Four-dimensional space-time is the lowest dimension that constitutes the real world, and our world happens to be four-dimensional. As for the high-dimensional real space, at least we can't perceive it yet. I mentioned an example in a post. When a ruler rotates in three-dimensional space (excluding time), its length remains unchanged, but when it rotates, all its coordinate values change and the coordinates are related. The significance of four-dimensional space-time lies in that time is the fourth coordinate, which is related to spatial coordinates, that is to say, space-time is a unified and inseparable whole, and they are a kind of "one change and one change" relationship.
Four-dimensional space-time is not limited to this. According to the relationship between mass and energy, mass and energy are actually the same thing. Mass (or energy) is not independent, but related to the state of motion. For example, the greater the speed, the greater the mass. In four-dimensional space-time, mass (or energy) is actually the fourth component of four-dimensional momentum, and momentum is a quantity that describes the motion of matter, so it is natural that mass is related to the state of motion. In four-dimensional space-time, momentum and energy are unified, which are called four vectors of energy momentum. In addition, four-dimensional velocity, four-dimensional acceleration, four-dimensional force and four-dimensional electromagnetic field equations are all defined in four-dimensional space-time. It is worth mentioning that the four-dimensional electromagnetic field equation is more perfect, which completely unifies electricity and magnetism, and the electric field and magnetic field are described by a unified electromagnetic field tensor. The physical laws of four-dimensional space-time are much more perfect than those of three-dimensional, which shows that our world is indeed four-dimensional. It can be said that at least it is much more perfect than Newtonian mechanics. At least because of its perfection, we can't doubt it.
In the theory of relativity, time and space constitute an inseparable whole-four-dimensional spacetime, and energy and momentum also constitute an inseparable whole-four-dimensional momentum. This shows that there may be a deep connection between some seemingly unrelated quantities in nature. When we talk about general relativity in the future, we will also see that there is also a profound relationship between the four vectors of space-time and energy momentum.
Matter moves forever in interaction, and there is no matter that does not move and there is no matter that does not move. Because matter moves in interaction, it is necessary to describe motion in the relationship of matter, and it is impossible to describe motion in isolation. In other words, motion must have a reference object, and this reference object is the frame of reference.
Galileo once pointed out that the motion of a moving ship is inseparable from the motion of a stationary ship. That is to say, when you are completely isolated from the outside world in a closed cabin, even if you have the most developed mind and the most advanced instruments, you can't perceive whether your ship is moving at a constant speed or at a standstill. There is no way to perceive speed because there is no reference. For example, we don't know the whole motion state of our whole universe, because the universe is closed. Einstein cited it as the first basic principle of special relativity: the principle of special relativity. Its content is: the inertial system is completely equivalent and indistinguishable.
The famous Michelson-Morey experiment completely negates the ether theory of light and draws the conclusion that light has nothing to do with the frame of reference. In other words, whether you stand on the ground or on a speeding train, the measured speed of light is the same. This is the second basic principle of special relativity, the principle of constant speed of light.
From these two basic principles, we can directly deduce all the contents of special relativity, such as coordinate transformation formula and velocity transformation formula. For example, the speed change is contrary to the traditional law, but it has been proved to be correct in practice. For example, the speed of the train is 10m/s, and the speed of a person on the train is also10m/s. People on the ground see that the speed of people in the car is not 20m/s, but (20- 10 (-65438). In general, this relativistic effect can be completely ignored, but it increases obviously when it is close to the speed of light, such as when the train speed is zero. 99 times the speed of light, people's speed is zero. 99 times the speed of light, then the conclusion of the ground observer is not 1. 98 times the speed of light, but 0. 999949 times the speed of light. The people in the car didn't slow down when they saw the light coming from behind, which was also the speed of light for him. So in this sense, the speed of light cannot be surpassed, because no matter in which reference system, the speed of light is constant. Velocity transformation in particle physics has been proved by countless experiments and is impeccable. It is precisely because of this unique property of light that it is chosen as the only scale of four-dimensional space-time.
Einstein's theory of relativity
Everyone can accept that the car is moving and the tree is still, but if the tree is moving and the car is still, many people will say you are crazy. In fact, these two statements are both correct in physics, but the selected reference frames are different. This is also the basic starting point of Einstein's great theory of relativity.
The first hypothesis created by the theory of relativity is that all frames of reference follow the same laws of physics. Whether on the ground or in a car driving at a constant speed, if you measure a board with a ruler or a pendulum 10 cycle with a stopwatch, the result is the same. However, if you put the board or pendulum on the car passing in front of the surveyor at a certain speed and repeat the above measurement, you will get different results. This difference is caused by all reference systems that follow the same physical laws.
The second hypothesis created by the theory of relativity is that the speed of light is constant in all reference frames. It seems to be the same as the first hypothesis, but if you think about it carefully, you will find the mystery. The second hypothesis means that whether you are sitting on a speeding train or a stationary couch, the speed of light remains the same, regardless of your state of motion. The reason is that when we deal with the speed of daily physical objects, we get the combined speed. For example, if you drive an off-road jeep at a speed of 25km/h and a passenger shoots the stones in front of you with a slingshot at a speed of 10km/h, then the actual moving speed of the marbles should be 35 km/h. But if you turn on the headlights, according to common sense, the speed of light is 334.8 million km/h, plus the moving speed of the car, the actual speed of light should be 334.8 million km/h.
To explain it, we must first start with the definition of speed. The distance traveled per unit time is called speed, that is, the distance divided by time gets the speed. According to the theory of length contraction, the measured length of a mass object in its motion direction is relatively shortened, and the length is shortened to zero when it reaches the speed of light. The basis of the theory is that the measurer and the measured object are in different reference frames, and they only occur in the direction of the object's movement, and will not affect the length perpendicular to the movement. That is to say, when you drive a car with a speed close to the speed of light, the length seen by a stationary observer is far less than its actual length, and the height direction has not changed. On the other hand, when you drive an express train through a doorway, the distance from your point of view is much shorter than the actual distance. This situation is often ignored in daily life, because the moving speed of objects is slow and the phenomenon of length contraction is not obvious. Time, like length, will change with the change of frame of reference, which is called time expansion. With the increase of movement speed, time will be relatively slow, generally weak and imperceptible. When it reaches the speed of light, time will stop completely. But this phenomenon only happens when the observer and the clock are not in the same frame of reference. In order to prove this conclusion, two atomic clocks are set exactly the same, one on the earth and the other on the high-speed space shuttle. When the plane lands, it will be found that the atomic clock on the plane is slower than that on the earth, and the slow time is the same as that calculated by Einstein's theory of relativity. That is to say, the time recorded by the atomic clock on the space shuttle has expanded relative to the time recorded by the stationary atomic clock on the earth.
It is not difficult to explain the light speed problem of headlights by understanding the change of length and time when moving at near light speed or at equal light speed, because the distance and time involved in light movement are different from those involved in our ordinary movement.
Another important concept of relativity is simultaneity. Different motion states can make people observe that the action sequence of objects is different. For example, there are two lights in the room. A stood between two lamps, and B stepped on the skateboard and moved to one lamp at a certain speed, just reaching the middle. When the two lights are turned on at the same time, A sees that the two lights are turned on at the same time, while B sees the first light facing him and the second light facing him.