Swampland

In physics

Physics

Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

, the term swampland is used in contrast to the term "landscape

String theory landscape

The string theory landscape or anthropic landscape refers to the large number of possible false vacua in string theory. The "landscape" includes so many possible configurations that some physicists think that the known laws of physics, the standard model and general relativity with a positive...

," to indicate physical theories or aspects of such theories which could be true if gravity wasn't an issue, but which are not compatible with string theory

String theory

String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything , a manner of describing the known fundamental forces and matter in a mathematically complete system...

. Recent developments in string theory suggest that the string theory landscape of vacua

False vacuum

In quantum field theory, a false vacuum is a metastable sector of space that appears to be a perturbative vacuum, but is unstable due to instanton effects that may tunnel to a lower energy state. This tunneling can be caused by quantum fluctuations or the creation of high-energy particles...

 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

Cumrun Vafa

Cumrun Vafa is an Iranian-American leading string theorist from Harvard University where he started as a Harvard Junior Fellow. He is a recipient of the 2008 Dirac Medal.-Birth and education:...

) 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 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
  Planck units
  In physics, Planck units are physical units of measurement defined exclusively in terms of five universal physical constants listed below, in such a manner that these five physical constants take on the numerical value of 1 when expressed in terms of these units. Planck units elegantly simplify...
  
   less than the gauge coupling strength. But that doesn't mean all charged particles ought to be light.
• This applies to magnetic monopole
  Magnetic monopole
  A magnetic monopole is a hypothetical particle in particle physics that is a magnet with only one magnetic pole . In more technical terms, a magnetic monopole would have a net "magnetic charge". Modern interest in the concept stems from particle theories, notably the grand unified and superstring...
  
  s as well.
• The sign of some higher order terms in the effective action
  Effective action
  In quantum field theory, the effective action is a modified expression for the action, which takes into account quantum-mechanical corrections, in the following sense:...
  
   is constrained by the absence of superluminal propagation.