The swampland and string theory

At very small distance scales, gravity becomes sensitive to quantum effects, implying that we need a quantum theory of gravity to describe physics. However, it is typically assumed that at larger distance scales this quantum gravitational physics is not important and can mostly be ignored. The Swampland programme is a field of contemporary research in theoretical physics which challenges this assumption, proposing that there can remain certain features of quantum gravity at long distances. Theories of physics which do not exhibit these features are then incompatible with quantum gravity and are termed to lie in a Swampland of inconsistent theories. Most of the evidence for such remnant features comes from string theory, our leading candidate for a quantum theory of gravity. This article introduces the Swampland programme, its connection to string theory, and its implications for how we construct new theories of physics, including those aiming to describe our universe.