Hawking radiation

The difference in the quantum vacuum near the edge versus far away would lead to a continuous emission of black body radiation.

This is known today as Hawking radiation, in his honor.

His calculations showed that this radiation would be composed almost entirely of photons, it would be inversely proportional to the black hole's mass, and would be emitted in a timeframe proportional to the mass of the black hole cubed.

This meant that black holes would, over time, eventually completely evaporate via

Hawking radiation.

This presented a problem because now it seemed that our book and everything else containing information that ever fell into the black hole would...

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eventually be emitted as photons.

When Stephen Hawking first proposed his theory of black holes emitting radiation in 1975, it was a monumental breakthrough.

Hawking radiation, as it came to be known, suggested that black holes are not completely black.

In fact, they emit faint radiation consisting of photons and potentially neutrinos and other particles.

But what if I told you that even Hawking's own book didn't accurately describe what's really going on? There's one crucial aspect of this theory that has become a bit too simplified in popular descriptions, specifically the idea that Hawking radiation comes from the spontaneous creation of virtual particle-antiparticle pairs near the event horizon of a black hole.

While this explanation is catchy and relatively easy to visualize, it turns out to be wrong.

or at least an egregious oversimplification of the actual quantum processes.

To understand the reality, we need to dig a little deeper into the physics of Hawking radiation and see what's really going on and why.

We're going to find the real explanation for Hawking radiation, what the mechanism is, and why it happens.

Exactly.

That escaping particle is what we now call Hawking radiation.

Okay, I've definitely heard of Hawking radiation.

Right.

And because of this process, black holes aren't entirely black.

Definitely not.

I mean, the glow has to be unfathomably faint, and these objects are thousands of light-years away.

The direct observation of Hawking radiation from an astrophysical black hole is currently beyond our technological capabilities.

As it turns out, what gives is the nature of the vacuum and, in fact, the notion of what a particle is becomes observer-dependent.

In coming episodes, we'll be delving into Hawking radiation and the Unruh effect, both of which are very similar and are results of this observer-dependent vacuum.

There are a couple of compelling but inaccurate explanations of Hawking radiation around, and here at Space Time, we wanna be as simple as we can, but no simpler.

So we're gonna take this episode to lay the final stepping stone we need to examine these effects in detail and accurately.

To get at this idea of observer-dependent particles and vacua, we're going to need some quantum field theory, and we're gonna need to draw heavily on this recent episode.

For example, in the Casimir effect.

Now, we've already seen how this effect reduces the energy of the vacuum between conducting plates.

In upcoming episodes, we'll apply these ideas to finally understand Unruh and Hawking radiation.

Empty space would look hot.

Lasinga makes a good point about the pop sci explanation of Hawking radiation.

If the event horizon swallows one half of a virtual matter/antimatter pair-

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Eh, then how does the black hole lose mass? After all, both matter and antimatter have positive mass.

So this is a good example of why these pop sci explanations can be more misleading than helpful.

In fact, as far as I know, there's no derivation of Hawking radiation that invokes the splitting of particle/antiparticle pairs, so it's not clear how meaningful that picture even is.

Very soon we'll be talking about the exact line of reasoning that Stephen Hawking used to hypothesize his radiation.

Our tribute is to bring you Stephen Hawking's most famous discovery.

I'm Matt O'Dowd, this is Space Time, and it's time for Hawking Radiation.

[upbeat music plays] Soon after Einstein revealed his great general theory of relativity in 1915, physicists realized that it allowed for the possibility of catastrophic gravitational collapse.

In places of extreme density like the dead core of a massive star, space and time could be dragged inwards to create a hole in the universe, a boundary in space time called an event horizon that could be entered, but from beyond which nothing could return.

Or so we thought, until 1974 when a young physicist named Stephen Hawking published a paper in Nature entitled Black Hole Explosions? In this, and in a follow-up 1975 paper, he attempted a new union of quantum mechanics and general relativity to show that black holes should not be so black after all.

They should leak.

They should emit what we now know as Hawking radiation.

There's a popular description of how Hawking radiation works.

Before they get to us, they'll find themselves in a patch of space that is moving away from us faster than the speed of light.

That horizon should produce a type of Hawking radiation, but its wavelength would be comparable to the distance to that horizon, so it's completely undetectable.

On the other hand, during the inflationary epoch in the extremely early universe, the cosmic event horizon was very close to every point.

The inflating universe should have been bathed in intense Hawking radiation.

Alex Korosntsev notes that the Bremsstrahlung radiation created close to the Schwarzschild radius of a kugelblitz might create the Zitterbewegung effect.

Nice.

Understanding how black holes launch these relativistic jets remains a crucial unsolved problem in astrophysics.

Hawking radiation represents a theoretical prediction that black holes are not entirely black, but emit radiation due to quantum effects near the event horizon.

Sometimes one particle falls into the black hole while its partner escapes.

The escaped particle becomes detectable radiation, carrying energy away from the black hole.

The smaller a black hole becomes, the hotter it gets and the faster it evaporates.

A stellar mass black hole would take far longer than the age of the universe to evaporate.

however primordial black holes formed in the early universe and small enough to have evaporated by now should produce detectable radiation no such radiation has been observed placing limits on the numbers of very small primordial black holes hawking radiation remains theoretical never directly observed, yet it represents a profound connection between gravity and quantum mechanics.

The information paradox presents one of the deepest puzzles in theoretical physics.