Abstract

Many models in physics focus on a small number of particles (often two) which are placed in an empty universe. But in solids and liquids, the number of particles is vast and many-particle interactions cannot be ignored. Moreover, the world is quantum-mechanical, not classical, and this further enriches the possible consequences of these interactions. New emergent phenomena appear, whose origin is hard to guess from the basic underlying equations (the Schrôdinger equation) but which can be described by new physical principles. I will describe some recent highlights of this exciting field of quantum matter, and in particular how such systems can be studied using muons, radioactive particles which can be implanted inside solids and allow a measurement of microscopic magnetic fields.