The Fate of Black Holes

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2017-08-06 49K 49 0 0

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. 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 Download Presentation

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The Fate of Black Holes




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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 radiation

Slide4

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?


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