It is this property that leads to many of the counter-intuitive behaviours accurately predicted by Einstein’s theory of special relativity (e.g. The speed of light is the upper limit for the speeds of objects with positive rest mass, and individual photons cannot travel faster than the speed of light. This means that the speed of light has exactly the same value for observers travelling at different speeds. Michelson set the early standard for measurements of the speed of light in the late 1870s, determining a speed within 0.02 percent of the modern value. The Italian physicist Galileo Galilee was among the first to try to. The speed of light in a vacuum is a constant. In ancient times, many scientists believed the speed of light was infinite and could travel any distance instantaneously.Nothing can travel faster than the speed of light in a vacuum (though some particles can exceed the speed of light in a transparent medium – resulting in Cerenkov radiation). It wasn't until later that Danish astronomer Ole Roemer calculated a finite speed of light by studying Jupiter's moon, Io.The speed of light in a vacuum is 299,792,458 metres/ second (though it is less in a transparent medium such as air, water or glass, depending on the refraction index).Although always referred to as the speed of light, this speed should be more properly (if not so poetically) called the ‘speed of a massless particle’ as it is the speed at which all particles of zero mass (not only photons, but gravitons and massless neutrinos if they exist) travel in a vacuum.Įinstein’s theory of relativity makes several important statements about the speed of light in a vacuum: The ‘ speed of light’ (commonly denoted by c) generally refers to the speed at which electromagnetic radiation propagates in a vacuum.
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