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 Oops Apparently not ID: 576447
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Slide1
The Fate of Black Holes
Slide2
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.Slide3
Oops!
Apparently not!
Black holes eventually
‘
evaporate
’
(in a sense) by a process known as
Hawking radiationSlide4
Stephen
Hawking
- the author of “A Brief History of Time”
in his words, “
the
best-selling unread book in history
”Slide5
Ordinary
Evaporation
[this does
not happen to black
holes!]
Slide6
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.Slide7
Like So:Slide8
Near the Event Horizon
What happens
if one
of the
particles should cross the
event horizon
in that brief moment
? Slide9
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?
Slide10
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. Slide11
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.Slide12
How It Ends
In
fact, the process
accelerates,
ending
with a spectacular burst of gamma rays
-- if you can
wait around long enough
.Slide13
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.)Slide14
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?Slide15
Maybe?
There are
‘
gamma ray
bursters
’
in large numbers, seen in all directions! What are they?Slide16
They Flare Up, then Fade Away Quickly
Varied
behaviour
.Slide17
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!]Slide18
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?Slide19
Wormholes?