After Einstein finally found the correct entrance to General Relativity , that is, equivalent principle , it took him several years to find the correct theory of general relativity. The theory of
can be summarized in one sentence, that is, time and space are both curved. If we summarize it in two sentences, it is that matter determines how time and space bend, and the bent time and space determines how objects move.
How did Einstein think that time and space are curved? We use the curved sphere as an analogy. Put a ball farther away, we see a sphere, but if we put the ball in front of us, we can only see one part of the ball. At this moment we noticed that a very small part was not that curved. The smaller this part, the less curved it is. Until we make the local part infinitely smaller, this local part infinitely closes to an infinitely small plane part.
We splice many small parts infinitely close to the plane and we get a spherical surface. Of course, other curved surfaces can also be done, such as treads or other more complex surfaces. This method was actually discovered by geometry .
In the mid-19th century, German mathematician Riemann extended this method of studying surfaces to spaces in any dimension. In fact, it is difficult for ordinary people to imagine what a curved three-dimensional space looks like, because we live in three-dimensional space ourselves. However, mathematicians can use abstract methods to study curved three-dimensional space, which is to splice together countless infinitely small flat three-dimensional Euclidean spaces. Of course, curved four-dimensional space and higher-dimensional space can be studied in this way.
So, how did Einstein think of the curved space and time? This is the credit for his equivalent principle. The so-called equivalent principle refers to the fact that in the reference system of free fall in the gravity field, the laws of physics we see are the same as those in 's special theory of relativity . In other words, the time and space seen by a freely falling person is the time and space in the special theory of relativity, that is, Minkovsky space-time. Einstein thought that Minkowsky's time and space were like space without curves. However, the reference system for free falls is different in different places in the gravity field. For example, a free whereabouts reference system in Beijing is completely different from a free whereabouts reference system in New York. Einstein called these different free-fall reference frames local inertial reference frames. Therefore, in a gravity field, there are many local inertial reference frames. If we splice these different local inertial reference frames, will we get a curved space-time? This approach is much like geometricians studying curved space.
So, how does an object move in a curved space and time? It's very simple. At every point in time and space, the way this object moves is to fall freely. For example, an satellite is constantly falling freely in the earth's gravity field. It’s just that in Einstein’s curved space and time, we use the bending of time and space itself to explain how objects move.
Einstein used the principle of equivalent to discover the curved space-time. Later he said that his discovery was actually related to the so-called Mach principle. What is the Mach principle? Remember when we mentioned the absolute space of Newton ? Galileo's principle of relativity points out that all inertial reference systems are equivalent, and no inertial reference systems is more special than other inertial reference systems. But Newton thought, is the space relatively stationary to all stars a little more special? Is this space absolute space? Mach went further to think, does the distribution of stars determine the inertial reference system? That is to say, the distribution of matter determines the inertial reference system. In fact, Mach's idea is very close to Einstein's idea.
Before Einstein, it was believed that time and space were fixed scaffoldings in which matter existed. When we come to Einstein, time and space themselves can also change. While thinking about quantum theory , Einstein took almost 8 years to completely solve this problem.His ultimate solution is simple: the existence of gravity makes time and space variable too. Since time and space are a whole in the special theory of relativity, is this whole fixed or like all objects, also variable itself? The existence of gravity makes time and space no longer a fixed scaffold. Before Einstein, many mathematicians had already jumped out of the fixed space established by Euclid and began to study various different curved spaces. When we arrived at Einstein, these curved spaces were not only mathematical imagination, but also physical reality.
The first key point in Einstein's theory is to bend space-time, and the second key point is how does bend space-time occur?
His answer is that when matter exists, or when energy and momentum exists more generally, time and space will become curved. The greater the energy and momentum, the more time and space will be curved. We usually say that black holes are the most curved places in space and time, which is not accurate. For a large black hole, although its mass is large, the "horizontal" containing this black hole is also very large. Outside the horizon and outside the horizon, time and space are not curved very well. The horizon here refers to where the clock moves infinitely slowly. There are two most powerful places where space-time bending is curvature. One is the " singularity " at the beginning of the Big Bang, and the other is the "singularity" of the collapse of black holes. At these two singularities, the curvature of space-time becomes infinite.
After replacing Newton's universal gravity with bent space-time, space-time can be bent even without energy, just like in electromagnetic theory, there can be an electromagnetic field without charge and current. Such an electromagnetic field is electromagnetic wave . The gravitational field corresponding to the bending space-time without energy is a gravitational wave.
With general relativity, physicists can study the fate of stars under the action of universal gravity. Why do you say so? Because stars rely on thermonuclear reactions to maintain their lives, once the thermonuclear reaction is terminated, if only Newton's universal gravitational theory is used, the stars will continue to collapse. When the gravity reaches a certain level, the gravity is too great, and Newton's theory will not be applicable.
Indians studying in the UK Chandrasekha found that when the mass of a star is less than a value, it collapses into something called a white dwarf. Why is it called a white dwarf? Because it emits white light, but the radius is very small, that is, the density is very large, white and short. If star mass is larger than this value, it will continue to collapse. What will it collapse into? Physicist Landau believes that as long as its mass does not exceed 2 suns, it will collapse into something with a smaller radius, called neutron star .
What happens if the mass of a star exceeds 2 suns? At this time, the gravity of all things is so strong that even the neutron star cannot protect itself. It must collapse and eventually form something called a black hole. So, what exactly is a black hole?
In the 1960s, physicists discovered that a black hole is a celestial body that does not emit light at all. There is a radius outside the black hole, where the gravity is so strong that even light cannot escape.
Why can’t even run out of it? We talked about space-time bending. What is space-time bending? It means that the closer you are to the center of mass, the slower the clock moves. Imagine that there is a clock placed near a black hole. Every second it walks, people in the distance have to wait for a year. This is the slowdown effect of the clock. The speed of light remains unchanged in Einstein's theory, that is, near that clock, although light ran 300,000 kilometers in one second, in our opinion, it only took 300,000 kilometers in a year. Okay, now move the clock closer to the black hole. At this time, every 100 millionth of a second of a clock, we have to wait for a year. In other words, in our opinion, walking only 3 meters in a year is much slower than a snail. In this way, the closer we get to the black hole, the slower the light walks, and we can't get out of it later.
Of course, this does not mean that a star's mass is twice as large as the sun and will eventually become a black hole, because the star will throw something out to the end. Therefore, physicists think that if a star has a mass of 20 times that of the sun, it will definitely become a black hole.
Einstein's theory applies not only to the earth, but also to the entire universe. For more than 200 years when Newton's view of space and time was dominant, people have always felt that the universe is static as a whole. However, when Einstein used general relativity to study the entire universe, he found that the universe could not be stationary, which made him very embarrassed. However, more than 10 years after Einstein proposed the theory of general relativity, Hubble discovered that other galaxies outside Milky Way are far away from the earth, and it can be seen that the universe is not static. A priest named Lemait said that if we assume that the universe is constantly expanding, then Hubble's discovery is exactly what Einstein predicted! Twenty years after Einstein proposed the theory of general relativity, Lemait finally convinced Einstein to believe in the theory of cosmic expansion.
Einstein's general theory of relativity believes that under the premise of the existence of matter, time and space are curved, which is the fundamental reason for the existence of universal gravity. Curved space-time causes objects to move freely and continuously, and the distribution of matter determines how time and space bend.
