Interesting discussion on relativity

There is an interesting story in the series. It is said that a handsome young man is a scholar. He is favored by a fairy named Princess Yunluo and has been married to her for several years. After Princess Yunluo married the scholar, she occasionally went back to her parents' house. The first time, they agreed to go back for only three days, but this time they went for several years. After the princess came back slowly, the scholar complained that the princess came back too late, but the princess said that she only stayed at her family for two and a half days and then hurried back. It turns out that a day in the sky is a year on earth!

In China's ancient fairy tales, there are some magical imaginations, such as "One day in the sky, one year on the earth" and "Seven days in the cave, and a thousand years on earth", which are recorded and described in some absurd stories and fairy tales. Just like the story of the famous Rotten Man, a woodcutter met a boy playing chess in the mountains, but he just looked at the chess game intently, only to find that the handle of the axe was rotten until he returned to the village.

It is said that Einstein once explained the theory of relativity to the public by taking lovers in love or lovers in honeymoon as an analogy. A more popular story is that Einstein once used this analogy to help readers understand the theory of relativity: sitting with a beautiful woman for two hours feels like one minute, but sitting on a hot stove for one minute feels like two hours. This statement was first reported in The New York Times 1929, but it has been made clear in the report that Einstein is said to have taught female secretaries to send away those curious visitors in this way. Since then, this statement has appeared in various popular versions.

Einstein's story is similar to Newton's discovery of gravity when he was hit on the head by an apple falling from a tree, or Galileo threw an iron ball from the leaning tower of Pisa in public to verify that the speed of falling objects has nothing to do with weight. They are also legends or human myths.

I think even if Einstein likes to compare the relativity of time with lovers or beautiful women in love, it is at best a joke. This analogy does not tell the essence of the relativity of time, but only the relativity of psychological time. At least for now, we can't equate the relativity of psychological time with the relativity of physical time in the physical sense.

If Einstein knew such fantastic ideas as "one day in the sky, one year on the earth" and "seven days in the cave, one thousand years on earth" in China's myths and legends, he would be greatly appreciated. He even used this absurd imagination in China's mythology to explain the theory of relativity to curious fans.

It's a pity that Einstein didn't understand China culture.

Of course, China's imagination of the fairy tale of going to heaven one day and coming to earth one year may also come from his perception of the relativity of psychological time, just as Qian Zhongshu said in On Happiness: "Heaven is more comfortable and happy than earth, so the gods pass quickly, and there is only one day in heaven."

However, even so, the imagination of the relativity of time between heaven and earth in China's myths and legends is still a good example to explain the relativity of time in a popular and vivid way, which just shows the essence of the relativity of time: the speed of clocks and watches in different worlds is inconsistent.

In fact, not only mortals imagine that immortals live happily, but also immortals in the sky envy earthly life. Therefore, in folk myths and legends, there are so many secular immortals to enjoy the happy life of ordinary people. It is no wonder that there are so many monsters in The Journey to the West who have to fall from the sky to be happy. Heaven and earth are yearning for each other, and the world thinks that the sky is "fast time", so must the sky. This is an unexpected encounter with special relativity here: two systems or worlds with relative motion look at each other's clocks and go very slowly.

Although the imagination of "fast time" in China's myths and legends is very creative, it is only limited to the imagination of myths, and it cannot be said that it has the bud of modern scientific thought. According to the general theory of relativity, the gravitational field slows down the clock, so the time living in the sky is faster than that on the earth, but it is a day when ordinary people on the earth enjoy the "fast time", because the gravitational field on the ground is stronger than that on the white clouds, and the time on the earth is slower than that from the celestial world, although this difference is subtle and difficult to detect.

However, we can still imagine that the celestial world is in another distant interstellar space with a much stronger gravitational field than the earth. Of course, this is a bit true. This is a sci-fi imagination, not a mythical imagination.

The magical thought of relativity arouses the infinite curiosity of human beings to explore the universe and inspires all kinds of wild fantasies: Star Trek, time travel and parallel universe, which are as fantastic as mythical imagination.

Relativity established the concept of relative time and space, subverted Newton's absolute view of classical mechanics, subverted our common sense cognition, marked a leap in human scientific thought and a victory of human rationality.

In the view of absolute space-time, the speed of time passing is absolutely constant: whether you are walking slowly on a pedestrian street or taking a high-speed express train, whether you are on a plane crossing clouds, or on a starship, whether you are on the top of white clouds, or on the moon, whether you are in the solar system or a distant galaxy, the speed of time passing is the same, and the measurement of time is the same. Time is a river flowing at a constant speed in the universe.

In the view of relative time and space, everything in the universe has no unified time base, and the speed of time passing between different systems and the world is different. Every system and every world has its own time scale, and everyone has his own time scale. Time does not flow at a constant speed, the scale of time will "expand" and the wheel of time will change speed.

In the world of absolute space-time view, "simultaneity" is of universal significance and will never be questioned, while in the world of relative space-time view, "simultaneity" has encountered challenges. In the world of absolute time and space, lovesick lovers can think of "the end of the world at this time"; However, in the world of relative time and space, visionary people have to sigh that they don't know when my moment is yours and when my dusk is yours.

In absolute space-time, space is also eternal, without starting point and boundary, it spreads evenly in all directions, without stretching and bending, and two parallel lines will unfold in parallel and never intersect; The measurement scale of space is the same in all directions, whether you measure horizontally, vertically, statically or dynamically.

In the view of relative space-time, space has a starting point and a boundary, and it is no longer even and flat. Space will be compressed and bent by the mass of the object, and parallel lines may also intersect. The sum of the angles of a triangle is not necessarily 180 degrees, and pi is not necessarily a number that we all recite. Space is one of the properties of material movement, and the measurement scale of space will change with the movement of objects, "the length of movement will be shortened", and the measurement scale of space between different worlds will not be consistent.

In the absolute world of time and space, the poet will happily "shine like lightning" and kill the enemy "invisibly" in his arms, while in the relative world of time and space, the lightning knight can only be horrified and embarrassed to find that his lightning-fast sword has shrunk into an "invisible sword" and almost disappeared at the moment of shooting, and he may have to pay the price of his life for his lightning speed!

In the absolute concept of time and space, time and space is the environment in which everything in the universe runs, allowing the wind to travel freely between heaven and earth, the light to gallop freely among the stars, and the universe to rotate freely. Time and space only constitute the reference system of material movement, and only measure the extensiveness and continuity of material movement.

However, in the view of relative space-time, the motion of an object is inseparable and intertwined with time and space. In physical events, time and space are not only the frame of reference of material motion, but also the attributes of material motion. Time and space are no longer spectators, but should participate in the process of physical events and play an important role.

In the view of relative time and space, the movement of objects affects the speed of time passing and the scale of space. The movement of an object slows down the clock and shortens the scale of space measurement. The mass of an object will bend time and space, and the accelerated movement of the object will arouse "ripples in time and space", just like a stone falling into the water, causing layers of waves. Time and space will in turn affect the way everything in the universe moves. Gravity is just a geometric effect of space-time bending. Objects do not follow the straight line of space, but follow the "straight line of time and space" and the shortest distance between time and space.

As the famous American physicist john wheeler famously said, "Time and space tell matter how to move, while matter tells time and space how to bend".

In the absolute space-time view, space and time are independent of each other, and there is no interference between them, and the speed of space movement and the speed of time passing will not be related.

In the view of relative space-time, time is no longer independent of space. Like the spatial dimension in three-dimensional space, time has become a dimension in four-dimensional space-time. In four-dimensional space-time, time and space can be transformed into each other.

Interestingly, from the viewpoint of relative time and space, not all physical quantities are relative, with one exception, that is, the speed of light. The speed of light in a vacuum is an absolute constant, which is the ultimate speed of the universe, regardless of the reference frame and the observer. Einstein deduced the relativity principle of time from the invariability of the speed of light. The faster space moves, the slower time passes, and space can change time. The faster space moves, the slower time passes. When space moves at the speed of light, time will stop passing.

Our world is a low-speed world, and the theory of relativity evokes our reverie and even imagination of the high-speed world, tempting us to imagine what the high-speed world will be like.

I have seen an interesting example to explain the relativity of time, which means that there was a gun battle on a high-speed Orient Express. Think of it as a gunfight between the police and the robbers. The frightened passengers on the train saw the robbers shoot at the police first, and then the police shot back. However, a railway worker who happened to be standing beside the railway subgrade outside the window saw a different scene: the police first shot at the robbers, and then the robbers shot back.

This example is a hypothesis of time relativity. Although the speed of time is relative to different observers, time is irreversible. Suppose the passengers on the train see the robbers and the police shooting one after another, then the observers outside the high-speed train see that the interval between the police and the robbers shooting at each other is longer, and they will not see the opposite shooting order, no matter how fast the train runs. When the speed of the train is getting closer to the speed of light, the time interval between the police and the robbers in the train will be infinitely lengthened for the observers beside the railway tracks. Assuming, of course, that the observers standing beside the tracks have time to see the gun battles on the passing trains. Even in time, trains traveling near the speed of light have become thin beams.

I wonder how many poets want to turn into a beam of light in their poems and fly freely in the starry sky of love. If the poet's wish comes true, he will be surprised to find that time has stopped passing and distance has lost its meaning. We may have imagined that roaming the universe like light would be the most romantic thing in the world, but in the eyes of light, this universe has no romantic time and space to roam. No matter how far the interstellar distance is for us, it is only zero distance for light. In our eyes, the vast universe is only equivalent to a singularity in time and space in the eyes of light.

Among the scientific fantasies produced by the theory of time relativity, the most fascinating one is the imaginary interstellar travel: imagine that you will become an astronaut one day in the future, and you will fly to the space station of the Milky Way in a hypervelocity spacecraft to perform space missions. Imagine that you will cross the universe. You are so excited that you can't even say goodbye to your friends and family. You and your companions will fly away from the earth in a spaceship near the speed of light, fly over the solar system and sail for the vast starry sky of the Milky Way. You arrive at the Galaxy Space Station in about a few hours' flight, stay there for a few days, complete your space mission, and then go home. After a few hours' flight, you returned to the earth. But you are surprised to find that friends and relatives on the earth are young and no longer gray-haired, although you have only been away from the earth for a few days. You seem to have crossed into the future, but you can't go back to the moment when you started from the earth. You have dreamily experienced the legendary "rotten people" wonderland. Fortunately, all your relatives and friends are still alive, just old.

There is a detail in the serial story mentioned at the beginning of the article: Princess Yunluo will return to heaven, and the maid will take out something that looks like a raft, spray it on the spot, spray clouds everywhere, and then disappear. We can naturally infer that Princess Yunluo traveled to and from the world on a raft, unlike the immortals in The Journey to the West who crossed the beast or traveled to and from the celestial world on Xiangyun. This detail seems to increase the rationality of the story arrangement. I don't know whether it is the author Pu Songling's careful conception or random description. Maybe this is too far-fetched. This detail inevitably makes us imagine Princess Yunluo's raft as a high-speed aircraft, just like a spaceship traveling in space. It takes only a few days for a princess to travel back and forth between heaven and earth, but it takes several years for scholars who live in a slow pace on the earth.

"One inch of time can't be light", knowing that the speed of time passing depends on the speed of movement, we may have a new understanding of life from a brand-new dimension, and "life lies in movement" has a new interpretation. Knowing the relativity of time and the relationship between time and sports, we should cherish every inch of time and make our life wonderful and rhythmic, so that maybe one day we can boast that we are still teenagers; Otherwise, we wasted half our lives, maybe just a few hours for others. Of course, you can take this as a joke.

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