The Quantum Structure of Space and Time (293 pages)

272 The Quantum Structure of Space and Time

dual description of some kind of string theory. Also, the fact that we have so many
different pictures of string theory, perhaps even different theories - we are not sure

that they are truly all connected in a dynamical sense ~ might be a hint that string

theory is a “framework” and not a theory, much as quantum field theory is a frame-

work in which other principles (such as symmetry principles or renormalizabiltiy)

must be appended to arrive at a theory. Maybe something is truly missing in our

understanding of string theory, which we cannot identify? Our present understand-
ing seems to me to be similar to the situation that physicists confronted in 1911.
If they had tried in 1911 to deal with the problem of radioactivity, to explain how

a nucleus could just sit there and randomly emit particles, they would have had

no chance at all. That is probably why they did not discuss it at the first Solvay

Conference. They were missing something absolutely fundamental in attacking that
problem, namely quantum mechanics. Once quantum mechanics came along, with
tunneling, radioactivity fell out. I have the feeling that the cosmological constant

and all the bagage that goes along with, such as the the landscape, might be a

problem of the same nature as radioactivity was in 1911, and that we are miss-
ing perhaps something as profound as they were back then. So, with that note of
confusion, I will end.

I would like to end by thanking Marc. I had some role in organizing the scientific

content of this conference, but the real hero behind this all is Marc. He has done an

incredible job both in reviving the Solvay Intitutes and revitalizing them. Hopefully

these wonderful conferences will continue all the way to the next century, so that

we can come back in 96 years when all will be clear. I would like to thank Marc

and I call for all of us to give him a big hand.

Answer by Marc Henneaux

Thank you very much. When two years ago, we decided to invigorate the Solvay

Conferences and to have one in 2005 according to the old format, I think it was

at the same time a very risky bet and a dream. The person who really convinced

me that the bet was not so risky was David: when I went to see him, he was very

supportive and immediately offered a tremendous help. The fact that you have

been in Brussels this week shows that indeed the bet was not too risky: we are

very grateful that you made the dream come true. And it will go on. Following the

success of this conference, there will be other Solvay Conferences in the future ~

as David just told us, we have at least 96 years to go.

To conclude, I would like to thank all of you, to thank all the chairs, to thank

all the rapporteurs who did an excellent job, and especially to thank the chair of

this conference, David.

And I would like to also warmly thank Isabelle, Dominique, Stkphanie and

Fabienne without whose faithful dedication the organization of the conference would

have been a disaster.