In the 18th century, an amateur astronomer, John Mitchell, proposed that an object with a radius 500 times the density of the Sun would not allow light to escape from it.

One thing to note is that we are talking about the density of the sun, not the mass of the sun. The density of the Sun is 1,410 kilograms per cubic meter (41% more than water); He proposed that if the density was 500 times the original radius, even light would not be able to escape.

Get out here, I still need to explain that I am freed first. If you want to escape from the gravitational field of a planet, you have to have enough velocity. Each planet has a surface escape velocity based on its own mass; The escape velocity of the earth is about 25,000 miles per hour. The escape velocity of the sun is about 14,000 miles per hour.

In order to leave the surface of objects with such a strong gravitational field, in order to permanently escape the gravitational field, there must be such a large amount of velocity. A rocket that wants to escape from the world’s gravity can travel to infinity beyond the Earth’s orbit only if it has a speed of 25,000 miles per hour. If we didn’t have that velocity, we would be trapped in the Earth’s gravitational field forever (I’ll post about escape velocity in a separate post).

## Escape Velocity

The escape velocity is calculated using the formula v = √ (2GM / R).

Since G is the gravitational constant, we will keep it this way.

Density formula in place of mass M

Substituting the formula M = ρV = 4ρπR^3 / 3, If so, we get v = √(8GρπR^2 / 3), which is John Mitchell’s calculation method.

## Gravity field

Let’s do the math. When John Mitchell calculated that way, he found that the escape velocity of an object 500 times the size of the Sun was greater than the speed of light. Since the speed of light itself cannot exceed that escape velocity, it will be forever trapped in the object’s gravitational field. The result is that any light that reaches it is trapped in the gravitational field and cannot get out. Since no light can come out, we can never see or detect these objects. That’s why John Mitchell called these space bodies Dark Stars. He went down in history as the first person to propose the existence of a black hole.

## Newton’s law

In the 20th century, Karl Schwarzschild calculated the radius of a black hole based on Einstein’s theory of relativity. The equation shown in that calculation is similar to the method of calculating the escape velocity of Newton’s law of gravity.

The Schwartz Shill radius calculation method is c = √ (2GM / R)

Newton’s radius calculation method is v = √ (2GM / R)

If you think about it simply, it is calculated by substituting c for v. What we know from that equation is that light can never go outside that radius.

If I have to explain, the speed of light is the highest speed in the entire universe. If even the light in that black hole cannot escape, then every other object will be the same way. That’s why we define a black hole as the radius where light can’t start escaping.

That radius is also called the Event Horizon. We can see the scene before the horizon, but we cannot see beyond the horizon. It’s like, before reaching the radius of the black hole, everything is normal in a way, but things beyond that can’t be detected anymore. Just as the horizon blocks our vision, the event horizon of a black hole blocks our detection. That’s why it’s called the Event Horizon in a physics-friendly way (every known law of physics is completely broken inside a black hole).

## Exit light in Black Hole?

The black hole is called the place where the Event Horizon starts. It will be dark because you can’t see the light coming from there (I wrote that it would be dark because I couldn’t think of how to use the lack of light), Outside the Event Horizon, everything is still normal, physics still works

A star called S2, orbits a point called Sgr-A; It has an orbital period of about 16 years and a major axis length of 970 AU (970 times the distance from the Sun to Earth); While rotating in such an elliptical orbit, the Sgr-A point is at one of the points of the ellipse, This indicates the gravitational pull of Sgr-A on that star, and when we calculate its mass according to the cosmic laws, it is found to be 44 million times the mass of the Sun. The real picture of this black hole containing 44 million suns has been released in 2022 by the Event Horizon Telescope (EHT Team).