The latest issue of Seed magazine (not yet available online, as far as I know), contains responses from various well-known physicists who were asked what they hoped to learn from the LHC. Here’s the gist of what they had to say:
Lisa Randall: The magnificent thing is that we know there should be an answer to the question of the weakness of gravity, and that it should be revealed at the LHC.
Leon Lederman: The long-simmering concern over the weakness of Einstein’s gravity may well be confronted. However, what is for sure is that the LHC, with its awesome reach, will answer all of our current astro-particle problems…
Alexander Vilenkin: If no trace of supersymmetry is found, this would be – necessarily indirect – evidence for the existence of the multiverse.
Sir Martin Rees: I’m hoping that it will clarify the nature of the particles that constitute the “dark matter” in the universe.
Edward Witten: The LHC wil tell us whether this notion [electroweak symmetry breaking] is correct, and if so how it works.
Max Tegmark: Our theory of particle physics has 26 pure numbers in it. Why do they have these particular values? How did the universe begin? Did it?
Leonard Susskind: I see only two possible outcomes of the LHC project – either there will be low energy supersymmetry or there won’t. … the big question is whether the gauge hierarchy fine-tuning is similar to the cosmological constant fine-tuning, or if it has a more conventional supersymmetric explanation.
Steven Weinberg: What terrifies theorists is that the LHC may discover nothing beyond the single neutral “Higgs” particle.. We fervently hope for some complicated discoveries.
John Schwarz: There are many speculative ideas for possible discoveries at the LHC. These include indications of extra dimensions, black holes, strings, magnetic monopoles, etc. I believe that all of these exist, and I would be thrilled to have experimental confirmation – but I am pessimistic about the prospects for finding them in the LHC’s energy range… I could be wrong. That’s why it is important that these experiments be carried out.
Sean Carroll: The beauty of science is that we don’t know what surprises may await us in these domains.
Gordon Kane: The LHC could discover the superpartners in a supersymmetric world. In addition to strong theoretical evidence for Higgs physics, there is strong experimental evidence that Higgs particles do exist with a mass implied by supersymmetry… Probably the main thing we have learned in the past two decades is that any understanding of nature at the most fundamental level (beyond a description) will require extending our thinking to embed our world in extra dimensions… An optimist (like me) can make the defendable argument that the LHC could test supersymmetry, establish string theory and move on to the remaining “why” questions.
My predictions: The LHC won’t see supersymmetry and won’t tell us anything about dark matter, dark energy or why gravity is weak. Witten has it exactly right, what the LHC will do will be to start to tell us what is causing electroweak symmetry breaking. It’s possible that Weinberg’s worry will be borne out, and, at LHC energies, we’ll just see what appears to be an elementary scalar, which will be depressing. But I think it’s equally likely that symmetry breaking is not coming from an elementary scalar, but from something much more interesting, quite possibly something we haven’t thought of, and the LHC may start to give us evidence for what this is.
I’ll also predict that we are still a few years from finding out what the LHC will tell us. These will be completely wasted years for particle theory if people just give up on looking for new ideas and sit on their hands (or wander pointlessly in the landscape), waiting for the LHC to revive the subject.