The speed of light c in vacuum is not measured. It has an exact fixed value in standard units. According to the international agreement of 1983, a meter is defined as the length of the path traveled by light in a vacuum in a time of 1/299792458 seconds. The speed of light is exactly 299,792,458 m / s. An inch is defined as 2.54 centimeters. Therefore, in non-metric units, the speed of light also has an exact value. Such a definition makes sense only because the speed of light in a vacuum is constant, and this fact must be confirmed experimentally (see Is the speed of light constant?). It is also necessary to experimentally determine the speed of light in media such as water and air.
Until the seventeenth century, it was believed that light spreads instantly. This was confirmed by observations of the lunar eclipse. At a finite speed of light, there should be a delay between the position of the Earth relative to the Moon and the position of the Earth's shadow on the surface of the Moon, but no such delay was found. We now know that the speed of light is too fast to notice a delay. Galileo doubted the infinity of the speed of light. He proposed a way to measure it by closing and opening a lantern several miles away. It is not known whether he tried such an experiment, but due to the very high speed of light, the measurement could not be successful.
The first successful measurement of c was made by Olaf Roemer in 1676. He noticed that the time between eclipses of Jupiter's satellites is shorter when the distance from Earth to Jupiter decreases, and longer when this distance increases. He realized that this is due to a change in the time it takes for light to travel from Jupiter to Earth as the distance between them changes. He calculated that the speed of light is 214,000 km / s. The inaccuracy is due to the fact that the distances between the planets at that time were not yet well defined.
In 1728, James Bradley estimated the magnitude of the speed of light by observing the aberration of stars (a change in the apparent position of a star caused by the movement of the Earth around the Sun). He observed one of the stars in the constellation Draco, and found that its apparent position changes throughout the year. This effect works for all stars, as opposed to parallax, which is more noticeable for nearby stars. Aberration is similar to the effect of motion on the angle of incidence of raindrops. If you are standing and there is no wind, then drops fall vertically on your head. If you run, it turns out that the rain comes at an angle and hits your face. Bradley measured this angle for starlight. Knowing the speed of the Earth's movement around the Sun, he determined that the speed of light is 301,000 km / s.
The first measurement of c on Earth was made by Armand Fizeau in 1849. He used the reflection of light from a mirror 8 km away. A beam of light passed through a gap between the teeth of a rapidly rotating wheel. The rotation speed was increased until the reflected beam became visible in the next gap. The calculated value of c turned out to be 315,000 km / s. A year later, Leon Foucault improved this method using a rotating mirror and obtained a much more accurate value of 298,000 km / s. The improved method was accurate enough to determine that the speed of light in water is slower than in air.
After Maxwell published his theory of electromagnetism, it became possible to determine the speed of light indirectly from the values of the magnetic and electrical permeability. Weber and Kohlrausch were the first to do this in 1857. In 1907, Rose and Dorsey got 299,788 km / s in the same way. At the time, this was the most accurate value.
Subsequently, additional measures were applied to improve accuracy. For example, the refractive index of light in air was taken into account. In 1958, Froome obtained a value of 299792.5 km / s using a microwave interferometer and a Kerr electro-optical shutter. After 1970, even more precise measurements became possible with the use of a highly stable laser and a precision cesium clock. Until that time, the accuracy of the standard meter was higher than the accuracy of measuring the speed of light. And now the speed of light became known with an accuracy of plus or minus 1 m / s. It is now more practical to use the speed of light in determining the meter. The 1 meter distance standard is now being determined using an atomic clock and a laser.
The table shows the main stages of measuring the speed of light (Froome and Essen):
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| date | Authors | Method | km / s | Error |
|---|---|---|---|---|
| 1676 | Olaus Roemer | Moons of Jupiter | 214,000 | |
| 1726 | James bradley | Aberration of the stars | 301,000 | |
| 1849 | Armand fizeau | Gear | 315,000 | |
| 1862 | Leon foucault | Rotating mirror | 298,000 | ± 500 |
| 1879 | Albert michelson | Rotating mirror | 299,910 | ± 50 |
| 1907 | Rosa, Dorsay | EM constants | 299 788 | ± 30 |
| 1926 | Albert michelson | Rotating mirror | 299 796 | ± 4 |
| 1947 | Essen, Gorden-Smith | Resonant resonator | 299 792 | ± 3 |
| 1958 | KDFroome | Radio interferometer | 299 792.5 | ± 0.1 |
| 1973 | Evanson et al | Laser interferometer | 299 792.4574 | ± 0.001 |
| 1983 | CGPM | Accepted value | 299 792.458 | 0 |