Sean Carroll, physicist and proponent of Everettian Quantum Mechanics, has just posted a new article going over some of the common objections to EQM and why they are false. Of particular interest to us as rationalists:
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. And the actual postulates of the theory are quite simple indeed:
- The world is described by a quantum state, which is an element of a kind of vector space known as Hilbert space.
- The quantum state evolves through time in accordance with the Schrödinger equation, with some particular Hamiltonian.
That is, as they say, it. Notice you don’t see anything about worlds in there. The worlds are there whether you like it or not, sitting in Hilbert space, waiting to see whether they become actualized in the course of the evolution. Notice, also, that these postulates are eminently testable — indeed, even falsifiable! And once you make them (and you accept an appropriate “past hypothesis,” just as in statistical mechanics, and are considering a sufficiently richly-interacting system), the worlds happen automatically.
Given that, you can see why the objection is dispiritingly wrong-headed. 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. This distinction between what is postulated (which should be testable) and everything that is derived (which clearly need not be) seems pretty straightforward to me, but is a favorite thing for people to get confused about.
Very reminiscent of the quantum physics sequence here! I find that this distinction between number of entities and number of postulates is something that I need to remind people of all the time.
META: This is my first post; if I have done anything wrong, or could have done something better, please tell me!
I have commented there, and I will quote my comment here. To clarify, I am not anti-MWI, I am pro- experimental evidence.
It seems to me that you strawman a bit the main objection. Indeed, as you say
However, this does not answer the objection that
if you phrase it the way the MWI opponents usually mean it:
or than any other interpretation favored by a particular MWI opponent, as long as that interpretation makes exact same predictions as the orthodox QM.
You are certainly right, MWI does not need an extra collapse postulate, it comes out in many possible ways from using the L^2 norm for probability in conjunction with, say, unitarity, or some other equivalent experimentally justified assumption.
Unfortunately your “rightness” is rather hollow, because you still have no definitive experiment that would convince your opponent. And so the argument becomes philosophical rather than physical, as it cannot be resolved by the scientific method.
Needless to say, a QM formulation which would lead to a testable prediction beyond those derivable from the orthodox approach, would be an exceedingly big deal. But one can hope.
To which the answer is "its not a theory , its an interpretation. Interpretations arent testable, they're philosophy. Including anti realism".