Session 7
Closing remarks, by David Gross
I originally decided not to prepare closing remarks beforehand, but to summarize the
mood of this conference and the conclusions which we have reached. Unfortunately,
I do not think we have any conclusions and my mood is one of total exhaustion.
Nonetheless I will say a few words.
We started this wonderful conference with history and we have been talking a
lot about history. We all respect enormously the history of the Solvay Conferences.
We look back at them with awe and try to learn from them. But I still wonder why
we have talked so much about history. Maybe it is because I invited Peter Galison
to start off the conference with the history of the Solvay Conferences; and he did
such a marvelous job. But also I think it is characteristic of a period of confusion
that we look to the past with the hope of getting guidance or learning lessons. So
I will also draw a few historical analogies from which we may learn lessons.
I used to say that the state of string theory is analogous to the state of quantum
theory between the Bohr atom (1913) and the development of quantum mechan-
ics(1924), a wonderful period of utter confusion. Physicists had part of the truth
and were faced with many paradoxes. If this historical analogy was correct then
we are very lucky, because the progress made in that period from the Bohr atom
to quantum mechanics relied very little on experiment. I would say that you could
have put Arnold Sommerfeld and all of his students on a desert island in 1913, and
they would have come up with quantum mechanics. It was inevitable, once they
had the semi-classical approximation to quantum mechanics, even though they did
not know what quantum mechanics was, that they would figure out the correct
theory. I think that is true that experiment played little role after the Bohr atom;
so far no historian has contradicted me. Without experiment they might not have
known about spin, but they would have come up with quantum mechanics.
But I am beginning to be a little more pessimistic about the state of affairs
in string theory. Maybe a better analog is not 1913 but 1911, which happens to
be the year of the first Solvay Conference. In 1911, physicists did not have the
Bohr atom and they were faced with many sources of confusion. When we look
back at the first Solvay Conference there were two big clouds in front of physics.
One was “quanta and radiation” (and the first conference was called “Radiation
and the Quanta”) and the threats to the classical picture - wave-particle duality- that Planck’s and Einstein’s revolutions entail. However, there was another big
problem that hung over their heads. This was the phenomenon of radioactivity
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