General relativity and quantum mechanics were crowning achievements of physics in the 20th century, and their unification has been left as our homework in the 21st century. Superstring theory is our best candidate for the unification. Although predictions of the theory are typically at extremely high energy and out of reach of current experiments and observations, several non-trivial constraints on its low energy effective theory have been found. Because of the unusual ultraviolet behavior of gravitational theory, the standard argument for separation of scales may not work for gravity, leading to robust low energy predictions of consistency requirements at high energy. In this talk, the speaker will discuss why the unification of general relativity and quantum mechanics has been difficult. After introducing the holographic principle as our guide to the unification, the speaker will turn our attention to its use to find constraints on symmetry in consistent quantum theory of gravity. The Speaker will also discuss the weak gravity conjecture, which gives a lower bound on Coulomb-type forces relative to the gravitational force, and consequences of the conjecture.

This lecture is a part of Kavli Asian Winter School (KAWS) on Strings, Particles and Cosmology 2018