A commenter in the previous posting pointed to an interview with Lenny Susskind that just appeared at the CERN Courier, under the title Lost in the Landscape. Some things I found noteworthy:
- He deals with the lack of any current definition of what string theory means by distinguishing between “String theory” and “string theory”. “String theory” is the superstring in 10 dimensions somehow compactified to have some large dimensions that are either flat or AdS. This can’t be the real world
I can tell you with 100% confidence that we don’t live in that world.
since the real world is non-supersymmetric and dS, not supersymmetric and AdS. He describes this theory as being “a very precise mathematical structure”, which one might argue with.
Something very different is “string theory”:
you might call it string-inspired theory, or think of it as expanding the boundaries of this very precise theory in ways that we don’t know how to at present. We don’t know with any precision how to expand the boundaries into non-supersymmetric string theory or de Sitter space, for example, so we make guesses. The string landscape is one such guess…
The first primary fact is that the world is not exactly supersymmetric and string theory with a capital S is. So where are we? Who knows! But it’s exciting to be in a situation where there is confusion.
- About anthropics and the landscape, he still thinks this is the best idea out there, but acknowledges it has gone nowhere in twenty years:
Witten, who had negative thoughts about the anthropic idea, eventually gave up and accepted that it seems to be the best possibility. And I think that’s probably true for a lot of other people. But it can’t have the ultimate influence that a real theory with quantitative predictions can have. At present it’s a set of ideas that fit together and are somewhat compelling, but unfortunately nobody really knows how to use this in a technical way to be able to precisely confirm it. That hasn’t changed in 20 years. In the meantime, theoretical physicists have gone off in the important direction of quantum gravity and holography.
- About the swampland, like everyone else I know, he can’t figure out what the argument is that is going to relate it to the real world:
The argument seems to be: let’s put a constraint on parameters in cosmology so that we can put de Sitter space in the swampland. But the world looks very much like de Sitter space, so I don’t understand the argument and I suspect people are wrong here.
His comments on Technicolor strike me as odd:
I had one big negative surprise, as did much of the community. This was a while ago when the idea of “technicolour” – a dynamical way to break electroweak symmetry via new gauge interactions – turned out to be wrong. Everybody I knew was absolutely convinced that technicolour was right, and it wasn’t. I was surprised and shocked.
I remember first hearing about the Technicolor idea around 1979 when Susskind and Weinberg wrote about it. It was a very attractive idea by itself, but the problem was that to match known flavor physics you needed to go to “Extended Technicolor”, which was really ugly (lots of new degrees of freedom, no predictivity). No idea when people supposedly were “absolutely convinced that technicolour was right”, maybe it was for the few months it took them to realize you needed Extended Technicolor.
- About the wormholes, he says:
One extremely interesting idea is “quantum gravity in the lab” – the idea that it is possible to construct systems, for example a large sphere of material engineered to support surface excitations that look like conformal field theory, and then to see if that system describes a bulk world with gravity. There are already signs that this is true. For example, the recent claim, involving Google, that two entangled quantum computers have been used to send information through the analogue of a wormhole shows how the methods of gravity can influence the way quantum communication is viewed. It’s a sign that quantum mechanics and gravity are not so different.
Unclear to me how this enthusiastic reference to the wormholes relates to his much less enthusiastic recent quote in New Scientist:
What is not so clear is whether the experiment is any better than garden-variety quantum teleportation and does it really capture the features of macroscopic general relativity that the authors might like to claim… only in the most fuzzy of ways (at best).