Swampland (physics)

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

It is natural to ask if this landscape is as vast as allowed by consistent-looking effective field theories. Some authors (like Cumrun Vafa^[1]) suggest that this is not the case, and that the landscape is surrounded by an even more vast swampland of consistent-looking semiclassical effective field theories, which are actually inconsistent. Some proposed swampland criteria^[2] are:

• If we have 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. But that doesn't mean all charged particles ought to be light.
• This 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.

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