In 1916, Albert Einstein published his General Theory of Relativity, following his 1905 publication of the Special Theory of Relativity. In many ways, Einstein's work on relativity was the most revolutionary change to physics since Isaac Newton worked out the gravitation during the Enlightenment. Einstein's general relativity is a synthesis of special relativity and Newton's Law of Universal Gravitation, describing gravity as a geometric property of space and time.
Einstein's work on general relativity began soon after publishing his 1905 work, as he sought to incorporate gravity into his formulations. By late 1915, he presented his work to the Prussian Academy of Science, where he outlined the Einstein field equations. This is a set of ten equations that describe the basic interaction of gravity as the result of space and time (spacetime) being curved by matter and energy.
One theoretical result of general relativity was the prediction of the existence of black holes. Black holes are regions of space where space and time become so distorted by the crushing weight of gravity (usually from a collapsed star) that nothing, not even light, can escape. Although black holes cannot be observed, scientists have been able to detect their existence through their gravitation interaction with surrounding matter. Scientists believe that there is a black hole at the center of the Milky Way galaxy.