1. The Fate of Black Holes

2. The Obvious Question
Once matter enters a black hole, is it fated never to reappear?
The answer seemed obvious – a clear “yes” – until about forty years ago.

3. Oops!
Apparently not! Black holes eventually ‘evaporate’ (in a sense) by a process known as Hawking radiation

4. Stephen Hawking - the author of “A Brief History of Time” in his words, “the best-selling unread book in history”

5. Ordinary Evaporation [this does not happen to black holes!]

6. What Happens Near a Black Hole: Virtual Particles
Even perfect vacuum is not truly empty: it is a frothing sea of things (‘virtual particles’) that come and go – but always in pairs, thanks to the conservation laws!
[This is quantum mechanics again – the unfamiliar behaviour of matter on the very smallest scales.]
The pair of particles spring into existence briefly, then annihilate one another and vanish. No net cost or gain.

7. Like So:

8. Near the Event Horizon
What happens if one of the particles should cross the event horizon in that brief moment ?

9. Surprise!
The remaining particle (or anti-particle!) has lost its partner, can no long annihilate, and has sprung into very real existence.
It goes on its way, with the net production of one particle outside the Black Hole.
But ‘building’ a particle in this way requires energy. Where did that come from?

10. From the Black Hole Itself! - Cosmic ‘Recycling’
The hole gives up some of its energy and mass, shrinking as it does so. Eventually all the material is returned and the black hole vanishes, having redistributed its material into space.
So black holes are not after all the permanent repositories (‘garbage cans’) we once thought.

11. But It’s a Slow Process!
For a black hole of one solar mass, it would take
100,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000
X the present age of the universe
to return all its material contents into free space.

12. How It Ends
In fact, the process accelerates, ending with a spectacular burst of gamma rays -- if you can wait around long enough.

13. The Relevance?
In a practical sense, this process is essentially irrelevant to the current and future structure of the universe. (It’s analogous to finding out that your plastic water bottles will indeed break down and recycle – after tens of billions of years! It’s not ecologically useful.)

14. But: Mini Black Holes?
Small black holes ‘evaporate’ faster than big ones.
If mini-black holes (with the mass of an asteroid, but smaller than a hydrogen atom) were made in large numbers in the ‘Big Bang’ 14 billion years ago, they would be ‘flaming out’ just about now.
Do we see any evidence of that?

15. Maybe?
There are ‘gamma ray bursters’ in large numbers, seen in all directions! What are they?

16. They Flare Up, then Fade Away Quickly
Varied behaviour.

17. But Probably Not Mini Black Holes
Increasing evidence suggests that these bursts may be caused by special kinds of supernovae, or even collisions between neutron stars… [Artist’s impression!]

18. Closing Thoughts
Do black holes exist? Yes: we are convinced; the evidence is quite compelling
Can we enter one? (Consider a rotating black hole in particular. There is a ‘safe zone’ – but beware tidal effects!)
Can we use them as portals to elsewhere and elsewhen? Or is this science fiction?

19. Wormholes?