Of all of the impressive objects
you could visit on a galactic vacation,
black holes should be at the bottom of your list.
You would accelerate past the event horizon
and be spaghettified as the gravitational tidal forces
stretched you apart.
There's no coming back from that destination.
You couldn't even send back your vacay photos.
And yet, even these colossal monsters
are theorized to die a slow death by evaporation,
an ending no more noble than the demise of a puddle.
But when it comes to black holes,
that evaporation is so very different.
And our current understanding of that death
brings up a whole lot of problems.
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OK.
So what is a black hole?
In short, it's a region of space where
there's so much concentrated mass
that gravity is so strong that nothing escapes,
not even light.
But light always travels at the speed of light.
So how could you possibly capture it?
Well, Einstein's theory of general relativity
taught us that mass bends space itself.
And that's what we perceive as gravity.
So as light passes a huge object whose mass
has warped spacetime, it travels in a curved path.
Black holes warp spacetime so much
that the light's path could never
lead out of the black hole.
And once you pass beyond the event horizon, which
is located at a specific distance from the black hole's
singularity, that's where nothing could escape.
Except black holes aren't completely black.
They leak particles, and thus energy, very slowly.
This is known as Hawking radiation,
and it's what eventually leads to the black hole's
evaporation.
Hawking radiation was, of course, first
proposed by physicist Stephen Hawking, who described
it something like this.
Imagine at the event horizon a virtual particle-antiparticle
pair pop into existence, as they do in the vacuum of space,
Weird as it is, this happens.
Then one of the virtual particles
that has negative energy gets sucked into the black hole
and, by the energy-mass equivalence,
decreases the black hole's mass, while the other particle
with positive energy escapes.
If this happens over and over, the black hole
will disappear, particle by particle.
This picture is relatively simple and compelling,
but it's misleading and can lead to trouble if taken seriously.
For example, all this radiation would be emitted right
outside the event horizon.
To escape, it would have had to start with very high energy
to overcome the black hole's pull.
And these highly energetic particles
would distort the spacetime near the horizon.
And that doesn't fit with the overall theory.
So what's the real picture?
It's hard to say.
Things get weird near such massive objects.
Not only does their mass distort space,
but they can be spinning, which also stretches and warps
spacetime.
Let's consider the birth of a black hole.
Say a star, maybe 10 times the mass of our sun,
collapses in on itself.
If we imagine the empty spacetime
around the former star as a fabric,
the gravity of the black hole puts a deep, drastic well
in the fabric.
And it turns out, when spacetime warps
that much, what looked like a vacuum
before is now no longer empty.
It now has particles, at least to an outside observer.
And those particles appear smeared out
over a distance of up to twice the radius of the event
horizon.
Those particles are what an outside observer
would see as Hawking radiation.
Sounds crazy, right?
Vacuum emptiness is relative.
Depending on the strength of gravity
or even how fast you're accelerating,
you could hypothetically observe particles in one reference
frame and emptiness in another.
This is called the Unruh effect.
And around a black hole, these particles
have energy, which has to come from somewhere.
So it comes from the mass of the black hole as it evaporates.
We have yet to directly observe a black hole,
let alone its death.
Because the amount of radiation it emits
is so little it's completely drowned out
by the cosmic microwave background.
But beyond that, the lifetime of a black hole is enormous.
The universe is around 10 to the 10 years old,
and a black hole would take 10 to the 67 years-- up to 10
to the 100 years for supermassive black holes--
to evaporate.
So why do we care about this thing
that we can't even measure?
Well, the most exciting thing about Hawking radiation
is that it's at the intersection of the two most
important theories in modern physics, quantum
mechanics of the very small and general relativity
of the massive objects.
When you combine these theories, the math becomes nonsensical.
The existence of Hawking radiation
would mean we're one step closer to a theory that
could describe everything.
Thank you so much for watching, and happy physics-ing.
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I wanted to give a special thank you
to physicist Sabine Hossenfelder, who
advised on the really tough topics in this video.
She's got an awesome blog at backreaction.blogspot.com
that you should check out for some other really
cool theoretical physics.