46 The Quantum Structure of Space and Time
I like what Hartle said about it in private though maybe not in his talks: it
is the word problem in physics. That is, it is not about the measurement
that you do or the calculation that you do, but the words that you say while
you are doing them. So that does not really seem so interesting to me, but
what is interesting to me is what I view as the likely possibility that quantum
mechanics is deeply wrong in some very fundamental way. For a long time it
looked as though in the context of black holes there might be some problems
with quantum mechanics. But now, though I do not think the nail is quite in
the coffin, it seems that all the behavior of black holes, at least when we use
results from string theory, is consistent with quantum mechanics. But I do not
see any reason why quantum mechanics should not, maybe relatively soon, go
the way of all our other cherished notions in physics, that is, need to undergo
some basic renovation. One reason why we might believe this is what I view
as a kind of white elephant standing in the room, which is the Big Bang. If
you believe in unitary evolution, you can take your quantum state and evolve
it forever. On the other hand we believe that the Universe had a beginning (of
course there is the ekpyrotic or other kinds of infinitely existing universes), but
that seems to me inconsistent with quantum mechanics which does not allow for
a beginning of time. More generally, I just think that because the Schrodinger
equation involves d/dt in such a preferred way is very much against the spirit
of general relativity. So I think the more interesting question is: is it time now
for quantum mechanics to be modified, or is that something that is still ahead
of us in the future? I think we should be open to the possibility of a very deep
modification of quantum mechanics.D. Gross To some extent, I think that Hartle dealt with some of those issues,
and I have not heard anything that refutes his statement in your discussion.
You do not need to formulate quantum mechanics in terms of the Schrodinger
equation with dldt. Normally, the initial conditions can be separated from the
kinematical framework.
J. Maldacena I like this hypothesis of radical conservatism, because we do not
have anything better to replace quantum mechanics with. We just have to
assume that it is correct and get as far as we can. I am not sure there is a
problem with time, because as Hartle said, time could emerge. In the example
of the Hartle-Hawking wave function, time is some kind of emergent property
and you can describe this de Sitter universe in a perfectly consistent fashion.
It looks like we need some framework which allows us to compute for example
quantum corrections to that. I think we probably need to put that whole
discussion in the string theory framework.D. Gross The one problem I wonder about is: in the absence of spacetime, how
do we recover a causal structure and in particular a sequence that would be the
equivalent of histories without this underlying spacetime?
M. Gell-Mann You said it yourself a little while ago, and I am sure Hartle would