Black holes are the weirdest objects in the known universe. In this article, we try to get right at the event horizon.
Instead of an object, it’s more accurate to refer to a black hole as a location in space. A very compact location where the gravitational field is so strong that not even… aghh… well, everybody heard about this “not even light can escape”… blla blla blla.
Theoretically, everything can become a black hole if it were small enough. The Earth, for example, would become one if you squeezed it to the size of a basketball.
But why do black holes look the way they do? I mean… what do they even look like?
Even though a black hole doesn’t emit, or reflect, any detectable type of light, humans can measure the radiation emitted by the surrounding area of them.
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Let’s take stellar-mass black holes. Don’t confuse them with the supermassive black holes in the middle of galaxies. Stellar black holes are much smaller. They are created when a very massive star collapses and usually weigh between five and ten times the mass of the Sun.
Astronomers can detect these black holes by observing stars that orbit around them.
Sometimes, when a star passes too close to a black hole, the black beast can tear the star apart as its gravity pulls it toward itself. As the star’s matter speeds up and gets heated, it churns out x-rays in the process. That radiation can then be detected here on Earth.
Stellar black holes are usually the harder targets to detect because there’s usually not much going on around them. And this is when we meet with a supermassive black hole. A relatively easier target to spot.
Supermassive black holes are the famous gargantuan monsters that lie at the centers of most, if not all, galaxies, including our Milky Way galaxy. These places pack a lot of mass, ranging from millions to billions of solar masses.
No one is quite sure how supermassive black holes form, but a lot of scientists suggest that they might’ve come as a result of numerous intermediate-mass black holes merging together.
In 2019, we got the first-ever picture of a supermassive black hole. This is the central supermassive beast of Messier 87 galaxy in the nearby Virgo galaxy cluster. It lies about 55 million light-years from Earth and has a mass 6.5 billion times that of the Sun.
An international network of radio telescopes called the Event Horizon Telescope created the image, and for the first time, it proved that indeed black holes caused the spacetime distortions that scientists have observed before.
Before this picture, we only had math that suggested black holes exist.
But what we are actually seeing in this image is not the black hole itself, but rather the shape of the accretion disk around the supermassive black hole and of the “shadow” produced by its event horizon, the dark region in the middle. So, the dark region that you are seeing here is not an object. It’s an absence. Something that has left our observable universe. This compact region affects its surroundings in extreme ways, warping spacetime itself and heating up the matter around it.
Closer To The Edge
Just above the event horizon, the boundary of a black hole where nothing can escape, there’s a region called the photon sphere. That’s where gravity is so strong that light can actually orbit around the hole if it has a tangential trajectory.
Light in the photon sphere can still escape if its direction is within some small angle of radially outward.
You wanna hear about a mind-blowing fact? If you were standing at the photon sphere, photons that start from the back of your head would orbit the black hole. Eventually, your eyes would capture them. You would literally see the back of your head. I know. Crazy stuff.
However, once light reaches the event horizon, that’s it. Once you get in, there’s no way out.
The event horizon acts as a membrane, but it’s not something that you can touch or anything like that. It’s a touchless sphere surrounding the black hole. And that’s as close as we can get to a black hole.
The region from a singularity, the center of a black hole, up to the event horizon, called the Schwarzschild radius, makes up the black hole. Everything inside it is hidden from us, forever. But whatever’s outside the event horizon can be within our reach. Because it’s impossible to get any information out of the event horizon, we will never know what’s inside it.
Spinning Black Holes
Spinning black holes, also known as Kerr black holes have another region just above the outer event horizon, called the ergosphere.
Big spinning objects drag spacetime around them. A black hole does that, too, but to a greater extent. It drags and twists spacetime in a circle around it. Scientists call this frame-dragging.
The ergosphere, within which an observer is forced to rotate with the black hole, touches the event horizon at the poles of a rotating black hole and extends to a greater radius at the equator. Due to this centrifugal force, the shape of the ergosphere is basically like a pumpkin.
The name ergosphere is due to the fact that energy can be extracted from the black hole in this region via the Penrose process. Ergo is a Greek word that means work.
But all that we’ve said here is based on what we know so far. It’s all a matter of perspective.
For example, the part where nothing can escape a black hole is not entirely true.
When Professor Stephen Hawking applied quantum field theory, a combination of classical field theory, special relativity, and quantum mechanics, to black hole spacetime, it showed that black holes can radiate particles in a process known as Hawking radiation.
Scientists suggest that very tiny particles spontaneously pop into existence all the time, everywhere in the universe. These spontaneous particles appear to crash into each other and vanish almost immediately after being created, and that’s why we don’t usually detect them.
So Hawking theorized that a virtual particle can pop into existence right at the edge of the event horizon with one entangled particle sucked into the black hole while the other particle survives. Because the particle no longer has a partner, it can’t vanish. These are the particles that can escape the black hole and are called the Hawking radiation.
Through this radiation black holes can lose energy and get smaller. However, the amount of Hawking radiation is extremely small, so it would take an inconceivable amount of time for the radiation to significantly reduce the size of a black hole.