Encyclopedia of popular science knowledge: "Time Knowledge"

Chapter 12 Time's Fun Story (5)
But in the microcosm, the opposite is true, and many things are exactly the same.You can't tell one electron from another, or find any difference between atoms of the same element.This is not because the measuring instruments are crude, but they are indeed identical, and in principle they cannot be mined separately.

People have already known the molecules, atoms, nuclei, and elementary particles in the microscopic world...they are as small as 10-13 centimeters, and we cannot see them with the naked eye anyway.

Although an atom is small, it is a complex world inside.Every atom has a nucleus, and the outer layer of the nucleus is arranged with high-speed electrons.When an atom (or molecule) is exposed to x-rays or other electromagnetic radiation, its orbital electrons can jump from one position to another (this is called a "jump" in physics), change direction, or pull like a spring The little ball kept beating like that.During the transition, the atoms will absorb or release electromagnetic waves of a certain energy.In essence, this kind of electromagnetic wave is the same as the simple pendulum, and it is also a periodic motion, except that its vibration period is shorter (it can reach billions of times per second), more precise and more stable.

In this case, can we put atoms on the clock and use its oscillation to make an atomic clock like making a pendulum clock?
This was the subject of intense discussion among physicists in the early 20th century. In 1927, G. Darwin, a descendant of Darwin, the great biologist and discoverer of the theory of evolution, was the first to theoretically discuss this issue, and then American physicists Phipus and Fritsch conducted experiments. In 1936, Professor Rabbi of Columbia University put forward the basic theory and method to obtain the oscillation frequency of atomic transition based on these experiments, and initially showed the possibility of using the atomic oscillation frequency to control the clock.Unfortunately, these experiments and studies were interrupted for several years due to the effects of World War II.

After World War II, relevant experiments and research work resumed and developed rapidly. In 1949, the US National Bureau of Standards first used the ammonia molecular transition to make an ammonia molecular clock. Five years later, the British Royal Physical Laboratory finally put cesium atoms on the clock, making the world's first cesium atomic clock.Since then, other types of atomic clocks have come out one after another, mainly hydrogen atomic clocks and rubidium atomic clocks.The time given by the atomic clock is called atomic time, which is recorded as AT in professional books, which is the abbreviation of the English name Atomic Time.

The universe of new challenges is endless, and people's understanding of it will not stay at one level.Like any other subject area, time measurement science is constantly developing with the progress of science and technology.

In fact, after the atomic clock gained dominance in defining time, time workers were not satisfied and stopped.

While further improving the performance indicators of the existing atomic clocks, they actively explored new timing standards and started a new "Long March".

According to the current laboratory work and theoretical analysis, the new principles and new methods mainly include: using thallium element to develop thallium atomic clock, using magnesium or calcium submillimeter beam to develop magnesium or calcium atomic clock, using the special structure of ions to develop "ion clock", Development of a "photonic clock" using laser frequency standards.

Photonic clocks hold great potential in these new quests, and they are challenging cesium atomic clocks.

We know that light itself is also an electromagnetic wave.Its frequency is much higher than that of the radio band.According to theoretical analysis, the stability of the laser frequency is 3 orders of magnitude higher than that of the cesium standard. If it is used to make a photon clock, the time measurement accuracy can be increased by 1000 times at the current level.This will be another major change.

Of course, there are many difficulties in realizing this change.However, scientists are never afraid of difficulties in scientific experiments, and they never go around them. "There are dangers in science, and hard work can pass." At present, they are working hard, and the laboratory experiments have achieved preliminary results, and the dawn of change has emerged on the horizon.

What are some of the consequences of implementing this transformation, and how will it affect metrology?Before the photon clock is completed, it is difficult for us to make a detailed and specific description, but we can theoretically speculate on the new problems it may raise.

We know that time, length, and quality are three basic physical quantities, and other physical quantities, such as speed, temperature, illuminance, voltage, power, etc., can be derived from these three basic quantities.Velocity, for example, is derived from length and time:
speed = distance time.

If the laser time standard succeeds, it will first shake the definition of the standard of length -- the meter.

The meter is widely used in countries all over the world, and it is also a relatively advanced unit of measurement of length. The length of 1 meter refers to one ten millionth of the meridian length of a quadrant (90°) on the meridian circle where Paris, France is located.Initially, people used high-hardness and oxidation-resistant platinum-iridium alloys to make so-called "rice prototypes" to maintain the standard length of meters.Although the expansion coefficient of this alloy is very small (about 1000×8.75-10/℃), it cannot guarantee that its length will not change with time.Since 6, it has been decided internationally to use the wavelength AL of an emission line of krypton (Kr1960) to define a meter, that is, 86 meter = 1λk, that is, the length of a meter is expressed in multiples of the wavelength.Using this method to determine the meter length, the accuracy is on the order of 1650763.73-10, that is, the error between two measurements is about 8 micron.

However, the accuracy of frequency measurement has been improved to above 10-13 at present.Here is an urgent question to be solved:

Wavelength and frequency are connected with each other through the speed of light. The speed of light c is equal to the product of wavelength and frequency f, c=f. In this way, the accuracy of the speed of light is limited by the wavelength standard.Therefore, in recent years, the international community is considering whether to redefine the speed of light.If the speed of light is redefined, then the meter is no longer an independent unit of measurement, and it will be unified with the definition of the speed of light and the second.In this way, the three basic quantities will become "two basic quantities".

Also, some other derived units may then depend on time.This may be the case, for example, with voltage measurements.

At present, electrical engineers use "standard batteries" to measure voltage, and the accuracy is on the order of 10-5 to 10-6.

But we know that the relationship between the frequency f of alternating current and the voltage V is:

f=2eV/h where e represents the charge of the electron and h is a constant called Planck's constant.By choosing an appropriate ratio e/h, the voltage measurement can be converted into a frequency measurement, that is, into a time measurement, because time and frequency are reciprocals of each other.

The huge potential of the laser time standard has attracted the attention of all countries in the world. The research work of many countries including my country is making progress, and another milestone in the history of time measurement has begun to break ground.We are eagerly looking forward to our country's time workers making greater contributions to its early unveiling.

The use of millionths of a second In the daily life of modern society, the time accurate to the second is enough.We have never found any civil aviation airport that sets the flight departure time at what time, minutes, and seconds, and we have not seen any school that sets similar regulations on the time when students start and leave school.Even the latest modern electronic wristwatches only give time down to the second.So, why do scientists have to measure time so accurately, accurate to one ten-thousandth of a second, one millionth of a second, or even one billionth of a second?Are they really strange people with a penchant for strange things?

totally not!

Scientists are most concerned about efficiency.If there is no need for production practice and scientific research, they will never waste their energy, waste time and social wealth.

Generally speaking, the time response of a person is about a few tenths of a second, and it takes about a few seconds from the reaction time to the beginning of a certain action.Therefore, in daily life, people's requirements for less than a second are not urgent.

However, in production activities and scientific research, the situation is completely different.

The simplest example is the [-]-meter race.At the level of modern sports, sometimes it is difficult to decide who is accurate to one-tenth of a second, and the winner must be accurate to one-hundredth of a second.

Another example is the study of lightning.Lightning is a natural phenomenon that everyone is familiar with.In the hot summer, there was a sudden strong wind, dark clouds and thunderstorms.The lightning that tears through the sky and the deafening thunder often destroy houses, bridges, forests, and dams, and even cause personal casualties.In the past, some people often described this natural phenomenon as the manifestation of thunder and punishment for the world.Later, American scientist Franklin released a kite in a thunderstorm to draw electric sparks from the clouds, which broke the superstitious legend about thunder and lightning.But for almost two centuries after Franklin, no one really knew what happened when the lightning flashed across the sky.The reason is that the accuracy of time measurement is not high, and it is difficult for people to distinguish the process of lightning occurrence.Now we know that every thunderstorm has a "main minefield", which emits a dull leading thunder, then branches in the cloud layer, discharges, and draws a flash of light to the ground. The time spent in each process is less than Ten-thousandth of a second.If the time measurement is accurate to less than one ten-thousandth of a second, it will be difficult for people to study the whole process of lightning, and it is impossible to find a way to avoid lightning strikes like today.

Another example is the study of the explosion process.Explosives are needed to open mountains and roads, and various weapons are required to defend national independence and security, among which conventional weapons also use explosives.The explosion process of explosives is very fast, glycerin explosives or yellow explosives (TNT), its explosion occurs in a short time of one millionth of a second (microsecond); modern torpedoes are detonated with a high-speed explosive, and it takes only 20 minutes from detonation to explosion. multiple microseconds.

Chemists and national defense technicians have to test and record the explosion speed of various substances. Without time measurement accurate to one millionth of a second, not only can they not find effective explosive substances, but sometimes even their lives cannot be saved.

As for space travel, its time requirements are even higher.Spaceship or satellite launch, human orbit, guidance, re-entry, safe recovery or landing, each process requires precise time measurement.From the launch site, the flight control center, to the recovery monitoring area, there is a need for a dedicated time control system—a unified time service system—to provide high-precision time signals to various parts to ensure a successful launch.It is said that before returning to the ground, the second manned spacecraft "Mercury" launched by the United States, due to a malfunction in the attitude control system of the spacecraft, the astronauts switched to manual control, which delayed the ignition of the braking rocket. Some, as a result, the spacecraft deviated from the normal orbit by more than 20 degrees, and deviated nearly [-] kilometers from the scheduled landing point, which almost caused danger.

Measurements of even shorter times than this take place in the wondrous realm of nuclear physics.Physicists have discovered that subatomic (smaller than atoms) particles move at close to the speed of light and have a particularly short life span of a few hundred millionths of a second.A German scientist said he had discovered the 109th new element.The new element (as yet to be named) has a lifetime of just a billionth of a second.It can be imagined that if there is no high-precision atomic clock, it will be difficult for people to study the microscopic change characteristics of matter, and there will be no discovery of new elements.

Scientists also predicted that the lifetime of some mesons is even shorter than this, only about 0.14×10-25 seconds.This is probably the shortest measurement of time that humanity will encounter in the near future.

exact time delivery

We have been able to obtain accurate time through astronomical observations and keep it in atomic clocks.But this is not enough, it must also be able to send accurate time to the user while maintaining the original accuracy as much as possible.

The service attitude of the staff of the time service system is quite good, and they always try their best to accurately transmit the time signal to each user.This process is called the transmission of accurate time, or "time service" and "time reporting".The user is "time synchronization" or "time synchronization" for the observatory.

Sound timekeeping From ancient times to the present, there are many ways to transmit time signals.At first, the transmission of time signals was always done by mechanical, acoustic and optical methods.For example, in ancient times, drums and cannons were used to tell the time; some clocks struck the hour on the hour, and they struck a few times when it was the hour, so that people could tell what time it was by hearing it.Now the clock at the Beijing Railway Station plays the beautiful Dongfanghong music on the hour, which is also an example of the use of sound to tell the time and it has been extended to the present.

Drop Ball Timekeeping Drop Ball - Another way to tell time. In 1884, the Xujiahui Observatory established a falling ball time signal station on the Bund of Shanghai. Boats moored in the water paid great attention to a special pole on the signal station.At 12:12 noon, the ball on the pole will fall, indicating that the time is 9:[-] noon.This is the so-called "falling ball timekeeping".This signal station also uses lights to announce the time at night. When the anchored ships see the flickering lights, they know that it is already [-] o'clock in the evening.

These mechanical, acoustic and optical timekeeping methods have relatively low accuracy; the highest timekeeping accuracy is 0.1 second.Therefore, they can only be used in some occasions that do not require high precision, and the application area is also very small, not exceeding the range of human hearing and vision.

There is another method for aircraft time synchronization, which is to use the aircraft to bring the time-frequency standard to the place where it is needed.For example, put the calibrated atomic clock on the plane, fly over the place where the time needs to be calibrated, notify the user by radio and perform time synchronization.This method is usually called "carrying the bell" method, or "airplane overhead".

In fact, the overhead time setting of the aircraft is similar to our usual time setting, except that the time setting accuracy is very high, which can reach more than ±1×10-6 seconds.This is one of the main methods of time synchronization at the microsecond level. This method uses fewer instruments, has higher precision, and takes less time for time synchronization.But it is more troublesome and requires long-distance air transportation.

The more convenient method of long-distance positioning method is to use the "Roland-C system" timing.Roland-C is a translated term, and its original meaning is "long-distance positioning".The Roland-C system was originally a "long-range precision navigation system" that provides precise navigation for aircraft, ships, and ships. At a distance of more than 1800 kilometers, the Roland-C system can provide users with a positioning accuracy of about 50 meters.The Roland-C system itself uses an atomic clock. As long as the cesium clock of the Loran-C main station is synchronized with the atomic clock of the observatory on the coordinated universal time, and each sub-station is synchronized with the main station, the Loran-C system can be used for time service.Because this system can serve time without adding any equipment, this time service method is more economical.Currently, the Loran-C system is capable of maintaining time synchronization of ±15 microseconds to UTC Coordinated Universal Time.

It is economical and affordable to use the broadcast system for time transmission, and the users are quite extensive.Every hour on the hour, the radio station announces the time with a specific sound, just like the one described at the beginning of this book.This is actually a kind of high-frequency timing.The signal after demodulation is sent out with sound, and after we hear it, we turn the watch by hand to tell the time.Since this kind of time synchronization does not use special instruments, it only relies on hearing and hand movements, so the accuracy is low, only accurate to 0.1 second.But it is enough for our daily life and work, so this time synchronization still plays an important role.

TV Time Telling With the popularization of TV and the development of TV technology, people began to use TV system to tell time. In 1962, the time signal was transmitted along the TV microwave transmission line in the Czech Republic. The loop was 800 kilometers long, and the time variation of the second pulse did not exceed 1 microsecond.The TV synchronization method is economical and practical, and the time can be synchronized without building another set of transmitting and receiving systems.The timing accuracy is also relatively high, about 50 nanoseconds within the line of sight and about 0.5 microseconds outside the line of sight.Of course, due to the different lines of TV microwave transmission lines, the delays of various factors on the propagation time are different, and they all need to be corrected through experiments.

Man-made satellite timekeeping Now it is possible to successfully launch a man-made earth satellite, which makes the satellite run at the speed of the earth's rotation, which is a "synchronous satellite".Geostationary satellites are helping people do more and more work: transmitting radio, television, telephone, telegraph around the world, performing radio fax, digital communication... We can hear or see what is happening around the world while sitting at home Important things.Time synchronization is also possible using satellites.The first satellite time synchronization experiment was carried out in August 1962. Through satellites, the atomic clock in Washington, USA and the atomic clock in Greenwich Observatory, UK were calibrated to about 8 microsecond.The use of satellites can also achieve global time synchronization. As long as three synchronous satellites are launched equidistantly, the entire surface of the earth can be covered.Setting up the necessary receiving equipment on the ground - "satellite ground station" can achieve global time synchronization.

In order to obtain the accurate time that people need, it will go through complicated steps such as measuring time, keeping time, and serving time.All countries in the world attach great importance to this work, and there is a special agency in the world - the International Time Bureau, and there are observatories and metrology bureaus at all levels in all countries to do this work.

what is time

Time is intuitively obvious but logically difficult to determine.It is like a galloping river, flowing continuously and moving forward, but it is as mysterious and inconceivable as described by the theory of relativity.

This strange property of time has driven many people throughout the ages to make various inferences and conjectures about it. In almost every age, philosophers and natural scientists have pondered its enigmatic nature.Both religious and scientific minds have tried to explain what time really is and where it's going.

(End of this chapter)