"Who ever would have guessed when SLAC began forty years
ago that understanding the vacuum, basically empty space in our frozen
epoch of cosmic evolution, would be the most challenging problem in
physics today? The discovery in 1998, totally unexpected, that the
expansion of the universe is accelerating, is both embarrassing and
exciting. How could we have missed something that big? There is nothing
in our current theories that even comes close to producing the right
order of magnitude for the term in Einstein's equation, the cosmological
constant, required for this effect. What the theory gives is a joke,
more than a hundred orders of magnitude off the mark.
"The vacuum plays an essential role in the inflation
theories, to which Stanford scientists have contributed many of the
most important ideas. And once again these theories are important
because they lead to phenomena that must be understood to relate observable
features of the universe to the structure and symmetries of microscopic
models models that may include strings, and that we hope will unify
gravity with the gauge forces of the Standard Model. We are going
to need all the help we can get to tie these future theories down
to empirical reality.
"The argument for building an accelerator beyond the
LHC, it seems to me, must be strongly linked to these ideas. At some
point we will simply have to stop building accelerators. I don't know
when that point will be reached, but we must start thinking about
what fundamental physics will be like when it happens. Theory, of
course, will continue to run on. But experimental physics at the frontier
will no longer be able to produce direct excitations of increasingly
massive parts of nature's spectrum, so it will have to do something
else. There are two alternatives. The first is to use the existing
accelerators to measure parameters of the standard model with ever-increasing
accuracy so as to capture the indirect effects of higher energy features
of the theory, much as BaBar is doing today at this laboratory. The
second is to turn to the laboratory of the cosmos, as physics did
in the cosmic ray era before accelerators became available more than
fifty years ago.
"Are we ready for this? When the last accelerator is
built, will there still be a gap in our knowledge that will prevent
us from working productively in the 'Laboratory of the Cosmos?' There
is no question that our ability to interpret what we see in the sky
depends on what we have learned about fundamental matter in our earthly
laboratories. How strong is this dependence? How much more do we need
from earth-bound accelerators before we can do without them? How can
we best prepare for the end of the accelerator era in fundamental
"However, and whenever, this transition occurs, it is
clear to me that the fates of deep space astronomy and particle physics
are strongly entwined. In the long run, the future of particle physics
lies in space-based experiments, and its productivity will depend
on having a model of nature that is complete enough to exploit cosmic
phenomena as a guide to theory. Now is the time to begin preparing
for the long run.
"I mentioned the 'ragged edge' of society's ability
to deliver big accelerators. 'Society' likes science. It is willing
to tax itself to provide funds for basic, discovery-oriented research.
It reads popular science books, watches educational television shows
on science, and encourages its young people to study such impractical
science topics as dinosaurs and black holes. In Congress, science
enjoys bipartisan support. All postwar administrations have supported
basic research, including the administration of President George W.
Bush. But there is a limit. Not, unfortunately, a well-defined or
clearly articulated limit. We saw this in the saga of the Superconducting
Super Collider. That project did not fail because of lack of love
for particle physics, or even for lack of understanding of the importance
of the Higgs mechanism. It failed, in my opinion, because the scale
of the project exceeded a critical size a size well within the ability
of society to pay, but placed within a domain of society's parameter
space that is unstable against chaotic behavior.
"If the SSC was beyond a threshold of stability, and
the LHC is beneath it, the Next Linear Collider [NLC] is already in
a gray area. I have expressed elsewhere my conviction, in agreement
with the High Energy Physics Advisory Panel, that the NLC is a logical
choice for a next big accelerator after LHC. I was always taken with
the simplicity of lepton-antilepton collisions, which create 'little
big bangs' with simple spatial structure and simple quantum numbers.
Moreover, I think a lepton collider is the right kind of machine to
do precision experiments of the sort that are going to be necessary
to probe mass regimes that are out of reach. Whether it will be the
'last big accelerator,' or whether a muon collider or something else
will have that honor, I don't know. Perhaps we will find a way to
keep building ever larger accelerators throughout the 21st century.
But already with the NLC we are going to have to change the way such
devices are financed. No single nation is likely to pick up as much
of the cost of the NLC as host countries have in the past. To be successful,
the project will need a new model of international support.
"What can the science community do to increase the inclination
of society to support these big machines? I think the best approach
-- and this is after a year in Washington, D.C. -- is to tell the
truth, the whole truth. But it must be told carefully, in language
that society can understand.
"The truth is that particle physics is as exciting as
it ever was. It is not dead. The fact that we are having trouble seeing
beyond the Standard Model is not bad news. It means that the next
discoveries will have a disproportionate impact on our understanding
of Nature. For the first time in a quarter-century experiment is driving
theory at the frontier, and not the other way around.
"The truth is that Nature functions in such a way as
to bring together the science of the very large with the science of
the very small, and that opportunities have emerged for discovery
about the fundamental nature of the universe that we never expected.
Technology places these discoveries within our reach, but we need
to focus efforts across widely separated disciplines to realize the
"The truth is that exploration of the new frontier will
attract the best young minds who will produce new technology to overcome
the barriers which define the limits of our perception. The excitement
of discovery, and the human will to see farther are powerful sources
of vitality in our society.
"What we should not do is give the impression that the
accelerators and other large scale apparatus are ends in themselves.
Only the search for the ultimate shape of Nature can justify such
large expenditures, and we must subordinate all other considerations
to that grand end. Nor should we over-emphasize the practical impact
of new technologies that will emerge from the search. Too few of us
are truly aware of the actual histories of previous impacts. To those
who know, the proposition that high energy physics was responsible
for magnetic resonance imaging devices, for example, is naive. And
above all we should never assume that the lay public will not be able
to appreciate what we are about. We need to support the science journalists
who care, and those among us who have the knack of translating the
fragmented and highly technical knowledge that is accumulating so
rapidly into a coherent story as appealing to the lay public as it
is to us."