The Quantum Structure of Space and Time (293 pages)

240 The Quantum Structure of Space and Time

term is the only term in the Lagrangian which does not have derivatives, so it is

important in the infrared. There could be a phenomenon similar to the Landau

zero charge picture when it is screened out as you go to large distances. So that

is another possibility, but I admit it does not solve the coincidence problem.

But it is still interesting to keep in mind.

N. Arkani-Hamed I just want to make some general comments about the angst of

predictivity in this picture. I think that part of the problem is that we continue

to confuse prediction of parameters with the more traditional accomplishment

of physics which is to predict and understand new dynamics. I do not think you

can even start talking about environmental or anthropic or whatever constraints

on parameters until you understand the dynamics well enough even to be able

to figure out what the relevant parameters are and how they vary. No one

would have been tempted to try to explain the phenomena of nuclear physics

anthropically. There is clearly major dynamics that was not understood, and

there was not even a question of talking about parameters to be tuned or not

tuned. Similarly, if we could go up to the string scale or the Planck scale

we would experimentally see what is going on, we would see weakly coupled

strings if we happened to be in the weakly coupled sector of the theory, and

all of that would be absolutely wonderful. It is only because of the practical

difficulty of not being able to do that, that we have psychologically replaced

it with being able to predict all of the parameters, which of course was never

promised. While in the history of physics, understanding dynamics always has

pushed us forward, focusing on parameters has some times pushed us forward,

and sometimes has not been the right question. The classic example of Kepler

trying to predict the distance of the planets from the Sun, is an example of

something that was just the wrong question.

I feel that what is different about our situation today is that, at least in our

understanding of long distance theories in terms of effective Lagrangians, we

are finally at a point where with a finite small number of parameters, we can

imagine in a controlled way what happens to physics as those parameters are

changed. Our questions are questions about those parameters. That dynamics,

at least at large distances, is basically governed by special relativity and quan-

tum mechanics, so that dynamics is under control in the infrared. Now, in that

context some parameters can be environmental, but not all of them. So the kind

of response that anything that you do not understand, there is an anthropic

explanation for it, is simply not true. There is no anthropic explanation for Vcb,

there is no anthropic explanation for bottom quark mass, there is no anthropic

explanation for QQCD or why there are three generations. And no one would

even attempt to come up with them, there are some things that are clearly

irrelevant to infrared physics. There are few other parameters, the relevant op-

erators and perhaps some other parameters, which are of great importance for

infrared physics and may have an environmental explanation, or not. I want to