Interpretation of basic concepts of general relativity;
The general theory of relativity is Einstein's theory of gravity field relativity, which was put forward 19 13 years after his special theory of relativity. This theory is completely different from Newton's theory of gravity. It attributed the gravitational field to the curvature of space-time around the object, and the motion of the object under the action of gravity to the free motion of the object along the short-distance line in the curved space-time. Therefore, the general theory of relativity is also called the geometric dynamics of space-time, which means that gravity is attributed to the geometric characteristics of space-time.
How to understand curvature of spacetime of general relativity? Here we borrow a model analogy to illustrate. If there are two massive steel balls, according to Newton's point of view, they will be close to each other because of the attraction of gravity. Einstein's general theory of relativity, on the other hand, does not think that there is gravity between these two steel balls. They are close to each other because when there are no steel balls, the surrounding space-time is like a flat net. Now two steel balls bend the space-time network, so the two steel balls roll along the curved network together. This is equivalent to an object moving along a short line due to the bending of time and space. So Einstein's general theory of relativity is a theory of gravity without "gravity".
In addition, the theory is based on two basic assumptions: equivalence principle and generalized covariant principle. The principle of equivalence is based on the basic fact that the inertial mass of an object is equal to the gravitational mass, and it is considered that the inertial force of gravity and acceleration system is equivalent, and they are inseparable in principle. The principle of generalized covariation can be regarded as a mathematical expression of the principle of equivalence, that is, all differential equations reflecting physical laws should be in the same form in all reference systems, and it can also be said that all reference systems are equal, thus breaking the special position of inertial systems in special relativity, hence the name of general relativity.
As we know, Newton's law of universal gravitation holds that all objects with mass attract each other, which is a long-distance static action.
In the general theory of relativity, the law of gravitational field produced by matter is expressed by Einstein's field equation, and the gravitational effect it reflects is dynamic and propagates at the speed of light.
General relativity is more generalized than Newton's theory of gravity, which is only a weak field approximation of general relativity. The so-called weak field means that the gravitational energy of the object in the force field is much smaller than the intrinsic energy, and the difference between them is manifested in the force field. At this time, we must apply general relativity to correctly deal with the problem of gravity.
After the establishment of 19 15 general relativity, Einstein proposed that its correctness could be tested from three aspects, namely, the so-called three experiments. This is the deflection of light near the sun, the precession of mercury perihelion and the frequency shift of spectral lines in the gravitational field, which was quickly confirmed by experimental observations at that time. Later, someone designed a radar echo delay experiment, which soon confirmed the general theory of relativity with higher accuracy. A series of new discoveries in astronomy in 1960s: 3K microwave background radiation, pulsars, quasars, X-ray sources and other new astrophysical observations all strongly supported the general theory of relativity, thus making people's interest in it turn from cold to hot. In particular, the application of general relativity in astrophysics and cosmology research has become a hot topic in physics.
Einstein always thought that general relativity was the most important scientific achievement in his life. He said: "If I don't find the special theory of relativity, others will find it, and the problem is ripe. But I think general relativity is different. " Indeed, general relativity contains more profound ideas than special relativity, and this brand-new theory of gravity is still the most beautiful one. Without bold innovative spirit, indomitable perseverance, keen theoretical intuition and solid mathematical foundation, it is impossible to establish general relativity. The great scientist Thomson once called the general theory of relativity one of the greatest achievements in human history.
Special relativity is
Special relativity is based on the theory of four-dimensional space-time view, so to understand the content of relativity, we must first have a general understanding of the space-time view of relativity. There are various multidimensional spaces in mathematics, but so far, the physical world we know is only four-dimensional, that is, three-dimensional space plus one-dimensional time. The high-dimensional space mentioned in modern microphysics is another meaning, which is only mathematical, so I won't discuss it here.
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. 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.