 Spontaneous collapse theories  seek to modify the Schrödinger equation, so that superpositions of different ‘everyday’ states do not arise or are very unstable. The best known such theory is the so-called GRW theory (Ghirardi Rimini and Weber 1986), in which a material particle spontaneously undergoes localisation in the sense that at random times it experiences a collapse of the form used to describe approximate position  measurements.[29]  In the original model, the collapse occurs independently for each particle (a large number of particles thus ‘triggering’ collapse much more frequently); in later models the frequency for each particle is weighted by its mass, and the overall frequency for collapse is thus tied to mass  density.[30]