In recent days, the Event Horizon Telescope (EHT) collaboration has put the name Sagittarius A* (Sgr A*) on everyone’s lips by showing its new results, the best and most detailed image of the hole super massive black that is located in the center of our galaxy. However, it is not easy to understand the concept of a black hole, since nothing similar exists in our daily lives.
One of the most practical ways to get an idea of what a black hole is is to understand the concept of escape velocity (VE), that is, the minimum speed an object needs to escape the gravitational attraction exerted by another. larger body above him.
If we throw a stone into the sky, it will fall again, but if we throw it with a speed greater than the EV of the Earth, the stone will not fall again and will go out into outer space.
The VE depends on the ratio between the mass and the size of the body, in this way more massive and smaller bodies will have higher VE than less massive and larger bodies. For the case of the Earth, the VE is 11 km/s (40,300 km/h), for the Moon it is 2.38 km/s and for the Sun it reaches 617.5 km/s. In the case of a black hole, we have a large amount of mass concentrated in an extremely small place, therefore, the VE is higher than the speed limit of bodies in the Universe.
I mean the speed of light in a vacuum, about 300,000 km/s.
Picky eaters might say that light has no mass, and therefore should be able to escape. However, VE can also be interpreted as the energy that something needs to be able to get out of the gravitational influence of a body. In this way, we can say that no photon of light can escape, because it loses all its energy before it can.
The EV of a black hole begins to decrease as we move away from it.
When we get far enough away, we will reach an area called the Event Horizon, which indicates the place where the VE is equal to the speed of light. This means that if we move a little further away from the black hole, it is possible to escape its gravitational influence.