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The theory of relativity is a philosophical idea that believes that everything in the world (including the universe) is relative, and there is no absolute.

For example, it makes sense to say that taller is relative to shorter. , there is no concept of height without shortness.

Length and shortness are interdependent and interdependent. Without shortness, there is no longness. Without bigness, there is no smallness. Without fastness, there is no slowness. Everything loses its contrast. It loses its meaning. As the saying goes, a ruler is shorter and an inch is longer. A single length value has no concept of length.

In physics, relativistic physics is an absolute concept that is different from classical physics. A major pillar of contemporary physics. It is the extension of classical physics to the field of large space and high speed.

Although classical physics also recognizes relativity, it does not exclude absolute concepts, such as speed. Classical physics believes that There must be an absolutely stationary substance (ether) in the universe, and the speed of motion relative to the ether is the absolute speed. Relativistic physics believes that there is no absolutely stationary substance in the universe, and all motion and all laws of motion are relative.

The basis of relativistic physics is a series of physical experimental conclusions:

1. All physical laws are equivalent in any inertial system. (Galilean transformation)

2. In a closed space, the speed of the system itself cannot be measured. (There is no concept of speed without other frames of reference)

3. The speed of light in any inertial frame Same. (The speed of light remains unchanged)

The constant speed of light is one of the important reasons why people question the theory of relativity, because the constant speed of light is contrary to the principle of speed superposition that people are accustomed to. However, this is exactly what the principle of relativity is used for. The most important manifestation in relativistic physics.

So when we mention the theory of relativity, we have to focus on explaining the principle of the invariance of the speed of light.

Because the invariance of the speed of light is too far from people's daily experience , so it is difficult for people to accept. Many people even stand on the standpoint of absolute speed when describing relativistic phenomena. For example, "When an object moves at a very high speed, time will slow down."

①. There is no speed without a reference object. Where does the very high speed come from?

②. If there is a reference system, is it the reference system that is moving or the object that is moving? Whose? Time slows down?

③. The reference system can be specified at will. If you change the reference system at will, will its time change accordingly? So how should time change?

For example, if we use the sun As a reference system, the earth has a time, taking the galactic center as the reference system, and the earth has another time, so which time do we use? In fact, we can choose countless reference systems, so isn’t the earth’s time meaningless?

Obviously the above statement itself is untenable.

Since there is no absolute speed, imagine:

Suppose there are no other celestial bodies in the universe, only one particle. How does this particle determine whether it is moving or stationary? How does it determine its speed? In classical physics, people will think of the ether, but in relativistic physics, speed is meaningless without other frames of reference. In fact, in this The speed of light measured on a particle is also an isotropic speed.

If there are only two celestial bodies in the universe and there are no other celestial bodies, the two celestial bodies may move relative to each other, but they cannot be distinguished. Are they moving in opposite directions or in the same direction, or are they moving in other directions at a certain angle at the same time? The only thing that can be determined about these two celestial bodies is whether the distance between them is moving away or approaching, and the speed of moving away or approaching.

Each celestial body can completely ignore the existence of another celestial body (thinking that it is stationary or floating in the universe at any speed). The speed of light measured on this system is also the same in all directions. Because there is no inevitable value for one's own speed (can be specified arbitrarily), there is no basis for the speed of light to be superimposed on one's own motion speed.

In other words: the speed of light seen in any inertial frame They are all c, constant. Relatively speaking, in the view of light

, all inertial systems are "stationary". In fact, the Mai-Moe experiment also verified that the speed of light does not superpose with any relative speed.

Therefore, the constant speed of light becomes the dividing line between relativistic physics and A watershed in classical physics (algorithm). The root of the theory of relativity is the concrete embodiment of the philosophical thought of relativity in physics.

Special theory of relativity:

The special theory of relativity is based on the inconsistency of the speed of light. The principle of variation is a theory of the conversion relationship between time and space measurements between two systems that move relatively uniformly in a straight line. It can be regarded as the differential form of the theory of relativity.

Special relativity does not involve forces and systems. The problem of interaction is just the impact of relative speed on mutual observations and the conversion relationship.

From the previous discussion, it can be seen that the constant speed of light is one of the characteristics of light, just like light propagates in a straight line.

Let’s take a very common example to illustrate the impact of light on observation.

Because light propagates in a straight line, the phenomenon of “large near and small at far” occurs, that is, we see The farther things are, the smaller they will be. If the automatic calculation function of our eyes and brain makes us feel that the size near is not obvious, we can use the camera to take pictures, and the objects far away will indeed become smaller. If we want to know the size of the objects in the distance, For the true size and dimensions of an object, we must multiply the measured (photographed) size by a factor greater than 1 to restore the true size.

Similarly, since the speed of light remains unchanged, when the relative speed is very high , when viewed from one system, the time on another system will "slow down". This is actually a common saying. In fact, it does not "slow down" but becomes faster. For example, when we see (measured) the time is t, we need Multiply it by a factor less than 1 to restore the real time t'.t'=t√(1-V2/C2). As long as the speed is not 0, √(1-V2/C2) must be less than 1, so it means that the actual time is The time seen on that system is slower than the time we observe, which means that the time we see is faster than the time of that system.

But we are still accustomed to saying that relative to our high-speed motion The time on the object will slow down. This is just a common saying and does not affect our correct use of transformation formulas to analyze time relationships. Because speed and time are originally relative, we think that our time has not changed, so it is It is not a mistake that time slows down.

General Relativity:

General relativity introduces the theory of relativity into the field of mechanics. The influence of force on speed (changing the state of motion) also has relative effects. properties. For all systems (not just inertial systems), all physical laws are still correct, so F=ma is correct within the scope of the theory of relativity.

F=ma; that is, a=F /m: The left side is acceleration, what is the right side? The gravity exerted by unit mass, this is the gravitational strength of the gravitational field. That is to say, acceleration and gravity are equivalent.

Then light propagates in a straight line , is bent in the strong gravitational field, indicating that space is bent. In the eyes of light, it still travels in a straight line.

Mass-energy formula:

F=ma ; Multiplying both sides by a distance is energy (or work). F × distance = m × V2

That is, energy = mass times the square of velocity, but what is the velocity here? Obviously it cannot be specified arbitrarily The arbitrary speed measured under the reference system is the speed of the mass body relative to light, which is C.

Therefore, the expression of the mass-energy formula is: E=mc2?