Every Part of a Black Hole, Explained

This very large star is right at the end of its fuel. Gravity finishes the job in about a second, and what's left behind gives off nothing at all. But gravity here is strong enough to bend light, so the far side of that ring arrives over the top of the shadow. Everything we know about black holes, we learned from whatever got too close. Particle jets, an accretion disk, a photon sphere, an ergosphere, the event horizon, and a singularity. Most of that isn't the black hole, cosmonauts. It's what the universe does when it gets too close. So, if there's no surface here, and nothing to stand on, what exactly is a black hole made of? Almost everything you've heard about black holes describes something that takes. This one throws. Charged gas in motion makes a magnetic field. Some of those lines thread right through the hole itself. Threaded through, they can't slip off. And a spinning black hole drags space around with it, so anything anchored in it winds up too. Then, it lets go. Two jets fire out along the poles. The only directions left open. They're leaving at about 99% of the speed of light. And that energy isn't coming from the gas. It's coming from the spin. The beam stays needle-thin the whole way out of the galaxy, and ends in radiolobes wider than most clusters of galaxies. Left alone, a black hole is almost impossible to find. There's just nothing there to detect, but they're rarely alone. Gas comes in from a nearby star, carrying angular momentum, a sideways swing it can't get rid of. Clumps on tilted orbits keep colliding. The up-and-down motion cancels. What survives is the motion going around. So the swarm flattens out into one plane, just like Saturn's rings. That's an accretion disk. Faster on the inside than the out. Much faster. The hole itself still gives off nothing. That light is gas, heated by its own fall, its friction. Neighboring rings scraping past each other, turning the energy of falling into heat. The inner edge runs at 100,000 degrees, and then the disk stops. Below this line, no stable orbit exists. Gas that crosses it doesn't circle anymore. It plunges, which is why the brightest objects in the universe aren't stars. Fusion converts less than 1% of a star's fuel. Falling into a spinning black hole releases tens of percent. So far, all of this has been matter falling in. Light does something stranger. Strip the gas away and send in a flat beam of light. How close a ray passes decides everything. Pass wide, and it bends a little. Closer, and it swings around the back. Closer still, and it never comes out. The photon sphere is where that switch happens. It sits 50% further out than the edge of the hole. Arrive at exactly that distance, and light neither escapes nor falls. It circles, but nothing collects there. It's a ridge, not a valley. That dark patch is a shadow, not the hole. It's 2.6 times wider, and it's made of every ray that passed too close. That edge is a photon ring, light that looped right around and came back. Its width depends on mass, not gas. One of the cleanest tests of Einstein we have. There's a second boundary out here. It's called the ergosphere, and it only exists because the hole is spinning. Its outer edge is the static limit. Stop the spin, and it collapses onto the horizon. Speed it up, and it swells right out. Out here, enough thrust holds you still. In there, you get dragged along anyway. Burn backwards at full power, and you still go forwards. And that's not thrust losing to a force. Standing still has stopped being a possible path. This still isn't the point of no return. Turn around, and you can leave. Penrose worked this out in 1969. You can leave with more than you brought, split something in two, and one half flies out with more energy than the hole arrived with. Nearly a third of a spinning black hole's energy is rotation, and you can watch that boundary shrink as it gets spent. Which is what those jets were, by the way. Nature just uses magnetic fields instead, draining the same battery. Now we reach the famous one, the event horizon. And the strange part is, there's nothing here. No surface, no shell. Cross it, and you'd feel nothing. The only way to show it is to send something in. This probe signals on a steady beat. For a hole like M87's, the fall takes 28 hours. From out here, it never arrives. The pulses come further apart, redder and fainter, and the image stalls at the edge. But it doesn't hang there. It fades out fast, straight through. That's what happened. What we watched was the view, not the event. The probe crossed at full speed and kept going, but its signals from inside never crawled out. They fell. Out here, you can fire light in any direction you like. And nearly all of it gets away. In there, every path bends back to the center, whichever way you aim. Light included. That's the boundary, really. Inside it, outward stops being a direction you can move in. The center isn't a place you travel to. It's a time you're already heading for. That leaves one last stop. The center. And the one part nobody has solved. Once the collapse starts, nothing stops it. Not even the pressure holding up a star. The equations follow it down to one point. Singularity. And there they report infinite density. And then they stop. And infinity isn't a discovery. It's a sign the theory has been pushed past where it works. So try again. Assume space isn't smooth, but grainy. Now it stops. A Planck star. A finite core. Held up by the graininess of space itself. And given long enough, it bounces back out. Or the inside was never there at all. A fuzzball is string theory's answer. No interior. Structure all the way through. And things land on it. Both are untested. And from outside, they look identical. One repairs the center. The other deletes both. Still unsolved, 60 years on. And if the fuzzball is right, the fall we just watched never happens. So, not matter. Shape. The disk. The ring. The jets. The drag. All of it is the universe reacting to a bend in space and time. The only part that might still be an object is the part we can't describe. So which would you rather be true? A bottom? Or no inside at all?