The shadow of a black hole: how is this possible?

It is known that photographing a black hole is very difficult. But when it is surrounded by material, we have the opportunity to see the hole carved by the event horizon. But what we see in the famous images of black holes is not the event horizon itself, but an enlarged version of it, known as the shadow of a black hole.

No matter, not even light, can leave the surface of a black hole, the boundary of which is called the event horizon. Because of this simple fact, black holes are extremely difficult astronomical targets. They do not emit their own radiation (except for a possible exotic quantum process known as Hawking radiation, which is too weak for us to detect). In addition, they do not reflect or refract ambient light, so we can only detect them based on their effect on the environment.

The most common way to do this is to find accretion disks, which are rings of material surrounding a black hole, made up of matter falling into the event horizon. As matter approaches the black hole, it heats up and emits intense, high-energy radiation. We have been able to observe accretion disks around black holes of all sizes, from stellar-mass black holes in the vicinity of our galaxy to supermassive black holes at the heart of most galaxies.

We have also been able to observe the gravitational waves emitted by black hole mergers and observe how stars revolve around central, invisible objects with strong gravity.

But a few years ago, the Event Horizon telescope was able to take a fresh look at black holes by directly visualizing material surrounded by two supermassive black holes, one in the Virgo galaxy and the other in our own. The images show a ghostly glow surrounding the void of complete nothingness.

This void of complete nothingness is the black hole, but the black hole itself is much smaller than the current image suggests. What we actually see in the gap in the images is known as the shadow. The very existence of the shadow is due to two reasons.

There must be an event horizon that will create a void. The event horizon absorbs any light emitted by objects behind the black hole, preventing that light from reaching us.

However, the black hole itself has such huge gravity that it can bend and enlarge background objects. This causes the black hole’s shadow to appear much larger than the event horizon itself due to this magnifying effect.

This means that if you were to get close to a black hole, it would appear much larger than it really is. In fact, due to the strong curvature of spacetime, you can see farther around a black hole than around a star or planet. It’s as if the sphere of the black hole is unfolding in front of you, allowing you to see it much larger.

The properties of the shadow are directly related to the properties of the black hole itself, especially its mass and rotational speed. There is no other astrophysical object that can create such a shadow, which is why the Event Horizon telescope images are direct evidence of the existence of black holes. In addition, they serve as an excellent test of general relativity, because it is in this language that we understand the nature of shadows.