What is a black hole?

A black hole is an astronomical object with a gravitational pull so strong that nothing, not even light, can escape from it. The “surface” of a black hole, called the event horizon, defines the boundary where the speed needed to escape exceeds the speed of light, which is the speed limit of the cosmos. Matter and radiation enter, but cannot leave.

Einstein's theory of general relativity provides the theoretical framework for understanding the nature of black holes. According to this theory, space and time are not absolute, but are combined into an entity called space-time, which is curved by the presence of matter and energy. Gravity is not a force, but a manifestation of the curvature of space-time. Black holes are the most extreme manifestation of this curvature, where the mass density is so high that it distorts the fabric of the universe significantly.

Black holes can form in a number of ways, one of the most common being the explosive death of a massive star, known as a supernova. During this process, the star's core collapses under its own gravity, compressing into an extremely compact region creating a singularity, a point of infinite density. The surface surrounding the singularity is called the event horizon, and marks the boundary beyond which nothing can escape the black hole.

There are different types of black holes, including stellar black holes, formed from the collapse of massive stars; supermassive black holes, which are found at the centre of galaxies and can have a mass equivalent to millions or even billions of times that of the Sun; and intermediate-mass black holes, whose masses are between those of stellar and supermassive black holes.

Detecting black holes is a challenge, as their dark nature makes them invisible to conventional telescopes; this has become an active area of research in astrophysics. Scientists have used different methods to detect the presence of black holes, including observing the effects of their gravity on stars and surrounding gas, as well as detecting gravitational waves.

In 2019, the Event Horizon Telescope Collaboration released the first direct image of a black hole, located at the center of the galaxy M87. This historic image provided visual evidence of the existence of black holes and confirmed many of the predictions of Einstein's theory of general relativity.

Black holes arise as hypotheticals from the formulas of physics and mathematics; however, thanks to technological advances and the evolution of knowledge, they are becoming more and more palpable, bringing with them many more questions that will force us to seek more answers in the verification and creation of new theories and the evolution of the exploration of the cosmos.