Something from nothing: linking molecules with virtual light

A new research field is emerging in which ensembles of molecules are collectively hybridised with light in a process known as strong coupling. This hybridisation leads to the formation of new states that are part light and part matter, states known as polaritons. Here we offer an entry point into the field of molecular strong coupling. We include an overview of the essential phenomena and an introduction to the conceptual framework – considerable use is made of simple classical physics models since they are helpful in developing an intuitive understanding. Open questions are identified and discussed, as well as some of the exciting experimental and theoretical challenges that lie ahead.