<Tr> <Td> 1983 </Td> <Td> 17th CGPM, definition of the metre </Td> <Td> 7005299792458000000 ♠ 299 792. 458 (exact) </Td> </Tr> <P> Until the early modern period, it was not known whether light travelled instantaneously or at a very fast finite speed . The first extant recorded examination of this subject was in ancient Greece . The ancient Greeks, Muslim scholars, and classical European scientists long debated this until Rømer provided the first calculation of the speed of light . Einstein's Theory of Special Relativity concluded that the speed of light is constant regardless of one's frame of reference . Since then, scientists have provided increasingly accurate measurements . </P> <P> Empedocles (c. 490--430 BC) was the first to propose a theory of light and claimed that light has a finite speed . He maintained that light was something in motion, and therefore must take some time to travel . Aristotle argued, to the contrary, that "light is due to the presence of something, but it is not a movement". Euclid and Ptolemy advanced Empedocles' emission theory of vision, where light is emitted from the eye, thus enabling sight . Based on that theory, Heron of Alexandria argued that the speed of light must be infinite because distant objects such as stars appear immediately upon opening the eyes . Early Islamic philosophers initially agreed with the Aristotelian view that light had no speed of travel . In 1021, Alhazen (Ibn al - Haytham) published the Book of Optics, in which he presented a series of arguments dismissing the emission theory of vision in favour of the now accepted intromission theory, in which light moves from an object into the eye . This led Alhazen to propose that light must have a finite speed, and that the speed of light is variable, decreasing in denser bodies . He argued that light is substantial matter, the propagation of which requires time, even if this is hidden from our senses . Also in the 11th century, Abū Rayhān al - Bīrūnī agreed that light has a finite speed, and observed that the speed of light is much faster than the speed of sound . </P> <P> In the 13th century, Roger Bacon argued that the speed of light in air was not infinite, using philosophical arguments backed by the writing of Alhazen and Aristotle . In the 1270s, Witelo considered the possibility of light travelling at infinite speed in vacuum, but slowing down in denser bodies . </P>

Who discovered that light is faster than sound