How long is one meter? Have you ever thought about this question seriously?

Source: China Science and Technology Museum

The unit of measurement "meter" plays a very important role in our daily life. "Meter" is indispensable in everything related to length and size. However, "meter" is not a product of nature, nor is it a concept created out of thin air. Why do we call a certain length "1 meter"? How did "meter" come into being?

Astronomical and geographical survey

In the late 17th century, France began measuring the arc length of the Earth. Further scientific research put forward new requirements for the length units in measurement experiments. The metrology community urgently needed to establish a unified length unit standard for the whole world.

On November 2, 1789, the French revolutionary government selected 15 scientists to form the "Technical Advisory Bureau" to conduct research on the formulation of new measurement units.

On April 14, 1790, the Technical Advisory Bureau finally decided to use the length of an arc of the meridian as a reference as the new standard for measurement units. The meridian is one of the "longitudes" we usually say. It is a large arc that people imagine connecting the north and south poles of the earth. The "0 degree meridian" is also called the prime meridian. It is the meridian that passes through the center of the meridian at the Greenwich Observatory in London, England, and is used as the starting point for calculating longitude. 1 meter is equal to one forty millionth of the circumference of the meridian. They believe that the length of the earth's meridian will never change, and the new standard based on it will also be eternal. At the same time, because the earth belongs to all mankind, the units formulated based on it will also belong to people all over the world.

In June 1792, Jean Baptiste Joseph Delambre and Pierre François André Méchain, academicians of the French Academy of Sciences, set out from Paris in two opposite directions to start a great scientific exploration in human history. Delambre was responsible for measuring the northern line from Paris to Dunkirk, while Méchain was responsible for measuring the southern line from Paris to Barcelona. They not only had to measure, but also calculated the arc length of the meridian based on the results, thus providing a basis for the formulation of new measurement units.

In June 1792, Jean Baptiste Joseph Delambre and Pierre François André Méchain, academicians of the French Academy of Sciences, set out from Paris in two opposite directions to start a great scientific exploration in human history. Delambre was responsible for measuring the northern line from Paris to Dunkirk, while Méchain was responsible for measuring the southern line from Paris to Barcelona. They not only had to measure, but also calculated the arc length of the meridian based on the results, thus providing a basis for the formulation of new measurement units.

The measurement work lasted for 7 years, and finally it was determined that the old French ruler 5130740 Toise would be the standard, and one forty-millionth of this length would be the new length standard "meter".

However, according to the latest satellite measurements, the actual length from the North Pole to the equator is about 9,980 kilometers, which means that the 1 meter at that time was actually a few millimeters less than 1/40 million of the meridian. Although there are errors, the scientific idea of ​​using the meridian to determine the unified standard of length is undoubtedly an unprecedented innovation.

Physical ruler measurement

In 1798, scientists made a 1-meter-long ruler with a minimum scale of 0.01 mm based on the measurement results. On June 22, 1799, the ruler was deposited in the Paris Archives and was called the "Archival Ruler". In December of the same year, the National Assembly confirmed the legal status of the Archival Ruler as the metric standard.

But it wasn’t long before the Archives Bureau discovered the problem. The end faces of the archival ruler were easily worn, which made measurement difficult. Secondly, the markings on the end points of the archival ruler were unclear, making it very troublesome to use.

In order to solve this problem, the International Bureau of Weights and Measures began to order 31 platinum-iridium alloy rulers in 1880. These rulers took another seven years to make and were finally completed in 1887. Two years later, with the advancement of industrial technology, countries had a common demand for a unified unit. Therefore, at the International Conference on Weights and Measures in 1889, a platinum-iridium alloy ruler numbered No. 6 was identified as the " International Prototype of the Meter ". Every country that joined the Meter Convention would have the same ruler and would regularly compare it with the "International Prototype of the Meter" to ensure the accuracy of the unit "meter".

Light wave definition

However, this meter prototype still has some shortcomings in use. First, as a physical object, it is inevitably affected by the environment. Second, wear and tear during use will greatly reduce the precision and accuracy of the measurement. Therefore, after the appearance of the meter prototype, scientists did not stop their research and continued to explore a more accurate method to define the meter.

At the end of the 19th century, the Michelson light wave interferometer was invented. It was the first time that the red spectrum radiation of cadmium was used to measure the "meter". The accuracy of the light wave measurement method has been greatly improved compared with the physical measurement, and it also shows superiority in stability. Later, more and more scientists began to try to measure length with light waves. In the late 1950s, people found that a krypton-86 isotope light source was more stable. As a result, in 1960, the definition of "meter" became: 1 meter is equal to the length of 1650763.73 wavelengths of the wavelength of the radiation corresponding to the transition between the 2p10 and 5d5 energy levels of the krypton-86 atom in vacuum.

Speed ​​of light measurement

With the development of the times, science and technology are also constantly improving. In the 1960s, the use of krypton-86 wavelength to measure distance could no longer meet the accuracy requirements of scientific research, and lasers with good directionality and brightness gradually entered the field of vision of scientists. In 1975, the 15th International Conference on Weights and Measures introduced a resolution to use the speed of light to represent the meter. This is also the current definition of "meter": the meter is the length of the distance traveled by light in a vacuum within a time interval of 1/299792458s. Because the speed of light in a vacuum is a fixed value, theoretically speaking, defining the meter using the speed of light is more accurate and more stable.

At the same time, the advantage of using the speed of light to describe the meter is that it is not a fixed wavelength of light source. As long as the frequency of light is known, light sources of different wavelengths can describe the meter within different accuracy ranges. In this way, there is no need to frequently change the definition of the meter, and the scope of application is wider. For now, using the speed of light to describe the basic unit of length can be said to be the most ideal definition.

You would never have thought that a small meter would have gone through so many changes. The process from nothing to something, and from something to excellence not only reflects people's in-depth understanding of the world, but also reflects scientists' spirit of exploration of the truth. How long is 1 meter? After reading this article, do you have the answer?

Creative team: China Science and Technology Museum New Media Team

Review expert: Liu Yuhang, postdoctoral fellow at Beijing International Center for Mathematical Research