The renormalization group provides a formal way to derive the running of a coupling, yet the phenomenology underlying that running can be understood intuitively. As explained in the introduction, the coupling constant sets the magnitude of a force which behaves with distance as 1 / r 2 . The 1 / r 2 -dependence was first explained by Faraday as the decrease of the force flux : at a point B distant by r from the body A generating a force, this one is proportional to the field flux going through an elementary surface S perpendicular to the line AB. As the flux spreads uniformly through space, it decreases according to the solid angle sustaining the surface S. In the modern view of quantum field theory, the 1 / r 2 comes from the expression in position space of the propagator of the force carriers. For relatively weakly-interacting bodies, as is generally the case in electromagnetism or gravity or the nuclear interactions at short distances, the exchange of a single force carrier is a good first approximation of the interaction between the bodies, and classically the interaction will obey a 1 / r 2 -law (note that if the force carrier is massive, there is an additional r dependence). When the interactions are more intense (e.g. the charges or masses are larger, or r is smaller) or happens over briefer time spans (smaller r ), more force carriers are involved or particle pairs are created, see Fig. 1, resulting in the break-down of the 1 / r 2 behavior. The classical equivalent is that the field flux does not propagate freely in space any more but e.g. undergoes screening from the charges of the extra virtual particles, or interactions between these virtual particles. It is convenient to separate the first-order 1 / r 2 law from this extra r -dependence. This latter is then accounted for by being included in the coupling, which then becomes 1 / r -dependent, (or equivalently μ -dependent). Since the additional particles involved beyond the single force carrier approximation are always virtual, i.e. transient quantum field fluctuations, one understands why the running of a coupling is a genuine quantum and relativistic phenomenon, namely an effect of the high-order Feynman diagrams on the strength of the force.