A Swampland Conjecture: Explained, Implications & Incompatibility

[Demystifier]

The swampland conjecture https://arxiv.org/abs/hep-th/0509212 is currently a very hot topic. Can someone explain, in simple terms, what exactly the swampland idea is? In particular, the conjecture states that the string landscape is surrounded by an even larger swampland of consistent-looking semiclassical effective field theories, which are actually inconsistent. What exactly does it mean that an effective theory is inconsistent?

 

[jedishrfu]

While I don't really understand it either, there's a lot references on Google:

https://www.google.com/search?q=Str...i57j0j69i61.2441j0j8&sourceid=chrome&ie=UTF-8

In particular, there were a few video with Prof Cumrun Vafa listed.

And this blog discussion about Cumrun Vafa's 2005 writings:

https://www.math.columbia.edu/~woit/wordpress/?p=10188

From Wikipedia, we get this definition of swampland:

In physics, the term swampland is used in contrast to the term "landscape" to indicate physical theories or aspects of such theories that could be true if gravity were not an issue but are not compatible with string theory. Recent developments in string theory suggest that the string theory landscape of False vacua is vast.

It is natural to ask if the landscape is as vast as allowed by consistent-looking effective field theories. Some authors (like Cumrun Vafa[1]) suggest that is not the case and that the landscape is surrounded by an even larger swampland of consistent-looking semiclassical effective field theories, which are actually inconsistent.

Some proposed swampland criteria[2] are the following:

• If there is a charge symmetry, that symmetry has to be a gauge symmetry, not a global one, and in the spectrum of charged particles, there has to be at least a particle with a mass in Planck units less than the gauge coupling strength. However, not all charged particles are necessarily light.
• That applies to magnetic monopoles as well.
• The sign of some higher order terms in the effective action is constrained by the absence of superluminal propagation.

https://en.wikipedia.org/wiki/Swampland_(physics)

 

[bapowell]

This thread (a few below this one) has been going for some time now: https://www.physicsforums.com/threads/no-metastable-type-iib-de-sitter-vacua.950271/

 

[Demystifier]

This thread (a few below this one) has been going for some time now: https://www.physicsforums.com/threads/no-metastable-type-iib-de-sitter-vacua.950271/

So does "inconsistent" really mean unstable?

 

[bapowell]

So does "inconsistent" really mean unstable?

I'm not sure...not an expert. Just a curious bystander. My limited impression is that in some of these instances, there simply is no corresponding dS solution in the UV-complete theory.

 

[ohwilleke]

The swampland conjecture https://arxiv.org/abs/hep-th/0509212 is currently a very hot topic. Can someone explain, in simple terms, what exactly the swampland idea is? In particular, the conjecture states that the string landscape is surrounded by an even larger swampland of consistent-looking semiclassical effective field theories, which are actually inconsistent. What exactly does it mean that an effective theory is inconsistent?

I think that the inconsistency is that subject to very broad and generic conditions (most notably that certain quantities which are naturally positive are indeed positive) that all vacua in the string landscape are necessarily anti-desitter, which is inconsistent with the observation that the actual universe is desitter.

So, to get the observed universe, you have to mess with what is inside it (basically the dark sector) to turn the natural topology of the string landscape into something that resembles the real universe.

Previous efforts to look at the landscape had determined that there was lots of swampland on an ad hoc basis, but failed to identify as the new conjectures do, overarching generic conditions that cause vacua to end up in swampland.

The optimists think that may be a small, finite number of exceptions to this general rule, narrowing string theory down to a handful a vacua that can be compared to reality one by one, rather than an intractably large landscape, but most commentators appear to conclude that all string vacua are in swampland thereby conceivably disproving all of the M-theory as a representation of physical reality, which would be a gigantic bummer to the entire string theory community.

But, I welcome the comments of anyone who understood it differently.

 

[Haelfix]

The Swampland arose in the mid 2ks as a sort of sociological response to the anything goes crowd. One of the things that was happening, was that any QFT coupled to gravity that you could write down, someone was instantly finding or postulating a stringy dual. So it seemed like String theory was merely a calculational tool and frequently superflous and was often discarded in favor of people using the more familiar effective field theory techniques.

But that was always based on a set of limits that weren't fully under control, and frequently pure string theorists were very skeptical of some of those constructions. Indeed String theory should be more than just EFT, and comes with various constraints and rules that might be invisible to the naive effective field theorist.

So the Swampland is an attempt (so far mostly conjectural) about formalizing the rules of the game which differentiate the set of all EFTs coupled to gravity from the set of consistent EFTs that have Stringy completions. The most famous example of this set of rules or constraints is called the weak gravity conjecture. Which is interesting in its own right, but seems to be fairly plausible (likely just waiting a precise mathematical statement). That just basically says that if you write down an effective field theory that includes coupling constants that are smaller than the gravitational coupling constant, then that theory does not have a stringy completion.

To give a classical analogy. Swampland rules are akin to energy conditions in GR. They are conjectural constraints about the possible forms of solutions the field equations can take. Almost certainly some of these conjectures will be too strong, but the hope is that at least some will ultimately be derived from the pure theory.

 

[Demystifier]

The Swampland arose in the mid 2ks as a sort of sociological response to the anything goes crowd. One of the things that was happening, was that any QFT coupled to gravity that you could write down, someone was instantly finding or postulating a stringy dual. So it seemed like String theory was merely a calculational tool and frequently superflous and was often discarded in favor of people using the more familiar effective field theory techniques.

But that was always based on a set of limits that weren't fully under control, and frequently pure string theorists were very skeptical of some of those constructions. Indeed String theory should be more than just EFT, and comes with various constraints and rules that might be invisible to the naive effective field theorist.

So the Swampland is an attempt (so far mostly conjectural) about formalizing the rules of the game which differentiate the set of all EFTs coupled to gravity from the set of consistent EFTs that have Stringy completions. The most famous example of this set of rules or constraints is called the weak gravity conjecture. Which is interesting in its own right, but seems to be fairly plausible (likely just waiting a precise mathematical statement). That just basically says that if you write down an effective field theory that includes coupling constants that are smaller than the gravitational coupling constant, then that theory does not have a stringy completion.

So "consistent" means consistent with string theory. Looks convincing, because that would explain why string theorists usually don't see a need to say that explicitly.

To give a classical analogy. Swampland rules are akin to energy conditions in GR. They are conjectural constraints about the possible forms of solutions the field equations can take. Almost certainly some of these conjectures will be too strong, but the hope is that at least some will ultimately be derived from the pure theory.

Let me offer another analogy. Since QCD is non-perturbative (and hence too hard for analytical treatment) in the low energy regime, physicists make up various models (e.g. linear and non-linear sigma model) which are supposed to be effective theories of hadron physics. But some of those models may in fact not be a low-energy limit of QCD, in which case the model is not consistent with QCD. Is that a good analogy?

 

[Demystifier]

Another question. Is swampland a problem or a solution to a problem? It seems to me that people mostly try to avoid the existence of a swampland, suggesting that swampland is a problem. If so, why exactly is it a problem?

 

[Demystifier]

I think that the inconsistency is that subject to very broad and generic conditions (most notably that certain quantities which are naturally positive are indeed positive) that all vacua in the string landscape are necessarily anti-desitter, which is inconsistent with the observation that the actual universe is desitter.

So, to get the observed universe, you have to mess with what is inside it (basically the dark sector) to turn the natural topology of the string landscape into something that resembles the real universe.

Previous efforts to look at the landscape had determined that there was lots of swampland on an ad hoc basis, but failed to identify as the new conjectures do, overarching generic conditions that cause vacua to end up in swampland.

The optimists think that may be a small, finite number of exceptions to this general rule, narrowing string theory down to a handful a vacua that can be compared to reality one by one, rather than an intractably large landscape, but most commentators appear to conclude that all string vacua are in swampland thereby conceivably disproving all of the M-theory as a representation of physical reality, which would be a gigantic bummer to the entire string theory community.

But, I welcome the comments of anyone who understood it differently.

That would mean that "consistent" means consistent with observations. Pretty odd definition of consistency for string theorists, I would say.

 

[Urs Schreiber]

So "consistent" means consistent with string theory. Looks convincing, because that would explain why string theorists usually don't see a need to say that explicitly.

Sounds funny (is funny!), but that's exactly what's happening.

 

[Urs Schreiber]

Haelfix in #7 has it right:

"landscape" = possible string theory vacua

"swampland" = field theories which are not low energy limits of possible string theory vacua.

The mathematician could say: the swampland is the co-kernel of the map that sends a string vacuum to its effective field theory.

 

[Urs Schreiber]

Another question. Is swampland a problem or a solution to a problem?

Notice that "landscape"/"swampland" is whimsical terminology for what in sober language are the predictions of string theory.

Landscape = solutions to string theory

Swampland = field theories that cannot approximate solutions of string theory.

(That's what the terms are meant to mean, not necesssarily what has been made sense of and proven.)

Hence the larger the swampland, the more predictive is string theory, compared to field theory.

 

[Demystifier]

Hence the larger the swampland, the more predictive is string theory, compared to field theory.

This means that swampland is a good thing from the point of view of true string theorists. But I can imagine that phenomenological model builders may view swampland as a bad thing, because their favored models (chosen by their own criteria, but not strictly derived from string theory) may look less appealing when there is a serious danger that those models could be in the swampland.

 

[Urs Schreiber]

their favored models may look less appealing when there is a serious danger that those models could be in the swampland.

Absolutely. I am guessing that Vafa chose the whimsical term "swampland" precisely in order to evoke such feelings.

I just wish he wouldn't associate all those sweeping conjectures now with the term. The field of string phenomenology has now had enough of whimsicalism and conjectures, what it rather needs is some solid professionalism and people who do the hard work of figuring out what the heck is actually the case.