Gravity is everywhere around us. According to Newton's law of universal gravitation, objects with mass will produce gravity, and everything has mass, so we can feel the existence of gravity at every moment. The reason why we can stand on the earth safely is because of the existen

Gravity is everywhere around us.

According to Newton The law of gravity, objects with mass will produce gravity, and everything has mass, so we can feel the existence of gravity at every moment. The reason why we can stand on the earth safely is because of the existence of gravity.

Newton proposed universal gravity, but did not explain what gravity is and how gravity is produced?

Newton himself certainly also knows that gravity itself has "flaws", but due to the limitations of the times, we cannot demand too much. It is difficult for the great Newton to reveal the essence of gravity. This limitation cannot be broken through, and any era has its limits that cannot be broken through.

This problem is left to another great physicist, Einstein .

1915, Einstein proposed the great general theory of relativity . Once this theory was proposed, it caused a sensation around the world because it was too subversive.

According to general relativity, the so-called gravity is essentially all things in space and time. Any mass object will cause bending to the surrounding space and time. In the curved space and time, all objects move along straight lines.

Note that the "linear motion" above is not what we often call a straight line, but the "line" in four-dimensional space-time , that is, the "gedoctoral". Everything can feel the bent space-time and space, and then move "gedotically" along the bent space-time, which reflects gravity.

To put it bluntly, the so-called gravity actually does not exist, it is just a manifestation of the curved space and time. This is the essence of gravity.

So how is time and space curved?

Our common description of space-time curves are some not very rigorous schematic diagrams, which are convenient for us to understand in a popular way. For example, describing space-time bending as the situation where the spring bed is compressed by heavy objects is actually not rigorous.

Because there is no distinction between directions in the universe and space, there is no concept of up and down, time and space cannot bend "downward".

To be more rigorous, the direction of time and space bending is the center of mass of the object, that is, the center of the object mass. Just like the figure below, the object will also pull the surrounding time and space movement together.

General Relativity is a more advanced embodiment of the law of universal gravitation. To put it bluntly, the law of universal gravitation is an approximation and special case of general relativity, and a special case in low gravitational conditions.

This is also why the law of universal gravitation cannot interpret Mercury's "precession" phenomenon, and general relativity can be interpreted well. Because Mercury is too close to the sun, it is affected by the bending of time and space than other celestial bodies!

However, for us who live on Earth, most of the time is still dominated by Newton's law of universal gravitation because we live in a low-gravity world. Once it rises to the cosmic level, general relativity theory needs to be launched!