1632, Galileo published his masterpiece "Dialogue between Ptolemy and Copernicus". "Salviati" of the earthquake school in the book gave a thorough answer to the above questions. He said, "Put you and some friends in the main cabin under the deck of the ship, let you take some small flying insects such as flies and butterflies, and put a big water bowl in the cabin with some fish in it." Then, hang a water bottle and let the water drop into the jar below. Ships and fish swim freely in all directions, and water drops drip into the jar below. When you throw anything to your friend, as long as the distance is equal, you don't have to use more force in one direction than in the other. Your feet jump together, and the distance in any direction is the same. When you carefully observe these things, and then let the ship move at any speed, as long as it moves evenly and does not sway from side to side, you will find that the above phenomenon has not changed at all. You can't tell whether the ship is moving or standing still from these phenomena. Even if the ship moves quite fast, you can't jump to the stern farther than the bow. Although when you jump into the air, the bottom of the boat moves in the opposite direction to the direction you jump. When you throw anything at your partner, whether he is at the bow or at the stern, as long as you stand on the opposite side, you don't need to use more force. The water drops will drop into the jar below as before, and not a drop will drop to the stern. Although the water drops are in the air, the ship has traveled a lot (the length from the big fingertip to the little fingertip is usually nine inches, which is an ancient unit of length). The force used by the fish before swimming to the water bowl is not greater than the force used after swimming to the water bowl; They swim leisurely to the bait placed anywhere on the edge of the water bowl. Finally, butterflies and flies continue to fly around carelessly. They will never focus on the stern, not because they may stay in the air for a long time, get rid of the ship's movement and look tired to catch up with it. "

Saviati's big ship tells an extremely important truth, that is, you can't tell whether the ship is moving or stationary from anything that happens on board. Now this paper is called Galileo's principle of relativity.

In modern language, Salviati's big ship is the so-called inertial reference system. In other words, ships that move at different uniform speeds without swinging from side to side are inertial reference systems. All kinds of phenomena that can be seen in one inertial system can certainly be seen in another inertial system, and there is no difference. In other words, all inertial reference frames are equal and equivalent. It is impossible for us to judge which inertial reference frame is at absolute rest and which is in absolute motion.

Galileo's theory of relativity not only fundamentally negates the criticism of the stationary party on the theory of ground motion, but also denies the concept of absolute space (at least within the scope of inertial motion). Therefore, in the transition from classical mechanics to relativity, many concepts of classical mechanics will be changed, but Galileo's principle of relativity not only does not need to be modified, but also becomes one of the two basic principles of special relativity.

Two principles of special relativity 1905, Einstein published the fundamental paper "On electrodynamics of moving objects". Regarding the basic principle of special relativity, he wrote: "The following considerations are based on the principle of relativity and the principle of invariance of light speed. We stipulate these two principles as follows:

1. The laws followed by the state changes of physical systems have nothing to do with which of the two coordinate systems with uniform motion is used to describe these state changes.

2. Any light moves at a certain speed c in the "stationary" coordinate system, no matter whether it is emitted by a stationary or moving object. "

The first is the principle of sex, and the second is the constant speed of light. The whole special theory of relativity is based on these two basic principles.

Einstein's philosophy is that nature should be harmonious and simple. In fact, his theory usually has a remarkable feature: simplicity and profundity. Special relativity is a system with this characteristic. The two basic principles of special relativity seem to be "simple facts" that are not difficult to accept, but their inferences have fundamentally changed the physics foundation since Newton.

We will start this inference later.

Einstein's Special Theory of Relativity

Relativity is one of the most important achievements in the history of physics in the 20th century, which includes special relativity and general relativity. Special relativity has changed the concept of time and space formed since Newton, promoted the unity and relativity of time and space, and established a new concept of time and space. General relativity extends the principle of relativity to non-inertial frame of reference and curved space, thus establishing a new theory of gravity. Einstein played a major role in the establishment of the theory of relativity.

Einstein was a German-born American physicist. 19 14 years, director of the Institute of Physics of Emperor William of Germany, academician of Prussian Academy of Sciences. 1933 moved to the United States because of persecution by Nazi regime, and served as the director of Princeton Institute for Advanced Studies. 1905, when he was 26 years old, the French scientific magazine Yearbook of Physics published his paper "On electrodynamics of moving objects", which was the first paper on relativity. It discusses the special theory of relativity quite comprehensively, and solves many problems about electrodynamics and the combination of mechanics and electrodynamics that many physicists have been unable to solve since the middle of19th century.

When it comes to special relativity, many people will think of the phenomenon that clocks go slowly and rulers become shorter. Many sci-fi works take this as the theme, describing a man who came back from a rocket flight in space and found that he was still young and his grandson had become an old man. In fact, the slow movement of the clock and the shortening of the ruler are just one of several conclusions of special relativity. It means that when an object moves at a high speed, the clock on the moving object slows down and the ruler becomes shorter. The phenomenon that the clock moves slowly and the ruler becomes shorter is the result of the change of time and space with the movement of matter. Another conclusion of the special theory of relativity is that the mass increases with the speed of motion. It is found in the experiment that the mass of electrons moving at high speed is greater than that of stationary electrons.

The most important conclusion of special relativity is that the conservation of mass has lost its independence. It is integrated with the principle of energy conservation, and mass and energy can be transformed into each other. If the mass of matter is m, the speed of light is c and the energy contained is e, then e = MC 2. This formula only shows that all the energy contained in an object with mass m does not mean that it can be released, and the mass disappeared in the nuclear reaction is converted into energy release according to this formula. According to this formula, the mass of 1 g is equivalent to the energy of 9 * 103 joules. This principle of mass-energy conversion and conservation is the theoretical basis of atomic energy utilization.

In the great special theory of relativity, although there is a conclusion that is completely incomprehensible from the viewpoint of Newtonian mechanics: space-time changes with the movement of matter, mass changes with the movement, and mass and energy transform each other, the special theory of relativity is not completely divorced from Newtonian mechanics. When the speed of motion is much lower than the speed of light, the conclusion of special relativity will not be different from Newtonian mechanics.

Decades of historical development has proved that special relativity has greatly promoted the scientific process and become one of the basic theories of modern physics.

1922, 12 On February 4th, Einstein gave a lecture entitled "How did I establish the theory of relativity? In his speech, he explained the emergence and development of his theory of relativity. He said: "It is not easy to say how I established the concept of relativity. My thoughts are inspired by so many mysterious and complicated things. At different stages of the development of the concept of happiness in life, the influence of each thought is different ... 17 years ago, I first put forward the idea of developing relativity. I'm not sure where this idea came from, but I'm sure it is contained in the optical characteristics of moving objects. Light travels through the ocean and the earth moves in the ether. In other words, the ether is relative to the earth. I tried to find obvious experimental evidence of etheric flow in physical literature, but Blue Sky didn't succeed. Later, I personally proved the movement of the ether relative to the earth, or the movement of the earth. When I first thought about this problem, I didn't doubt the existence of the ether, nor did I doubt the movement of the earth through the ether. "So, he conceived an experiment using two thermocouples: some reflectors were set to make the light emitted by a single light source reflect in two different directions, one parallel to and in the same direction as the earth's movement, and the other opposite. If you imagine the energy difference between two reflected beams, you can measure the difference in heat generated by two thermocouples. Although the idea of this experiment is very similar to Michelson's experiment, he didn't get the result.

Einstein said: He first considered this question when he was a student. At that time, he already knew the wonderful result of Michelson's experiment, and he quickly came to the conclusion that if he believed Michelson's zero result, then the idea about the movement of the earth relative to the ether was wrong. He said: "This is the first way to lead me to the special theory of relativity. From then on, I began to believe that although the earth revolves around the sun, the movement of the earth cannot be detected by any optical experiment, and the movement of the earth cannot be detected by any optical experiment. "

Einstein had the opportunity to read the paper published by Lorenz in 1895. He discussed and satisfactorily solved the higher-order term of u/c (U is the speed of a moving object and C is the speed of light). Then Einstein tried to give a lecture on free soul experiment by assuming that Lorentz's electron equation holds in the vacuum frame of reference and should also hold in the frame of reference of moving objects. At that time, Einstein firmly believed that Maxwell-Lorenz electrodynamics equation was correct. In addition, it is assumed that these agendas are effective in the frame of reference of moving objects, which leads to the concept that the speed of light is constant. However, this is contrary to the principle of speed addition in classical summation.

Why these two concepts contradict each other. In order to explain Lorenz's theory, Einstein spent nearly a year trying to revise it. An accidental opportunity. He solved the problem with the help of a friend. Einstein went to ask him and talked about all aspects of this difficult problem. Suddenly, Einstein found a solution to all the difficulties. He said, "I finished the principle of special relativity in five weeks."

Einstein's theory denies the concept of ether, affirms that electromagnetic field is an independent and special material existence form, and deeply analyzes the concepts of space and time, thus establishing a new space-time relationship. His 1905 paper is recognized as the first paper on relativity in the world and the first real relativistic physicist.