from infinite density. Although the densities of typical stellar-mass fuzzballs are quite great—about the same as neutron stars.—their densities are many orders of magnitude less than the Planck density (), which is equivalent to the mass of the universe packed into the volume of a single atomic nucleus. Fuzzballs become less dense as their mass increases due to fractional tension. When matter or energy (strings) fall onto a fuzzball, more strings are not simply added to the fuzzball; strings fuse together, and in doing so, all the quantum information of the infalling strings becomes part of larger, more complex strings. Due to fractional tension, string tension exponentially decreases as they become more complex with more modes of vibration, relaxing to considerable lengths. The "mathematical beauty" of the string theory formulas Mathur and Lunin employed lies in how the fractional tension values produce fuzzball radii that precisely equal Schwarzschild radii, which Karl Schwarzschild calculated using an entirely different mathematical technique 87 years earlier. Due to the mass-density inverse-square rule, fuzzballs need not all have unimaginable densities. There are also supermassive black holes, which are found at the center of virtually all galaxies. Sagittarius A*, the black hole at the center of our Milky Way galaxy, is 4.3 million . If fuzzball theory is correct, it has a mean density that is "only" 51 times that of gold. At 3.9 billion (a rather large super-massive black hole), a fuzzball would have a radius of 77 astronomical units—about the same size as the termination shock of the Solar System's heliosphere—and a mean density equal to that of the Earth's atmosphere at sea level (1.2 kg/m3). Irrespective of a fuzzball's mass and resultant density, the determining factor establishing where its surface lies is the threshold at which the fuzzball's escape velocity precisely equals the speed of light. Escape velocity, as its name suggests, is the velocity a body must achieve to escape from a massive object. For Earth, this is 11.2 km/s. In the other direction, a massive object's escape velocity is equal to the impact velocity achieved by a falling body that has fallen from the edge of a massive object's sphere of gravitational influence. Thus, event horizons—for both classic black holes and fuzzballs—lie precisely at the point where spacetime has warped to such an extent that falling bodies just achieve the speed of light. According to Albert Einstein, via his special theory