Nothing can travel faster than the speed of light. Black holes can trap everything, including light.

So, how does gravity escape from a black hole? This question is very interesting and can make most people really understand gravity.

The answer is that gravity doesn't work in the way that most people think.

The most intuitive understanding of gravity is nothing more than the force between two objects.

For example, the earth exerts gravity on the moon, and so does the moon on the earth.

Newton used this gravity when he discovered the law of universal gravitation? Mechanical model? .

It was not until early 1900 that it was defined as the theory of gravity that we still use today.

However, some assumptions were used in the construction of this gravity model.

Suppose we have a universe with only a single mass.

Imagine a space that extends in all directions and all the mass is concentrated in its center. Let's call it ..

Does this mass have gravity? If gravity is only the interaction between one object and another, then the answer is no.

No other mass can exert gravity on A, so there is no gravity.

If you add a mass to this universe, and write it as B, then B and A will exert force on each other, so that gravity will exist.

But gravity only exists between a and b, and other empty places in the universe do not exist.

One problem with this mechanical model is that it requires one mass to exert force on another mass across space.

This? Long-range operation French mathematician and astronomer Pierre Simon solved this problem. Pierre-Simon Laplace was partially solved in the early1800s.

His idea is that mass must be related to another mass through some kind of energy, which he calls a field.

Another group feels that this field is a force acting on them.

So if we imagine the mass A in a lonely universe, we will say that there is a gravitational field around A, even if there is no other mass in the universe.

This eliminates the necessity of distance effect, because when we put mass B into the universe, it just feels the gravitational field at this position and is affected by gravity.

We know that this gravitational field is because A is at a certain distance, and B just feels that there is a gravitational field in his position.

This mechanical model and Newton's gravitational field model both give the same prediction, so there is no difference between them in the experiment.

But the concept of field is easier to deal with by mathematics, and the field is also used to describe things like electricity and magnetism, so we generally regard Newton's gravity as a field.

But this will lead to another problem.

Suppose in the universe of A and B, we suddenly change the position of A, how long will it take B to feel this change? In other words, if we change the position of A, how fast will this change propagate in the gravitational field? When Laplace thought about this problem, he found that the change of gravitational field would happen immediately.

? The speed of gravity? Will be infinite.

For example, if gravity travels at the speed of light, the center of gravity around the earth will be the position of the sun 8.3 minutes ago (it takes 8.3 minutes for light to travel from the sun to the earth).

So the earth's orbit will become unstable over time.

At that time, the idea that the speed of gravity was infinite was not considered a problem.

In fact, it is used as an argument to refute the viewpoint of alternative gravity put forward at that time.

But in the early 1900 s, Einstein put forward the special theory of relativity, pointing out that nothing can exceed the speed of light.

If so, then our theory of gravity will be wrong.

19 15, Einstein put forward a new theory of gravity, which is the general theory of relativity, which can satisfy both Newton's gravity model and special theory of relativity.

At the beginning of this year, LIGO successfully detected gravitational waves.

According to the theory, when two massive celestial bodies (such as neutron stars) surround each other, they will produce gravitational waves and radiate outward.

These gravitational waves will travel at the speed of light.

The existence of gravitational waves was successfully detected at the beginning of this year.

But if gravity moves at the speed of light, does this mean that the orbit of the celestial body must be unstable? Actually, it's not.

When Laplace studied gravity with finite velocity, he only considered the influence of gravitational velocity.

In special relativity and general relativity, the limitation of light speed will lead to other effects, such as the time expansion effect caused by relative motion and the obvious change of mass.

Mathematically, these effects are derived from Poincare's invariance.

Because of this invariance, the time delay of gravity and the speed-dependent effect of time mass cancel each other out, so that the effective mass will attract places with mass.

This cancellation effect means that gravity seems to act instantaneously for orbital motion.

But wait a minute, since the gravitational field has a finite speed, how can it produce an instantaneous effect? In fact, the gravitational field can't do this.

But in general relativity, gravity is not an energy field.

Long before Newton, it was generally believed that matter and energy field would interact in space at a specific time.

In this way, time and space can be regarded as the background of a thing.

Space and time are regarded as a measurable cosmic grid.

When developing the special theory of relativity, Einstein found that space and time cannot be absolute backgrounds.

Newton believed that for all observers, two things that happened at the same time happened at the same time.

But what does Einstein need to find that the speed of light is constant? Now? This concept is relative.

Different observers moving at different speeds will have different views on the sequence of events.

Distortion of time and space near the earth

This principle was carried forward in Einstein's theory of gravity.

In general relativity, gravity is not an energy field.

On the contrary, mass distorts the relationship between space and time.

If we go back to the previous example, if we put mass A in an empty universe, the relationship between space and time around it will be distorted.

When we put mass b nearby, the distortion of space-time around it means that it will move towards mass B.

It seems that B is pulled towards A by a force, but in fact it is because time and space are distorted.

Like physicist John? Wheeler once said: time and space tell us how matter moves; Matter tells space-time how to bend. ?

This is why gravity seems instantaneous, but gravitational waves travel at the speed of light.

Gravity is not something that propagates in time and space. Gravity is time and space.

A black hole is an extremely distorted space-time because of its huge mass.

This distortion of time and space can prevent light and matter from escaping.

But the distortion of time and space is also gravity, and there is no need to escape from the black hole, because it is a black hole.