Principle of Relativity (Relativity)
Relativity is the basic theory about space-time and gravity, which was mainly founded by Einstein and divided into special relativity (special relativity) and general relativity (general relativity). The basic assumptions of relativity are the principle of invariance of light speed, the principle of relativity and the principle of equivalence. Relativity and quantum mechanics are two basic pillars of modern physics. Classical mechanics, which laid the foundation of classical physics, 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 has greatly changed the common sense concepts of the universe and nature, and put forward new concepts such as simultaneous relativity, four-dimensional space-time and curved space.
The concept of general relativity
When the theory of relativity came out, people saw the following conclusions: four-dimensional curved space-time, finite boundless universe, gravitational wave, gravitational lens, big bang cosmology, black hole, the main theme of 2 1 century, and so on. All this comes too suddenly, which makes people feel that the theory of relativity is mysterious. Therefore, in the early years of the advent of the theory of relativity, some people threatened that "only twelve people in the world understand the theory of relativity." Some people even say that "only two and a half people in the world understand the theory of relativity". What's more, the theory of relativity is compared with spiritualism and idealism. In fact, the theory of relativity is not mysterious. It is the most down-to-earth theory, a truth that has been tested thousands of times, and not unattainable.
The geometry applied by relativity is not ordinary Euclidean geometry, but Riemann geometry. I believe many people know non-Euclidean geometry, which can be divided into Roche geometry and Riemannian geometry. Riemann unified three kinds of geometry from a higher angle, which is called Riemann geometry. Non-Euclidean geometry has many strange conclusions. The sum of the internal angles of a triangle is not 180 degrees, and the pi is not 3. 14. So when it was first put forward, it was ridiculed as the most useless theory. It was not until its application was found in spherical geometry that it was paid attention to.
If there is no matter in space and space-time is flat, then Euclidean geometry is enough. For example, the application in special relativity is four-dimensional pseudo-Euclidean space. Because there is an imaginary unit I in front of the time coordinate, a pseudo word is added. When matter exists in space, the interaction between matter and space-time bends space-time, which means using non-Euclidean geometry.
Relativity predicted the existence of gravitational waves, and found that both gravitational fields and gravitational waves travel at the speed of light, denying the distance effect of the law of universal gravitation. When light comes from a star and meets a massive celestial body, it will converge again, that is, we can observe the stars blocked by celestial bodies. Generally speaking, what you see is a ring called Einstein ring. When Einstein applied the field equation to the universe, he found that the universe was not stable, and it either expanded or contracted. Cosmology at that time believed that the universe was infinite, static and the stars were infinite. So he did not hesitate to modify the field equation, added the universe term, got the stable solution, and put forward the finite infinite universe model. Soon Hubble discovered the famous Hubble law and put forward the theory of cosmic expansion. Einstein regretted this and gave up the cosmological term, calling it the biggest mistake in his life. In later research, physicists were surprised to find that the universe was not only expanding, but also exploding. The very early universe was distributed in a very small area. Cosmologists need to study the content of particle physics to put forward a more comprehensive model of the evolution of the universe, and particle physicists need cosmologists' observations and theories to enrich and develop particle physics. In this way, the two most active branches of physics-particle physics and cosmology-are combined with each other. As the preface of high school physics says, it's like a strange python biting its tail. It is worth mentioning that although Einstein's static universe has been abandoned, its finite boundless universe model is one of the three possible fates of the future universe and the most promising. In recent years, the cosmological term has been revalued. The problem of black holes will be discussed in a future article. Although black holes and big bang are predictions of relativity, their contents have gone beyond the limitation of relativity and are closely combined with quantum mechanics and thermodynamics. I hope the future theory can find a breakthrough here.
The concept of special relativity
The philosophy of Mach and Hume had a great influence on Einstein. Mach believes that the measurement of space-time is related to the movement of matter. The concept of time and space is formed through experience. Absolute time and space, no matter what experience is based on, can't be grasped. More specifically, Hume said: Space and extension are just visible objects that are filled with space and distributed in a certain order. And time is always discovered through the perceptual changes of changeable objects. 1905, Einstein pointed out that Michelson and Morey's experiments actually showed that the whole concept of "ether" was redundant and the speed of light was constant. Newton's concept of absolute space-time is wrong. There is no absolutely static reference object, and the measurement of time varies with different reference frames. He put forward Lorentz transformation based on the principle of invariance of light speed and relativity. Created the special theory of relativity.
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 its space-time view. 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. 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.