This episode was made possible by generous supporters on Patreon.
Hey Crazies.
Black holes are insane!
Did you know they can spin?
What?!
I know, right?!
Let's compare and contrast.
According to general relativity, a non-rotating black hole has two parts:
An event horizon, a boundary we can't see beyond
and a physical singularity, a point of infinite density and infinite curvature.
A rotating black hole has two event horizons and ring singularity, or ringularity,
which is a ring of infinite density and curvature.
Now, all of this might leave you wondering.
How can anything possibly have infinite density?
And, wait, two event horizons? What does that even mean?!
All of which are, not only valid questions, but they're good ones.
So, let's keep something in mind.
Unless it can be experimentally verified, you probably shouldn't call it science.
Having points or rings of infinite density is fun math, but cannot be reality.
So all this really shows us is a place where the math doesn't match our universe.
Not where the universe is broken.
Whether the black hole is rotating or not,
our clear scientific knowledge ends at the event horizon.
We'll be ignoring any results the math gives us inside it.
That being said, anything outside the horizon is fair game
and there are three things we can detect there.
Well, at least if we ignore quantum mechanics.
One: mass or better yet energy,
two: charge,
and three: angular momentum.
Mass is the easiest thing to detect about a black hole.
It shows up in the curvature of spacetime near a black hole,
and, farther away, we can sometimes see stars orbiting it
like the supermassive black hole at the center of our galaxy.
Charge is detectable in a similar same way.
It leaves an electric field outside the event horizon.
Full disclosure though: while charged black holes are possible, they're not very likely.
They would quickly attract a bunch of opposite charge and become neutral.
The same cannot be said for angular momentum or spin.
Unlike charge, spin is something black holes definitely have.
How do we know that?
Conservation principles!
Just like the amount of energy and the amount of charge stay the same over time,
so does angular momentum.
You've seen this before with figure skaters.
If they bring their mass in closer to the xis of rotation, they speed up.
Stars are spinning too.
When their cores collapse into black holes they speed up just like the figure skater.
And a black hole is so much smaller than the stellar core that made it.
Tiny by comparison.
They go from the size of a star to the size of small city.
That is a dramatic change in size,
which means there's a dramatic change in rotational speed.
How fast are they spinning?!
Fast fast!
Let's say we're dealing with a small one, like, 3 times the mass of the Sun.
Its event horizon will be spinning at about 1/10th of a percent the speed of light.
That might not sound like much, but it's around 1 million miles per hour.
Fast fast!
And those are the slow ones.
If they've been around for a while, they can absorb angular momentum from the stuff they eat
and they'll speed themselves up over time.
The supermassive one at the center of our galaxy is spinning at over 30% the speed of light.
There is an upper limit though.
They seem to max out at the speed of light,
which makes sense since that's the universal speed limit.
Also, if a black hole spun any faster, the event horizon would disappear,
exposing the physical singularity to the rest of the universe.
It would be called a naked singularity if it could happen,
which it can't.
Anyway, if you want to know more about how we actually measure this speed,
Veritasium did a great video on this recently.
I'm not going to talk about it.
I'm not an experimentalist.
Hey, you're getting side tracked. Don't forget to talk about the ergosphere.
Oh! Right right. I almost forgot. Thanks Nerd Clone.
The ergosphere is a weird region of space just outside the event horizon.
Here's the outer edge of it compared to the event horizon itself.
But to really understand what happens in there, we need the spacetime metric.
This is the familiar metric around a non-rotating black hole.
Like any metric, it's basically just Pythagorean Theorem for 4-dimensional spacetime.
If you're subscribed and have been watching for a while, you've seen this before.
Subscribe!
Are you ready?
This is the metric around a rotating black hole.
Angular momentum really messes everything up.
It's nasty!
Thankfully, we don't need to do anything crazy math with that thing.
We just need to look for some patterns.
This thing here is in a denominator, so when it's zero, this fraction blows up to infinity.
So this tells us where the event horizon is.
This term goes to zero before we get to the event horizon.
Where that happens is the outer edge of the ergosphere.
If the math is getting overwhelming, just remember this diagram from earlier.
The ergosphere is outside the event horizon
and, let me tell you, some weird [beep] happens in there.
As the black hole spins, it drags the surrounding spacetime along with it
as well as any matter and light in that spacetime.
We call it frame dragging because your entire frame of reference can get dragged along
with the black hole's spin.
Inside the ergosphere, your frame is dragged so much that,
that it's physically impossible for you to sit still, no matter how much thrust you have.
Stationary paths just aren't possible.
Hmm, this might make more sense with a spacetime diagram.
If we include time in our picture, a stationary path looks like this.
Near a non-rotating black hole, you can maintain that with a good amount of thrust.
But if the black hole is rotating, it doesn't matter how much thrust you have
or even what direction you point that thrust inside the ergosphere.
You're going to rotate with the black hole no matter what.
Being in there does give you an opportunity though.
You're still outside the event horizon, so you can escape with enough thrust.
If you go in on just the right path and break off part of your ship just the right way,
you could escape with more energy then you had when you entered.
Does that create energy?
No, no, don't be ridiculous!
Conservation of energy shall not be violated!
The only possibility is that you took energy from the black hole.
The black hole would slow down a little bit, losing rotational energy.
We already know that nothing can escape the event horizon,
so this rotational energy must be in the spacetime outside it.
And I mean all of it.
We can keep doing this until we've taken all the black hole's rotational energy.
In the end, it'll be a non-rotating black hole.
So how do rotating black holes work?
Rotating black holes have an event horizon just like non-rotating ones.
It's just that the rotating event horizon is smaller and flatter.
But what makes rotating black holes special is the ergosphere.
It's a weird place where your entire frame of reference is dragged along with the rotation of the black hole
and, since that rotational energy is outside the event horizon,
we can steal it from the black hole.
After all the stars in the universe stop burning and all the white dwarfs cool into black dwarfs,
Rotating black holes will probably be the only source of energy left in the universe.
So, do you think humans will survive long enough to be a black hole civilization?
Please share your thoughts in the comments.
Thanks for liking and sharing this video.
A special thanks goes out to Patreon patrons like LT Marshall Faulds
who help make this show possible.
Don't forget to subscribe if you'd like to keep up with us.
And until next time, remember, it's OK to be a little crazy.
The featured comment comes from Rock Shop who asked:
If the energy comes from the field, why does the battery matter?
Because the electric current is the reason the energy transfer is happening
and you don't get a current without the electric field from the battery.
There's no free lunch.
All debts must be repaid.
Anyway, thanks for watching!
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