But what is time? It is difficult for us to answer this question, but can we measure it? It? Because the easiest way to study one thing is to go? Measurement? So people thought of a method to measure time: period. We can use some periodic movements as the standard of time and mark time with the number of cycles. For example, a pendulum uses a swing as a second to measure time.
But the tools for measuring time all use the number of cycles to measure time. What if the time of the first cycle is different from that of the second cycle? We can't find out whether the last second and the next second are the same time interval, so at this time, we try to choose an object that can move many times in a short time within the scope of daily intuition. So what? Cesium atomic clock? It was born that cesium atoms can vibrate for one second 9. 19? 10 9 times, which is an astronomical number, naturally becomes the most accurate timing tool for human beings at present.
With time measuring tools, we can study time machines. What physical theories are closely related to time at present? Obviously, Einstein's theory of relativity came in handy at this time. Specifically, many solutions can be obtained through the field equation of Einstein's general relativity. I talked about the field equation in the special issue before. If you haven't read the advice, you can read it first.
Through Einstein's field equation, we can get a solution, called Godel solution, which corresponds to a strange Shi Kongtu. What is Shi Kongtu? In fact, it is to draw a function diagram with space as the abscissa and time as the ordinate, as shown below.
It can be seen that although the Shi Kongtu in the conventional universe (that is, our universe) is a curve (of course, it can also be a straight line), on the whole, time and space are developing forward, while Shi Kongtu in the Godel universe is allowed to form a? Closed loop? This? Closed loop? How to understand? We can see that the Shi Kongtu of Godel Universe began to move forward in space and time, but to some extent, space began to move backward. Of course, our space can also move backwards. For example, I started with x=0, then I was at x= 10, and then I was at x=6, which is equivalent to going back to 6 from 10. It's nothing. But if you look closely at Shi Kongtu in Godel's universe, you will find that not only the space has gone back, but also the time has gone back, and it will intersect with the previous trajectory, as shown in the following figure.
Intersection is very magical, because it shows that not only space has gone back, but also time has gone back, so Godel's universe is allowed to travel through time and space. However, according to the cosmic data we have observed so far, it does not conform to Godel's model, so if there are parallel universes, then other universes may conform to Godel's model.
Although Godel's model of the universe does not conform to our model of the universe, it also conforms to the field equation of general relativity, so it is correct in a sense and lays the foundation for us to explore time and space travel.
But that's not enough. We need to continue to explore the theory of traveling through time and space, which means that we should mention another solution of the field equation of general relativity: black holes. My previous article about black holes was also specifically introduced. If you haven't seen it, you can go and have a look first. Why can black holes travel through time and space? Because general relativity integrates time and space, general relativity basically has no separate concept of time, no separate concept of space, only an integrated concept? Time and space? Therefore, the general theory of relativity does not need to do force analysis to describe the motion of an object, but only needs to describe the space-time where the object is located in geometric language, so the object moves with it? Geodesic? Just do the simplest unstressed inertial motion (several articles have written about this equivalent process in detail), what is geodesic? In fact, geodesic is a straight line between two points in time and space, but Einstein thinks that time and space are curved, so geodesic is a curve between two points A and B. If A and B are directly connected, time and space will be broken, but we can only walk on the surface of time and space, but we can't break through it directly, as shown in the figure below, so objects in curved time and space basically move along curved trajectories.
With the above theoretical knowledge, we will continue to analyze. Because mass will distort the surrounding objects in time and space, if an object is in a place with a great degree of distortion in time and space, it will slow down time, just as each of us passes through the river of time. Newton's view of time and space thinks that each of us has the same speed of time passing, that is, side by side? Dragon boat race? Everyone will always walk on the river of time at the same speed, so the result of the competition must be that everyone's score is the same.
But Einstein unified time and space, so there will be some places where time and space are greatly distorted. Dragon boat race? There will be winners and winners, as shown in the figure below.
When time itself is divided into speed and speed, crossing time becomes feasible. Actually, I talked about it when I wrote the special theory of relativity before. We can let a person walk on a high-speed train and then return to the ground. In this way, this person's high-speed travel time slowed down, so he got off the train and found that the technology on the earth was more developed. His former peers became white-haired and his children were older than himself. In other words, this man crossed into the future.
So if you want to go to the future, you just need to generate time with others according to relativity? Speed difference? Yes, so we still have a way to the future, that is, to let this man get close to the black hole in a spaceship. Because the mass around a black hole will distort space-time and slow down time, this person's time speed is definitely not as fast as that of people on earth. So when he stays in a black hole for 1 hour, it may be equivalent to staying on the earth for seven years. When he returned to the earth for three hours, he found that the earth had passed 2 1 year, in other words.
In fact, the above scene is the scene of the interstellar movie. In order to make this film, the director of this film really consulted several physicists who studied general relativity at that time. Don't think that the above scenes can only appear in movies. This is a conclusion derived from real scientific theory, but our current technology can't accelerate a train to the speed of light, nor can we get a person on a spaceship to play around a black hole.