[LINK] The Wrong Objections to the Many-Worlds Interpretation of Quantum Mechanics

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!

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 like to think of it as an extension of the conjunction fallacy; the probability of A and B being true can't be higher than the probability of either A or B; adding new conditions can only make the probability stay the same or go down. So the probability of a theory once it has an extra postulate, must be equal to or lower than the probability of the same theory with fewer postulates. Of course, that assumes the independence of the postulates.

The probability of the postulates all being true goes down as you add postulates. The probability of the theory being correct given the postulates may go up.

3Lumifer11y

Not only it assumes independence, it also assumes that the two competing theories have exactly the same postulates except for a single extra one. That is typically not how things work in real life.

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[LINK] The Wrong Objections to the Many-Worlds Interpretation of Quantum Mechanics

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[LINK] The Wrong Objections to the Many-Worlds Interpretation of Quantum Mechanics

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