New Event Horizons - What is a Black Hole?

A rendering of what a black hole might look like up close. Note how the gravity of the black hole actually bends the light of the stars behind it, creating what is known as an "Einstein Ring."

One of the most simultaneously perplexing and fascinating topics in astrophysics is undoubtedly the black hole. They’re holes in the very fabric of space and are famous for how they defy all conventional logic. And, most defining, once something goes in, it’s never coming out.

To understand a black hole, you first have to understand gravity. We know the Earth has gravity, but actually, all objects with mass have it as well. The more massive and denser an object is, the more gravity it has. Smaller objects tend to have less mass, so the effect of their gravity isn’t noticeable, especially when compared to that of the Earth or Sun.

Stars are both extremely massive and quite dense. This means they have extreme gravity. In fact, they have so much gravity that a star’s own gravity wants to cause the entire star to collapse into itself and condense. But inside the star, it’s producing massive amounts of energy via nuclear fusion, and this energy wants to push outwards. The outwards forces of a star caused by its nuclear fusion is enough to balance the inward pull of gravity and keep the star from collapsing. This is called hydrostatic equilibrium. But what about when the star dies? When a star is no longer able to produce energy, gravity wins out in the tug of war and the star collapses. Usually, this just means it collapses into a smaller stellar remnant, such as a white dwarf. But, if the star was massive enough, then the gravity will keep on pulling until the star has shrunk down to a single, dimensionless point. That means all the mass of the massive star has been condensed to an infinitesimally small point in space. The star is now a singularity and has infinite density. That is the making of a black hole.

But what exactly makes the hole black? Imagine throwing a ball into the air. It’s going to fall right back down thanks to the Earth’s gravity. But imagine throwing it a little harder. Then it travels a little faster and makes it higher into the air before falling. Throw it even harder, it makes it even further. Eventually, if you could somehow throw it hard enough, it would make it high enough to have escaped Earth’s gravity, in which case it wouldn’t fall back down. In order to escape from an object’s gravity, you have to travel at its escape velocity. On Earth’s surface, that’s about seven miles per second, which is the speed rockets have to travel to launch into space.

Well, gravity affects everything, even light. Light, despite appearing to our eyes to move instantly, actually moves at a finite speed of 300 million meters per second, and it is the fastest thing there is. When the force of gravity becomes so strong that its escape velocity is faster than the speed of light, that means that not even light can escape it. As light will never leave this strong point of gravity, the area around that point is no longer visible, as we can only see an object when light is emitted by it or reflects off of it. That leaves a cloak of pure darkness around the singularity. This is known as the event horizon, and it’s the point of no return. As nothing is faster than light, nothing can escape the black hole.

However, black holes are not the cosmic vacuum cleaners that sci-fi tends to make them out to be. While it is impossible to escape once you pass the event horizon, they don’t suck you up from a distance as Hollywood would have you believe. Before you make it to the event horizon, it is very possible to escape. If the Sun were to spontaneously transform into a black hole, the Earth would continue to orbit in the same way it always has. The black hole Sun may have infinite density, but it would still have the same mass it always had, which means that at the distance of the Earth, nothing would change. The only thing that changes is how close an object can get to the Sun’s center of mass. Escape velocity decreases at greater distances from the center of mass, meaning that anything past where the Sun’s current surface wouldn’t be affected any differently by the black hole’s gravity. It’s only at those close distances near the event horizon where the black hole becomes a problem. If the Sun were to become a black hole, the radius of its event horizon would be only 3 kilometers.

But don’t start panicking about the Sun becoming a black hole. It won’t ever happen. Black holes only form from stars even more massive than the Sun. A star has to be somewhere around 20 times more massive than the Sun to collapse into a black hole at the time of its demise. Anything less massive than that will either become a white dwarf or a neutron star.

Black holes are everywhere in the universe. There’s even a supermassive black hole in the center of our galaxy, which the stars of the Milky Way orbit around like the planets around the Sun. Despite how much we know about them, there’s so much we don’t know. Past the event horizon, physics itself seems to break down. There are aspects of black holes that could always be a mystery to us. But that’s exactly what makes them so fascinating to study.

The first ever actual picture taken of a black hole (M87). Image credit: Event Horizon Telescope Collaboration (CC BY 4.0)