The speed of light in a vacuum ... and not only

Man has always been interested in the nature of light, as evidenced by myths, legends, philosophical disputes and scientific observations that have come down to us. Light was always an occasion for discussions of ancient philosophers, and attempts to study it were made even at the time of the Euclidean geometry - 300 years BC. Even then it was known about the straightforwardness of the propagation of light, the equality of the angles of incidence and reflection, the phenomenon of refraction of light, and the reasons for the occurrence of the rainbow were discussed. Aristotle believed that the speed of light is infinitely large, and therefore, logically arguing, and the measurement of the speed of light is not subject to discussion. A typical case when the problem is deeper than the era of understanding the answer.

Some 900 years ago, Avicenna suggested that no matter how large the speed of light, it still has a finite value. This opinion was not only him, but no one could prove it experimentally. The ingenious Galileo Galilei proposed an experiment of mechanistic understanding of the problem: two people standing several kilometers apart give signals, opening the flap of the lantern. As soon as the second participant sees the light from the first flashlight, he opens his damper and the first participant fixes the time of receiving the reciprocal light signal. Then the distance increases and everything repeats. It was expected to fix the increase in the delay and, on this basis, to calculate the speed of light. The experiment ended with nothing, because "everything was not suddenly, but extremely quickly."

The first to measure the speed of light in vacuum in 1676 by astronomer Ole Remer - he used the discovery of Galileo: he discovered in 1609 four satellites of Jupiter, which for six months the difference in time between two eclipses of the satellite was 1,320 seconds. Using the astronomical information of his time, Remer received a speed of light of 222,000 km per second. It was amazing that the method of measurement itself is incredibly accurate: the use of now known data on the diameter of the orbits of the Earth, Jupiter and the delay time of the satellite's obscuration gives the speed of light in a vacuum at the level of modern values obtained by other methods.

At first, there was only one complaint to the experiments of Remer - it was necessary to make measurements by terrestrial means. Nearly 200 years passed, and Louis Fizeau built a witty setup in which a ray of light reflected from a mirror over a distance of more than 8 km and came back. The subtlety was that it passed along the road back and forth through the hollows of the cogwheel, and if the speed of rotation of the wheel increases, then the moment will come when the light will cease to be visible. The rest is a matter of technology. The result of the measurement is 312000 km per second. We now see that Fizeau was even closer to the truth.

The next step in measuring the speed of light was made by Foucault, who replaced the cogwheel with a flat mirror. This allowed to reduce the dimensions of the installation and increase the measurement accuracy to 288,000 km per second. No less important was Foucault's experiment, in which he determined the speed of light in the medium. For this purpose, a pipe with water was placed between the mirrors of the installation. In this experiment, it was found that the speed of light decreases as it propagates in the medium, depending on the refractive index.

In the second half of the 19th century, it was Michelson who devoted 40 years of his life to measurements in the field of light. The crown of his work was the installation on which he measured the speed of light in a vacuum using an evacuated metal pipe more than a mile long. Another fundamental achievement of Michelson was the proof of the fact that for any wavelength the speed of light in vacuum is the same and as a modern standard is 299792458 +/- 1.2 m / s. Such measurements were made on the basis of refined values of the reference meter, the definition of which has been approved since 1983 as an international standard.

The wise Aristotle was wrong, but it took almost 2000 years to prove this.