Richard Marshall interviews Alastair Wilson in 3:AM Magazine:
3:AM: One of the things you’ve dedicated yourself to contemplating is the Everettian multiverse. Before we go further into your thoughts can you sketch out what this is , how it solves Schrodinger’s cat puzzle and other issues? Is it science? And why do you thinkscientists like it more than philosophers?
AW: The Everettian multiverse is one of the most beautiful ideas in the history of science. If its wrong, at least it’s gloriously, elegantly, ambitiously wrong. My approach to Everettian quantum mechanics (EQM) builds directly upon that of the ‘Oxford Everettians’ – David Deutsch, Simon Saunders, David Wallace, and Hilary Greaves. Wallace’s presentation of the view has become canonical, and any seriously interested readers should start by ignoring me and reading his lovely book The Emergent Multiverse.
EQM involves a straightforward scientific realist attitude to the quantum-mechanical formalism. The best explanation of the empirical success of quantum mechanics is that it’s tracking some real structure in the world; and if the theory seems to describe superpositions of macroscopic states, we should at least explore the possibility that there are superpositions of macroscopic states. Everett’s remarkable idea was to reconcile macroscopic superpositions with the ‘manifest image’ by interpreting superpositions not as indeterminacy but as multiplicity. Instead of a single cat in an indeterminate state – half-alive-half-dead – we have two cats each in a determinate state – one alive and one dead.
Is it science? I don’t see why not, at least if our theories about quarks and gluons and about gravitational waves and about galaxies beyond the visible universe count as science. Such things are all posited on the basis that they allow for theories with superempirical theoretical virtues; they’re testable indirectly through the empirical generalizations that they help to elegantly explain.
There are lots of reasons to worry about EQM, some of them worth taking seriously. Amongst the interesting objections are the various aspects of the probability problem (on which more below) and concerns about emergent ontology and lack of determinate identity conditions for worlds. Technical difficulties (the ‘preferred basis problem’) which loomed large in older discussions have largely been resolved by decoherence theory, though this remains pretty controversial. To my mind the less interesting objections include ontological extravagance and departure from common sense. If we can solve the probability and ontology problems in satisfactory ways, then objections based on intuition or aesthetics aren’t going to carry much weight.