Encyclopedia of popular science knowledge: "Time Knowledge"

Chapter 11 Time's Fun Story (4)
Some people think that the reason for the slowing down of the earth's rotation is caused by tidal friction, and others think that it is related to the change of natural conditions at the two poles of the earth.In recent years, it has been found that the average temperature of the earth has a rising trend. In this way, the huge glaciers in the two poles are slowly melting, the ice at the two poles is decreasing, and the water level of the ocean surface near the earth's equator is rising. If the earth wants to maintain its original speed, it requires Increase torque.And the rotation moment of the earth's rotation is formed by the sun, the earth, and the moon according to their own laws, and relatively speaking, it is constant.The balance of forces can only be achieved by slowing down the rotation of the earth.Of course, such changes are very small and cannot be measured without precise instruments.

It can be seen that the three forms of universal time: UT3, UT0, and UT1 are all affected by unpredictable and long-term changes in the earth's rotation.

Due to the above-mentioned reasons, the second length of universal time formulated according to the rotation of the earth still has a large error, sometimes up to the order of 10-7, which is equivalent to a difference of 3 second every 1 and a half months.With the development of modern science and technology, such a large error is not allowed.In addition, it is always not ideal to correct and correct on the basis of the earth's rotation, so people are looking for a new method to define the length of a second.

Use the revolution of the earth to determine the length of a second—we all know in the almanac that the earth has a revolution besides its rotation.The time for the earth to revolve around the sun is one year, and the earth revolves around the sun, which can also be imagined as a huge clock.The connection line between the sun and the earth is equivalent to the pointer, just like an alarm clock with a "small satellite" on the second hand.However, the time it takes for a "small satellite" to make one revolution is a second, and it takes one year for the earth to make one revolution around the sun.

It is true that the speed of the earth's revolution is not constant, but the revolution period of the earth is quite stable.If a fraction of the earth's revolution period is fixed as 1 second, the length of such a second is quite uniform.

In 1952, a timekeeping system based on the revolution period of the earth around the sun was established, called "almanac time", denoted as ET.

In order to use the almanac time in practice, when defining the almanac time ET, the connection between ET and UT should be considered, so as not to cause confusion and unnecessary troubles during application. The method is as follows:
(1) Make the transition from universal time to almanac time without interruption of time; (2) Make the second length of almanac time and universal time as consistent as possible.

According to the above principles, when the almanac was adopted in 1960, it was stipulated that:
The starting time of almanac time is 1900:1 on January 1, 0, universal time, which is strictly connected with universal time in terms of time.

历书时的秒即是上述1990年1月1日0时正开始的回归年长度的1／31556925。9747。

Since the length of the tropical year is not affected by the speed of the earth's rotation, the ephemeris time is uniform in seconds.

For technical reasons, ephemeris time is generally determined by observing the moon.

From 1960 to 1967, the almanac obtained by using the improved Brownian calendar was called ET0; from 1968 to 1971, the almanac measured after using a new astronomical constant system and correcting an error in the Brownian series was called It is ET1; and since 1972, the new expansion of Brownian series has been studied, and the obtained almanac is called ET2.

Theoretically, almanac time is a kind of uniform time, but it is not easy to obtain; the accuracy of ±1×10-9 can be obtained only after several years of astronomical observations.

Things are always divided into two. Compared with the second in ephemeris time and the second in universal time, the accuracy is much improved.For the accuracy of ±1×10-9, it takes 30 years of accumulation to produce an error of ±1 second.But this accuracy cannot meet the needs of modern scientific development. At the same time, the almanac requires long-term astronomical observations, which makes people look for and define new time benchmarks.

Nevertheless, almanac time is still preserved as an astronomical constant, and it still has important reference value in the research of geodesy and astronomy.

Atomic time At the No.1967th World Conference on Weights and Measures in 13, it was decided to use atomic time, recorded as AT.

The second length of atomic time is determined by the period of atomic transition frequency.

The second in modern atomic time is defined as follows:

When the cesium 133 atom is in the zero-field transition between the two hyperfine energy level structures of the ground state, the time interval of 9192631770 cycles of the radiation frequency is 1 second.

People are accustomed to using the world time. In order not to cause troubles in the use, the atomic time and the world time must be well connected.

选取1958年1月1日UT2的0时为原子时的时刻起点，即要求满足：

(AT-UT2) 1958·0=0 Due to technical reasons, when implementing this provision, only:

(AT-UT2) 1958·0=0.0039 seconds This value is saved as a historical constant, and it is enough to deduct this small correction amount when applying it.

The second length of atomic time is reproduced by the atomic clock we discussed earlier. Compared with the earth and the sun, the volume of the atomic clock is much smaller, and it can be well preserved in the laboratory.We already know the principle of the atomic clock, which is rarely affected by external conditions, and is a more objective and constant time reference.The second length of the atomic time stored in the atomic clock is easy to measure and apply, and does not require long-term astronomical observations.However, its stability and accuracy are very high, generally up to ±1×10-12 or higher, just as we have said before, which is equivalent to a difference of 30 second in 1 years.

Coordinated Universal Time, Leap Second Universal Time, Almanac Time, and Atomic Time are three timekeeping systems that define the length of a second by looking for a uniform motion period. Since the earth's rotation and revolution periods are very long, the universal time and ephemeris time The second length is obtained by dividing the long period equally.However, the period of atomic transition frequency is very short, so the second length of atomic time is obtained by multiplying the short period.

In addition, we already know that UT is defined by the earth's rotation period, and the speed of the earth's rotation is not uniform, so, strictly speaking, UT is not "uniform time".Among the three universal times UTo, UT, and UT3, although UT2 has undergone three corrections and is relatively uniform, it can only be called "quasi-uniform time" because there are long-term changes in the earth's rotation and random jumps that cannot be corrected.The cycle of the earth's revolution around the sun is uniform, and the cycle of the atomic transition frequency is also uniform, so both ephemeris time and atomic time can be called uniform time.

Some people may ask, since the second length of atomic time is the most accurate, then universal time and almanac time can be omitted.

Not so.Universal time UT and ephemeral time ET cannot be abolished, because they each have their own uses.There are differences and connections between these three timing systems. They can be converted to each other, but they cannot replace each other.In a sense, they complement each other's deficiencies.

Universal time UT is most closely related to people's life. If UT is canceled, people's life will feel very inconvenient, and UT is also indispensable in navigation and aviation; for this reason, the time starting point of atomic time must also be consistent with The universal time is strictly aligned.

How to apply these three timing systems depends on the occasion of use.When the requirements are not high, use the universal time UT.When the requirements are relatively high, atomic time is used.Almanac time is generally only used in astronomy, geodesy and other occasions.Of course, atomic time is the most accurate timekeeping system of our time.

The problem is that a new contradiction arises after using the unified atomic time second length, because the atomic time second length is not exactly equal to the universal time second length.Over time, the atomic time deviates from the universal time. For example, from the time when the atom was established in 1958 to the end of 1971, the universal time lagged behind the atomic time by nearly 10 seconds, and the difference became larger and larger. This is a big opinion on the usage department.As a result of the negotiation, "Coordinated Universal Time" is produced, which is recorded as UTC.

Coordinated Universal Time is not an independent timekeeping system, but a service method. The three timing systems UT, ET, and lights retain their own definitions respectively. When conversions between them or conflicts arise in applications, a method of jumping seconds is artificially used to "coordinate" to facilitate applications. This is The essence of "Coordinated Universal Time".

Generally speaking, we make the second length of Coordinated Universal Time faithfully reflect the second length of atomic time. When |UTC-UT|>0.9 seconds (1974 seconds before 0.7), we make an integer jump of 1 second, which is called is the leap second.The leap second is notified by the International Time Bureau (BIH) and usually occurs on New Year's Day every year.If it is still not enough, another leap will be made on July 7.

这种协调世界时从1972年1月1日开始实行。1971年12月31日做了-0.107758秒的特殊跳秒，这样到1972年1月1日时，AT与UTC整差10秒，也作为一个历史常数保存下来。

Coordinated Universal Time can be understood in this way: universal time is used at all times, which is convenient to use; the time interval (that is, the length of a second) is atomic time, which improves accuracy.When doing this coordination, it is necessary to deviate from the universal time, and then use the leap second method to correct it.

Of course, the method of leap second is not without disadvantages. Sometimes, leap second will disrupt the continuous work of the timing system, so the calls for canceling leap second are growing.

Just to determine this short 1 second, people don't know how much hard work they spent, how many international professional conferences were held, and special international institutions were established to study it.For example, at the Tenth International Conference on Weights and Measures in 1954, the "Second Definition Advisory Committee" was specially established... After long-term efforts, the definition of the second was gradually improved.Even so, until now, there are still many theoretical and technical problems to be further resolved on the definition of the second.

International organizations that coordinate time In order to effectively coordinate time work, some specialized organizations and institutions have been established in the world. According to their own needs, capabilities, official requirements and traditional habits, they are concerned with different aspects of the time field, thus forming a complex system.Here, it is impossible for us to describe their activities in detail, but we can only give a brief introduction to their main tasks.

International organizations that coordinate time (including frequency) services are usually divided into two categories: intergovernmental organizations and non-governmental organizations.The former generally receive some form of official support from governments, while the latter is very indirect.

Intergovernmental organizations are:
(1) International Conference on Weights and Measures (CGPM) This is an international conference attended by government representatives.The International Meter Convention is signed and revised by it.

(2) International Committee of Weights and Measures (CIPM) It is the administrative body of the International Conference on Weights and Measures between sessions.

(3) Executive agencies and laboratories of the International Bureau of Weights and Measures (BIPM) International Weights and Measures Congress and the International Committee of Weights and Measures.

(4) The Second Definition Advisory Committee (CCDS) was established in 1956 and is composed of scientists nominated by the International Committee of Weights and Measures.

(5) The International Telecommunication Union (ITU) is composed of officials and telecommunication experts from the administrations of the member states.

(6) International Radio Consultative Committee (CCIR) is an advisory body in the International Telecommunication Union responsible for dealing with radio communication services.Its Study Group [-] deals with standard time and frequency broadcasting services.Many regulations in the current radio time service are formulated by it.

Non-governmental organizations mainly include:

(1) The International Union of Science (ICSU) is equivalent to the general coordination bureau among various international academic societies.

(2) The International Astronomical Union (IAU) dealt primarily with the coordination of time in the early days of its establishment in 1919.It currently plays a role in time through its Committee 31.

(3) The International Radio Science Association (URSI) is responsible for dealing with various issues in radio science.Its group A (electromagnetic metering group) includes time metering.

(4) The International Time Bureau (BIH), which is responsible for time standards such as International Atomic Time (TA), Coordinated Universal Time (UTC) and Universal Time (UT1), is also the only permanent institution in the international time work. mechanism.

Looking for a constant second length As mentioned earlier, mean solar time is uneven, and its second length can be stretched and contracted, which is a kind of "rubber second".It is true that this kind of expansion and contraction is so small that it is even difficult to detect it in our daily life, but it is absolutely unacceptable for some precise scientific measurements.Therefore, people have to continue to explore in order to find a constant second length standard.

At the beginning of this century, both atomic physics and quantum mechanics were in the initial stage of development, and people's understanding of the microscopic world was still very superficial; astronomers were still looking for better time measurement standards in the macroscopic world.

Astronomers have discovered through long-term astronomical observations that although the Earth's revolution speed varies in different seasons of the year, its revolution time is quite stable. They deduce that if a fraction of the Earth's revolution period is fixed as one second , such seconds may be fairly uniform.

However, to obtain such a time, one must accurately grasp the law of the earth's revolution.That is to say, the annual apparent motion of the sun must be accurately measured.

As early as the end of the 19th century, Newcomb compiled a solar calendar based on the earth's revolution around the sun.In this calendar, Newcomb calculated and listed the position of the sun by using the so-called "Newton time" (that is, the ideal uniform time) according to the laws of celestial mechanics.In this ideal system of time, at a given moment, a corresponding position of the sun can be found from the watch.

In this case, can the problem be reversed, and the average time corresponding to this position can be deduced from the observed position of the sun?
Doing so is possible, at least in theory.After demonstration, the International Astronomical Society decided in 1956 to define an ideal time scale based on the Newcomb solar calendar, which is what the academic circles call "almanac time".Its second length is equal to 1960/1 of the year length at 1:0 on January 1, 31556925.9747.It is also stipulated internationally that since 1960, ephemeris time will replace mean solar time as the basic time measurement standard.

In this way, we theoretically have a uniform unit of second length.But in fact, it is quite difficult to obtain such a second length, because it is difficult to observe the sun, and people can only measure the ephemeris time by observing other celestial bodies such as the moon.

The moon is a celestial body with a relatively large apparent circular surface, and its edges are not very neat. Using astronomical instruments such as modern meridian rings, transit instruments, and moon cameras, after several years of observation, the accuracy of the almanac time can only reach 10-9 order of magnitude; compared with Pingtai, the accuracy of solar time is only less than ten times higher, which still cannot meet the requirements of modern science and technology for time accuracy.

Put the atoms on the clock So far, we have roughly followed the original sequence of the development of things, and introduced the development history of human beings to understand and measure time starting from the laws of material movement in the macroscopic world.We have seen that human beings have gradually developed the concept of "sun" in the long-term process of "working with the sun and resting with the sun"; and from the moon's eclipse and recovery, we have recognized a longer unit of time - the month; After humans knew that the sun was a star, the cycle of the earth's movement became the scientific standard for measuring time.During this long period of time, mankind has invented sundials, hourglasses and all kinds of ingenious clocks to measure shorter intervals of time.With the development of physics, people learned to hang a simple pendulum on the clock, and made a pendulum clock, which improved the timing accuracy; after that, they used quartz crystal oscillation to pull the clock face, and made a quartz clock, which made the timing accuracy much higher. Great improvement.The time measured by these clocks is based on the macroscopic movement period of celestial bodies, and the movement period of celestial bodies is determined by astronomical observations.Therefore, for a long time, people have been accustomed to associate time work with astronomy, and this is the reason.

Under the conditions of modern science and technology, human beings' understanding of the macroscopic world has far exceeded the scope of people's horizons, and has expanded to more distant stars, galaxies, galaxy clusters, quasars... reaching a sky area of ​​more than 100 billion light-years.It is true that this is a remarkable progress, but we cannot but see that human beings' current understanding of the laws of motion of various celestial bodies is far from perfect, even for the earth where human beings live and lives. Find out its laws of motion.

Under such circumstances, there are two difficulties in determining the time through astronomical observations; the first is the theoretical difficulty, that is, the law of motion of celestial bodies on which time measurement is based has not yet been clarified; The light reaches the observation instrument through the earth's atmosphere, and the refraction of starlight by the atmosphere greatly limits the accuracy of ground observation.At present, when using optical telescopes on the ground to observe stars and determine the world, its accuracy can only reach the level of a few thousandths of a second.

When the macro time standard (the movement of celestial bodies) cannot meet the needs of scientific development, human cognition develops towards another aspect—the micro world, and a new process of understanding and measuring time begins.

We know that no identical individuals can be found in the macroscopic world.There are more than 40 billion people in the world, that is, there are more than 40 billion appearances. Even if they are twin brothers, they look similar, but if you observe carefully, you can always find the difference.A certain electrical product produced by the same manufacturer with the same type of components can be exactly the same in appearance, but the quality will have its own advantages and disadvantages.

(End of this chapter)