Where has the time gone? The history of timekeeping technology

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Produced by: Science Popularization China
Produced by: National Time Service Center of the Chinese Academy of Sciences, Time Traveler Team
Supervised by: Computer Network Information Center of the Chinese Academy of Sciences

Today is the first day of 2016, and the new year has begun. But without timing, we would have no concept of time, let alone the new year and the old year. Today, let's talk about human timing technology.
The four directions above and below are the universe, and the past and present are the universe.
The origin and extinction of all matter in the universe, the birth and end of all things in the world, the birth and extinction of all life on earth, all of these are closely related to time. The sun and the moon alternate, the stars move, the flowers bloom and fall, and the thousands of years of human civilization flow slowly in the long river of time. Time has witnessed all these changes, and the increasingly advanced human civilization enables us to perceive time more accurately. As early as the beginning of human civilization, humans began to describe the changes of everything by measuring time, and thus a series of increasingly accurate timekeeping instruments were born.
1. The development of time measurement In
ancient times, people used the rising and setting of the sun as a measure of time, so people lived a life of "working at sunrise and resting at sunset"; in the second century BC, sundials appeared, which improved the accuracy of time measurement; more than a thousand years ago, skilled craftsmen in the Northern Song Dynasty of China designed water clocks; more than six hundred years ago, mechanical clocks appeared; in the seventeenth century, simple pendulums were used in the design of mechanical clocks, which increased their timing accuracy by nearly a hundred times; in the 1930s, quartz clocks were born, and timing accuracy was further improved.

These timekeeping instruments are what we call "clocks". If we count the development of these clocks, the oldest clocks can be traced back to the sun clock, such as the gnomon and sundial. The first sun clock with a well-documented history began in the Yao Emperor period of 2357-2258 BC. "The Zhou Li: The Official of the Land: The Great Minister of the Law" states that "the depth of the earth is measured by the method of the gnomon, and the shadow of the sun is corrected to find the center of the earth". "The Spring Official: The Book of Auspicious Omens" also mentions that "the gnomon is used to determine the sun and the moon in the four seasons, and the land is used to determine the state".
Later, the water clock appeared, which is a type of fluid clock. In addition to the water clock, the fluid clock also includes the sand clock. Both the water clock and the sand clock use a certain amount of fluid to indicate a fixed time interval by measuring the time required for the fluid to flow in a specific way. This type of "clepsydra" timekeeping instrument that does not rely on astronomical phenomena has a history of at least 4,000 years. The earliest mechanical clock in China also appeared in the Northern Song Dynasty, while the mechanical clock in Europe appeared in the 13th century. The earliest mechanical clock introduced to China was presented to Emperor Wanli during the Wanli period of the Ming Dynasty.

With the needs of social development, the requirements for clock accuracy are getting higher and higher, so quartz clocks came into being. It uses a stable quartz oscillator inside the quartz clock to time. With the requirements of scientific and technological development and the persistent pursuit of precision measurement by mankind, quartz clocks gradually cannot meet the requirements and are gradually replaced by clocks with higher precision, such as atomic clocks.
Atomic clocks use electromagnetic waves emitted when atoms absorb or release energy to time. Because the period of this radiation electromagnetic wave is very stable, coupled with the use of a series of precision instruments for control, the timing of atomic clocks can be very accurate. The elements that were first and most commonly used in the development of atomic clocks are alkali metals such as hydrogen, cesium, and rubidium. The accuracy of atomic clocks can reach an error of 1 second every 1 million years. In 1967, the cesium atomic clock was used to define the "second", which is 9,192,631,770 times the period of the microwave frequency transition between the two hyperfine structure sub-energy levels of the ground state of the 133Cs atom without interference, which is what we usually call the International Atomic Time. From then on, the era of the atomic "second" as the time measurement standard began, and the definition of "second" is still maintained by the cesium atomic fountain clock.

1955: First atomic clock

NIST's Cesium Fountain Reference Clock

Pulsars are another unique time measurement instrument. They are compact neutron stars that rotate at high speeds and have a very stable rotation period. By observing the timing of pulsars, a high-precision space-time reference frame can be established. The integrated pulsar time system established and maintained using pulsar clocks may have higher long-term stability than the current atomic time system and can independently detect the systematic errors of atomic time. This unique time measurement method is being studied by scientists.

Scientists have spared no effort to explore atomic clocks and have achieved great research results. They have developed a clock with higher accuracy than the current reference clock, the cesium atomic clock, namely the optical clock. The status of the cesium clock as a reference clock has been seriously impacted. The research on optical frequency standards has been vigorously developed. This clock, which uses the optical band resonance frequency standard of atoms as the time frequency standard, has greatly improved the accuracy of human measurement of time and frequency!
Because when an atomic clock measures time frequency, its "ruler" is the wavelength emitted when the atom resonates. The shorter the wavelength, the finer the scale of the "ruler", and the more accurate the measurement. The wavelength of atomic resonance in an optical clock is 5 orders of magnitude shorter than that in a microwave atomic clock! At present, the measurement accuracy of the latest optical clock is more than 100 times higher than that of a microwave atomic clock! With the deepening of research on optical clocks, people have widely predicted that optical clocks will be used as a new definition of time in the second decade of this century.

2. Why is Beijing time released by Shaanxi?
In the 1950s, developed countries such as the United States, the Soviet Union, and Japan began to establish their own standard time and time service systems. At the beginning of the founding of the People's Republic of China, all industries were waiting to be developed. Realizing the far-reaching significance of the standard time frequency broadcast, Chairman Mao pointed out: "China must have its own standard time, and China's time cannot be controlled by foreigners!" On March 26, 1966, Premier Zhou Enlai personally approved the establishment of its own time broadcasting system in the hinterland of China. China has a
vast territory, spanning five time zones from east to west. The capital Beijing is located in the East Eighth Zone in the international time zone division, and the time service station must be built in the central area of ​​China. Therefore, the release of "Beijing Time" is not in Beijing but in Shaanxi, that is, the standard time issued by the Central People's Broadcasting Station is broadcast by the National Time Service Center of the Chinese Academy of Sciences. Every hour, the radio that is listening to the radio will broadcast a "beep, beep" sound. People use this to calibrate the speed of their clocks.

3. What is the significance of improving the accuracy of time measurement?
From ancient times to the present, what impact has the continuous improvement of time measurement accuracy brought to people's living needs and national activities?
In daily life, 1 second is very short, and it is enough to be accurate to 1 second, such as clocking in at work, the bell for class and dismissal at school, the bell for handing in exam papers, and the Double Eleven flash sales in shopping. But in some fields, using seconds to measure time is too long. In our sports events, athletes' performance needs to be accurate to one hundredth of a second, that is, a difference of 0.01 seconds determines the ownership of the champion; in means of transportation, such as cars and airplanes, their engines rotate thousands of times per minute. In order to change the frequency of the engine and increase its running speed, the time measurement must be accurate to 0.01 seconds.
For scientific research activities, with the improvement of the accuracy of time and frequency measurement, people can explore the laws of nature at a deeper level and promote the progress of basic scientific research. For example, the measurement of the Rydberg constant, the stability of the fine structure constant, the measurement of the Lande factor, the measurement of the charge-to-mass ratio, the measurement of the gravitational redshift, the detection of gravitational waves, etc., all depend directly on the measurement accuracy of the time frequency. These measurements and studies are important methods for testing the basic theories of physics (relativity, quantum electrodynamics, gravitational field theory, etc.). For example, to test the general theory of relativity by measuring the change of the fine structure constant over time, the frequency measurement accuracy needs to be better than 10-17; to test the existence of gravitational waves by laser interferometry, the frequency measurement accuracy needs to be better than 10-18. In addition, people also indirectly improve the measurement accuracy of other physical quantities by converting the measurement of other physical quantities into the measurement of time frequency. At present, the definition or measurement conversion of physical quantity units such as length, current, voltage, luminous intensity and temperature has been completed.

For national activities, take satellite navigation as an example. In order to improve the accuracy of GPS, the time of "clocks" must be unified. However, even if the atomic clock has an error of one second in 30,000 years, due to the relativistic effect, it will have a time difference with the ground clock, which will eventually affect the positioning effect of GPS. According to the special theory of relativity, because the artificial satellite is in motion, time passes slowly from the ground. Since the flying speed of the artificial satellite is slow compared to the speed of light, there is a little error, and the clock on the artificial satellite will be 7 microseconds slower than the ground clock every day. In addition, according to the general theory of relativity, the stronger the gravity, the slower the time. The faster the rotation speed of the space station, that is, the greater the artificial gravity inside it, the greater the degree of time slowing down. On the contrary, from a place with strong gravity to a place with weak gravity, time will appear to be faster. Therefore, from the surface of the earth, the clock on the artificial satellite subject to the weak gravity of the earth runs faster. The error here is 46 microseconds faster every day, minus the 7 microseconds of artificial satellite time lag caused by the special theory of relativity, the artificial satellite clock is 39 microseconds faster every day. This time difference has a great impact on GPS. The error in distance is equal to the error in time multiplied by the speed of light. A time difference of only 39 microseconds causes an error of 12 kilometers in GPS positioning, making GPS unusable and unable to serve as a navigation system. However, we correct this error based on the special theory of relativity and the general theory of relativity, so that the satellite and ground clocks are consistent, and GPS can be used with confidence.

Today, with the strengthening of its comprehensive national strength, China has played an important role in the world. The National Time Service Center is also developing at a leapfrog speed to provide the most basic time guarantee for our country's scientific research and people's livelihood. We sincerely hope that in 2016, we can all cherish time and have a wonderful year.

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