September 2015 saw the historic discovery of gravitational waves, almost exactly 100 years after Einstein predicted their existence as a consequence of his theory of general relativity. Gravitational waves are a literal stretching and compressing of the fabric of space. Even the most sensitive instrumentsβcapable of sensing that the path of a 4-km-long laser beam has lengthened by one-thousandth the diameter of a protonβcan detect waves created by only the most extreme cosmic events. The first detection was due to the collision of two black holes more than 750 million light years from earth. Although a full description of gravitational waves requires knowledge of Einstein's general relativity, a surprising amount can be understood with the physics you've already learned. (d) Two black holes collide and merge when their Schwarzchild radii overlap; that is, they merge when their separation, which we've defined as 2r, equals 2RSch . Find an expression for ΞE=EfβEi , where Ei β 0 because initially the black holes are far apart and Ef is their total energy at the instant they merge. This is the energy radiated away as gravitational waves. Your answer will be a fraction of McΒ², and you probably recognize that this is related to Einstein's famous E=mcΒ² . The quantity McΒ² is the amount of energy that would be released if an entire star of mass M were suddenly converted entirely to energy.