Last week’s public lecture at the Institute for Advanced Study by Nati Seiberg is now available online. He was speaking with the title What’s Next? and promoting a story about where particle physics is and where it is going pretty much identical with that coming from his IAS colleagues. Despite the overwhelming failure of string theory unification and the dramatic evidence from the LHC ruling out popular ideas about SUSY, there was no admission of any discouragement about string theory or SUSY.
String theory was described as the best candidate for a fundamental theory, one that has been making “enormous and exciting progress with amazing new insights” and “all signs are that we will continue to make progress.” For more details Seiberg points to talks given by Witten such as this one. According to Seiberg, string theory has not problems and failures, but “challenges”. One challenge is that “we do not understand the principles” of string theory. Another is that “we need experimental confirmation”, which makes it sound like the problem is one of experiments not done yet, rather than the real problem, which is a “theory” that predicts nothing.
The hierarchy problem is emphasized as the central problem for particle theory, with almost exactly the same point of view as that of Nima Arkani-Hamed, which I’ve discussed here many times (see for example here and here). We’re told not to think of the LHC results as providing evidence against SUSY, but to interpret LHC results as choosing between two possibilities:
- SUSY exists at LHC scales and arguments about SUSY solving the hierarchy problem are vindicated. Things don’t look good for this so far, but hope is held out for the next run, with an admission that if it doesn’t turn up then, that’s it for SUSY as a solution to this problem.
- No SUSY at LHC scales just means it is at higher scales, and the multiverse is now brought in to deal with the hierarchy problem. In a recent Science Weekly podcast, Arkani-Hamed says he’s still willing to bet several years salary that SUSY exists, but now he thinks maybe it only shows up at higher energies than he’ll see in his lifetime. He’s willing to bet that SUSY will show up at the next LHC run, but just $50.
Since even enthusiasts who have devoted their career to the cause are now only willing to put up $50 in favor of SUSY at 13 TeV, it’s pretty clear that hardly anyone is now expecting to see this. We’re already in the era of trying to understand the implications of no SUSY at the LHC, with the multiverse the main argument now being deployed in favor of not giving up on cherished speculation about SUSY and strings, no matter what experiments say.
Seiberg does give a different historical analogy for the hierarchy problem, likening it to a fine-tuning problem that Newton was worried about, that of the stability of planetary orbits. Why does a small perturbation of such an orbit not lead to exponentially large changes, destabilizing the orbit? Seiberg lists three possible solutions to such fine-tuning problems:
- There really is no problem if you understood the theory well-enough.
- You need to invoke new physics as a stabilizing mechanism.
- The answer is “environmental”: the orbits are generically unstable, we just happen to live in an unusual place where they are stable.
The odd thing about his use of this historical analogy is that the lesson to be drawn is that of course the answer is the first alternative, but he quickly passes that one by as not worth talking about. I doubt the last alternative ever occured to Newton as anything other than a joke, and don’t know of any evidence that he tried to come up with models of things like new unseen planets to solve this supposed problem. Newton surely realized there was plenty that he didn’t understand about what Newtonian mechanics had to say about celestial mechanics. It’s just as clear that our best model of the Higgs, with its large number of undetermined parameters, is such that we just don’t fully understand where the Higgs potential and Yukawas come from.
The Seiberg talk seems to be one of a series (others listed here) of talks associated with the Milner Fundamental Physics Prize. IAS director Dijkgraaf introduced Seiberg as one of the four IAS winners of the $3 million Milner prize, with this leading his list of honors awarded to Seiberg. The talk was a public one of a sort that has for the IAS not just an educational role, but also a fund-raising one. Something is being sold here, the idea that SUSY and string theory are great successes, with the IAS faculty well-deserving the multi-million-dollar checks awarded to them for their work on these topics. Later this week they’ll be getting together in San Francisco to decide how to split up $3.6 million in new checks among five other string theorists (the announcement of the winner of the 2014 prize will be made Thursday). All of this I fear has something to do with why we’re not hearing from those at the IAS a truer picture of what no SUSY at the LHC means: the collapse of ideas that don’t work and evidence that we don’t yet have any viable conceptual framework for going beyond the Standard Model. This summer the IAS will host its usual PiTP program to train grad students and postdocs in what they need to know to face the future. The topic? String theory.