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The original title: "Measurement of the speed of light"

The speed of light is one of the most important constants in physics, which determines the scale of time and space, and is also the basis of relativity and quantum mechanics. However, human understanding of the speed of light is not achieved overnight, but after thousands of years of exploration and experiments. This article will review the history of the measurement of the speed of light, from ancient philosophers to modern scientists, and show how they use wisdom and creativity to gradually solve the mystery of the speed of light.

The earliest thinking about the nature of the speed of light can be traced back to ancient Greece. At that time, there were two views: one was that Aristotle and others believed that light propagated infinitely fast without substance, and the other was that Empedokler and others believed that light was made up of tiny particles and had a limited speed. But these ideas have no experimental basis and are just pure reasoning.

It was not until the early 17th century that the Italian physicist Galileo carried out the earliest experiments to measure the speed of light. He devised a simple method: let two people stand on the top of a mountain a few kilometers apart, holding screens to block the lights. One of them first opened the screen to reveal the light, and the other opened his own screen as soon as he saw it. The speed of light can be calculated by recording the time difference and distance difference between the two signals. However, Galileo did not get satisfactory results because there was no equipment such as precise timers and binoculars at that time.

Then, in the middle and late 17th century, Romer, a French astronomer, successfully measured the speed of light using astronomical phenomena for the first time. He found that the time of occultation of Jupiter's moon is related to the distance between Earth and Jupiter: occultation occurs early when Earth is close to Jupiter, and later when Earth is far away from Jupiter. Romer speculates that this is because it takes time for light to travel from the moons of Jupiter to the eyes of observers on Earth, and estimates the speed of light at about 210000 kilometers per second.

In the early 18th century, the British physicist Bradley further improved Romer's method and obtained a more accurate speed of light of 300000 kilometers per second by observing the tiny wobble of the star's position with the seasons.

In the mid-18th century, French physicists Foucault and Foy respectively used ground experiments to measure the speed of light. Their method is to use rotating gears and mirrors to break and reflect a beam of light and calculate the speed of light according to the time difference between different positions of light. Foucault and Foy got similar results, about 298000 kilometers per second.

In the 1860s, Maxwell was using experimental data to determine the speed of electromagnetic waves, which he calculated at 310000 kilometers per second. And he thinks this is the speed of light, because light is only a form of electromagnetic waves. This means that if we want to know the speed of light, we don't have to actually measure light. In 1907, two scientists used this idea to determine the speed of light using permittivity and permeability.

At the end of the 19th century, Michelson, an American physicist, devoted his life to the precise measurement of the speed of light. He worked with Morey to try to find a medium that would allow light to travel. However, their experiments got the opposite results, proving that the ether does not exist and that the speed of light seems to be a constant. In 1879, he improved Foucault's method, using a lens to extend the optical path and obtained a speed of light of 299910 ±50 kilometers per second. In 1882, he improved the measurement accuracy and obtained a value of 299853 ±30 kilometers per second.

In 1905, the German physicist Albert Einstein put forward the special theory of relativity. Inspired by the Michelson-Morey experiment, he assumed that the speed of light is a constant, the same in any inertial reference frame, and not affected by the state of the observer's motion. This theory has become one of the cornerstones of modern physics.

In the middle and later period of the 20th century, with the development of electromagnetic wave, radar, laser and other technologies, human beings have more new methods and means for measuring the speed of light. For example, radar is used to transmit signals to the planets in the solar system and receive echoes to calculate the planetary distance and signal propagation time; laser interferometers or cavity resonators are used to measure laser wavelengths and frequencies.

After hundreds of years of continuous efforts and innovation, human beings have made a very accurate and stable measurement of the speed of light. In 1973, at the 14th International Metrology Conference held in Paris, France, an important resolution was adopted in the revision of the Metric Treaty: the distance of light traveling in the vacuum within 1max 299792458 seconds was defined as 1 meter. In this way, the meter is associated with the speed of light in a vacuum, making one meter an accurate constant, eliminating the need for experimental measurements. This resolution also marks a new stage in the history of the measurement of the speed of light.

In short, the history of the measurement of the speed of light is not only a history of human wisdom and creativity, but also a history of scientific methods and technological progress. From ancient philosophers to modern scientists, from astronomical observations to ground experiments, from relativity to quantum mechanics, human understanding of the speed of light has been continuously deepened and developed, which provides an important basis and inspiration for physics and other scientific fields.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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