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!
The comments below the article evoke my "Read the Sequences" emotions.
But where are those other universes?
Doesn't splitting the universe by making a decision contradict the conservation of energy?
And I feel like: "The conservation of energy is a rule within the universe; it does not apply to splitting universes. Even the notion of 'where' only applies within the universe. And the universe does not split 'when you make a decision'; it keeps splitting all the time regardless of the content of your neurons. Duh! Could we just skip this level of debate and give more space to meaningful comments, please?"
But then I realize that's me, spoiled by reading the Sequences. For most readers those questions probably feel completely relevant, and they would wonder if no one would mention such obvious objections.
Going a bit more meta -- because I really don't care about the quantum physics that much -- this seems like a problem of debates in general. You can't have a debate on different levels of expertise simultaneously. Bad arguments drive out the good ones. The easiest way to protect the good arguments is to forbid the bad ones; but again, to an outsider this will seem like those bad arguments are actually the good ones you don't have an answer to! Or maybe there could be multiple "levels" of the debate where moderators could move individual comments to a "beginner" or "expert" column. (Any question that was already answered on the website and does not bring any new argument is classified as "beginner".)
Also I realize that if I had a blog with discussions below the articles, this (having to answer to the same trivial objections again, and again, and again) might drive me crazy. But that's a fact about me.
One way of interpreting the math is even more basic: from the perspective of conservation of energy, there's still just one universe with different parts of the wave function not interacting.