Albert Einstein’s General Theory of Relativity showed that massive objects in the universe, like Earth or the Sun and other stars, created curvatures in space-time, very similar to what a heavy object would do when placed on a taut rubber sheet. And, that the force of gravity is nothing but the drag that another object feels when inside this curvature. It is because of this curvature that an object either falls on the Earth or keeps orbiting it. To escape this curvature, an object needs to move at fast-enough speeds, called the escape velocity. The escape velocity to move out of Earth’s gravity is 11.2 km per second.
The more massive a body, the bigger and deeper the size of the curvature it creates in space-time. Consequently, the higher is the escape velocity required to pull away from its gravity.
EXPLAINED | What black hole image tells us
It was soon evident that this reasoning would lead to the concept of black holes, areas from where nothing could ever hope to escape. That is because there is an upper speed limit in the universe. Nothing can move faster than light, or electromagnetic waves in general. But there is no upper limit, as yet known, on the massiveness of a heavenly body.
Stars that are billions of times more massive than our Sun are known to exist. The Sun itself is about 1.4 million km in diameter, and has a mass of about 2×10^30 kg. Massive stars, when they are dying, are known to collapse under their own gravity, forming extremely dense spheres of astronomical masses. They pack the mass of thousands or millions of Suns into a radius of a few kilometres.
Such mindboggling bodies create incredibly narrow and deep curvatures in space-time, from where even light, travelling at nearly 300,000 km per second, is unable to escape. Once an object falls inside this curvature, there is absolutely no hope of coming out, ever. That is why black holes do not send out any signals or radiation, because nothing can escape from it.
Initially, many scientists, including Einstein himself, were skeptical about black holes. Over the years, however, scientists have gathered several evidences of the existence of black holes. For example, the observed orbits of several heavenly bodies could be explained only by the presence of a black hole nearby.