Albert Einstein's theory of General Relativity, published in 1915, extended his theory of Special Relativity to systems that are accelerating. One of the primary causes of acceleration in the universe is gravity, and Einstein showed that the effects of acceleration are actually the same as those of the force of gravity; in fact, they are locally indistinguishable. For instance, both in an accelerating rocket in space and in a rocket standing on its launch pad on Earth, the astronauts are pushed back into their seats. Unlike Newtonian physics, which views gravity as an attractive force between all bodies in the universe, General Relativity describes the universe in terms of a continuous space-time fabric that is curved by masses located within it. In the space-time continuum of General Relativity, events are defined in terms of four dimensions: three of space, and one of time, with one coordinate for each dimension; we continuously move along the time dimension. What does it mean, though, for space-time to be curved? One way of conceptualizing this is to imagine just a two-dimensional space-time, with one spatial dimension and one time dimension. But instead of an infinite plane, imagine a tube, with an object's position in time defined by a coordinate of length along the tube, and position in space by a coordinate around the circumference of the tube. An object traveling uniformly through space then describes a helix along this tube, eventually returning to its starting space-coordinate position, but at a different time. (It is an open question in cosmology as to whether our universe has a similar curvature in three dimensions; if so, traveling in one direction long enough would bring you back to where you began.) An important consequence of the notion of curved space-time is that the curvature should affect all motion; thus, even light, which has no mass, should follow a curved path wherever gravity has warped space-time. An important verification of this-which made headlines around the world-took place during a solar eclipse on May 29, 1919, when it was observed that light from stars near the Sun was bent by an angle exactly predicted by the expected curvature of space-time near the massive Sun. Space-time can in principle be warped so strongly by a huge mass that any radiation emitted from the mass curves back in again and cannot escape. These huge masses are thought to exist as black holes.