Tuesday, April 10, 2018

A Notable Paper Potentially Falsifying String Theory

There are vast numbers of mathematically possible concrete theories called vacua within the class of theories that make up string theory. This makes finding the needle in the haystack that could match reality difficult. This has been one of the main obstacles to developing a string theory phenomenology that can be tested with experiments.

But, it turns out that all, or almost all, of the vacua are topologically inconsistent with observed reality which has a "de Sitter" topology:
De Sitter space and anti-de Sitter space are named after Willem de Sitter (1872–1934), professor of astronomy at Leiden University and director of the Leiden Observatory. Willem de Sitter and Albert Einstein worked in the 1920s in Leiden closely together on the spacetime structure of our universe. 
In the language of general relativity, de Sitter space is the maximally symmetric vacuum solution of Einstein's field equations with a positive cosmological constant (corresponding to a positive vacuum energy density and negative pressure). When n = 4 (3 space dimensions plus time), it is a cosmological model for the physical universe; see de Sitter universe
De Sitter space was also discovered, independently, and about the same time, by Tullio Levi-Civita.
The class of topologically inconsistent vacua are called the "Swampland" and if all or most String theory vacua are in the Swampland then the task of finding a String theory vacuum consistent with our reality, or ruling out String theory entirely becomes much easier.

Woit notes an important new pre-print on "Swampland" string theory vacua (paragraph breaks  and emphasis in bold added), which suggests that all of the Sting theory vacua are in the Swampland.
I haven’t paid much attention to the Swampland business since then, but noticed last night a new preprint with the title What if string theory has no de Sitter vacua?. The authors summarize their argument:
From this analysis we conclude that string theory has not made much progress on the problem of the cosmological constant during the last 15 years. There is a general agreement that the presence of dark energy should be an important clue to new physics. So far, string theory has not been up to the challenge. Or to be more precise, string theorists have not been up to the challenge. 
The well-motivated introduction of the anthropic principle and the multiverse, was a big relief. The mathematical standards were lowered, and unconstrained model building could set in exploring a wild and free landscape of infinite possibilities. But beyond this suggestive connection between a possible multiverse and the rich mathematical structures of string theory not much solid results have been achieved. 
We reviewed some fraction of the mounting evidence that most, if not all of this landscape, is a swampland and we refer to [14,16,149] for similar lines of thought. We believe it makes more sense to listen to what string theory is trying to tell us, then to try to get out of the theory what one would like to have. In recent years, especially with the program of the Swampland [14, 150–152], there is luckily a growing community that embraces this idea. Perhaps this program really already made its first prediction: no measurable tensor modes in the CMB. 
From what we have seen so far, we believe that the most sensible attitude is to accept there are no dS vacua at all because string theory conspires against dS vacua. 
The suggestion here is basically that effective field theories on a deSitter background are in the Swampland, so can’t be derived from string theory. Since we seem to live in a deSitter space, the obvious conclusion to draw from this is that string theory is falsified: it can’t be the fundamental theory we are looking for. The authors discuss various unconvincing ways to try and avoid this conclusion.
Physicists have been working on String Theory for basically my entire life. It has turned out to be a dead end and a distraction for the most part, although it has revealed some important mathematical insights including some relevant to quantum gravity. It is time to look for alternatives.

For what it is worth, it is also possible that gravity and the cosmological constant observed are not actually topological effects as in General Relativity, but are instead has a mechanism that involves the behavior of gravitons in a spacetime that is itself, fundamentally flat rather than being deSitter or anti-deSitter, even though that graviton behavior is similar to and in most circumstances almost exactly equivalent to, a topologically curved spacetime.

3 comments:

neo said...

"Physicists have been working on String Theory for basically my entire life. It has turned out to be a dead end and a distraction for the most part, although it has revealed some important mathematical insights including some relevant to quantum gravity. It is time to look for alternatives."


I actually agree and i think it's time for top universities physics departments to start hiring QG researchers in other fields, along with post docs grad students, and offer physics majors undergraduate courses in these alternatives, such as the usual list LQG, AS, CDT, emergent gravity etc

one specific example is a child prodigy named Jacob Barnett, he's with Lee Smolin at Perimeter researching LQG, when i last heard from him.

what's your fav alternative?

Mitchell said...

I am EXTREMELY skeptical. Plenty of people have argued that there are no eternally stable de Sitter vacua in string theory, but these authors even want to argue against metastable de Sitter vacua. That would mean that you cannot even temporarily have a positive cosmological constant in string theory.

So I would regard the technical difficulties that they point to, not as a sign that the whole landscape is a phantasm, but as a sign that there is more to learn about how these things really work in string theory.

I could compare it to calculating the particle masses for a specific string vacuum. The calculation has not been completed for any remotely realistic vacuum, but that doesn't mean that the calculation has no answer, that there is no such calculation. It just means that we can't figure out the equilibrium values of the geometric parameters.

I see these "problems of stringy de Sitter space" as a similar phenomenon on a deeper level: de Sitter space almost certainly DOES exist within string theory, we just haven't figured out all the nuances of it yet.

By the way, towards the end they also make some comments about difficulties of plain old QFT in de Sitter space.

neo said...

Mitchel have you thought about bringing this to Woit's attention?