Azara Blog: The Coming Revolution in Fundamental Physics

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Date published: 2007/10/08

David Gross (of UCSB) gave a talk this afternoon at the CMS entitled "The Coming Revolution in Fundamental Physics". Gross is in the majority camp of theoretical high energy physicists which believes in String Theory. Indeed, he was quite dismissive about people (e.g. Lee Smolin) who do not adhere to this particular belief. Funnily enough, he said at one point that he could not have given his talk thirty years ago, because of advances since then, but he could have given an almost identical talk twenty years ago. Not that much has happened the last couple of decades except for the theoretical elaboration of String Theory, with no experimental evidence to support it or not.

Gross' talk was at least accessible to a wide audience. He went through the usual litany of the unification of electromagnetic, weak and strong interactions, which is supposedly giving good predictions down to 10^{-18} cm, indeed all particle physicists believe (with no real evidence except dimensional analysis) that it will be good down to 10^{-33} cm, the so-called Planck length. And general relativity gives answers good out to the size of the universe. So between these various forces of nature, we can make predictions over 60 orders of magnitude. And you can make simple-minded extrapolations which indicate that although the forces of nature are of vastly differing strengths in the "low energy" world in which we now inhabit, at much higher energies (so in the early universe) the forces would (or could) all be the same strength. This comes about because symmetries that are manifest at high energies are "broken" at low energies.

But it turns out that these simple-minded extrapolations indicate the various strengths of the forces do not all become equal at the same energy. Enter supersymmetry, which is a symmetry which relates particles of odd spin with particles of even spin. This allegedly saves the day. So far there is no experimental evidence for supersymmetry, but with the new LHC ring at CERN in Geneva, this evidence might be forthcoming.

Superstring theory is a particular supersymmetric theory, and most theoretical physicists believe this is the most promising approach. In particular, it might help "explain" the dark matter problem, namely that around 90% of the matter in the universe can allegedly not be seen (very easily). It might also explain the ratios of masses that are observed.

Superstring theory treat objects as 1-dimensional strings instead of 0-dimensional particles, and with certain variants it avoids the singularities you find when you do ordinary quantum field theory of point particles. (Each harmonic frequency of the string represents a specific particle.) But it does this by insisting there are extra dimensions other than the four space-time ones that Einstein talked about. The story goes that these dimensions are small enough that you would not notice them, hence we just think that we live in four dimensions. Sure, but why are we left with four? And there are plenty of other problems. Superstring theory allegedly has no arbitrary constants, but in fact it implicitly does, because why is one solution (e.g. for what form the extra dimensions take) any better than any other solution.

Gross said that the problem with String Theory is that nobody is sure what it is. There is no law and no dynamical principle, just solutions of equations. Of course he is convinced enough that it will all come right in the end. Having worked a few decades on the subject it would be surprising if he said anything else. These people are relying on the LHC to provide some evidence for supersymmetry, a basic pre-condition for superstring theory to even be plausible. It will be interesting to see what is found.

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