[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:

  1. The world is de­scribed by a quan­tum state, which is an el­e­ment of a kind of vec­tor space known as Hilbert space.

  2. 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!