speed of light
The "speed of light" can also mean the speed at which light travels in a given medium. For example, light travels only two-thirds as quickly in glass as it does in a vacuum. The change in velocity when light enters a medium such as air, water or glass causes the light to bend on entering a different medium and refraction occurs. The refractive index of a medium is the ratio of the velocity of light in a vacuum to its velocity in the medium. For example, the refractive index of water is 1.333 or 4/3, and thus the velocity of light in water is only three-quarters of its velocity in a vacuum.
If something, such as a subatomic particle, travels faster through a medium than light does, the result is a kind of electromagnetic shock wave known as Cerenkov radiation. However, there is no violation of the laws of physics, since the universal speed limit is how fast light travels in a vacuum.
The speed of light also limits the maximum rate at which information can be transferred from one place to another. Although nonlocal effects in quantum mechanics, such as teleportation, may be instantaneous even over great distances, they do not violate the special theory of relativity because no matter, energy, or information is moved faster than light. However, various ways by which the "light barrier" might be broken or circumvented have been proposed by theoretical physicists.
Any particles of zero rest mass, such as photons, travel at the speed of light. Massive particles approach the speed of light when their energy is very large relative to their rest energy.
Measuring the speed of light
The first reasonably accurate value was calculated by the Danish astronomer Olaus Römer in 1676. He noticed that Io, one of the moons of Jupiter, was blocked from the Earth's view at certain intervals of time. These eclipses occurred when Io was behind Jupiter which stopped the light from reaching the Earth. When the Earth was nearest to Jupiter he calculated the times when he expected the eclipses to happen, but he found that when the Earth was on the far side of the Sun the eclipses occurred some time later. He reasoned that the delay was due to the light take longer to travel the extra distance. As he knew the distance involved he was able to calculate the speed of light.
Another astronomical determination of the velocity was made by the English astronomer James Bradley in 1728. He observed that the stars are seen in slightly different directions depending on the position of the Earth in its orbit. This phenomenon, called stellar aberration, is caused by the Earth's motion and the differences in direction are simply related to the difference between this motion and the velocity of light. Bradley was therefore able to obtain a value for the velocity of light, and it was of the same order as Römer's figure.
It was until almost 200 years later that Römer's result was verified by the Frenchmen, Fizeau and Foucault. Fizeau in 1849, and Foucault in 1862, succeeded in measuring the speed of light using comparatively short light paths. Neither made use of astronomy.
Light from the source passes through a plate of glass and then through a converging lens which concentrates it into the reflecting surface of a plane (flat) mirror. Another mirror, this time concave, is placed so that light reflected from the plane mirror is brought to focus on its surface and reflected back along the same path by which it came. But when the light reaches the plate of glass, although some of it passes through, some is also reflected by its surface in just the same way that a glass window can give a reflection. These reflected light rays converge forming an image of the light source. This image is viewed through an eyepiece. Now the plane mirror is set rotating as fast as it can be made to move. When the light it reflects falls back on it, the mirror is in a different position and consequently the image viewed through the eyepiece is slightly shifted. The light speed can be worked out from the speed of rotation, the extent of the shift, and the distances between the mirrors.
There are many modifications to this basic idea from which more accurate results have been found.
The largest error in Foucault's method lay in measuring the small shift made by the image. It would have been much better if he could have rotated the mirror so fast that it was back again in the same position to receive the returning light. This would give an image in the same position and only the speed of rotation and the distances between the two mirrors would be needed to calculate the speed of light. Michelson overcame this problem by replacing the plane mirror with a glass octagon. Different facets of the octagon reflected the light on both its outward and return journeys. Hence light could only reach the observer when the octagon was in one particular position. This would be the case if the octagon were stationary or if it were rotating at such a speed that one facet exactly replaced the next while the light was traveling over its measured path to a distant reflector and back again. The speed of rotation which fulfils this condition could be found accurately and used in calculating the speed of light.
Michelson's methodA rotating mirror was used by Michelson to measure the velocity of light in 1927. Light traveled from one face of the mirror to a plane mirror 35km (22 miles) away and then back to another face and an eyepiece. An image of the light source was obtained with the mirror first stationary and then rotated at sufficient speed for the image to be seen in the same position. At such a speed the mirror turned so that the next face moved into position as the light made its 70km (44 miles) journey to and from the plane mirror. Velocity of light was calculated from the speed of rotation.
The mystery of the ether
As the medium through which light moved could not be seen to exist, one was invented; it was called the ether and it was supposed to pervade the whole universe. Thorny problems surrounded the ether. Known wave motions move more rapidly in denser, more elastic substances and a wave motion as fast as light should theoretically need a medium denser than steel. Yet the planets continue to sail through space, unimpeded by the ether. There were many other contradictions and so an experiment was made to detect the motion of the Earth through the ether.
In the 1880s two American physicists, Albert Michelson (1852-1931) and Edward Morley (1838-1923), made a simple instrument to detect the ether (see Michelson-Morley experiment). In it a beam of light was split into two beams at right-angles and the two beams reflected from mirrors before recombining. Combined beams show interference effects if one travels a slightly longer path than the other. Michelson and Morley observed the combined beams in one direction and then turned the instrument at right-angles and observed the beams again. If the light were traveling in an ether it would have to move over a different path in the direction of the Earth's motion from at right-angles to it. Turning the instrument at right-angles should show a difference in the interference effects if the ether existed. None was observed and none had been observed in many repeats of this classic experiment.
The basis of relativityThe conclusion of the Michelson-Morley experiment was that ether does not exist and light does not need a medium for its propagation, or that the ether can never be detected. Without a stationary ether there is no basis in the universe against which the absolute motion of everything can be measured, except for light. The Michelson-Morley experiment showed that the velocity of light is the same in the direction of the Earth's motion as at right-angles to it and is always the same whatever the observer's motion. These conclusions had profound implications but to realize them it took a genius – Albert Einstein – who used them as a basis for the theory of relativity.
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