What is a black hole, and how are we seeing one for the first time?

This image is an actual photo of the supermassive black hole at the center of the Messier 87 galaxy, roughly 55 million light years away, and the black hole is so huge that it's about the size of our entire solar system, and is several billion times the mass of our sun. This is the first direct image of any black hole in human history.

I've been waiting for today for a long time. This is the day that humankind was able to see an actual image of a black hole for the very first time.

It's a big news story this morning that will probably fade from public consciousness pretty quickly, and that's a shame, in my opinion. It's far too easy to become immersed in things that, ultimately, are extremely temporary. It's not to say those things aren't important; each of our lives, while understandably meaningful to us, occupies an infinitesimal amount of time on a cosmic scale. I do understand how something that's so relatively far away, extraordinarily complex, and hard to comprehend can be easy to dismiss. But I also think that people are smarter than they realize, and that I may be able to help explain what a black hole is in a way that people might be able to grasp. We're going to do this without the incredible math and science aspects that probably cause most people to assume that they can't get it. I think they can.

This was the recorded livestream of the media announcement unveiling the first image of a black hole.

Gravity Is Everything
To understand the universe, you should have a basic idea about gravity. Everything that has mass -- you, me, a cat, a car, a banana, a guitar, a planet, a star -- has a force known as gravity. Gravity pulls things with mass or energy toward each other. Don't worry about how gravity works; just accept that it does. The more mass an object has, the greater the force of gravity it exhibits. The most massive thing near you right now is planet Earth, and the gravity it has pulls you down toward its center, which is why you're not floating away right now.

Earth is pretty big compared to you or me or a cat. But it's not very big compared to a lot of other things in the universe. The sun -- that star that is the basis for our solar system -- is so massive that a bunch of planets orbit around it for millions and millions of miles due to its gravity. The sun and every other star were actually made from gravity. Billions of years ago, there was a big cloud of gas -- mostly hydrogen -- that started gathering together in empty space (why? gravity!), and eventually there was so much mass in the middle that pressure caused the atoms of hydrogen to turn into helium through nuclear fusion. That made a big explosion, but the gravity of the mass was so great by then that this ball of exploding matter that we call plasma is held together in a ball. That's what a star is. It's a big nuclear explosion that keeps on exploding, often for billions of years, but the force of gravity keeps it together. Neat, huh?

Size Matters
Much like our planet, our sun seems pretty big. But again, as far as stars go, it's really not. There are stars that are much, much larger than our sun. Insanely large, in our way of thinking. There are stars that are so big that if they sat where our sun currently sits, the edge of the star would be where planet Saturn is now.

Back in 1915, an amazingly smart guy named Albert Einstein developed a scientific theory called general relativity. Not long afterwards, using the math that Einstein put together, other scientists (including a guy named Karl Schwarzschild) noted that at a certain point eventually known as a singularity, the equations reached infinity. They calculated that if a star of a certain size and mass were to truly exist, the math stated that its gravity would be so great that literally nothing that came within a certain distance from it could escape from it... not even light. Even time would cease to function in this area of immense mass based on these equations.

It's very difficult to even imagine such a concept, and frankly, no one thought that these things could exist in reality. It was just some interesting math. But then, through technological development, our ability to observe evidence of our surrounding universe increased dramatically. Like many scientific discoveries, the black hole wasn't confirmed through direct observation -- after all, how do you observe something that by definition you can't see? Instead, it was through observing the behavior of things close to the black hole... gas, dust, planets, stars... that showed the evidence of some phenomenon that acted as if there was an incredibly massive object there. Once the measurements were done, the effect of that mass met the definitions of a black hole. By the way, all this stuff is so relatively new that the term "black hole" wasn't in use until 1967.

If you want to learn more details about black holes, the Wikipedia article is very thorough.

How Do You See Something That You Can't See?
As a lover of science and space, I've been fascinated by the idea of the black hole my entire life. As someone who had a basic understanding of what a black hole was, I never actually expected to see a black hole. After all, no light comes from a black hole by definition. To be clear, by "light", I mean all forms of electromagnetic energy, not just visible light. No x-rays, no microwaves, no radio waves, no gamma rays. Nothing. How can one expect to "see" this thing?

I'd followed the observational experiments of scientists like Andrea Ghez, who used lasers to improve the optics of telescopes used take photos of the center of our galaxy and thereby was able to confirm the existence of a supermassive black hole at the center of the Milky Way where we live. You can see things like stars orbiting the black hole, watching them accelerate to insane speeds as they approach. But the thing they were orbiting remained a theoretical mystery of sorts.

Beginning a number of years ago, scientists around the world came up with a plan to produce an actual image of a black hole, but to do it, they'd need a telescope the size of the Earth. Seems impractical, so what they did was coordinate a global array of radio telescopes in many locations, and then combine the data they received individually into a single image. That image, released on April 10, 2019, is what you see at the top of this page.

This woman is Katie Bouman. She is standing behind stacks of hard drives that were used to collect the data from the many telescopes involved in imaging the black hole. Katie is the computer scientist who created the algorithm that allowed this project to happen.

What Am I Seeing?
This image is a picture made from aiming all of these radio telescopes at one region of space called the Messier 87 galaxy. A black hole doesn't only suck in all of the light and everything else that comes within a certain range. It also bends the light that's coming toward it from all directions. At the same time, there's something called an accretion disk that circles around it. This is gas and matter that is moving so fast due to the black hole's gravity that it becomes superheated and glows.

The dark spot in the middle of the image is a shadow of what's called the event horizon. The event horizon of a black hole is the spherical point in space from which the power of the black hole overwhelms everything else, and from where nothing can escape once it crosses that point. The light that you see around the shadow area is a combination of the light being bent around the black hole and the accretion disk of material around it. Derek Muller, who runs an excellent YouTube science channel called Veritasium, explains this in detail.


What's Inside of the Event Horizon?
I'd say that the two most important aspects of human nature are our compassion and our curiosity. It's difficult to accept that there are some things we may never know. It is, from our perspective today, quite possible that we will never know what's beyond the event horizon of a black hole. No information that passes it can come back out. Our best science says that even the laws of physics that govern everything else in the universe are not applicable inside the event horizon. We have no framework in which to understand what might happen there. Even concepts such as time and space might have drastically different meanings beyond the event horizon.

If humans are to ever understand this question, it almost certainly won't happen within the lifetimes of anyone living today. But perhaps that's okay. There are certain boundaries that, like it or not, we may need to accept are impermeable on a bi-directional basis, and we can just live with that... but it probably won't stop us from trying to figure it out anyway. We are, after all, apes who want to know it all.