Otherwise known as GTR, the general theory of relativity is a concept in geometry that relates to gravitation. Einstein published on the subject of GTR in 1915, revolutionising human understanding on the topic, as previously laid out by Newton. Einstein introduced a new concept, known as space-time, which Newton had not included, that allowed him to explain why gravity varies according to its relative position to its observer. Special relativity is also attributed to Einstein, but it was a theory developed by him from the work of Hendrik Lorentz. In this theory, an attempt is made to define the relationship between time and physical space. Famously, it postulates the notion that the speed of light is a constant for all observers regardless of their relative position, so long as it is travelling in a vacuum.