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The speed of light in vacuum, commonly denoted c, is a universal physical constant that is exactly equal to 299,792,458 metres per second (approximately 300,000 kilometres per second; 186,000 miles per second; 671 million miles per hour).
The two-way speed of light is the average speed of light from one point, such as a source, to a mirror and back again. Because the light starts and finishes in the same place, only one clock is needed to measure the total time; thus, this speed can be experimentally determined independently of any clock synchronization scheme.
In the context of this article, "faster-than-light" means the transmission of information or matter faster than c, a constant equal to the speed of light in vacuum, which is 299,792,458 m/s (by definition of the metre) [3] or about 186,282.397 miles per second.
Physicists generally believe faster-than-light travel is impossible. Relativistic time dilation allows a traveler to experience time more slowly, the closer their speed is to the speed of light. [34] This apparent slowing becomes noticeable when velocities above 80% of the speed of light are attained.
The flash of light is given off at a point exactly halfway between the two ends of the train, and again form two 45° lines, expressing the constancy of the speed of light. In this picture, however, the points at which the light flashes hit the ends of the train are not at the same level; they are not simultaneous.
At a sufficient distance, the speed at which the beam "moves" may exceed the speed of light. The lighthouse paradox is a thought experiment in which the speed of light is apparently exceeded. The rotating beam of light from a lighthouse is imagined to be swept from one object to shine on a second object. The farther the two objects are away ...
Eddington's 1919 measurements of the bending of star-light by the Sun's gravity led to the acceptance of general relativity worldwide. Around 1904–1905, the works of Hendrik Lorentz, Henri Poincaré and finally Albert Einstein's special theory of relativity, exclude the possibility of propagation of any effects faster than the speed of light ...
The limitations of phase velocity beyond the speed of light later led him to develop his Dirac communicator. Larry Niven used hyperwave in his Known Space series as the term for a faster-than-light method of communication. Unlike the hyperdrive that moved ships at a finite superluminal speed, hyperwave was essentially instantaneous.