Encyclopedia of popular science knowledge: "Time Knowledge"

Chapter 14 Time's Fun Story (7)
"If we want to describe the motion of a particle, we give its coordinates as a function of time. Now we must remember that such a mathematical description can only be described if we understand very clearly what 'time' here refers to What has physical meaning after. We should take into account that all our judgments in which time plays a role are always judgments about simultaneous events. For example, if I say, 'The train arrived here at 7 o'clock', this probably It means: 'The short hand of my watch points to 7 and the arrival of the train is a simultaneous event.'"

It might be thought that by substituting "the position of the short hand of my watch" for "time", it might be possible to overcome all the difficulties arising from the definition of "time".In fact, if it were only a matter of defining a time for the place where the watch is, then such a definition would suffice.But if the problem is to relate in time a series of events occurring in different places, or--the result remains the same--to time events which occurred in places remote from the watch, then Such a definition is not enough.

Einstein realized that time and signal velocity are inextricably linked, that the simultaneity of two events at different distances is related to the relative positions of the events and the way in which an observer perceives them as connected.If the distance of the event and the velocity of the signal linking it to the observer are known, the observer can calculate when the event occurred and relate it to a moment previously experienced by him.This calculation is different for different observers.However, before Einstein raised this question, people have always adhered to such a principle: the time at which an event is perceived depends only on the time at which it occurs, which is the same for all observers.Einstein pointed out that the above principle is based on the premise that all observers should get the same time for the same given event if their calculations are correct.However, Einstein demonstrated convincingly that this premise does not generally hold.He found that different observers in uniform relative motion generally always measure different times for the same event.If two clocks are in constant motion relative to each other, they will keep different times, and you cannot say which clock is "accurate".A moving clock is always slower than a relatively stationary clock.This effect is negligible at the speeds of motion we encounter every day, but as the clock moves closer to the speed of light, the effect of slowing the clock becomes more pronounced.

To illustrate this further, let's do a "thought experiment".This is an "experiment" that does not have to be carried out in the laboratory, but is only imagined through the mind. It is also a form of scientific experiment and is quite popular among physicists.In fact, even middle school students often use it when doing exercises in physics classes.

The experiment looks like this:

Assume that there are two clocks A and B with the same quality in the satellite building of the Capital Airport. After calibration and synchronization, let A clock stay in the satellite building, and install B clock on the plane.When the plane flies from Beijing to Shanghai and then returns to the Capital Airport, compare clock B with clock A, will the time indicated by their pointers be the same?
Some readers may blurt out: same.but it is not the truth.If the two clocks are sufficiently precise, we will find that clock B is slower than clock A.

This is the "clock contradiction" predicted by Einstein's theory of relativity.The contradiction mentioned here is not a contradiction in the logical sense, but refers to a way of thinking that is contrary to common sense, that is, the so-called "paradox".

According to special relativity, two synchronized clocks, one of which moves along ten closed curves with speed V, and returns to the original position after one second, then it is 12 (V/c) slower than the clock that has never moved 2, where c is the speed of light.From this, it can be deduced that for the same experience process, the time interval measured by clock B on the plane is △τ, and the time interval measured by clock 4 in the satellite building is △t. It will exceed the speed of light, and the value of √1-(V/c)2 is always less than 1, so compared to clock A, clock B slows down.When clock A passes 1 second, clock B only passes 1-(v/C) 2 seconds.

Usually, the V/c value is much smaller than 1, 1-(v/C) 2 is approximately equal to 1, and the degree of slowing down of the clock is negligible.However, if we can launch a spaceship and make it fly at a speed of 0.98 times the speed of light relative to the earth, from the perspective of people on the ground, the speed of the clock in the spaceship will only be 1/5 of the speed of the clock on the ground.In this case, if we let the elder brother of the 25-year-old and 28-year-old brothers fly in a spaceship for 5 years, then when he returns to the ground, the younger brother will find that he is 1 year older than the elder brother.Because these 5 years refer to the 5 years on the ground, the younger brother is already 30 years old.But during this time, the clock in the spacecraft only passed one year, and my brother was only one year older, only 1 years old.

Some physics books call this phenomenon the "twin paradox".

This wonderful phenomenon predicted by the theory of relativity has long been a hotly discussed topic among physicists.However, it was not possible to make a positive experimental verification of it until the advent of the atomic clock.

In 1971, the U.S. Naval Observatory installed four cesium atomic clocks on a plane and departed from Washington, respectively, to fly east and west around the world.It was found that there was a nanosecond difference in readings between the cesium clocks flying east and those parked at the observatory; when flying west, the difference was 4 microseconds.Although the effect of the earth's gravity was not deducted in this experiment, the measurement results show that the "twin paradox" does exist.

The clock on the equator runs slowly. Einstein was a great physicist, and many of his predictions were confirmed by experiments one after another.But are every one of his predictions correct?The answer is no.

In his first paper on the theory of relativity, "On the Electrodynamics of Moving Bodies," Einstein deduced that a clock on the equator would be more efficient than a clock of exactly the same mass placed at the poles of the earth, all else being equal. , to walk slowly.In other words, clocks run at different speeds at different latitudes on the Earth's surface.At the equator, the earth's rotation speed v=0.46 m/s, V2/c2≈1.8×10-12; while at the poles, v=0.During the course of a day, the equatorial clock will be about 102 nanoseconds behind the polar clocks.

Obviously, Einstein only considered the speed of time here, but did not take the gravitational effect into consideration at the same time.We know that although the linear velocity on the surface of the earth is different at different latitudes, the linear velocity becomes smaller the farther away from the equator, and zero at the two poles, but the earth is an ellipsoid, and the two poles are closer to the center of the earth than the equator, so the gravitational potential at the two poles larger than at the equator.The effects of these two factors on the clock speed cancel each other out, and the combined effect is exactly zero.

In order to verify this conclusion, Avery and others used C-1977 long-distance transport aircraft to carry out a flight between Washington (latitude 6′ north latitude) and an air force base in Greenland (latitude 144′ north latitude) in June 38049. Flight clock test.The measured difference between the flying clock and the ground clock is 76032 nanoseconds, which is consistent with the theoretical calculation value (38 nanoseconds), thus proving that the clock speed has nothing to do with latitude.This shows that Einstein's deduction at the time was wrong, and the clock at the equator will not run slower than the clock at the poles.

It has also been suggested that at certain times of the year, such as the summer solstice, the North Pole is closer to the Sun than the South Pole due to the tilt of the Earth's axis of rotation.In this way, if the principle of relativity is applied within the gravitational potential of the sun, will the clock in the northern hemisphere be faster than that in the southern hemisphere?

The math proves no.In order to verify the correctness of the mathematical proof, the C-144 aircraft made two flights between Washington and Christridge, New Zealand in July 1977. The measured results also showed that the clock speed has nothing to do with its latitude.

It can be seen that any great scientist will inevitably be limited by the technical conditions at that time when they create new scientific theories.Later explorers have the responsibility to test these theories according to their own practice, or to sublate, or inherit, or revise, and must not be superstitious.

Gradually Slowing Gravitational Clock There are 4 fundamental forces at work in nature.These are: gravity, electromagnetism, nuclear force, and the weak force that arises when atoms decay.

Gravity was discovered by Newton, but it was Einstein who opened the secret door of gravity.Einstein pointed out in the general theory of relativity that the universe is full of gravitational waves, which is a phenomenon in which the curvature of space generated around objects takes the form of waves and propagates at the speed of light.This is a rather esoteric question, although more than half a century has passed since it was raised, it still attracts the interest of many physicists. In 1938, the British physicist Dirac and the American physicist Dick successively put forward the hypothesis of the weakening of gravity after making some modifications to Einstein's gravitational theory.

According to this hypothesis, the gravitational constant G is decreasing slowly, relative to the electric constant, about 1×10-11 per year.

Is gravity really weakening?

This is another question that needs experimentation to answer.After the appearance of the atomic clock, someone suggested that the reduction of gravitational force could be measured directly by comparing the atomic clock with the "gravitational clock".

Atomic clocks use electron vibrations inside atoms to replace the pendulum of a normal clock.The force that determines the size of the electron vibration period is the electromagnetic force between the electron and the nucleus in the atom.The electromagnetic force is constant, so the rate of the atomic clock does not change.

So-called gravitational clocks are artificial satellites.The rate of the gravitational clock can be calculated from the period of a satellite orbiting the earth.This period increases when gravity weakens, indicating that the rate of the gravitational clock slows down.If gravity does gradually weaken over time, the gravitational clock will also gradually slow down.Comparing the rates of atomic and gravitational clocks over several years could, in principle, test the hypothesis of a weakening of gravity.

(End of this chapter)