorry, but you are not you – I can’t say what it is you are, but it’s not what you think you are” is obliged to take questions afterwards. I wrote my article in Aeon to try to get those questions, so determinedly overlooked in many expositions of Many Worlds (though clearly acknowledged, if not really addressed, by one of its thoughtful proponents Lev Vaidman) on the table.
But no. We’re not having that, apparently. Sean Carroll’s response doesn’t even mention them. Perhaps he feels as Chad Orzel does: “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’m sort of hoping that isn’t the case. I’m hoping that when Carroll writes of an experiment on a spin superposition being measured by Alice, “There's a version of Alice who saw up and a version who saw down”, he doesn’t really think we can treat Alice – I mean real-world Alices, not the placeholder for a measuring device – like a CCD camera. It’s the business of physics to simplify, but we know what Einstein said about that.
All he picks up on is the objection that I explicitly call minor in comparison: the matter of testing the MWI. His response baffles me:
"The MWI does not postulate a huge number of unobservable worlds, misleading name notwithstanding. (One reason many of us like to call it “Everettian Quantum Mechanics” instead of “Many-Worlds.”) Now, MWI certainly does predict the existence of a huge number of unobservable worlds. But it doesn’t postulate them. It derives them, from what it does postulate."
(I don’t quite get the discomfort with the “Many Worlds” label. It seems to me that is a reasonable name for a theory that “predicts the existence of a huge number of unobservable worlds.” Still, call it what you will.)
I’m missing something here. By and large, scientific theories make predictions, and then we do experiments to see if those predictions are right. MWI predicts “a huge number of worlds”, but apparently it is unreasonable to ask if we might examine that prediction in the laboratory.
But, Carroll says, “You don’t hold it against a theory if it makes some predictions that can’t be tested. Every theory does that. You don’t object to general relativity because you can’t be absolutely sure that Einstein’s equation was holding true at some particular event a billion light years away.” The latter is a non-sequitur: accepting a prediction that can’t be tested is not the same as accepting the possibility of exceptions. And you might reasonably say that there is a difference between accepting a theory even if you can’t get experimentally at what it implies in some obscure corner of parameter space and accepting a theory that “predicts a huge number of unobservable worlds”, some populated by other versions of you doing unobservable things. But OK, might we then have just one prediction that we can test please?
I was dissatisfied with Carroll’s earlier suggestion that you can test MWI just by finding a system that violates the Schrödinger equation or the principle of superposition, because, as I pointed out, it is not a unique interpretation of quantum theory in that regard. His response? “So what?” Alternatives to MWI, he says, have to add to its postulates (or change them), and so they too should predict something we can test. And some do. I understand that Carroll thinks the MWI is uniquely exempt from having to defend its interpretation in particular in the experimental arena, because its axioms are the minimal ones. The point I wanted to raise in my article, though, was that the wider implications of the MWI make it less minimal than its advocates claim. If a “minimal” physical theory predicted something that seemed nonsensical about how cells work, but a more complex theory with an experimentally unsupported postulate took away that problem, would we be right to assert that the minimal theory must be right until there was some evidence for that other postulate? Of course, there may be a good argument for why trashing any coherent notion of self and identity and agency is not a problem. I’d love to hear it. I’d rather it wasn’t just ignored.
“Those worlds happen automatically” – sure, I see that. They are a prediction – sure, I see that. But this point-blank refusal to think any more about them? I don’t get that. Perhaps if Many Worlders were to stop, just stop, trying to tell us anything about how those many unobservable worlds are peopled, to stop invoking copies of Alice as placeholders for quantum measurements, to stop talking about quantum brothers, to say simply that they don’t really have a clue what their interpretation can mean for our notions of identity, then I would rest easier. And so would many, many other physicists. That, I think, would make them a lot happier than being told they don’t understand quantum theory or that they are being silly.
I’m concerned that this sounds like a shot at Sean Carroll. I really don’t want that. Not only is he a lot smarter than me, but he writes so damned well on such intensely interesting stuff. I’m not saying that to flatter him away. I just wanted to get these things discussed.