The Quantum Structure of Space and Time (293 pages)

238 The Quantum Structure of Space and Tame

went over that very rapidly, and since I spoke for Hartle, and one of the things

on one of the slides was “many worlds not equal to many bubbles”, so I am

asking his question: why is that?

J. Polchinski I put that in there partly because Hartle did, but also because

Susskind said it at “Strings” and other people said it in other language. You

can read that in two directions. The more ambitious direction is to say that

eternal inflation explains quantum mechanics. I do not think anybody believes

that. The less ambitious way to read it is simply that holography tells you that

you do not talk about things that are out of causal contact, they are folded back

into your own wave function, and so in that sense the many worlds of eternal

inflation really are the many worlds of quantum mechanics.

M. Gell-Mann In Everett’s work of the late 1950’s when he was a graduate stu-

dent of Wheeler, he formulated the ideas that were ancestrial to a lot of things

that we do today. Bryce De Witt called them - or somebody else, not Everett

• called them “many worlds”. What they are, and what they continue to be

today, is a set of ideas about multiple alternative histories of a Universe. If

you want to believe that for every such history there is a universe somewhere,

then if the universes do not communicate with one another, it does not change

anything. But the many-worlds idea is not in any way important for this idea of

many histories of the Universe. The other idea having to do with the budding

off of new universes in a multi-verse is completely different. That might have

some actual consequences for the individual universe we study. If it used to be

part of a bigger system which broke up, the state vector would become a density

matrix of a certain kind, for example. There might be some consequences. But

referring to the other theory, the many-histories idea, as many worlds is just

misleading, I think.

N. Seiberg I really enjoyed your nuclear physics analogy. I would like to pursue

it a little bit further. If at the time people faced nuclear physics with all its

resonances, they had adopted the anthropic principle, you could have repeated

your talk with minor changes at that time and it would have had a very negative

effect on the development of science. The standard model would not have been

discovered. Returning to our time, there could be all sorts of rich physics that

we should understand, and adapting the anthropic principle will prevent us

from finding it. I think it is premature to declare defeat.

M. Douglas May I answer this? This is sort of a standard answer. There are

different metaphors that I like, such as the one that says that we live in a big

crystal and we have to figure out which one it is. The situation here is sort

of historically reverse. Back then you could have said that the real interesting

thing is to identify the protons and quarks and so forth, while here we are

actually working backwards. We really believe in the fundamental nature of

the supersymmetric pieces, and now we are trying to assemble this into the

complicated mishmash that looks more like our world. From that point of