Introduction to Black Holes

Table of Contents

Background

Black holes are one of the more mysterious aspects of our universe. However, even though there is a lot that we still don’t know, there is a lot that we do know too. Before we get into black holes, let’s start with a core aspect of black holes: Gravity.

What’s Gravity?

Gravity is an invisible force created by all matter. The more mass an object has, the “stronger” the gravity is.

Gravity & Space-Time

Draw a stick figure on a piece of paper. That figure can only experience life within that piece of paper; they can’t see or sense you. They are living on a 2D surface, like a square, where they can only move up, down, left, and right.

In a cube, which is a 3D space, we can move up, down, left, right, back, and forward.

Zero dimensions is a single dot. One dimension is a line. Two dimensions is a square. And three dimensions is a cube.

Image Source: MathIsFun.com

Picture your bed and its flat surface. The surface of your bed represents a two-dimensional (2D) world. The bowling ball represents three-dimensional (3D) space. Now, imagine placing a bowling ball in the middle of the bed. What happens to the bed?

A GIF of a bowling ball being dropped onto a flat bed. When the bowling ball lands on the bed, the bed sinks down due to the bowling balls weight/mass.

Image Source: JupiterScientific.org

As the above image shows, the bed dips to accommodate the weight/mass of the bowling ball. Those living within the 2D surface would not see any difference in their world, but the fabric of their universe has moved.

Picture this same scenario, except the bed represents 3D space and the bowling ball represents 4D space-time. In this representation, the bowling ball’s gravity is bending the fabric of space-time (4D). In 3D, we cannot see this bending of space-time.

A digital image with a grid layout representing the fabric of space-time of our universe. Sitting atop that, are three yellow spheres, representing three stars. The fabric of space-time is curving where the stars sit, to accommodate their mass. This curve represents gravity.

Image Source: Encyclopedia Britannica

What Are Black Holes?

A black hole is an invisible object* that holds significant mass within a tiny volume, giving it immense gravity. There are different types of black holes, but this introduction focuses on stellar black holes.

*Black holes are not technically objects because they are not composed of anything. The “shell” of a black hole is simply the fabric of space-time. So, while it would be more accurate for me to refer to it as an area of space-time, that’s harder to imagine and conceptualize, so I’ll use “object.”

Every black hole has:

This is the first photo ever taken of a black hole: An image of a round, somewhat blurry ring of red-yellow-orange light in front of the black background of space. The ring of fire-colored light is the light of a star orbiting the entrance to a black hole, which is evident by the black circle at the center of the ring of light.

Image Source: Event Horizon Telescope Collaboration

I thought black holes weren’t visible. What’s that light?

That light is coming from a star that the black hole is “eating.” When an object, like a star, gets caught in a black hole’s gravity, it will often orbit the black hole while it gets stretched apart due to the black hole’s immense gravity. (Imagine if Hans Christian Anderson created a dark, twisted spiral wishing well.)

A GIF of a green spiral wishing well. Someone has placed two coins on it, which are now spiraling down the sides of the well towards the bottom, where there's a hole that leads to somewhere we can't see. But we can assume it's some sort of collection box to collect all the coins. The camera adjusts its angle and we see a silver elephant on the edge of the well for decoration.

GIF found at MakeAGif.com

How big are black holes?

Bigger than a molecule but smaller than the universe 😆 Just kidding, kind of. A black hole can, in theory, be any size. Although, very tiny black holes don’t contain enough energy to sustain themselves for long, so they dissipate quickly. The more mass a black hole has, the longer it can sustain itself. So, if a black hole “eats” a lot of stars or other matter, it can also grow in size and mass.

It’s important to note that the mass of a black hole is directly proportional to its “surface” size. So, even a very tiny black hole still has immense mass: An atom-sized black hole would have the mass of an entire mountain.

How do black holes form?

There are a few ways we know they can form, and some additional ways scientists theorize they can form, and it differs based on the type of black hole. Here are how two of the most common black holes form:

Stellar black holes form when a massive star dies. This is because a very large star can become so unstable that it collapses in on itself and implodes into a black hole. The specific mass and circumstances of the star’s death determine if a black hole will form, or not. For instance, the larger the star, the more likely a black hole will form during the star’s collapse.

Supermassive black holes are another common type of black hole, but they’re more mysterious. One theory is that they formed at the beginning of the universe during the Big Bang due to the sudden, rapid expansion and warping of space-time. In 2023, scientists discovered the oldest supermassive black hole we’ve ever found: A 13.33 billion-year-old supermassive black hole. (For comparison, today, we exist 13.8 billion years after the Big Bang.) This discovery has led scientists to be more confident in the theory that supermassive black holes formed during the Big Bang. With continuing advances in technology, scientists expect to eventually discover even older supermassive black holes.

How do we find black holes?

Since black holes are essentially invisible due to their ability to trap everything, including light, there are only two ways that scientists are currently able to detect them.

Gravitational Lensing

When gravity is “strong” enough, it can bend even light itself. Before LIGO, scientists were able to locate black holes by looking for gravitational lensing surrounding a black spot. This meant that a black hole was causing the illusion.

Digital illustration reads: Hubble Measures Deflection of Starlight by a Foreground Black Hole. The image shows a diagram of a black hole in space with a star located to the back right of the black hole. An arrow shows the star's light traveling towards the black hole and then getting caught by the gravity, causing the light to start moving to the left (aka the light is being deflected to the left side of the back hole). The light proceeds to move towards up (the observer) on the left side of the black hole. Thus, to the observer, it would look like the star is on the back left of the black hole even though it's actually on the back right.

Image Source: NASA, ESA, STScI, Joseph Olmsted

Animated simulation of gravitational lensing:

Animated simulation of gravitational lensing caused by a black hole with a galaxy passing by behind it. The black hole sits in the middle of the image and appears static. As the galaxy passes behind the black hole, the galaxy appears to grow in size around the black hole. In reality, the light of the galaxy is being reflected in multiple places around the black hole, which gives the illusion that the galaxy suddenly grew in size. Once the galaxy finishes passing behind the black hole, the galaxy appears normal size again.

Image Source: Wikipedia

Gravitational Waves

In 2019, the Laser Interferometer Gravitational-Wave Observatory (LIGO) successfully detected gravitational waves from merging black holes. With multiple partner facilities located around the world, they were able to triangulate where the black holes were located in the universe. How all that works is for a Black Holes 201 lesson, but if you’re curious, you can read more about it here.

An artist visualization of gravitational waves caused by two merging black holes. In the image, we see two black circles that represent two black holes orbiting each other as they begin to run into each other. We also see wave-like ripples emanating from the black holes. These ripples represent the movement of the fabric of space-time itself.

Image Source: LIGO at CalTech

What’s the Point of Black Holes?

Despite their reputation as the mysterious, destructive beasts of the universe, black holes are an essential part of the universal ecosystem. Here are two examples:

They’re the universe’s recycling system

Similar to how microorganisms decompose deceased plants/animals, some scientists believe that black holes are universal recycling bins, meaning that they consume and convert matter into electromagnetic radiation. Then, they later release a small amount of electromagnetic radiation back out into the universe (picture a black hole burping after a good meal).

This emitting of radiation is called Hawking Radiation, named after Stephen Hawking. Over time, this process causes the black hole to eventually “evaporate” and leave behind nothing but leftover radiation.

An artist's depiction of a black hole emitting Hawking Radiation. A black sphere is surrounded by orbiting light from a sun it is devouring. There is also a light blue and white, spiraling stream that the black sphere is spewing out, which is the Hawking Radiation.

Image Source: NASA/JPL-Caltech

Black holes help spread elements like seeds

When a star dies and collapses, it can explode to create a supernova. During this event, it is common for the core of very large stars to implode as well, forming a black hole. During this implosion and explosion, some of the star’s elements (e.g. carbon and oxygen) will spew out into the surrounding universe. The released elements will then float through space indefinitely until they come across existing celestial objects to combine with or until they help form new celestial objects.

According to scientists at the Chandra X-Ray Observatory at Harvard, “The shock waves from the explosions may also trigger the formation of new stars and new solar systems, so our existence on Earth might be due to an explosion that formed a black hole.”

An artist's depiction of a star death featuring a supernova and a black hole formation. A black hole is surrounded by spinning white, blue, teal, red, orange, and yellow elements from the dying star.

Image Source: INGIMAGE

Glossary

Electromagnetic Radiation

The spectrum of radiant energy, which includes visible light, UV light, radio waves, and X-rays.

Event Horizon

The gravitational “boundary” of a black hole that not even light can escape from.

Mass

The amount of matter an object contains. This differs from weight, which measures the force of gravity on an object. For instance, a ball with a mass of 1kg would weigh about 2.5 lbs on Earth and 0.4 lbs on the moon since the gravity is stronger on Earth than it is on the moon.

Matter

Anything that has mass and takes up space. For example, clouds are matter since water vapor has mass and takes up space. Sound waves aren’t matter since they don’t have mass and don’t take up space.

Singularity

The term for the scientifically puzzling black hole’s core: The heart of every black hole contains infinite gravity, mass, and density within an infinitely small space.

Space-Time

A theoretical and mathematical concept that considers space and time to be inherently connected. In this concept, space is dimensions (3D) and time is one. This means space-time is four dimensions (4D).

Stellar Black Hole

These are the black holes that many think about when they hear the term “black hole.” These “regular” black holes are 5-20 times more massive than our sun.

Supermassive Black Hole

A black hole of extremely large size and even more extreme mass. These have a mass of millions to billions of times more massive than our sun. Note: A supermassive black hole sits at the center of most galaxies, including our own Milky Way galaxy.

Supernova

A massive and extremely bright explosion of a star.

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