How do black holes forged a shadow?

It is notoriously difficult to take a picture of a black hole. But when they are surrounded by material, we have a chance to witness the hole that the event horizon has carved. But what we see in the famous black hole images is not the event horizon itself, but an enlarged and magnified version known as the shadow.

No light can leave the surface of a black hole, a boundary known as the event horizon. Because of this simple fact, black holes are frustratingly difficult astronomical targets. They don’t emit any radiation of their own (except for the possibility of the exotic quantum process known as Hawking radiation, but that’s far too faint to be meaningful). Also, they don’t reflect or refract any ambient light, so we can only recognize them by their impact on their surroundings.

The most common way to do this is to look for accretion disks, which are rings of material surrounding a black hole made up of matter flowing into the event horizon. As matter approaches the black hole, it heats up and glows with intense, high-energy radiation. We have observed accretion disks around black holes of all sizes, from stellar-mass black holes in our galactic neighborhood to the supermassive black holes that reside at the heart of galaxies.

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We’ve also been able to observe the gravitational waves emitted when black holes merge and watch stars orbit around central, unseen centers of gravity.

But in recent years, the Event Horizon Telescope has been able to offer a new look at black holes, directly imaging the material around two supermassive black holes, one in the Virgo galaxy and one in our own. The images show an eerie glow surrounding a void of utter nothingness.

This void of utter nothingness is indeed the black hole, but the black hole itself is much smaller than the hole suggests. What we actually observe in the gap in the images is known as the shadow. The shadow itself is caused by two things.

First, there must be an event horizon to create a shadow. The event horizon absorbs any light emitted by objects behind the black hole and prevents that light from reaching us.

The very first true image of a black hole, captured in 2019. Image credit: Event Horizon Telescope Collaboration

But the black hole itself has such extreme gravity that it can warp and enlarge wallpapers. This makes the shadow appear much larger than the event horizon itself due to this magnifying effect.

This means if you were to approach a black hole, it would appear much larger than it really is. Essentially, because of the extreme curvature of spacetime, you can see further around the black hole than around a star or planet. It’s as if the black hole’s sphere is unfolding in front of you, allowing you to see more of it.

The properties of the shadow are directly related to the properties of the black hole itself, specifically its mass and rate of rotation. There is no other astrophysical object that can cast such a shadow, and so the Event Horizon Telescope images provide direct evidence for the existence of black holes. They’re also great for testing general relativity, because it’s the language that helps us understand the nature of shadows.

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