Stronger Gravity - PowerPoint Presentation

Slide1

Stronger Gravity Black Holes and theCurvature of Space-Time

Slide2

Black HolesThe name “black hole” was coined by John Archibald Wheeler, of Princeton University, in the 1960s. At that time, such objects were matters of idle speculation, not real astrophysical research.Here we see Wheelerwith Einstein and Yukawa, a year before Einstein’s death.Slide3

The Extreme Case If an object of mass M shrinks to a size given by the Schwarzschild radius: RS = 2 G M / c2 the escape velocity is

c

(the speed of light), an

d

neither light nor material particles can escape. Slide4

Some NumbersNote that Rs is proportional to the mass. For the Earth, Rs = about 1 cm

For the Sun

(300,000 times more massive),

R

s

= 300,000 cm = 3000 m =

3 kmFor a ten-solar-mass star, Rs = 30 km (bigger than a neutron star!)Slide5

It Doesn’t Stop There!Once the material attains this small size and enormous density, nothing can stop the further complete infall. Gravity overwhelms all

resistance; the ‘fall’ continues.Slide6

SingularityAs noted, the collapse continues within the Schwarzschild radius, to zero volume and infinite density: a singularity – or at least that’s what the mathematics tells us.What ‘really’ happens

there? Physics can not yet tell us: we need a theory of

quantum

gravity

,

or perhaps

string theory.

http://cjwainwright.co.uk/maths/physicscube/Slide7

What is a Singularity?First, consider burrowing through the Earth. It may not be possible for practicalreasons (for example, the extreme

pressure overwhelms our tools)

but

there

is

nothing

in principle

that

disallows this. You can imaginemaking

your way, inch by

inch

.Slide8

But in the Singularity…In a singularity like a black hole, there is no ‘connection’ from one side to the other through the central point.

Imagine climbing down one side of a well, intending later to climb back up the other – and then discovering there is no bottom!

Slide9

Infinitely Deep!The points on opposite sides of the hole are always separated by an infinite distance, if you choose to go through the hole. (You can of course always go around the hole, out where space is not so distorted.)

This is because of the infinite elasticity (the

‘

stretchiness

’

) of space itself.Slide10

Trapping People in a SingularitySlide11

Running Down is Easy…but try climbing! http://www.physics.queensu.ca

/~

hanes

/Movies

/Climbing-Dunes.mp4Slide12

TheBehaviour of LightNote that light follows curvedpaths, and can even orbit the black hole.Slide13

An Important Proviso:The Material has Vanished - But is Not GoneNote that the mass is still there! For objects far away, it has exactly the same

gravitational influence as it always

did.Slide14

Black Holes Do Not Suck!Suction is a manifestationof pressure.The motor and fan expelair out the top, creating avacuum in the canister. Air in the room rushes inthrough the hose, carrying

d

ust and dirt with it.Slide15

The Fate of the EarthIf the Sun became a black hole, the Earth would not suddenly get ‘sucked into’ it.There is no external pressure to push it into the Sun in the way that air in your bedroom pushes the dust into the vacuum cleaner. And the gravity we feel would not change.Very close to

the black hole, however, the situation is different.Slide16

Here’s a Thought Experiment Move to the moon for your own safety. Now compress the Earth to the size of an olive, so it collapses to a black hole. What next?The moon would continue in its orbit, around an invisible Earth.If you looked directly towards the shrunken Earth, the new black hole would have an occasional pronounced

effect on beams of light from

stars directly behind it.

(

“

lensing

”

) Slide17

Nothing Else Would Change…But you’d never be able to go home!There

’

s no way to

‘

re-expand

’

the

Earth.Slide18

Now Suppose You (A Keen Scientist!) Decide to Look at the Black Hole ‘Close Up’ This is not a good idea!Slide19

Let’s Consider Some NumbersRight now your body is more than 6 million metresfrom the centre of

the Earth.

This gives rise to an overall

n

et downward

gravitational

f

orce that

you experience as your weight. Slide20

But Near a Shrunken Earth Suppose you descend towards the grape-sized black hole until your feet are just one metre from it. 1 metreWhat gravitational pull would you now experience? Obviously a large one,

because you are closer to all the Earth’s mass, but

just how big?Slide21

Inverse-Square LawThe atoms in your feet are now only 1 meter from the material in the hole, rather than 6 million meters. Because of the inverse-square law, the force on the atomsin

your feet would be

36 trillion

times

( =

6 million x 6 million) as strong as what they are feeling right now. Slide22

That’s Not Necessarily a Problem!If all your atoms felt the same force, and if you were coasting freely through space, no problem! (Touching down on a surface would hurt, though! Your own weight

would

crush you.)

Slide23

But Your Head is Farther Away!If you are of average height (~1.6 metres), your head would be about 2.6 metres from the hole. So your head does not feel as strong a force.Slide24

What Matters is the Difference!Both these forces are unimaginably strong, and pull you towards the hole. But the gravitational force on the atoms in your feet is fantastically stronger than the force on the atoms in your head.You’d get ripped

apart, atom by

atom.Slide25

Spaghettification! This is a tidal effect. See the ASTR 101 notes for a reminder of tides.]Slide26

Meet a Brave Astronaut Find a volunteer willing to fall into the black hole for the sake of science.

Watch

her go, and ask her

to

report

regularly by radio.

What

would we

see and learn?Slide27

Two Very Different PerceptionsHer experience: quick and nasty  spaghetti! Our observation: infinitely drawn out, with a slow fade to

invisibility

. Slide28

Her Signals Arrive Ever More Slowly(‘News From the Pit’)The last few escaping photons……take forever to get out, and barely make it at all, losing almost all their energy