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Hey Crazies.
You’ve been asking for a good solid video on black holes for a while
and there’s a reason I waited until now to do it.
I had to cover some relativity stuff first or this video wouldn't have made any sense.
Anyway, here we go!
It seems pretty natural to start this at the beginning.
No no! Too far!
Not the beginning of the universe, the beginning of a black hole.
A black hole forms when you get enough mass into a small enough volume
and the best mechanism the universe has for that is gravity.
Say you’ve got a star with a sufficiently large mass.
Anything greater than 20 times the mass of our Sun ought to do it.
Now, not all of that mass is going to form a black hole.
Only the stellar core will, which is only about 3 times the mass of our Sun.
The other 17 solar masses are blasted into space by a supernova.
At the end of a large star’s life, the core is mostly made of iron,
but it doesn’t stay iron for much longer.
The core’s own gravity collapses it into a neutron star.
That’s a ball of neutrons about the size of a small city.
Neutrons are capable of exerting some outward pressure thanks to quantum mechanics,
but this thing is just too massive for it to do any good.
The neutron star continues to collapse under its own gravity.
Eventually, the mass is compressed into such a small volume that
the space-time around the neutron star is curved to an extreme.
A perfectly-black bubble forms around it called an event horizon.
This is a black hole.
Except, it’s not actually a hole.
This graphic is not what a black hole looks like.
Ignore these.
A black hole is a sphere.
It’s a ball.
But it’s so black, you can’t tell that it’s three dimensional.
It just looks like a circle from all directions.
Black holes have been a part of general relativity since it was published in 1915,
but the term “black hole” wasn't coined until the mid-1960s.
Black ball isn’t nearly as catchy, exciting, or scary.
I mean, tell me what you think:
Black balls in space!
vs
Black holes in space!
Look, I’m not saying black holes aren’t scary.
Because they are.
They’re terrifying!
But only because they let you get closer than usual.
Once those outer layers of the star get blasted away, the collapse
of the core has no effect on the gravity outside the original core’s volume.
If anything is still orbiting, it will keep orbiting.
Nothing is going to suddenly get sucked in just because it’s a black hole now.
Black holes don’t suck!
All the weirdness that happens around a black hole
happens inside the volume of the original core.
As the core collapses, it makes more space accessible around itself.
You can get a lot closer to that mass than you could before.
Now, before we move forward, we’re going to make a big assumption.
For simplicity, we’re going to assume this black hole is not rotating.
Yes, I know that’s not very realistic.
But rotating black holes really deserve their own video.
Making that assumption, the first thing you’re going to notice
is a sphere of orbiting light, which we naturally call a photon sphere.
Yeah, you heard me right!
Light.
The thing that always travels the straightest path possible through space-time
can orbit the black hole at this place.
The diameter of this sphere is 1.5 times the diameter of the event horizon.
That’s about 16.5 miles across for our 3 solar-mass black hole.
Outside that sphere, stable orbits are possible.
Inside, orbits are destined to collapse into the black hole.
The next thing you’ll notice is that perfectly-black and terrifying event horizon.
Let’s talk a little more about that.
The size of the event horizon is called the Schwarzschild radius.
For our 3 solar-mass black hole, it’s about 5.5 miles.
Meaning it’s about 11 miles across, which is a very small space to have 3 solar-masses
crammed into.
And, as far as general relativity is concerned, this is the lower limit for the size of a
black hole.
The more mass you cram into a black hole, the bigger the event horizon.
Black holes grow as they eat.
Why do they call it a horizon?
Excellent question! It's actually a very good name.
A horizon is a boundary we can’t see beyond.
Kind of like the horizon when you stand on the Earth.
The curvature of the Earth has hidden those places from view.
Likewise, an event horizon is a boundary hiding events from view.
As in space-time events.
Yes, the same kind of events we’ve been talking about in these relativity videos.
To the space-time diagram!
Right before the neutron star turns into a black hole, it’s moving forward in time.
But these axes don’t belong to that neutron star.
They belong to a distant observer, like maybe us on or near the Earth.
To form a black hole, the neutron star must collapse inside this boundary.
A boundary that will eventually be the event horizon, but isn’t yet.
Speaking of which, once that event horizon forms, we call that
the outside edge of black hole instead of the matter still collapsing behind it.
Here’s why.
As the last bit of neutron star collapses inside the boundary,
the event horizon comes into being.
Everything inside it is immediately separated from the rest of the universe.
It’s no longer part of the normal flow of time.
Once the event horizon forms, none of the "now" that happens inside it
will ever be part of our "now."
A black hole is what it looks like when events have been completely severed from the universe.
So, in a way, it's kind of like a hole.
It’s a piece of the universe that’s missing.
So what’s a black hole?
A black hole is a black spherical object with the mass of at least 3 Suns.
That mass is so compressed and the space-time around it so curved
that all events inside its horizon are forever separated from the universe.
A black hole is a missing piece of space-time.
So, got any questions about black holes?
Please ask in the comments.
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Oh, one more thing.
Thanks to continued support from patrons on Patreon,
I’ll be at VidCon next week.
If any of you are there and see me, don’t hesitate to come up and say hi.
If you’re not, that’s cool.
I’m hoping to post a vlog about it in early July.
Anyway, thanks for watching!