2.19  The Speed of Light

Throughout the history of Greek, Muslim, and Medieval European philosophy, many scholars debated whether light traveled instantaneously or at finite speed. Empedocles (490-430 BC) argued that light has a finite speed because it is something in motion, and therefore must take some time to travel. Aristotle argued to the contrary, that light is due to the presence of something, but it is not a movement. Euclid and Ptolemy suggested that light is emitted from the eye, thus enabling sight. Based on that theory, Heron of Alexandria (10-70 AD) argued that the speed of light must be infinite because distant objects, such as stars, appear immediately upon opening the eyes.

In 1021, Al-Hazen published the Book of Optics, in which he dismissed the emission theory of vision in favor of the now accepted intromission theory, in which light moves from an object into the eye. This led Al-Hazen to propose that light must have a finite speed, and that the speed of light decreases in denser mediums. In the 11th century, Abu Rayhan Muhammad Al-Biruni (973-1048 AD) agreed that light has a finite speed, and observed that the speed of light is much faster than the speed of sound.

In the early 17th century, Kepler believed that the speed of light was infinite, since empty space presents no obstacle to it. Descartes argued that if the speed of light were to be finite, the Sun, Earth, and Moon would be noticeably out of alignment during a lunar eclipse. Since such misalignment had not been observed, Descartes concluded the speed of light was infinite. Descartes speculated that if the speed of light were found to be finite, his whole system of philosophy might be demolished.

Many experimental physicists have attempted to measure the speed of light throughout history, with increased accuracy starting from the 17th century. 15th General Conference on Weights and Measures in 1975 recommended using the value:for the speed of light in vacuum. Since 1983, the meter as the base unit of length in the International System of Units (SI), was redefined in terms of this standard value of the speed of light, so that it equals exactly:seconds.

The effective velocity of light in various transparent substances containing ordinary matter, is less than its value in vacuum. Some teams of physicists were said to bring light to a “complete standstill” by passing it through a Bose-Einstein condensate of the element rubidium. However, in these experiments, light stored in the excited states of atoms, then re-emitted at an arbitrary later time, as stimulated by a second laser pulse, but during this time it had ceased to be light.

In 1856, Wilhelm Weber (1804-1891 AD) and Rudolf Kohlrausch (1809-1858 AD) measured the ratio of the electromagnetic and electrostatic units of charge:and found that its numerical value was very close to the speed of light. One year after that, Gustav Kirchhoff (1824-1887 AD) calculated that an electric signal in a resistanceless wire travels along the wire at this speed. Then in the early 1860s, James Clerk Maxwell (1831-1879 AD) showed that, according to the theory of electromagnetism he was working on, as we shall review in section 20, electromagnetic waves propagate in empty space at a speed equal to the above ratio, so he proposed that light is in fact an electromagnetic wave.

It was thought at the time that empty space was filled with a background medium called the luminousness aether in which the electromagnetic field existed. Some physicists thought that this aether acted as a preferred frame of reference for the propagation of light and therefore it should be possible to measure the motion of the Earth with respect to this medium, by measuring the isotropy of the speed of light.

Beginning in the 1880s several experiments were performed to try to detect this motion, the most famous of which is the experiment performed by Albert Michelson and Edward Morley in 1887. The detected motion was always less than the observational error. Modern experiments indicate that the two-way speed of light is isotropic (the same in every direction) to within 6 nanometres per second. Because of this experiment Hendrik Lorentz (1853-1928 AD) proposed that the motion of the apparatus through the aether may cause the apparatus to contract along its length in the direction of motion, and he further assumed, that the time variable for moving systems must also be changed accordingly, which led to the formulation of the Lorentz transformation. Based on Lorentz’s aether theory, Henri Poincaré (1854-1912 AD) showed in 1900 that this local time is indicated by clocks moving in the aether, which are synchronized under the assumption of constant light speed. In 1904, he speculated that the speed of light could be a limiting velocity in dynamics, provided that the assumptions of Lorentz’s theory are all confirmed. In 1905, Poincaré brought Lorentz’s aether theory into full observational agreement with the principle of relativity.

In 1905 Einstein postulated from the outset that the speed of light in vacuum, measured by a non-accelerating observer, is independent of the motion of the source or observer. Using this and the principle of relativity as a basis he derived the special theory of relativity, in which the speed of light in vacuum featured as a fundamental constant, also appearing in contexts unrelated to light. This made the concept of the stationary aether (to which Lorentz and Poincaré still adhered) useless and revolutionized the concepts of space and time. We shall discuss Special and General Relativity in the following chapter III.