Now back from a short vacation, and there seems to have been a lot happening on the debate over fundamental physics front. From the experimentalists, news that the Standard Model continues to resist falsification:
- At ICHEP, as expected, new data from ATLAS and CMS ruled out the supposed 750 GeV state that would have indicated new physics.
- Also at ICHEP, significantly stronger bounds on SUSY: gluinos ruled out up to 1.9 TeV, stops up to 900 GeV.
Recall also the recent results from LUX and PandaX (discussed here) putting stronger bounds on the sort of WIMP dark matter supposedly a feature of SUSY models.
For what it all means, you should of course consult Resonaances (and read a profile of Jester, The Rogue Blogger Who Keeps Spoiling Physics’ Biggest News) as well as Tommaso Dorigo (and some coverage from Physics World featuring him).
From an even wider perspective, see Sabine Hossenfelder for The LHC “nightmare scenario” has come true and Natalie Wolchover’s piece on the “nightmare scenario”, What No New Particles Means for Physics (by the way, congratulations to Wolchover on some well-deserved awards, including this one). My own views on this are well known: this was in some sense a major theme of my book, and among other places I’ve written about this, see for example a 2013 Edge essay.
My perspective on this is in some ways similar to Hossenfelder’s, but I draw very different conclusions, strongly disagreeing with her criticism of “reliance on gauge symmetry”, and “trust in beauty and simplicity”. This hasn’t been what theorists have been doing for 30 years. The string theory unification ideology has led to an emphasis on extremely complex and ugly models, with gauge symmetry not a fundamental feature at all. Yes, she’s right to point to “A failure of particle physicists to uncover a more powerful mathematical framework to improve upon the theories we already have”, but I’d argue that that failure is due to an insistence on looking in the wrong place.
Wolchover’s piece captures some of the current angst well, for instance quoting Maria Spiropolu about SUSY as follows:
“We had figured it all out,” said Maria Spiropulu, a particle physicist at the California Institute of Technology and a member of CMS. “If you ask people of my generation, we were almost taught that supersymmetry is there even if we haven’t discovered it. We believed it.”
Arkani-Hamed is as quotable as ever, saying the lesson of all this failure is
“There are many theorists, myself included, who feel that we’re in a totally unique time, where the questions on the table are the really huge, structural ones, not the details of the next particle. We’re very lucky to get to live in a period like this — even if there may not be major, verified progress in our lifetimes.”
I suspect that there are quite a few physicists, of my generation and later, who don’t necessarily feel “very lucky” to get to live in a period of no significant progress in the past 40 years, with the prospect of none in our lifetimes. It would be great if the “huge, structural” questions really were on the table, but I’ve seen little interest among theorists in such questions, beyond whatever the fad of the day related to AdS/CFT might be.
Also finishing up while I was away was the Strings 2016 conference, and for the state of the subject you might want to watch David Gross’s “Vision” talk (video here, I had to download the whole thing to watch it, streaming was unusable). I think Gross was right in pointing to work on the so-called “Sachdev-Ye-Kitaev” model as perhaps the most interesting thing being discussed at the conference. This is an exactly solvable large-N quantum mechanical model exhibiting features of holography. A good place to start learning about it is Kitaev’s KITP lectures here and here.
Gross also went over some history, noting that some current topics in string theory echo back to 1967 work of Mandelstam (and that Gross himself had written a paper at that time on this material, so next year will be a 50th anniversary of his engagement with the subject). As for the current state of the theory, his take is much the same as that he has talked about at many earlier Strings conferences: string theory is now a “framework” encompassing QFT and most of the rest of fundamental physics, but we don’t really know “what string theory is”. To me it’s very unclear why one is supposed to believe so strongly in the overarching role of a set of ideas that aren’t well-defined and have been an utter failure at explaining anything about particle physics. Soon he’ll have to pay off his SUSY bets, but this doesn’t seem to have changed his conviction that superstring theory unification is on the right track. He ended with his usual sign-off “The best is yet to come”, but this time added a parenthetical “I hope it comes quick”.
Another new Wolchover piece well worth reading is about Miranda Cheng and her work on modular forms and string theory on K3 surfaces. I think this sort of work on the boundary of mathematics and physics makes clear the problem with string theory and mathematics: things are complicated and ugly if you try and make the theory look like the real world. You get beautiful ideas and great mathematics when you ignore the supposed connection to the real world and go in another direction. The problem is that often, instead of pursuing good mathematical ideas where they lead, people doing this feel the need to stick to some connection to the failed idea. Here’s what Cheng has to say:
I personally always have the real world at the back of my mind — but really, really, really back. I use it as sort of an inspiration for determining roughly the big directions I’m going in. But my day-to-day research is not aimed at solving the real world. I see it as differences in taste and style and personal capabilities. New ideas are needed in fundamental high-energy physics, and it’s hard to say where those new ideas will come from. Understanding the basic, fundamental structures of string theory is needed and helpful. You’ve got to start somewhere where you can compute things, and that leads, often, to very mathematical corners. The payoff to understanding the real world might be really long term, but that’s necessary at this stage.
For another physicist still enthusiastic about string theory, see this interview with Brian Greene, somehow motivated by a sci-fi horror series, Netflix’s “Stranger Things”.