Earth is really floating in space ?
If you look at the earth from space, the earth seems to be floating in space. There have been many probes taking pictures of the Earth in space. Let’s give a few examples:
On December 7, 1972, the US Apollo 17 was on its way to the moon and turned around to take a picture of the Earth called "" Blue marbles " photo.
In 1968, Apollo 8 took a picture named " out of " in its orbit around the moon.
On October 2, 2017, the probe OSIRIS-REX took a family portrait toward the earth and the moon at a distance of 5 million kilometers from the earth.
On February 14, 1990, Voyager 1 took a picture of 6.4 billion kilometers away from the earth and towards the direction of the earth. This picture was named: the dim blue dot, which is only 0.12 pixels in size.
If you look at these four photos carefully, without exception, the earth is floating in space. We must know that for people on earth, we can always feel a "downward" force, which is related to our own weight. This "force" firmly presses us to the ground. The weight of an ordinary person is generally under 100 kg, and people who can exceed 100 kg can be said to be very fat. Now the global population has reached 7 billion. If everyone counts on 100 kilograms, then the global population is 700 billion kilograms at full capacity, which is 700 billion tons. However, the weight of the earth has reached 60 trillion tons, which is more than 10 orders of magnitude higher than the total weight of all mankind. Then why can the earth float in space without falling down?
Why does the earth float in space?
To understand this problem, we must first start with Newton. Newton can be said to be the founder of modern science. His main achievements are the three laws of mechanics and the law of universal gravitation. Among them, the law of universal gravitation tells us that the laws followed by everything in the universe are actually the same as those on the earth, which unifies the dynamics of space and the earth. The three laws of mechanics are the foundation of this part of physical mechanics.
According to Newton's theory, , if the object is not under force, or when the resultant force is zero, it should be in a static or uniform linear motion. On the earth, human beings are subject to the gravitational force of the earth, so when a person jumps up, he will be subject to the combined action of the gravitational force of the earth and the force of the earth’s self-turning heart. Therefore, humans will always feel a "downward" force, as does the force on objects on the earth. When people see that everything will fall toward the surface of the earth, common sense has formed the fallacy that everything will "fall down".
Let's take a look at the situation of the earth again, does not have any force to pull the earth down. The earth is affected by the sun's gravitational force. If you want to clarify some details, then the earth will also be affected by the gravitational force of the moon and other celestial bodies in the solar system. Because the earth has a certain initial velocity, the earth moves in an elliptical orbit around the sun, and the sun is a focal point on the ellipse. Therefore, through Newton's theory, we will know that the earth will not fall because it is not subject to a downward force, and the force it receives is directed toward the sun. If you have to specify a "downward" direction for the earth, then this direction should be towards the sun. If the speed of the earth drops to zero, the earth will "fall" into the sun.
Similar phenomena also exist on the earth. If the speed of an object on the earth reaches 7.9km/s, it can revolve around the earth, which is like the earth revolving around the sun. The principle of the satellite and the detector is in this way.
Gravity in the eyes of Einstein
Newton’s theory of mechanics and the law of universal gravitation are called classic physics . However, physics has undergone earth-shaking changes in modern times. Today, the mainstream view of gravity comes from Einstein's general relativity . We can also understand from Einstein's general theory of relativity: Why does the earth seem to float in space?
In the special theory of relativity , Einstein unified time and space. He believed that time and space should be combined into space-time, and human beings live in a four-dimensional space-time. In the general theory of relativity, Einstein made a further promotion of this special theory of relativity. He believed that gravity does not exist, and the essence of gravitational phenomenon is the curvature of time and space.
The moon revolves around the earth because the earth bends the surrounding time and space, and the earth is moving along the "geodetic line" of time and space. The "geodesis" here refers to the shortest distance between two points.
Similarly, the earth revolves around the sun, in fact, because the sun is bending the surrounding space and time, the earth is moving along the geodesic line. In other words, the earth is a "straight line" in a four-dimensional space-time to some extent, and conforms to Newton's first law: the earth is not subject to force, but moves in a straight line at a uniform speed. From our perspective, its trajectory is just an ellipse. Therefore, from the perspective of relativity, the bending of space does not make the earth move downward, and there is no force to push the earth downward, and the earth will not fall.
