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. APlanck 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 10-35meters. That’s one onehundred 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
theergosphere, 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…
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.
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.
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
Disk of DOOOOOM!
Image credit: NASA/CXC
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.
Having said that, let me ask you a question:
if I were to take the Sun and replace it with 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
I’m right there with you, dude.
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 everincreasingly 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.
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 quadrillionkilometers), 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.
Hmmm.
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 π 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
togethereight 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 expertRoger 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.