Speed of light: the speed at which light waves (electromagnetic waves) expand in a vacuum.
For a long time, light was thought to travel at infinite speed. Galileo Galilei was the first to challenge this assumption, and around 1600 he made an attempt to determine the speed of light with more scientific accuracy. He placed two men on top of two hills who were to send a light signal back and forth between each other using bull's-eyes. Although theoretically this seemed an appropriate method, in practice it was doomed to fail due to an unacceptable degree of measurement imprecision. The first more or less exact measurement was performed by the Danish astronomer Ole Rømer in 1676. He measured the time intervals between various eclipses of Jupiter's moons at different distances from the Earth. As a result, he estimated the speed of light to be between 200,000 and 300,000 km/s.
In 1849, Hippolyte Fizeau measured the speed of light employing Galilei's method, though he replaced the men and bull's-eyes with a gearwheel and mirror. The experiment was conducted on the outskirts of Paris. A ray emitted from a light source S passed the teeth of a fast rotating gearwheel F and was reflected back to the gearwheel though the objective of a telescope L4 by a mirror M2 located at a distance of almost 9 kilometers from the light source. Whether the light ray becomes visible in the telescope or is blocked on its way back by a tooth of the gearwheel depends on the gearwheel's rotating speed and on how long it takes the light to get to the mirror and back. Fizeau calculated a speed of light of 313,000 km/s.
In 1887 Abraham Michelson and Edward Morley conducted their famous ether experiment. They used an interference apparatus to measure the speed of light in the direction of the Earth's motion as well as at a right angle to it. In doing so they discovered that the speed of light is equal in all directions, hence independent of the light receiver's motions. Since previous observations of binary stars had already shown that the observed speed of light does not depend on the light source either,* this was an unexpected and — given the state of physics at the time — seemingly inexplicable result. It led to the development of ►Special Relativity Theory and to the insight that the speed of light is the maximum limit of all possible speeds.
Today, the estimated value of the speed of light in a vacuum is 299,792.458 km/s; this speed is regarded as a ►natural constant. Light itself has a dual nature. It is both an electromagnetic wave, that is, a rhythmic vibration of the electric and magnetic fields in space, and a stream of massless particles, the so-called photons. Depending on the kind of experiment that one conducts with light, it can display either its wave or its particle nature. The energy of a light particle corresponds to the vibration frequency of the light's source, while the number of particles corresponds to the strength (amplitude) of the wave. The human eye is able to discern an incoming stream of merely 200 photons or so as a weak flash of light.
* Binary stars are stars that circle one another, taking turns in moving toward the Earth and away from it. If the speed of light were dependent on the motion of its source, then light emitted by such a star would reach us sooner when it was moving toward us, later when it was moving away; this would show up to us as an "aberration" of the starlight.