Where does "Beijing Time" come from? Where does it go?|Read+

Time, both familiar and abstract, is as silent as air. The code word at the window is until 3:00 in the morning, everything is silent, and the clock is ticking, telling us that time is fleeting and never stops:

In one tick, hummingbirds flapped their wings 55 times, cheetahs galloped 28 meters across the grassland, the earth has revolved 30 kilometers around the sun, the spacecraft flew nearly 8 kilometers, about 4 babies were born in the world, and humans ate super 85 tons of food, the world consumes about 150,000 liters of oil...

There are countless famous sayings about time in ancient and modern China and abroad. However, what exactly is time? Augustine, an ancient Roman thinker, said: "I am clear when no one asks me; if someone asks me, I want to explain, but I am at a loss."

Beijing time, which is uniformly used by 1.4 billion Chinese people, is not actually a "Beijing specialty". At the foot of Lishan Mountain in Lintong District, Xi'an, Shaanxi, in the mysterious underground clock room of the National Time Service Center of the Chinese Academy of Sciences, there are many atomic clocks. These treasures are working all the time, and "generate" Beijing time with an accuracy of not less than one second in 30 million years .

The Time Science Museum (located in Xi'an, my country) built by the National Time Service Center of the Chinese Academy of Sciences.

After careful preparation, in 2016, the National Time Service Center of the Chinese Academy of Sciences prepared to build the "Time Science Museum", which is currently the only popular science exhibition hall with the theme of time science in my country. In order to show the mysteries of time to the general public and create a portable and mobile science museum, Dou Zhong and Liu Yongxin of the National Time Service Center of the Chinese Academy of Sciences jointly created the book "Time Science Museum", which was rated as an excellent popular science book in 2022 by relevant departments . Recently, they accepted an exclusive interview with Changjiang Daily's "Reading +" weekly magazine to talk about the story behind Beijing time.

"Time Science Museum" edited by Dou Zhong and Liu Yongxin Science Press

How does the time come?

The sun, moon, and stars rise in the east and set in the west, reflecting the rotation of the earth. The earth rotates once to make a day, the moon revolves around the earth to make a month, and the earth revolves around the sun to make a year—these common senses that people are familiar with today are the result of thousands of years of exploration between the heavens and the earth by the ancients.

In the Time Science Museum, a huge tree ring with a diameter of 1.4 meters is displayed, recording the changes of 200 years: it broke ground in 1815 during the Jiaqing period of the Qing Dynasty, and witnessed the French physicist Foucault's single pendulum experiment to prove the earth's rotation. In 1851, the 1911 Revolution when the Qing Dynasty came to an end, two world wars, and 1969 when humans first landed on the moon...until it was cut down in 2015.

Two hundred years of vicissitudes, if placed in the vast cosmic calendar, it is but an instant. If the 13.8 billion years of cosmic evolution history are condensed into one year, the five thousand years of human beings is equivalent to the last 10 seconds, and the human life span is only 0.2 seconds, which is fleeting!

Strolling through the Time Science Museum, you can see that human beings have been measuring time since ancient times, and have been diligently pursuing the limit of time measurement accuracy.

There was no writing in ancient times, and people tied ropes to keep track of the day; gradually, our ancestors evolved a solar clock that "sees the shadow on the watch, and knows the time by watching the shadow" according to "sunrise to work, sunset to rest".

The Taosi ruins in the middle reaches of the Yellow River are considered by archaeologists to be a city in the late Neolithic age. It is likely to be the capital of Emperor Yao. The ancient observatory in the city is more than 4,000 years old. Through the gaps in the pillars on the platform, observe the tower in the east direction. The sunrise on the mountain determines the season of solar terms.

Stonehenge in England was once considered to be a mysterious alien relic. Archaeologists speculate that it was built between 3000 BC and 1500 BC. It may have been built by ancient Britons to observe celestial phenomena such as sunrise and sunset, to determine solar terms and seasons, and to guide farming. , which is similar to the ancient observatory at the Taosi site.

From the guibiao more than 4,000 years ago to the sundial of the Zhou Dynasty; from the fire clocks that burn candles and spices, "burning incense to know the night, burning candles is difficult to change", to fluid clocks such as water clocks, hourglasses, and lamp leaks; From mechanical clocks with intersecting watches to electronic clocks composed of electronic components, from radio clocks with built-in chips to atomic clocks, to the most stable and accurate astronomical clocks in the universe - pulsar timing... The evolution of timing instruments is constantly innovating.

Quantum physics studies have shown that atomic clocks can be made by using the electromagnetic waves emitted by atoms when they absorb or release energy, and the timing is within 1 second for tens of millions or even hundreds of millions of years. It has been widely used in scientific research, astronomical observation, navigation and space navigation. and other fields. In addition to time-keeping ground atomic clocks, scientists are also developing spaceborne atomic clocks and optical clocks on satellites or other spacecraft. Among them, the Zhang Shougang team of the National Time Service Center of the Chinese Academy of Sciences and the Wuhan Institute of Precision Measurement Science and Technology Innovation of the Chinese Academy of Sciences jointly developed the chip atomic clock with a volume of only 2.3 cubic centimeters.

According to measurements, the timing accuracy of pendulum clocks is 10-120 seconds/day, that of quartz clocks is 0.001-0.2 seconds/day, that of cesium atomic clocks is 1 second/1 million years, and that of strontium optical clocks is 1 second/16 billion years.

Dou Zhong, a well-known popular science writer, said with emotion: Human research on time measurement technology has always stood at the forefront of technological development and promoted the progress and development of science and technology; the timing instruments of each era represent the highest technology of that era level.

High-precision time matters

The time is generated through time measurement and timing, and the time must be sent out by various means for use by various users. This is "time service".

"Chicken Hudan" (human learns from rooster crowing), beat watch, morning bell and evening drum are the methods of time telling in ancient times; Two decommissioned cannons were erected to tell the time at noon every day. The sound of the noon cannon was so loud that two nearby houses collapsed when the cannon was fired for the first time.

In the 20th century, wireless time service was born; Internet time service, which began in the 1980s, developed rapidly in the world at the beginning of the 21st century, with a huge number of users; Russia's GLONASS, China's Beidou, and EU's Galileo are four major satellite navigation systems, and the satellite timing accuracy is on the order of 10 nanoseconds.

High-precision time is of great significance to national life, scientific research, economy, military, aerospace and other fields. Electricity, aircraft, high-speed rail and other command and operation systems are required to maintain high-precision time synchronization; 5G mobile communication base stations require time synchronization accuracy that is better than a hundred nanoseconds; high-frequency stock transactions are more than 300 transactions per millisecond, with an increase of 1 millisecond Time delay will cause a lot of losses; aerospace projects such as China's space station and Mars exploration have reached the nanosecond and picosecond level for time synchronization.

Quick battle and quick decision is the core idea of ​​"Battle Chapter" in "Sun Tzu's Art of War". High-precision and high-stability time-frequency systems are extremely important for modern military and national security applications. Informationized combat platforms, electronic reconnaissance systems, friend-or-foe identification systems, precision strikes, and air interception systems all require extremely high time synchronization accuracy. After the Gulf War in 1991 and the Kosovo War in 1999, military experts exclaimed: In modern warfare, the atomic clock is more important than the atomic bomb! In recent years, "scheduling war" has become the focus of military struggles under the conditions of today's informationization.

【Interview】

How was Beijing Time born?

Read+: Everyone is curious why the National Time Service Center is located in Xi'an?

Dou Zhong: There are historical reasons for this. The earliest time work in our country was mainly in Shanghai, and later moved to the inland due to various reasons.

In the early days of the founding of New China, both social operations and people's lives needed to tell everyone the accurate time, so time service was put on the agenda. The main method at that time was to transmit time information through radio, and the characteristic of radio was to radiate from the center to the surroundings, so it was better to choose the time service station in the geometric center of the country, so as to cover the whole country.

The second factor is also because of the importance of time work. Time has a greater impact on national security and people's lives, so we must have our own independent research time. As a technology related to national security, it has been moved to the internal land area, away from the southeast coast.

In the 1960s, due to the needs of large-scale economic construction and national defense construction, the country decided to quickly build a dedicated timing station in the hinterland. The Chinese Academy of Sciences organized experts from the Shanghai Astronomical Observatory, Zijin Mountain Observatory, and Beijing Astronomical Observatory to conduct multi-site surveys and select sites. According to the requirements of "relying on the mountain and entering the cave", Jinzhi Mountain near Tang Xianzong's Jingling Mausoleum in Pucheng County, Shaanxi Province was selected on the Weibei Plateau. Build the Northwest Time Service Station. Construction began in 1966, and it was basically completed in 1970 and named Shaanxi Astronomical Observatory.

In 1970, Zhou Enlai, then Premier of the State Council, personally instructed Shaanxi Astronomical Observatory for the trial broadcasting work. The document written by Premier Zhou on the test broadcast of the short wave station is now a precious exhibit of the Time Science Museum.

In 2001, the Chinese Academy of Sciences, approved by the Central Committee, renamed the Shaanxi Astronomical Observatory as the National Time Service Center of the Chinese Academy of Sciences. Based on time service, the time service center carries out time service work and related research, making my country's time service and time frequency research level rank among the world's advanced as a whole.

Read+: I heard that the core of the National Time Service Center is the bell room in the basement. Is there a time generation system in it?

Liu Yongxin: The generation of time is that the atomic clock group jointly generates a second-long signal to obtain the atomic time, which is combined with the astronomical time to form the time used in our daily life. Therefore, the atomic clock constitutes an important device for time. In order to avoid being damaged by blows, and at the same time ensure the stability of the operating environment and other comprehensive factors, the atomic clock group was finally located in the basement.

Read+: How did Beijing time come about?

Dou Zhong: When the People’s Republic of China was founded, it was agreed that the time in the time zone where Beijing is located should be used as my country’s standard time. On September 27, 1949, the first plenary session of the Chinese People's Political Consultative Conference adopted a resolution: The People's Republic of China adopts the Gregorian calendar and changes "Central Plains Standard Time" to "Beijing Time". On September 28 of that year, Xinhua Broadcasting Station (later changed to Central People's Broadcasting Station) broadcast in Beijing time for the first time.

Beijing time involves the concept of time zone, because the earth is a sphere, and with the rotation of the earth, each meridian receives sunlight differently in the morning and evening, so it was stipulated at the Washington International Longitude Conference in 1884 that the starting point of the world time zone is the meridian passing through the Greenwich Observatory in England ——The prime meridian draws a time zone every 15 degrees of longitude from east to west. There are 24 time zones in the world. The time zone where Beijing is located is Dongba District.

Due to the inhomogeneity of the earth's rotation, Greenwich Mean Time was gradually replaced by the "Coordinated Universal Time" based on atomic time. Since 1972, it has officially become the international standard time, and it is still the universal time in the world. How to coordinate it? Using the second length of atomic time and the moment of astronomical time at the same time, the time calculated in this way is the time in zero time zone. Each country is in a different time zone, plus the time difference in each region, it forms the time of each country.

After the founding of New China, Xujiahui Observatory was assigned to Zijinshan Observatory, and Shanghai Zhenru short-wave radio station was rented to provide time service to the whole country; Renamed as the National Time Service Center of the Chinese Academy of Sciences) successively established a complete time service system, my country's independent atomic time system, and the coordinated universal time system.

At present, the world-recognized Coordinated Universal Time, the data of at least 500 atomic clocks from more than 80 time laboratories around the world, through the time transfer link and the German Federal Physical Laboratory (PTB) as the hub, regularly aggregated to the International Bureau of Authority in Paris, France (BIPM) weighted calculation, resulting in Coordinated Universal Time.

Read+: Regarding time, what are the different regulations in different countries? Can you give some vivid examples?

Liu Yongxin: At present, most countries in the world use the time zone where their capital is located as their national standard time, and maintain an integer hour difference from Greenwich Mean Time. There are also a few countries whose standard time differs from the zone time by half an hour according to their needs or national traditions, such as India and Iran.

In 2018, North Korea launched the new Pyongyang time, and the clock was adjusted forward by 30 minutes, and changed to the East 9th zone time based on 135 degrees east longitude. As a result, North Korea and South Korea have the same time zone, and the Korean peninsula has a unified time zone.

my country's international status in the field of time service is among the top three in the world

Read+: What is the significance of maintaining the time of each country independently?

Dou Zhong: At present, there are only more than 80 laboratories participating in "Coordinated Universal Time", but there are far more than these countries in the world. Not every country has the ability to independently maintain its own time. Some countries rely on receiving The standard time sent by the international convention organization.

However, in a special period, once the international standard time signal is blocked, some standard time-related equipment used in this country may be paralyzed and make mistakes, especially applications that require high time accuracy. For example, power grids, communications, military affairs, and national security are all closely related to high-precision standard time. If the standard time is not provided by the outside world, the entire system may have some unpredictable effects.

In the 1990s, the United States shut down GPS satellite signals in the Asia-Pacific region, paralyzing some equipment in the region.

Standard time maintenance and service is a basic project related to the national economy and the people's livelihood. Any major country strives to have its own independent time standard and service system that maintains the highest level of the times. Beijing time cannot simply rely on the outside world. While participating in international time work , we must do a good job of independent local atomic time system.

Time is not a point, it is a line that keeps flowing. We want to keep this line running smoothly, the closer to the average the better. More than 80 time laboratories around the world regularly provide data to the International Bureau of Authority. Everyone contributes their respective strengths. The greater the weight, the greater the contribution to the generation of UTC; the more stable the deviation from UTC, the more stable the UTC. The higher the level of time retention.

What makes us very proud is that the standard time maintained by the National Time Service Center of the Chinese Academy of Sciences currently ranks among the top three in the world in terms of weight in international standard time. This also means that the current international status of our country in the field of time service is basically in the top three levels in the world. We started later than European and American countries. Through continuous exploration and scientific research progress, we have independently developed various types of atomic clocks to get rid of the shackles of "stuck necks".

Read+: We often say that tens of millions of years and hundreds of millions of years are within one second, and the impact on our daily life may not be obvious. What is the significance of pursuing such precision?

Liu Yongxin: If we compare time to a ruler and want to measure something 1 cm long, the scale of the ruler needs to be at least 1 mm in order to measure accurately. If you want to measure smaller things, should the scale of the ruler be smaller?

Exploring the frontier of science and technology will require higher time precision. For example, research on quantum physics, dark matter, dark energy, etc. requires a higher-precision time "ruler" to measure.

Different industries have different requirements for time accuracy. For example, the time accuracy required by the power grid may not be as high as that required by satellite navigation. Therefore, China's current timing method also serves users through multiple means and multiple precision. For example, we will use optical fiber to deliver the highest precision time to users in need, and directly transmit time signals point-to-point through optical fiber.

Read+: I have seen the atomic clock appearing at the National Science and Technology Achievement Exhibition many times. What "black technology" is contained in it?

Liu Yongxin: Atomic clocks are currently the most accurate tool for measuring time, and their research and development involves multiple disciplines such as quantum physics, electricity, materials science and thermal science.

The atomic clock is very close to people's daily life, and it can be called a "time magic box". For example, when we take out our mobile phone and open the navigation software, we can see our geographical location and enter the destination. The navigation software can help us plan the route and tell us how to go in real time. Among them, the atomic clock plays a "heart" role in the satellite navigation system.

Because the key to navigation and positioning is the measurement of time, each satellite and ground station is equipped with a set of atomic clocks, and the time signal is sent together with the ephemeris. In the process of calculating the satellite position, time information and distance can also be obtained at the same time . In this way, our position can be calculated. Electronic products such as mobile phones in daily life usually have built-in navigation modules, which can receive signals transmitted by satellites in real time and perform calculations based on certain algorithms. Thus, we have real-time control of our own position.

The complexity of an atomic clock is far beyond people's imagination. It may require hundreds of electronic components. After thousands of tests and verifications, it condenses the efforts of countless scientific researchers.

With the continuous development of science and technology, the demand for time accuracy in various industries is also increasing, so we are also developing the next generation of frequency standards. Atomic clocks are more accurate than traditional timing instruments. The next generation of higher-precision timing instruments is an optical clock based on optical frequency. The strontium atomic optical clock developed by the National Time Service Center has been in closed-loop operation in the laboratory. The indicators have reached the international advanced level, and many countries are conducting research in this area.

Read+: Have all the time-generating equipment of the National Time Service Center been localized?

Dou Zhong: We have been working hard on the road of localization. In the early days, our country was not capable of developing atomic clocks and other devices. The country made a lot of deployments in this area. Our time service center also took the research of domestic atomic clocks as its own responsibility and made some achievements. For example, small optically pumped cesium atomic clocks have been put into commercial use, and cesium atomic fountain clocks and strontium atomic optical clocks have been operating stably in laboratory environments. Now the reference clocks we use are all developed by ourselves, and we are constantly conducting and improving the research on the reference clocks. In this area, we are fully capable of doing better on our own.

Read+: What bottlenecks or "stuck necks" have we encountered?

Dou Zhong: In the past, because our basic science was relatively weak, high-end finished equipment from some developed countries were prohibited from being exported to us. If we want to buy it, we can’t buy it at all. We can only research and explore by ourselves. For example, cesium atomic clocks, two major suppliers in the world have banned sales to my country many years ago. We started research from the source. The principle of the self-developed cesium atomic clock is different from that of foreign countries, but the final performance indicators are basically similar, and it has now been put into application.

There are also some international cutting-edge research, which we are also carrying out simultaneously. For example, strontium atomic clocks, next-generation standards, etc., we are now at the same level as European and American countries, basically running side by side. People will not sell these cutting-edge technologies to us, we can only rely on ourselves.

Display the Chinese view of time

Read+: China has a vast territory and spans five time zones from west to east. Why is there no distinction between Eastern Time and Western Time?

Liu Yongxin: The United States is divided into Eastern Time Zone, Central Time Zone, Mountain Time Zone, Pacific Time Zone Eastern Time, Western Time, etc.; while our country is a united country, we uniformly stipulate the use of Beijing time, which is actually an increase The convenience of communication and communication.

Read+: Regarding the time, we feel it is very simple to use today, but in fact it is not simple behind the simple.

Liu Yongxin: In our daily life, we usually do not feel the complexity behind time. But in fact, time can be compared to air. When life is full of air, we hardly feel its existence, or we don’t pay much attention to its existence; but when the air suddenly disappears, the impact will be very great.

Read+: As the curator of the Time Science Museum, can you tell us about the "treasure of the museum"?

Liu Yongxin: The goal of the Time Science Museum is to create a unique and refreshing time science museum. scientific truth.

Among them are some special exhibits. For example, there is a water transport instrument platform in the Northern Song Dynasty. It is a large-scale astronomical timing instrument built by the scientist Su Song in the Northern Song Dynasty. 12 meters, about 3 meters wide, divided into 3 layers, is a symbolic astronomical instrument in ancient China, which integrates testing, punctuality and time service. The original objects have not been preserved in the long river of history, and we have restored the crystallization of wisdom of the ancients based on historical materials.

There is also a horse running leak, which is hung in a carriage for timekeeping. This is a timer invented by the ancient Chinese that can be taken away, leading the world at that time.

The Time Science Museum has made a detailed display of the methods and instruments used to perceive time in ancient China. Ancient Chinese civilization paid attention to the unity of man and nature and the five elements. For example, the king emphasized the power of the king, and the emperor would first change the year name after he ascended the throne to prove his legitimacy. There are also twenty-four solar terms, astronomy and geography, etc., which all reflect the time concept of the Chinese nation.

The Time Science Museum has been open to the outside world for 7 years and has become a national and provincial science education base. We focus on explaining the concept of time with Chinese characteristics, and introduce to you the Chinese time culture from ancient times to the present, as well as the vigorous development of Chinese science and technology in the new era, which not only enhances cultural confidence, but also enhances technological self-confidence. We hope that, like the air, we can moisten things and show the science behind time silently, and show that through the hard work of scientific researchers, we can easily use high-precision time today and enjoy the convenience brought by the development of science and technology.

(Changjiang Daily reporter Ke Li)

[Editor: Zhang Jing]

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