The Swampland : Constraints from String Theory on low-energy theories

Physical theories are typically associated to a range of length, or energy, scales in which they are valid. Separation of scales in physics implies a certain decoupling between high-energy scales and low-energy scales. One aspect of this decoupling is that a single theory at high scales, say the Standard Model of particle physics, can lead to many different lower-energy theories, for example describing different materials in condensed matter. This leads to the question: given a high-energy theory, what are the possible low-energy theories that it could lead to? I will describe work over the past decade or so attempting to answer this question in the context of String Theory as the high-energy scale starting point. The results are a certain set of conjectured constraints that differentiate between low-energy theories that could arise from String Theory, the so-called Landscape, from those which cannot, the Swampland. I will then describe recent work developing the idea that the microscopic principle which differentiates between the two types of theories is that the dynamical degrees of freedom in the low-energy theory must be an emergent phenomenon associated to many interacting high-energy degrees of freedom.