(bright music) - Of all 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 spaghettfied as the gravitational tidal forces stretched you apart.

There's no coming back from that destination.

You couldn't even send back your vaycay 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.

Okay, 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 have 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 Steven 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've 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 hundred 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.

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.