The basic hypothesis of relativity is the principle of relativity, that is, the choice of physical laws and reference systems is related to the fact that massive objects do not distort in time and space and change their direction of travel. The difference between special relativity and general relativity is that the former discusses the physical laws between reference frames (inertial reference frames) with uniform linear motion, while the latter is extended to reference frames with acceleration (non-inertial reference frames), which is widely used in gravitational fields under the assumption of equivalence principle. Relativity and quantum mechanics are two basic pillars of modern physics. Classical mechanics is based on classical physics and is not suitable for high-speed moving objects and microscopic fields. Relativity solves the problem of high-speed motion; Quantum mechanics solves problems under microscopic subatomic conditions. Relativity subverts the common sense concepts of the universe and nature, and puts forward new concepts such as relativity of time and space, four-dimensional time and space, and curved space. The special theory of relativity was put forward in 1905, and the general theory of relativity was put forward in 19 15 (Einstein completed the establishment of the general theory of relativity at the end of 19 15, and published relevant papers at the beginning of16).
Because Newton's law poses difficulties for the special theory of relativity, that is, any object will be subjected to force in any spatial position. Therefore, there is no inertial observer in the whole universe. Einstein put forward general relativity to solve this problem.
The most famous inference of special relativity is the mass-energy formula, which shows that mass increases with the increase of energy. It can also be used to explain the huge energy released by the nuclear reaction, but it is not the reason for the birth of the atomic bomb. The gravitational lens and black hole predicted by general relativity are consistent with some astronomical phenomena.
According to the equation of mass and energy, it is easy to deduce that "the speed of light is the fastest speed in the universe". Because when an object reaches the speed of light, its mass will become infinite, which is not true. However, it has also been suggested that there are two universes, namely, the "fast universe" and the "slow universe". All elementary particles are faster than the speed of light in the fast universe, that is, superluminal particles, so the substances they make up are also faster than the speed of light, and vice versa. In addition, some astronomers have surprisingly observed superluminal phenomena, including the separation speed of galaxies, the waste of quasar expansion and so on. But so far, there is no convincing argument, and there is no argument to overthrow the theory of relativity.
Absolute space-time view
The so-called concept of time and space is the understanding of the physical properties of time and space. Galilean transformation is a mathematical description of the principle of relativity in mechanics. It reflects the concept of absolute time and space in classical mechanics.
1. The time interval has nothing to do with the choice of inertial system.
If two events occur one after another, the time interval measured by observers in two different inertial systems is the same.
2. The space interval has nothing to do with the choice of inertial system.
The distance between any two points in space has nothing to do with the choice of inertial system.
We can see that in classical mechanics, the coordinates and velocity of an object are relative, and so is the same place. But time, length and quality are absolute, and at the same time, they are also absolute. This is the concept of absolute time and space in classical mechanics.
Ether?
/kloc-in the middle of the 9th century, Maxwell established the electromagnetic field theory and predicted the existence of electromagnetic waves propagating at the speed of light C./kloc-at the end of the 9th century, experiments completely confirmed Maxwell's theory. What is electromagnetic wave? To whom is its propagation speed c? The popular view at that time was that the whole universe was filled with a continuous medium called "ether", in which light and radio signals fluctuated. A complete theory needs to carefully measure the elastic properties of ether. To this end, Harvard University established the Jefferson Laboratory. The whole building does not need any nails to avoid interfering with magnetic survey. However, due to the planner's neglect of the large amount of iron contained in chestnut bricks, it is expected that the experiment will not be carried out as scheduled. By the end of the century, the concept of deviating from penetrating all etheric began to appear. If we think that the earth is moving in the static ether, then according to the principle of velocity superposition, the speed of light propagating in different directions on the earth must be different, but the experiment denies this conclusion; If we think that the ether was taken away by the earth, it is obviously inconsistent with some astronomical observations. In this regard, people find that this is a theory full of contradictions.
Schematic diagram of Michelson Morley's experiment 1887 albert michelson and Edward Morley made a very accurate measurement by using the interference phenomenon of light, and still did not find any movement of the earth relative to the ether. In this regard, H.A. Lorenz put forward a hypothesis that all objects moving in the ether should contract along the moving direction. From this, he proved that even if the earth moves relative to the ether, Michelson could not find it. Einstein studied this problem from a completely different way of thinking. He pointed out that all difficulties can be solved as long as Newton's concept of absolute time is abandoned, and there is no need for ether at all.
★ Note: Ether: put forward by Greek scholars and considered as the medium of light propagation.
Fixed Ether Theory: If light is a wave in an elastic substance called ether, the speed of light seems to be higher for the person (a) moving towards it on the spaceship, and lower for the person (b) moving in the same direction with the light on the spaceship.
Two basic assumptions
1. The laws of physics have the same form in all inertial systems.
2. In all inertial systems, the propagation rate of light in vacuum has the same value c.
The first is called the principle of relativity. That is to say, if the coordinate system K' moves at a constant speed relative to the coordinate system K without rotating, it is impossible to distinguish which coordinate system is K and which coordinate system is K' in any physical experiment made relative to these two coordinate systems.
The second principle is called the principle that the speed of light is constant, which means that the speed of light c (in vacuum) is constant, and it does not depend on the moving speed of the luminous object.
On the surface, the constant speed of light seems to conflict with the principle of relativity. Because according to the classical law of mechanical speed synthesis, the speed of light should be different for the two coordinate systems, k' and k, which move at a relatively uniform speed. Einstein believes that in order to admit that these two hypotheses are not in conflict, we must re-analyze the physical concepts of time and space.
However, it can be proved that Michelson-Morey experiment is wrong for the simple reason that the experimental accuracy is far from enough. Assuming that the wavelength of the experimental light wave is 600 nm (1 nm is equal to the negative 9th power of 10), in order to obtain the interference phase difference of 90 degrees, the two interference waves need to obtain the time difference of 150 nm. The problem is that 150 nm is very small, and the instrument vibration caused by environmental background noise far exceeds 65438.
Lorentz transformation
The law of velocity composition in classical mechanics actually depends on the following two assumptions:
1. The time interval between two events has nothing to do with the motion state of the clock used to measure time.
2. The spatial distance between two points has nothing to do with the motion state of the ruler used to measure the distance.
Einstein found that if the principle of light speed invariance and the principle of relativity are recognized to be compatible, then both hypotheses must be abandoned. At this time, the simultaneous events of one clock are not necessarily simultaneous for another clock, and they are relative at the same time. In two coordinate systems with relative motion, the values obtained by measuring the distance between two specific points are no longer equal, and the distance is relative.
If an event in the K coordinate system can be determined by three spatial coordinates X, Y, Z and a time coordinate T, and the same event in the K coordinate system is determined by X', Y', Z' and T', Einstein found that X', Y', Z' and T' can be solved by a set of equations. The relative velocity of the two coordinate systems and the speed of light c are the only parameters of the equation. This equation was first derived by Lorentz, so it is called Lorentz transformation.
Using Lorentz transformation, it is easy to prove that the clock will slow down because of movement, the ruler will be shorter when it is moving than when it is at rest, and the sum of speeds satisfies a new law. The principle of relativity is also expressed as a clear mathematical condition, that is, under Lorentz transformation, the spatio-temporal variables X', Y', Z' and T' will replace the spatio-temporal variables X, Y, Z and T, and any expression of natural laws will still take the same form. What people call the universal law of nature is covariant for Lorentz transformation. This plays a very important role in exploring the universal laws of nature.
The connection between time and space
Besides, in classical physics, time is absolute. It has always played an independent role different from the three spatial coordinates. Einstein's theory of relativity involves time and space. It is believed that the real world of physics is composed of various events, and each event is described by four numbers. These four numbers are its space-time coordinates T and X, Y, Z, forming a four-dimensional rigid continuous space-time, usually called Minkowski flat space-time. In relativity, it is natural to examine the real world of physics in a four-dimensional way. Another important result caused by special relativity is about the relationship between mass and energy. Before Einstein, physicists always thought that mass and energy were completely different and were separately conserved quantities. Einstein found that in the theory of relativity, mass and energy are inseparable, and the two conservation laws are combined into one. He gave a famous formula of mass and energy: e = MC 2, where c is the speed of light. So quality can be regarded as a measure of its energy. Calculations show that tiny masses contain enormous energy. This was proved in later nuclear reaction tests.
Most physicists, including Lorenz, the founder of relativistic transformation relation, find it hard to accept these new concepts introduced by Einstein. The obstacle of the old way of thinking made this new physical theory not familiar to physicists until a generation later. Even in 1922, when the science prize was awarded to Einstein by the Royal Swedish Academy, it just said, "Because of his contribution to theoretical physics, it is also because he discovered the law of photoelectric effect." Not a word about relativity.
Establish general relativity
Einstein further established the general theory of relativity in 19 15. The principle of relativity in a narrow sense is limited to two coordinate systems with uniform motion, while the principle of relativity in a broad sense cancels the restriction of uniform motion. He introduced an equivalence principle, that is, gravitational effect and non-uniform motion are indistinguishable, that is, any acceleration and gravity are equivalent. He further analyzed the phenomenon that light will be bent by gravity when passing near a planet, and thought that the concept of gravity itself was completely unnecessary. It can be considered that the mass of the planet makes the space around it curved, and the light takes the shortest path. Based on these discussions, Einstein derived a set of equations, which can determine the curved space geometry caused by the existence of matter. Using this equation, Einstein calculated the displacement of the perihelion of Mercury, which was completely consistent with the experimental observation, and solved a long-term unexplained problem, which made Einstein excited. In a letter to Allen Fast, he wrote: "The equation gives the correct value of perihelion. You can imagine how happy I am! For several days, I was so happy that I didn't know what to do. "
experimental verification
1915165438+1On October 25th, Einstein submitted a paper entitled "Equation of Gravitation" to the Prussian Academy of Sciences in Berlin, which fully discussed the general theory of relativity. In this article, he not only explained the mystery of the perihelion motion of Mercury's orbit found in astronomical observation, but also predicted that the starlight would deflect after passing through the sun, and the deflection angle was twice that predicted by Newton's theory-only observed during the total solar eclipse. Einstein prepared funds, and astronomer freundlich went to Crimea to observe. Unfortunately, Germany declared war on Russia, and freundlich was suspected as a spy. He was detained until the end of August, when he and his team exchanged with senior Russian officers detained in Germany. [ 1]
Eddington's solar eclipse data (3 pieces)1965438+the total solar eclipse on May 25th, 2009 provided people with an observation opportunity. Eddington, an Englishman, went to principe island on the west coast of Africa and made this observation. 165438+1On October 6th, Thomson solemnly announced at the joint meeting of the Royal Society and the Royal Astronomical Society that Einstein, not Newton, had proved this result. He praised "this is one of the greatest achievements in the history of human thought." Einstein discovered not an island, but a brand-new continent of scientific ideas. "The Times reported this important news under the title of" Revolution in Science ". The news spread all over the world, and Einstein became a world-famous celebrity. General relativity has also been elevated to a mythical sacred position.
Since then, people have shown more and more interest in the experimental test of general relativity. However, because the gravitational field of the solar system is very weak and the gravitational effect itself is very small, the theoretical result of general relativity deviates very little from Newton's gravitational theory, which makes the observation very difficult. Since 1970s, due to the progress of radio astronomy, the observation distance has far exceeded the solar system, and the accuracy of observation has been greatly improved. Especially in September of 1974, Taylor of MIT and his student Hall observed with a large radio telescope with a diameter of 305 meters and found a pulse binary star. It is a neutron star and its companion star revolve around each other under the action of gravity, with a period of only 0.323 days. The gravity on its surface is100000 times stronger than that on the surface of the sun. This is a laboratory where it is impossible to test the theory of gravity on earth or even in the solar system. After more than ten years of observation, they got a very good result, which is in line with the prediction of general relativity. Because of this great contribution, Taylor and Hall won the 1993 Nobel Prize in Physics.