The Quantum Structure of Space and Time (293 pages)

256 The Quantum Structure of Space and Time

that be a running away horizon? I am not quite happy with it, but that could

be a conceivable answer. So, is it a horizon which runs away from us or is there

really a membrane? Would it be something like in science fiction, a kind of wall

at the end of the universe and you just have to go through it to see the other

part of the universe, anything like that?

A. Linde As I said, this is a model-dependent issue, and it also depends on the

nature of the state. If you are sitting in de Sitter space, you never even come

to this part of the Universe. It will run away from you. If you are sitting in

Minkowski space, then you would have two possibilities: the one is that the

boundary travels away from you with the speed equal to the speed light and

then you will never touch it. There is another possibility that this part travels

with the speed of light towards you, and then you will never see it because at

the moment you see it, how to say, there will be nobody to respond and nobody

to report. NSF will not support your further work.

G. ’t Hooft Maybe the question to the other people here is the following: is that

the same notion of landscape as seen in other talks?
A. Linde We are talking about the same landscape, but this landscape is an “an-
imal with many faces”. All of these things are very much different. The
parts of the universe which are de Sitter have some properties, parts which
are Minkowski are rare animals, and they have some other properties. Parts
which we are supposed to associate with anti-de Sitter, they are actually not
anti-de Sitter but collapsing Friedman Universes. All of them can be part of
this picture.
S. Shenker One of the things about these issues I find the most interesting (that
is probably because I just started thinking about them) is this issue that Guth,
Linde and others discussed, about putting a measure on the space of “pocket
universes”. Now that it seems more and more likely that string theory contains

some kind of landscape, and it seems that there is a well-defined quantum

gravity. For this mysterious bubbling phenomenon, there should be some kind

of question about how likely is every kind of bubbling universe. And it is

incredibly hard, as Guth mentioned, to figure out what that question means.

Now, we think we have a well posed theory. Either this question is, for some

reason, completely nonsensical or we should be able to sharpen it up. This

problem is one to think about. It has the psychological advantage that you

can think about issues of landscape and bubbles and never have to think about

anthropics, which I find psychologically appealing.

M. Douglas I agree with the importance of this. There is another version of this

question that I have read in the existing literature. It just seems that if at some

point of your calculation you get infinity over infinity, or a limit of quantities

which is going to become infinity over infinity, then your definition is inherently

ambiguous, and at that point you have already lost. Was there any kind of

suggestion or hope in the existing works, of a definition that would make at