Why can black holes bend space-time?

Black holes are one of the most fascinating objects in the universe, but human understanding of black holes still has many aspects that cannot be clearly answered.

Between 1907 and 1911, Einstein worked on general relativity (non-inertial frames of reference). He published a paper titled "On the influence of gravity on the propagation of light" in 1911. It predicted that time is relative, and relative to an observer in relation to its gravitational field.

He also proposed the equivalent theory that gravitational mass is the same as inertial mass. Einstein also predicted time dilation due to gravity.

Gravity causes a distortion of space-time. Two events in different regions will have different times. The greater the distortion, the slower time passes.

Another important result of his theory was the prediction of the existence of black holes and the expansion of the universe .

Gravity causes distortions in space-time.

In 1915, a few months after Einstein published his general theory of relativity, German physicist and astronomer Karl Schwarzschild provided a solution to Einstein's field equations. Now known as the Schwarzschild radius , it describes the escape velocity of matter at the surface of a solid spherical object equal to the speed of light.

In 1931, Indian-American astrophysicist Subrahmanyan Chandrasekhar used special relativity to calculate the critical mass for the collapse of spinless electron degenerate matter.

In 1939, Robert Oppenheimer and others agreed with Chandrasekhar's analysis that neutron stars passed a critical value and collapsed into black holes.

General relativity predicts that the universe is either expanding or contracting. In 1929, Edwin Hubble confirmed that the universe is expanding. At the time, this seemed to disprove Einstein's theory of the cosmological constant.

The cosmological constant was to ensure that the universe was static. In response, Edwin Hubble used redshift measurements to discover that galaxies were moving away from the Milky Way.

He also discovered that galaxies farther away from Earth recede faster , a phenomenon later known as Hubble's Law . Hubble set the Hubble constant (expansion factor) at 500 km/(s.Mpc).

Black holes cannot be observed directly, but their masses can be known indirectly.

According to general relativity, gravitational fields bend space-time.

For a given mass, the larger the size of a star, the lower its density. When the volume of a star is very large, its gravitational field has almost no effect on space-time, and light emitted from a given point on the surface of the star can travel in a straight line in any direction. However, for that mass, the smaller the radius of the star, the more it will curve the surrounding space-time, and light emitted at certain angles will return to the surface of the star along the curved space. For example, super-dense neutron stars can achieve time dilation coefficients of 10-20%.

When the radius of a small star reaches a certain value (in astronomy called "Schwarzschild radius"), even the light emitted from the vertical surface is captured, at this time the star will become a black hole. That is, it is like a bottomless hole in the universe, once any matter falls into it, it can no longer escape.

Black holes cannot be observed directly, but their existence and mass can be known indirectly and their effects on other things can be observed.

Information about the existence of a black hole can be obtained through "edge information" emitted by X-rays and gamma rays due to friction caused by the acceleration brought about by the black hole's gravity before the object is sucked in.

It is speculated that the existence of a black hole could also be obtained indirectly by observing the orbits of stars or interstellar clouds, and its position and mass could also be obtained.

 Black holes are likely to evolve from stars.

So how do black holes form? In fact, like white dwarfs and neutron stars, black holes likely evolved from stars.

As a star ages, its fusion reactions run out of fuel (hydrogen) at the center and the energy produced by the center also runs out, in this way it no longer has enough energy to support the enormous weight of its outer shell.

So under the weight of the outer shell, the core begins to collapse until a small, dense star is formed and is able to rebalance the pressure.

These newly formed stars mainly evolve into white dwarfs, however for stars with particularly large masses, they can form neutron stars.

According to scientists' calculations, the total mass of a neutron star cannot be more than three times the mass of the Sun. If it exceeds this value, there will be no more Force that can compete with its own gravity, thus causing another massive collapse.

At this point, according to scientists' conjecture, matter will move towards the center until it becomes a "point" whose volume tends to zero and density tends to infinity. Accordingly, the enormous gravitational force at this point will prevent even light from being emitted to the outside, thus cutting off all connections between the star and the outside world, which is when a black hole is born.

Around a black hole, the distortion of space-time is very large.

Compared to other celestial bodies, black holes are very special , for example, black holes have the ability to "invisible" , humans cannot directly observe them, even scientists can only make various guesses about their internal structure.

We all know that light travels in straight lines. This is basic common sense. But according to general relativity, space is curved under the influence of gravity.

Now, although the light still travels along the shortest distance between any two points, it is no longer a straight line but a curve. To put it vividly, it seems that the light originally wanted to travel in a straight line, but the strong gravitational force changed its path.

On Earth, because the effect of the gravitational field is very small, this type of bending is negligible, but around the black hole, the distortion of space-time is very large. In this way, although part of the light emitted by the star is blocked by the black hole and will fall into the black hole, the other part of the light will go around the black hole in the curved space and reach the Earth.

Therefore, we can easily observe the starry sky behind the black hole, as if the black hole did not exist, this is the invisibility of the black hole. (For example, the accretion disk around the gravitational force of the black hole is often used to determine the magnitude of the black hole).