Speed of lightThe speed of light in vacuum is exactly equal to 299,792,458 metres per second. This is approximately 300,000 kilometres per second, or 186,000 miles per second. At present, the speed of light is a definition, not a measurement, as the metre is defined in terms of the speed of light and not vice versa.
RefractionIn passing through materials, light is slowed to less than , by the ratio called the refractive index of the material. The speed of light in air is only slightly less than . Denser media such as water and glass can slow light much more, to fractions such as 3/4 and 2/3 of .\nOn the microscopic scale this is caused by continual absorption and re-emission of the photons that compose the light by the atoms or molecules through which it is passing."Faster-than-light" experimentsRecent experimental evidence shows that it is possible for the group velocity of light to exceed c. One experiment made the group velocity of laser beams travel for extremely short distances through caesium atoms at 300 times . However, it is not possible to use this technique to transfer information faster than ; the product of the group velocity and the velocity of information transfer is equal to the square of the normal speed of light in the material. Exceeding the group velocity of light in this manner is comparable to exceeding the speed of sound by arranging people in a distantly spaced line of people, and asking them all to shout "I'm here!", one after another with short intervals, each one timing it by looking at their own wristwatch so they don't have to wait until they hear the last person shouting. The speed of light may also appear to be exceeded in some phenomena involving evanescent waves. Again, it is not possible that information is transmitted faster than ."Slower-than-light" (i.e., slowing light) experimentsIn 1999, a team of scientists led by Lene Hau were able to slow the speed of a light beam to about 17 meters per second. In 2001, they were able to momentarily stop a beam. See Bose-Einstein condensate for more information. In 2003, Mikhail Lukin, with scientists at Harvard University and the Lebedev Institute in Moscow, succeeded in completely halting light by directing it into a mass of hot rubidium gas, the atoms of which, in Lukin's words, "[behaved] like tiny mirrors", due to an interference pattern in two "control" beams.HistoryUntil relatively recent times, the speed of light was largely a matter of conjecture. Empedocles maintained that light was something in motion,\nand therefore there had to be some time elapsed in traveling. Aristotle said that, on the contrary, "light is due to the presence of something, but it is not a movement". Furthermore, if light had a finite speed, it would have to be very great; Aristotle asserted "the strain upon our powers of belief is too great" to believe this. One of the ancient theories of vision is that light is emitted from the eye,\ninstead of being reflected into the eye from another source. On this theory,\nHeron of Alexandria advanced the argument that the speed of light must be infinite, since distant objects such as stars appear immediately when one opens one's eyes. The Islamic philosophers Avicenna and Alhazen believed that light has a finite speed, although most philosophers agreed with Aristotle on this point.\nJohannes Kepler believed that the speed of light is infinite since empty space presents no obstacle to it. Francis Bacon argued that the speed of light is not necessarily infinite, since something can travel too fast to be perceived, such as a musket ball. Rene Descartes argued that if the speed of light were finite, the Sun, Earth, and Moon would be noticeably out of alignment during a lunar eclipse. Since such misalignment had not be observed, Descartes concluded the speed of light is infinite. In fact, Descartes was convinced that if the speed of light were finite, his whole system of philosophy would be demolished. Isaac Beeckman, a friend of Descartes, proposed an experiment (1629) in which one would observe the flash of a cannon reflecting off a mirror about one mile away. Galileo proposed an experiment (1638) to measure the speed of light by observing the delay between uncovering a lantern and its perception some distance away. This experiment was carried out by the Accademia del Cimento of Florence in 1667, with the lanterns separated by about one mile. No delay was observed. Robert Hooke explained the negative results of Descartes and Galileo by pointing out that such observations did not establish the infinite speed of light, but only that the speed must be very great. The first quantitative estimate of the speed of light was made in 1676 by Ole Rømer, who was studying the motions of Jupiter's satellite Io. Rømer observed that eclipses of Io by Jupiter appeared sooner when Earth was approaching Jupiter and later when Earth was moving farther away. Rømer correctly deduced that this discrepancy was due to the time it took for light to cross the lesser or greater distance between the planets. On the basis of his observations, Rømer estimated that it would take light 22 minutes to cross the diameter of the orbit of the Earth (that is, twice the astronomical unit); the modern estimate is closer to 16 minutes and 40 seconds. Around the same time, the astronomical unit was estimated to be about 140 million kilometres. The two results were combined by Christiaan Huygens, who estimated the speed of light to be 16 and 2/3 Earth diameters per second. This is about 220,000 kilometres per second, well below the currently accepted value, but still very much faster than any physical phenomenon then known. The finite speed of light was not conclusively established by these observations, as it could be argued the differences in the times of eclipses were due to perturbations of the orbits of the satellites. However, after the observations of James Bradley (1728) the hypothesis of infinite speed was considered discredited. Bradley deduced that starlight falling on the Earth should appear to come from a slight angle, which could be calculated by comparing the speed of the Earth in its orbit to the speed of light. This "aberration of light", as it is called, was observed to be about 1/200 of a degree. Bradley calculated the speed of light as about 185,000 miles per second. This is only slightly less than the currently accepted value. The abberation effect has been studied extensively over the succeeding centuries, notably by Friedrich Georg Wilhelm Struve and Magnus Nyren. The first successful measurement of the speed of light using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Fizeau's experiment was conceptually similar to those proposed by Beeckman and Galileo. A beam of light was directed at a mirror several thousand meters away. On the way from the source to the mirror, the beam passed through a rotating cog wheel. At a certain rate of rotation, the beam could pass through one gap on the way out and another on the way back. But at slightly higher or lower rates, the beam would strike a tooth and not pass through the wheel. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, the speed of light could be calculated. Fizeau reported the speed of light as 313,000 kilometres per second. Fizeau's method was later refined by Marie Alfred Cornu (1872) and Joseph Perrotin (1900). Leon Foucault improved on Fizeau's method by replacing the cogwheel with a rotating mirror. Foucault's estimate, published in 1862, was 298,000 kilometres per second. Foucault's method was also used by Simon Newcomb and Albert A. Michelson. Michelson began his lengthy career by replicating and improving on Foucault's method.See alsoReferencesHistorical references
Modern references
External links
|
||
"Sometimes a scream is better than a thesis." - Ralph Waldo Emerson (1803-1882) |
