Wilson’s renormalisation group is often regarded as the most important idea in physics from the second half of the twentieth century. It has had profound influence in fields as disparate as condensed matter physics and high-energy particle physics. One of the key ideas of the renormalisation group is that rather than study the full Hamiltonian of the system of interest, which is usually intractable, one should study an effective low energy Hamiltonian. An effective Hamiltonian is derived by ‘integrating out’ the high-energy degrees of freedom of the system....Read more