10 things you don’t know about black holes

Written by Phil Plait

Well, they’re black, and they’re like bottomless holes. What would you call them?
-Me, when a friend asked me why they’re named what they are

Ah, black holes. The ultimate shiver-inducer of the cosmos, out-jawing sharks, out-ooking spiders, out-scaring… um, something scary. But we’re fascinated by ’em, have no doubt – even if we don’t understand a whole lot about them.

But then, that’s why I’m here. Allow me to be your tour guide to infinity. Or the inverse of it, I suppose. Since it’s Halloween this seems appropriate… and my book Death from the Skies! just came out, and there’s lots of ways a black hole can destroy the Earth. Mwuhahahaha.

So below I present ten facts about black holes – the third in my series of Ten Things You Don’t Know (the first was on the Milky Way; the second about the Earth). Regular readers will know a few of these since I’ve talked about them before, but I’m hoping you don’t know all of these. And if you do, then feel free to leave a comment preening about your superior intellect. Mind you, this list is nowhere near complete: I could have picked probably 50 things that are weird about black holes. But I like these.

1) It’s not their mass, it’s their size that makes them so strong.

OK, first, a really quick primer on black holes. Bear with me!

The most common way for a black hole to form is in the core of a massive star. The core runs out of fuel, and collapses. This sets off a shockwave, blowing up outer layers of the star, causing a supernova. So the star’s heart collapses while the rest of it explodes outwards (this is the Cliff’s notes version; for more details on the process – which is way cool, so you should read it – check out my description of it).

As the core collapses, its gravity increases. At some point, if the core is massive enough (about 3 times the mass of the Sun), the gravity gets so strong that right at the surface of the collapsing core the escape velocity increases to the speed of light. That means that nothing can escape the gravity of this object, not even light. So it’s black. And since nothing can escape, well, read the quotation at the top of the page.

The region around the black hole itself where the escape velocity equals the speed of light is called the event horizon. Any event that happens inside it is forever invisible.

OK, so now you know what one is, and how they form. Now, I could explain why they have such strong gravity, but you know what? I’d rather let this guy do it. I hear he’s good.

So there you go. Sure, the mass is important, but sometimes it’s the little things that count.

2) They’re not infinitely small.

So OK, they’re small, but how small are they?

I was writing about black holes in my previous job, and we got in a fun discussion over just what we meant by black hole: did we mean the object itself that collapses down to a mathematical point, or the event horizon surrounding it? I said the event horizon, but my boss said it was the object. I decided she had a point (HAHAHAHAHA! A “point”! Man, I kill me), and made sure that when I wrote about the event horizon versus the black hole itself I was making myself clear.

Like I said above, to the collapsing core, its clock keeps ticking, so it sees itself collapsing all the way down to a point, even if the event horizon has some finite size.

What happens to the core? The actual mass that collapsed?

Out here, we’ll never know for sure. We can’t see in, and it sure enough isn’t gonna send any info out. But our math in these situations is pretty good, and we can at least apply them to the collapsing core, even when it’s smaller than the event horizon.

It will continue to collapse, and the gravity increases. Smaller, smaller… and when I was a kid I always read that it collapses all the way down to a geometric dot, an object with no dimensions at all. That really bugged me, as you can imagine… as well it should. Because it’s wrong.

At some point, the collapsing core will be smaller than an atom, smaller than a nucleus, smaller than an electron. It’ll eventually reach a size called the Planck Length, a unit so small that quantum mechanics rules it with an iron fist. A Planck Length is a kind of quantum size limit: if an object gets smaller than this, we literally cannot know much about it with any certainty. The actual physics is complicated, but pretty much when the collapsing core hits this size, even if we could somehow pierce the event horizon, we couldn’t measure its real size. In fact, the term “real size” doesn’t really mean anything at this kind of scale. If the Universe itself prevents you from measuring it, you might as well say the term has no meaning.

And how small is a Planck Length? Teeny tiny: about 10-35 meters. That’s one one-hundred quintillionth the size of a proton.

So if someone says a black hole has zero size, you can be all geeky and technical and say, not really, but meh. Close enough.

3) They’re spheres. And they’re definitely not funnel shaped.

The gravity you feel from an object depends on two things: the object’s mass, and your distance from that object. This means that anyone at a given distance from a massive object – say, a million kilometers – would feel the same force of gravity from it. That distance defines a sphere around an object: anyone on that sphere’s surface would feel the same gravity from the object at the center.

The size of an event horizon of a black hole depends on the gravity, so really the event horizon is a sphere surrounding the black hole. From the outside, if you could figure out how to see the event horizon in the first place, it would look like a pitch black sphere.

Some people think of black holes as being circles, or worse, funnel-shaped. The funnel thing is a misconception from people trying to explain gravity as a bending in space, and they simplify things by collapsing 3D space into 2D; they say the space is like a bed sheet, and objects with mass bend space the same way that a massive object (a bowling ball, say) will warp a bed sheet. But space is not 2D, it’s 3D (even 4D if you include time) and so this explanation can confuse people about the actual shape of a black hole event horizon.

I’ve had kids ask me what happens if you approach a black hole from underneath! They sometimes don’t get that black holes are spheres, and there is no underneath. I blame the funnel story. Sadly, it’s the best analogy I’ve seen, so we’re stuck with it. Use it with care.

4) Black holes spin!

It’s kind of an odd thought, but black holes can spin. Stars rotate, and when the core collapses the rotation speeds way, way up (the usual analogy is that of an ice skater who brings in his arms, increasing his rotation rate). As the core of the star gets smaller it rotates more rapidly. If it doesn’t quite have enough mass to become a black hole, the matter gets squeezed together to form a neutron star, a ball of neutrons a few kilometers across. We have detected hundreds of these objects, and they tend to spin very rapidly, sometimes hundreds of times a second!

The same is true for a black hole. Even as the matter shrinks down smaller than the event horizon and is lost to the outside Universe forever, the matter is still spinning. It’s not entirely clear what this means if you’re trying to calculate what happens to the matter once it’s inside the event horizon. Does centrifugal force keep it from collapsing all the way down to the Planck length? The math is fiendish, but do-able, and implies that matter falling in will hit matter inside the event horizon trying to fall further but unable to due to rotation, This causes a massive pile up and some pretty spectacular fireworks… that we’ll never see, because its on the other side of infinity. Bummer.

5) Near a black hole, things get weird

The spin of the black hole throws a monkey in the wrench of the event horizon. Black holes distort the fabric of space itself, and if they spin that distortion itself gets distorted. Space can get wrapped around a black hole – kind of like the fabric of a sheet getting caught up in a rotating drill bit.

This creates a region of space outside the event horizon called the ergosphere. It’s an oblate spheroid, a flattened ball shape, and if you’re outside the event horizon but inside the ergosphere, you’ll find you can’t sit still. Literally. Space is being dragged past you, and carries you along with it. You can easily move in the direction of the rotation of the black hole, but if you try to hover, you can’t. In fact, inside the ergosphere space is moving faster than light! Matter cannot move that fast, but it turns out, according to Einstein, space itself can. So if you want to hover over a black hole, you’d have to move faster than light in the direction opposite the spin. You can’t do that, so you have to move with the spin, fly away, or fall in. Those are your choices.

I suggest flying away. Fast. Because…

6) Approaching a black hole can kill you in fun ways. And by fun, I mean gruesome, horrifying, and really really ookie.

Sure, if you get too close, plop! You fall in. But even if you keep your distance you’re still in trouble…

Black hole, down the drain

Gravity depends on distance. The farther you are from an object, the weaker its gravity. So if you have a long object near a massive one, the long object will feel a stronger gravitational force on the near end versus a weaker force on the far end! This change in gravity over distance is called the tidal force (which is a bit of a misnomer, it’s not really a force, it’s a differential force, and yes, it’s related to why we have ocean tides on Earth from the Moon).

The thing is, black holes can be small – a BH with a mass of about three times the Sun has an event horizon just a few kilometers across – and that means you can get close to them. And that in turn means that the tidal force you feel from one can get distressingly big.

Praying to this guy won’t help.

Let’s say you fall feet first into a stellar-mass BH. It turns out that as you approach, the difference in gravity between your head and your feet can get huge. HUGE. The force can be so strong that your feet get yanked away from your head with hundreds of millions of times the force of Earth’s gravity. You’d be stretched into a long, thin strand and then shredded.

Astronomers call this spaghettification. Ewwww.

So getting near a black hole is dangerous even if you don’t fall in. Evidently, there really is a tide in the affairs of men.

7) Black holes aren’t always dark

The thing is, black holes can kill from a long way off.

Disk of DOOOOOM!
Image credit: NASA/CXC

Matter falling into a black hole would rarely if ever just fall straight in and disappear. If it has a little bit of sideways motion it’ll go around the black hole. As more matter falls in, all this junk can pile up around the hole. Because of the way rotating objects behave, this matter will create a disk of material whirling madly around the hole, and because the gravity of the hole changes so rapidly with distance, matter close in will be orbiting much faster than stuff farther out. This matter literally rubs together, generating heat through friction. This stuff can get really hot, like millions of degrees hot. Matter that hot glows with intense brightness… which means that near the black hole, this matter can be seriously luminous.

Worse, magnetic and other forces can focus two beams of energy that go plowing out of the poles of the disk. The beams start just outside the black hole, but can be seen for millions or even billions of light years distant.

They’re bright.

In fact, black holes that are eating matter in this way can glow so brightly that they become the brightest continuously-emitting objects in the Universe! We call these active black holes.

And as if black holes aren’t dangerous enough, the matter gets so hot right before it makes the final plunge that it can furiously emit X-rays, high-energy forms of light (and the beams can emit even higher energy light than that). So even if you park your spaceship well outside the event horizon of a black hole, if something else falls in and gets shredded, you get rewarded by being fried by the equivalent of a gazillion dental exams.

I may have mentioned this: black holes are dangerous. Best to stay away from them.

8) Black holes aren’t always dangerous.

I’m right there with you, dude.

Having said that, let me ask you a question: if I were to take the Sun and replace it with Folgers crystals a black hole of the exact same mass, what would happen? Would the Earth fall in, be flung away, or just orbit like it always does?

Most people think the Earth would fall in, sucked inexorably down by the black hole’s powerful gravity. But remember, the gravity you feel from an object depends on the mass of the object and your distance from it. I said the black hole has the same mass as the Sun, remember? And the Earth’s distance hasn’t changed. So the gravity we’d feel from here, 150 million kilometers away, would be exactly the same! So the Earth would orbit the solar black hole just as nicely as it orbits the Sun now.

Of course, we’d freeze to death. You can’t have everything.

9) Black holes can get big.

Q: What happens if two stellar-mass black holes collide?

A: You get one bigger black hole.

You can extrapolate from there. Black holes can eat other objects, including other black holes, so they can grow. We think that early on in the Universe, when galaxies were just forming, matter collecting in the center of the nascent galaxy can collapse to form a very massive black hole. As more matter falls in, the hole greedily consumes it, and grows. Eventually you get a supermassive black hole, one with millions or even billions of times the mass of the Sun.

However, remember that as matter falls in it can get hot. It can be so hot that the pressure from light itself can blow off material that’s farther out, a bit like the solar wind but on a much grander scale. The strength of the wind depends on many things, including the mass of the black hole; the heftier the hole, the windier the, uh, wind. This wind prevents more matter from falling in, so it acts like a cutoff valve for the ever-increasingly girthy hole.

Not only that, but over time the gas and dust around the black hole (well, pretty far out, but still near the center of the galaxy) gets turned into stars. Gas can fall into a black hole more easily than stars (if gas clouds collide head-on their motion relative to the black hole can stop, allowing them to fall in; stars are too small and too far apart for this to happen). So eventually the black hole stops consuming matter because nothing more is falling into it. It stops growing, the galaxy becomes stable, and everyone is happy.

Don’t panic!
OK, maybe a little.

In fact, when we look into the Universe today, we see that pretty much every large galaxy has a supermassive black hole in its heart. Even the Milky Way has a black hole at its core with a mass of four millions times that of the Sun. Before you start running around in circles and screaming, remember this: 1) it’s a long way off, 26,000 light years (260 quadrillion kilometers), 2) its mass is still very small compared to the 200 billion solar masses of our galaxy, and therefore 3) it can’t really harm us. Unless it starts actively feeding. Which it isn’t. But it might start sometime, if something falls into it. Though we don’t know of anything that can fall into it soon. But we might miss cold gas.


Anyway, remember this as well: even though black holes can cause death and destruction on a major scale, they also help galaxies themselves form! So we owe our existence to them.

10) Black holes can be low density.

Of all the weirdnesses about black holes, this one is the weirdest to me.

As you might expect, the event horizon of a black hole gets bigger as the mass gets bigger. That’s because if you add mass, the gravity gets stronger, which means the event horizon will grow.

If you do the math carefully, you find that the event horizon grows linearly with the mass. In other words, if you double the black hole’s mass, the event horizon radius doubles as well.

That’s weird! Why?

The volume of a sphere depends on the cube of the radius (think way back to high school: volume = 4/3 x ? x radius3). Double the radius, and the volume goes up by 2 x 2 x 2 = 8 times. Make the radius of a sphere 10 times bigger and the volume goes up by a factor of 10 x 10 x 10 = 1000.

So volume goes up really quickly as you increase the size of a sphere.

Now imagine you have two spheres of clay that are the same size. Lump them together. Is the resulting sphere twice as big?

No! You’ve doubled the mass, but the radius only increases a little bit. Because volume goes as radius cubed, to double the radius of your final clay ball, you’d need to lump together eight of them.

But that’s different than a black hole. Double the mass, double the size of the event horizon. That has an odd implication…

Density is how much mass is packed into a given volume. Keep the size the same and add mass, and the density goes up. Increase the volume, but keep the mass the same, and the density goes down. Got it?

So now let’s look at the average density of matter inside the event horizon of the black hole. If I take two identical black holes and collide them, the event horizon size doubles, and the mass doubles too. But volume has gone up by eight times! So the density actually decreases, and is 1/4 what I started with (twice the mass and eight times the volume gives you 1/4 the density). Keep doing that, and the density decreases.

A regular black hole – that is, one with three times the Sun’s mass – with have an event horizon radius of about 9 km. That means it has a huge density, about two quadrillion grams per cubic cm (2 x 1015). But double the mass, and the density drops by a factor of four. Put in 10 times the mass and the density drops by a factor of 100. A billion solar mass black hole (big, but we see them this big in galaxy centers) would drop that density by a factor of 1 x 1018. That would give it a density of roughly 1/1000 of a gram per cc… and that’s the density of air!

A billion solar mass black hole would have an event horizon 3 billion km in radius – roughly the distance of Neptune to the Sun.

See where I’m going here? If you were to rope off the solar system out past Neptune, enclose it in a giant sphere, and fill it with air, it would be a black hole!

That, to me, is by far the oddest thing about black holes. Sure, they warp space, distort time, play with our sense of what’s real and isn’t… but when they touch on the everyday and screw with that, well, that’s what gets me.

I first thought of this at a black hole conference at Stanford a few years back. I was walking with noted black hole expert Roger Blandford when it hit me. I did a quick mental calculation to make sure I had the numbers right, and related to Roger that a solar system full of air would be a black hole. He thought about it for a moment and said, “Yes, that sounds about right.”

And that, me droogs, was one of the coolest moments of my hole life. But thinking about it still makes my brain hurt.


Well, what can I say? Black holes are weird.

As it so happens, there was a lot more that could be said about them, of course. What about wormholes? What about how they form? what about Hawking radiation? Can black holes totally evaporate?

You can find answers to these and other questions elsewhere on the web (and even on this very blog); I couldn’t cover everything in just ten sections! But I’ll note (shocker) that chapter 5 of my book Death from the Skies! talks in detail about how they form, and what they can do if you get too close to them. Later chapters also talk about the black hole in the core of the Milky Way, and what will happen to black holes a long time from now… literally, 1060, 1070, even a googol years from now.

But even then, that’s not the scariest thing about black holes. I almost didn’t put this in the post, it’s so over the top mind-numbingly horrifying. But I’m a scientist, and we’re skeptics here, so we can take it. So I present to you, the worst thing about black holes of all:

“If there’s any justice at all, the black hole will be your grave!”

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