Why are all the planets floating in the air?
If we look at a picture of the universe, we may see that there are many celestial bodies floating on it in the void space. Of course, there are only a few real pictures that have been shot. However, seeing these pictures will make many people wonder why these objects are "floating" in the universe and will not fall. It's not surprising that we have this question. In fact, if we only observe on the earth without any modern scientific knowledge, we will have such a question. The ancient Greeks were very puzzled about this, because on earth, we saw that all things on the earth were falling towards the ground, but in space, the celestial bodies did not have a downward trend, but revolved around a certain celestial body. Therefore, people at that time put forward a set of theories. They believed that the world below the moon, including the earth, was composed of four elements: water, fire, earth and air. Water and soil are relatively heavy, and tend to fall to the ground. Fire and gas are relatively light and will be far away from the ground, but they will not run to the world above the moon.
Above the moon is made of ether. This separates the physical laws of the celestial sphere and the earth. The celestial sphere does not need to conform to the laws of the earth. Therefore, there will be no falling. In fact, this theory was eventually incorporated into the theory of geocentrism, which is a part of geocentrism. We can also think of it as the earliest theory of gravity.
This is also known as Aristotle's world view, and one of the elements is that force is the reason for maintaining the state of motion of objects. If you want to run all the time, you have to work hard all the time.
Later, with the gradual development of science, a group of scholars represented by Copernicus put forward different views, which was also known as the Copernican Revolution in history. However, a lot of scientific theories appeared in that era, which have not been unified. It was not until Newton appeared that the new knowledge was organically combined. At the same time, Newton also proved the law of universal gravitation.
And unify the physics of the earth with the laws of the motion of celestial bodies in the universe. This set of theory can explain the vast majority of physical phenomena seen at that time, and can also predict. Newton was one of the greatest scientists in the history of science, and only Einstein could match it.
According to the law of universal gravitation and Newton's three laws, we know that force is the reason for changing the state of motion of objects. Things on the earth fall to the ground because of the gravity of the earth. The reason why the earth revolves around the sun is that there is gravity between the sun and the earth, and because the earth has a certain initial velocity, it makes the earth rotate around the sun.
We always have the feeling of "falling" on the earth, which is essentially caused by the action of gravity and powerful force. If we analyze the earth, the earth is actually subject to gravity, of which the largest gravity comes from the sun, and then other celestial bodies. Generally speaking, the force direction of the earth is pointing to the sun, that is, for the earth, "falling" is pointing to the direction of the sun. Our vision of the earth's "below", in fact, there is no force, the earth will not naturally fall down.
Of course, we can understand this problem from another angle. As we all know, today's gravitational theory is actually general relativity. Newton's gravitational theory is only applicable to weak gravitational field, while Einstein's theory is more universal. How to understand this problem with general relativity?
This should start with special relativity. Einstein derived special relativity through the principle of constant speed of light and the principle of relativity. In this theory, he unified time and space. He believed that both of them would be affected by the state of motion. Time and space should not be separated from each other, but should be combined and called space-time. And we live in four-dimensional space-time, which is one-dimensional time and three-dimensional space.
Later, Einstein wanted to further promote the special theory of relativity, he found that there is no gravity, in fact, the essence of gravity is the curvature of space-time. Let's take the earth as an example. The earth bends the space-time around it. The moon actually moves along the geodesic line of space-time. (geodesics are similar to straight lines in a two-dimensional plane, the shortest distance between two points in space-time.) To put it bluntly, it is moving along the "straight line" of time and space.
In the same way, the reason why the earth revolves around the sun is because the sun has bent the space-time around it. The earth is moving along the geodesic line of space-time.
Therefore, the earth and other celestial bodies are moving along the geodesic line of space-time, so they have no possibility of going down. They are already moving along the "ground" of space-time.
summary
Therefore, celestial bodies appear to be suspended in the universe, essentially because of the illusion that we live on earth. In Newton's system, force is the reason for changing the motion state of objects. The earth's attraction to objects on the earth's surface makes us feel that all objects will "fall". However, in space, the direction of gravity is not below our vision, but on the line between celestial bodies. The earth's force direction is actually towards the sun. In fact, there is no force at the bottom of the earth's vision, so it will not fall. If we understand from Einstein's theory, in fact, the celestial bodies are already moving close to space-time.
