David Deutsch, welcome to Within Reason.

Hi.

I'm excited to talk to you.

I was reading your book just recently on an aeroplane actually,

which is why for the viewers watching watching they may notice it's a

slightly different background to usual I'm in a hotel room or a motel room in Miami and

you're currently balanced on top of a microwave in a suitcase so that accounts

for the change in circumstance.

I was reading your book on the plane over here, the beginning of infinity and right

at the very beginning you talk about the nature of our universe, you talk about our planet being part of a solar system and the solar system being part of a galaxy, the galaxy part of a universe, and then rather confidently you say that this universe is part of a multiverse, just one universe among many others.

I hear people talk about this as a matter of conjecture all the time, but the confidence with which

you painted a picture of this multiverse into the very beginning of your book made me want to start by asking

What gives you such confidence that this multiverse exists?

I don't know if confidence is the right word. I have as much confidence in quantum

theory as I do in any of the other superb series of physics, which are our best knowledge of the world.

The fact that all these decades after, it's almost a century since the theory was proposed and half that time it has been known

that it describes a multiverse and yet that proposition is still disputed by most physicists.

When I say most people ask me how many I don't know but but those who adopt the many universes form of

quantum theory or Everettian quantum theory as it's called after Hugh Everett, who proposed it in 1957 are perhaps 10% of theoretical physicists but the

proportion is much higher in certain branches of physics where you have to have to actually ask what is happening

to bring about the predictions that quantum theory makes rather than just using the

predictions as a sort of almanac. And philosophically I think that And yes, so one of the excuses that's commonly made is to call this an interpretation of quantum theory rather than calling it quantum theory.

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