A black hole is an extraordinary object in the universe, named for its defining characteristics: it is black because not even life can escape its immense gravitational pull. The term hole and reflects how it seemingly pulls everything in its vicinity toward it. Click here to discover how black holes arise as a part of the lifecycle of stars.

Theory of general relativity

To get a brief understanding of black holes, one must be familiar with Einsteins theory of general relativity: it explains that matter or energy curve space-time, which is a combination of space and time as an additional dimension. 

While the concept of curved dimensions is difficult for humans to visualize, it can be facilitated by imagining a two- dimensional flat blanket that deforms, or curves, when a heavy object, like a ball, is placed on it. 

Moving the object on the blanket causes wave-like deformations in the blanket. Similarly, gravitational waves are a result of accelerating masses in the universe, causing vibrations of space-time.

Properties of black holes

A black hole begins where the escape velocity-the speed required to break free from its gravitational pull- exceeds the speed of light. The equation for the escape velocity (v=√(2×G×M/r)) illustrates this relationship. 

The boundary of a black hole, known as the Schwarzschild Radius, marks the region where not even light can escape. 

Interestingly, a black hole doesn’t require an immense mass, it simply needs its mass to be compressed into a very small volume. For example, if earth were compressed into a sphere with diameter of approximately 1.8cm, it would form a black hole.

Singularity

According to Roger Penrose and Stephen Hawking, a black hole contains a singularity at its core. The singularity is a point of infinite density and extreme space-time curvature where the laws of physics break down. 

Singularities remain a mystery because infinities have bot been observed in nature, leading many physicists to question their existence. They always lie in the future as time ends in it and you can only move towards it. 

For physicists, a singularity represents a point in an equation where calculations break down and no longer yield meaningful results. Some theoretical solutions propose avoiding the singularity altogether, such as the idea of falling through a wormhole and emerging in another part of the universe.

Surrounding many black holes is an accretion disk-rotating matter drawn toward the black hole by its immense gravity. The intense rotational motion of this matter generates heat, emitting X-rays and gamma rays that we can detect. 

When scientists captured the first photograph of a black hole, they did not actually photograph the black hole directly as it does not emit light, but rather the glowing disk around it.