Strings 2021 started today, program is available here. Since it’s online only, talks are much more accessible than usual (and since it’s free, over 2000 people have registered to in principle participate via Zoom). Talks are available for watching every day via Youtube, links are on the main page.
As has been the case for many years, it doesn’t look like there will be anything significantly new on the age-old problems of getting fundamental physics out of a string theory. But, as has also been the case for many years, the conference features many talks that have nothing to do with string theory and may be quite interesting. I notice that Roger Penrose, a well-know string theory skeptic, will be giving a talk on the last day of the conference next week.
Another series of talks that I took a look at and that I can recommend is Nima Arkani-Hamed’s lectures on Physics at Future Colliders at the ICTP summer school on particle physics. He never actually gets anywhere near discussing the topic of the title for the talks, but does give a very nice leisurely introduction to computing amplitudes for zero-mass particles. What he’s doing is emphasizing ideas that are often not taught in conventional QFT courses (although they should be). His second talk explains how to think of things in terms of classifying representations of the Poincare group, an old topic that unfortunately is often no longer taught (see chapter 42 of my QM textbook). His third talk emphasizes thinking of space-time vectors as two by two matrices (see section 40.4 of my QM book). This is a truly fundamental idea about space time geometry that gets too little attention in most physics courses.
Update: At String 2021, yesterday Nima Arkani-Hamed gave a talk on “Connecting String Theory to the Real World We See Outside Our Windows”, where he sometimes sounds like me, contrasting the pre-LHC claims of string theorists:
1. LHC will discover SUSY
2. String Theory Loves SUSY + Unification
to what they are saying now that the LHC has found no SUSY
CICADAS [i.e. crickets]. (Anyway, String Theory is mainly about Quantum Gravity).
He goes on to explain the “landscape philosophy”, which he sees string theorists (and himself) as now adopting. According to this philosophy, “connection to particle physics appear[s] hopeless/”parochial”/unimportant”. As a result, he sees the current situation as
- String theorists are for the most part no longer actively pursuing connecting to particle physics of the real world.
- Understandable as a short-term strategy
- But in my view a real mistake in the long run…
One reason for this being a real mistake is that, divorced from input from the real world, theory becomes sterile:
Questions Posed by Nature are Vastly Deeper and more fruitful than ones we humans tend to pose for ourselves.
Unfortunately I don’t think Arkani-Hamed has any compelling argument against “string theory implies landscape implies nothing to say about particle physics”. He discusses the “swampland philosophy”, but gives as a challenge to theorists just making more precise the sort of empty question that this philosophy deals in (he asks whether D=9 SU(2021) to the power 2021 is in the swampland).
Update: In the final discussion section, Witten emphasizes that “What is string theory?” still has no answer, that we have “little idea what it really is”. He lists two main things we know about the supposed theory:
1. General string perturbation theory using 2d conformal field theory. He mentions that one basic problem with this is that there is no understanding of what happens in time-dependent backgrounds, so, in particular, this is useless for addressing the big bang, which is the one place people now point to as a possible connection to real world data.
He notes that to get at non-perturbative string theory we seem to need some more general understanding of quantum theories, going beyond the usual quantization starting with an action, and ends by saying maybe quantum information theory can help. In the discussion section, Vafa challenges him on this, saying he sees no indication that quantum information theory gives any insight into dualities he sees as the central aspect of the non-perturbative theory. Witten’s answer is that this was just a vague hope, that the duality ideas are now 25 years old, we haven’t progressed beyond them, need something new.