physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...

, the term swampland refers to effective low-energy physical theories which are not compatible with

quantum gravity Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics; it deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the vi ...

. This is in contrast with the so-called "

string theory landscape The string theory landscape or landscape of vacua refers to the collection of possible false vacua in string theory,The number of metastable vacua is not known exactly, but commonly quoted estimates are of the order 10500. See M. Douglas, "The ...

" that are known to be compatible with

string theory In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and interac ...

which is believed to be a consistent quantum theory of gravity. In other words, the Swampland is the set of consistent-looking theories with no consistent ultraviolet completion with the addition of gravity. Developments in string theory suggest that the string theory landscape of false vacua is vast, so it is natural to ask if the landscape is as vast as allowed by anomaly-free effective field theories. The Swampland program aims to delineate the theories of quantum gravity by identifying the universal principles shared among all theories compatible with a gravitational UV-completion. The program was initiated by

Cumrun Vafa Cumrun Vafa ( fa, کامران وفا ; born 1 August 1960) is an Iranian-American theoretical physicist and the Hollis Professor of Mathematics and Natural Philosophy at Harvard University. Early life and education Cumrun Vafa was born in Tehran ...

, who argued that string theory suggests that the Swampland is in fact much larger than the string theory landscape.

Swampland conjectures

The swampland conjectures are a set of

conjecture In mathematics, a conjecture is a conclusion or a proposition that is proffered on a tentative basis without proof. Some conjectures, such as the Riemann hypothesis (still a conjecture) or Fermat's Last Theorem (a conjecture until proven in 19 ...

d criteria for theories in the string theory landscape. Some proposed swampland criteria: * If there is a charge symmetry, that symmetry has to be a gauge symmetry, not a global one, and at least one charged particle must have a mass in Planck units less than the gauge coupling strength. However, not all charged particles are necessarily light. The same applies to

magnetic monopole In particle physics, a magnetic monopole is a hypothetical elementary particle that is an isolated magnet with only one magnetic pole (a north pole without a south pole or vice versa). A magnetic monopole would have a net north or south "magneti ...

s as well. This criterion would imply the weak gravity conjecture. * The sign of some higher order terms in the

effective action In quantum field theory, the quantum effective action is a modified expression for the classical action taking into account quantum corrections while ensuring that the principle of least action applies, meaning that extremizing the effective ac ...

is constrained by the absence of

superluminal Faster-than-light (also FTL, superluminal or supercausal) travel and communication are the conjectural propagation of matter or information faster than the speed of light (). The special theory of relativity implies that only particles with zero ...

propagation. * There are finitely many types of

massless particle In particle physics, a massless particle is an elementary particle whose invariant mass is zero. There are two known gauge boson massless particles: the photon (carrier of electromagnetism) and the gluon (carrier of the strong force). However, g ...

References

External links

* Lecture by Cumrun Vafa
String Landscape and the Swampland
March 2018 String theory Quantum gravity {{string-theory-stub