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Hey Crazies.
I’ve been skeptical of Hawking radiation for a very long time.
If nothing can escape a black hole,
how could a black hole possibly radiate anything?
It’s ridiculous!
But after reading the scientific papers on the subject,
I’m kind of on board with it now.
Hawking radiation has a lot more in common with strings than particles.
Let’s get into this.
If you’ve heard of Hawking radiation before,
then you’ve probably heard the following explanation:
Particle/anti-particle pairs are popping in and out of existence everywhere all the time.
If one of these pairs forms on the event horizon of a black hole,
one partner inside and one outside,
then one can escape and the other can’t.
Without being able to recombine, they can’t annihilate,
which means they’ve popped into existence, but can’t pop back out.
The particle outside the event horizon needs a lot of energy to escape.
That energy must come from somewhere
and the only thing around is the black hole.
If the black hole loses energy, then it loses mass,
which means it shrinks
until eventually it’s gone.
Except this explanation leads to a lot of questions.
Shouldn’t the infalling particle add to the mass of the black hole?
But then the pair of particles came from nothing,
so is energy not conserved for black holes?
Why does the outside particle even have enough energy to escape?
These are the questions that always came to my mind.
It’s why I always thought Hawking radiation was bogus.
In fact, I had every intention of this video being a debunking extravaganza.
But then I read several scientific papers while researching
including Hawking’s original work,
the title of which is solid YouTube clickbait.
As it turns out, black holes do radiate away their mass.
It’s just that particles popping in and out of existence is a bad explanation.
This is not how Hawking radiation works.
Good ole Hawking made up an incorrect explanation
because he thought it was easier for the general public to swallow.
But the real explanation makes a lot more sense
and doesn’t raise so many questions.
As long as you understand two things:
black holes and quantum fields.
Quicky review time!
[Title Screen: "What is a black hole?"]
A black hole is what you get when you cram a lot of mass into a very small space.
They’re so black that we can only see them when they have material falling in
or when light-emitting stars are orbiting them.
Getting that much mass into such a small space is not easy,
which is why these only tend to only form at the end of a large star’s life.
The outer edges of the star get blasted away by the supernova,
but the core collapses in on itself.
If there’s enough mass in that core, it’ll collapse to form a black hole.
You mean it’ll collapse into a black hole, right?
No, I mean what I said. It’ll collapse to form one.
The collapsing core material is not the black hole.
The event horizon that forms around it is.
Black holes aren’t material things.
They’re spacetime itself.
The event horizon isn’t a solid surface.
It’s just a place we can’t see beyond.
It’s a collection of points in space and moments in time
that have been separated from the rest of the universe.
Once an event horizon forms, it doesn’t matter what happens to anything inside it.
Black holes are like spherical blemishes in spacetime.
Black balls in space!
They’ll bend the paths of anything inside that spacetime, including light.
An accretion disk is what happens to material as it falls into a black hole.
This is what it would look like if you try to see it from different angles.
If it can do that to light, image what it can do to a quantum field.
[Title Screen: "What is a quantum field?"]
[Dramatic exhale]
Ok, this one is tough and it’s definitely something I learned more recently.
When I feel motivated, I read through a little more of this textbook.
I’ve learned enough so far that I think I can explain what we need to know for this video.
Here’s the gist:
We like to imagine that particles are little bits of matter flying around through space.
But, in quantum field theory,
there’s just a field filling space with quantum properties.
Each point has a little uncertainty, but they all hover around zero for the most part.
A "particle" is just what it looks like when one of those points isn’t hovering around zero.
There’s a little concentration of energy there.
As that energy moves around, it looks like a particle is moving around.
That isn’t the whole story though.
It understates the weirdness that is a quantum field.
This field is filling space with quantum properties,
but what makes those properties "quantum" is that they’re probabilistic.
Every point in space is teeming with everything that could possibly happen there.
We only see a particle somewhere when we reduce those possibilities to one.
The thing is though, it’s possible for those non-particle points to become particles.
[Title Screen: "What do black holes do to quantum fields?"]
Considering all the possibilities at once in such an abstract way can be a bit overwhelming.
Unless you’re an Observer from Fringe.
So we try to lean on visuals that we already know.
Maybe you can try imagining all this empty space isn’t empty,
but is filled with particles:
virtual particles.
That’s where that popping in and out of existence idea came from.
It’s complete nonsense though.
We call them virtual because they’re not real.
But nonsense is ok,
if it’s helpful.
For Hawking radiation,
virtual particles are the opposite of helpful.
Too many questions.
We need better nonsense.
Believe it or not,
Hawking radiation has a lot more in common with strings than particles.
Consider a string stretched between two heavy blocks.
If you pluck the string, it’ll vibrate.
But some vibrational frequencies are easier for the string than others.
They’re the string’s natural frequencies,
also known as normal modes.
These are the ones where the wave loops fit nicely on the string.
There are an infinite number of them,
but the first several examples are the most dominant.
One loop, two loops, three loops, and so on.
Ooo yeah, real nice.
Anyway, they’re all available to the string.
Unless we pinch it.
If you pinch the center of the string, that point must remain stationary,
which eliminates some of the available frequencies.
You pinching the string changes what the string can do.
Black holes do something similar for quantum fields.
Now, rather than having a flickering box represent the point in space,
we’ll just take snapshots of it across time.
This pattern can be expressed by a combination of positive and negative wave modes.
The positive modes go forward in time and the negative modes go backward in time.
Hopefully, you can see now why a quantum field is kind of like a wave on a string.
Usually, nothing weird happens.
The positive and negative modes tend to cancel out.
When someone says virtual particles, this is actually what they’re talking about:
the quantum wave activity of the vacuum.
For contrast, a real particle looks like this
and a real anti-particle looks like this.
What does this have to do with Hawking radiation?
It’s all about that vacuum activity.
[Spring Noises]
Like I said, these vacuum modes usually cancel out,
but not when there’s a black hole.
The formation of a black hole kind of pinches the quantum field,
like your fingers did with a string.
Remember how pinching the string made some of the vibrational modes unavailable?
Well, the black hole does the same thing with the waves in the quantum field.
Some of the modes before the black hole’s formation aren’t available after.
By making part of the quantum field unavailable, the modes don’t cancel anymore.
The black hole has turned the vacuum into a particle-filled region of space.
The keyword there being "region."
These new particles are not localized to the event horizon.
They’re emerging from a large region of space around the black hole.
A region that’s a few times larger than the event horizon.
And, let me tell you,
it’s a lot easier for particles to escape from way out here
than it is for them to escape from in here.
They’re going pretty fast though, right?
Oh yeah, definitely.
Fast fast!
Hawking radiation is almost exclusively flowtons,
Flowtons. What’s a flowton?
Hawking radiation is almost exclusively photons,
otherwise known as light.
And that light has a gigantic wavelength.
It’s upwards around 80 times the radius of the black hole by the time it finally escapes.
That’s not visible light.
That’s extreme radio waves.
I’m serious when I say it isn’t localized.
These new particles can’t just come from nowhere though.
All particles have energy and that energy has to come from somewhere.
Conservation of energy shall not be violated!
And the only thing around is that black hole.
[Title Screen: "Black hole evaporation"]
Remember that a black hole is not a physical object.
It’s pure unadulterated spacetime curvature.
The energy for Hawking radiation comes from that curvature,
which means the curvature will decrease.
And, according to Einstein’s equation, less curvature means less mass.
A radiating black hole will lose mass and shrink.
But, unless there are some micro black holes roaming around,
not a single black hole in the universe has begun shrinking yet.
They’re emitting Hawking radiation so slowly,
that what they’re eating still makes them grow.
Even the isolated ones are eating the cosmic microwave background fast enough to grow.
Huh. It’s no wonder we haven’t detected it yet.
In the unimaginable far future,
the universe will cool off enough to allow black holes to shrink.
It’ll start slowly, but speed up as it shrinks.
The energy of the radiation will grow until eventually it’s visible.
In the black hole’s final moments, it’ll erupt violently
with massive particles, not just photons.
Black hole explosions indeed, Mr. Hawking.
When this eruption happens depends on the black hole’s size once it starts to shrink.
For stellar mass black holes,
it’ll take an octodecillion times the current age of the universe.
That’s 10 to the 67 years.
For supermassive black holes,
it could easily take a million googol years.
The bigger it is, the slower it evaporates.
No matter what though, all black holes will evaporate eventually.
But not because particles are popping in an out of existence at their event horizons.
Just like pinching a string makes vibrational modes unavailable,
a black hole kind of pinches spacetime,
which makes quantum modes unavailable.
Doing so, the quantum vacuum doesn’t cancel itself anymore
and you get particles.
Those particles, which are almost certainly photons,
are called Hawking radiation.
And that radiation carries energy away from the black hole.
Eventually, so much energy will get carried away
that the black hole completely disappears.
A black hole might be a blemish in spacetime,
but it has a mechanism to correct itself.
Which is kind of beautiful actually.
So did this help you understand Hawking radiation a little better?
Let us know in the comments.
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Hey! What about Mars in Venus’s orbit?!
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Basically, nothing would change.
Anyway, thanks for watching!