258 The Quantum Structure of Space and Tame
about the way this anthropic principle is used here. It unavoidably makes one
think about the principle of natural selection in biology. Now, I do not want
to discuss natural selection in biology but I think the transcription of it frombiology to physics is a little bit dangerous. I just want to give a simple example.
Suppose that we do not know the theory of gravity and suppose that someoneasks why do apples fall on the ground (the apple had a historical importance in
gravity, that is why I choose apples). The answer is extremely simple: because,
if they do not fall on the ground, they do not give rise to trees and therefore
those apples that do not fall on the ground have disappeared. You can very
easily generalize that statement: not only apples but, of course, everything falls
on the ground because what does not fall on the ground is no more there. The
morality of this is that, of course, we would like to say that maybe one shouldnot reason too simplistically on this, but one should better look for a theory
of gravitation at that moment. Maybe one should look for a decent theory for
explaining that particular element which is the cosmological constant.B. Greene Just a quick remark relevant to the question that was asked about
finding the measure on the landscape. I guess I am not still quite convinced
that this is a really interesting question. And the reason I am not convinced
is the following. If you would ask the same kind of question in the context of
ordinary field theories - look at the landscape of all possible field theories and
write some measure on that space - you are never going to find the Standard
Model as some generic field theory in this space of field theories. It is a veryspecial field theory, and yet it is the one that is right. Although I understand
the motivation for having a measure on the space of the landscape from stringtheory, that is to have a possible anthropic solution to the cosmological constant,
but what if you go beyond that and talk about the rest of phenomenology? The
basic question is: since we all know that very special theories are sometimesthe right theory, why try to have some sense of genericity as a guide to finding
the right model?
S. Shenker Now we are going to have the last couple of talks.6.4.1 Paul J. Steinhardt: A modest proposal for solving the cos-
mological constant problem
Probably all of the participants at the Solvay Conference have dreamed of solving
the cosmological constant (A) problem. And probably all have, at one time or an-
other, sought the same solution: a dynamical relaxation mechanism that gradually
cancels all contributions to A, whether due to physics at the Planck scale, the elec-troweak scale, the QCD scale, etc. In this way, the universe could begin with a
natural value for A of order the Planck scale but have an exponentially small value
today. During the last quarter century, though, a serious roadblocks has been placed