Black holes

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# Black holes - PowerPoint PPT Presentation

Black holes. Philip Freeman Roberta Tevlin. first… (no, not a word from our sponsors). What do we already know about Black Holes?. University of Hollywood:. Everything I need to know about physics I learned from movies…. <video cut to decrease posting size>. White Board Exercise 1:.

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### Black holes

Philip Freeman

Roberta Tevlin

first… (no, not a word from our sponsors)

• <video cut to decrease posting size>
White Board Exercise 1:

Draw the earth and sun on your whiteboards:Nice and big but leave room because we’ll be drawing some orbits!

White Board Exercise 1a:
• Draw the present orbit of the earth around the Sun (with a dotted line)
White Board Exercise 1b:
• How would the orbit change if the sun were to suddenly implode into a black hole? Draw the new orbit with a solid line. Be ready to explain your reasoning!
White Board Exercise 1c:
• What path do you think your students might predict after the Sun imploded? Draw the path(s) with a dashed line. Be ready to explain the reasoning!
White Board Exercise 1:
• Around theblack holeNo change in gravity, so no change in orbit!
White Board Exercise 1:
• Somecommonresponses
So… what is a black hole really?
• And what are they like?
The Big Idea: A history of the concept
• Classical Black Holes (dark stars)
• Outlandish Results from Relativity
• Our current ideas (a sampling)
Newton’s gravitation
• If light is made out of ‘corpuscles’ (little bits)
• Then gravity should affect light
• And since light has a finite speed…
• If a star is big enough light will not be able to escape!
•  A DARK STAR!
Light can’t escape if r is small enough

Michell (1783), Laplace (1796): “Look! Particles of light can’t escape from a really big star!”

Young (1803) “But light’s a wave.”

Everybody: “Oh, never mind!”

General Relativity
• GR reunites gravity and light
• Schwarzchild solves General Relativistic equations for stars…… but there seems to be a glitch.
• For certain circumstances (large mass, small radius) the equations contain singularities.
Singularities?
• The equations blow up!
• At a certain radius (the “Event Horizon”) time as seen from the outside STOPS. But not seen from someone falling in (huh?)
• Deep inside everything goes to infinity, and nothing makes any sense!

“Black Holes are Where God Divided by Zero”

What’s with this picture anyway?Curved Spacetime
• Recall that General Relativity shows that spacetime is “curved”...
How to show this curvature?
• Curvature of the Earth’s surface:a circle on the earth has more area inside than the outside suggests:
• Curvature near a black hole:a sphere around a black hole has more volume inside than area suggests:
Embedding diagram:
• Take a flat slice through the star IN ANY DIRECTION.More area insideThan outside suggestsMore area insideThan outside suggests
A word of warning:

from xkcd (www.xkcd.com)

Black Holes
• The critical radius turns out to be exactly that of the earlier ‘dark stars’:
• But if light can’t escape then NEITHER CAN ANYTHING ELSE, particle, wave, ANYTHING.
• This is a point of no return. Exercise: Calculate r for the sun (m =21030 kg, G=6.710-11Nm2/kg2, c=3108m/s)
Einstein ignores, then tries to disprove
• Einstein and Eddington, the two major figures in early GR agree: Singularities are nonsense!
• Einstein and Eddington turn out to be wrong.
• By the 1960s&70s black holes are a major focus of investigation (the golden age)
Some important things we know about black holes:
• Form when a large enough star collapses
• Or any other time enough mass-energy is squeezed into a small enough region (big bang? LHC?)
• Contain singularities (places where spacetime stops existing -- whatever that means!)
• Are surrounded by event horizons, so that these singularies can’t be seen (cosmic censorship)
Anatomy of a very simple black hole:

Event horizon: no return past this point

Photon sphere: no orbits possible past this point (light speed orbit!)

Inside: strong tidal effects (mixmaster physics, spaghettification.

Singularity: where spacetime ends… Here be not yet understood quantum effects

Schwarzschild Black Hole

How much more complex can a black hole get?
• Black holes have no detailed structure, only mass, charge, and spin.
• “BLACK HOLES HAVE NO HAIR!”.
• Over the past 50 years deep results have been discovered about black holes
• Hints of the connection between gravity and QM
• Hints that the nature of the universe may be of LOWER dimension than we think
Journey to a black hole
• Turning the sun into a black hole at the start of all this turned out mostly to be a lot of nothing (so to speak).
• Let’s try a journey to a bigger black hole.
From a distance
• We already know that from the outside the black hole is no different than any other mass.
• But because it is so much more compact things can get a lot more intense
• And that makes for some more intense effects, but only up close.
Gravitational Lensing (bending of light)
• Recall that an earlytest of GeneralRelativity was thebending of starlightby the sun, changingThe apparent position of stars:
Tidal effects: Exercise 2

Seen from an accelerating observer there are “forces” of gravity pulling the person toward the hole.

Remember Alice&Bob: gravity is ‘imaginary’ force. What will this look like if we subtract off gravity? Sketch remaining force on your whiteboards.

X

Tidal effects

Seen from a free falling observer (subtract off the imaginary ‘force’ of gravity) there are tidal forces stretching and squishing

Geometrically this is the CURVATURE of spacetime.

The closer to the hole the stronger this effect (spaghettification)

X

Whiteboard exercise 3

What would you see, looking up at noon, if the sun really did implode into a black hole?

Describe or sketch on your white board.

The sky at noon (post black hole)

Regular night sky (except for season)

Why don’t we see anything?
• Black holes are black
• The sun would be a small black hole
• Effects from intensity are significant only very close to event horizon (around 3km!)
Time slows down (changing space changing time)
• <Alice & Bob video> <cut to decrease posting size>
BUT effects are strong only near the event horizon
• So, how can we identify a black hole if they are different only up close?
• We look for stuff falling in!
Infalling material heated by friction and accelerated by twisted magnetic fields

Jet

Accretion Disk

OR We could look for a REALLYBIG Black Hole (galactic centre = 4 million suns!)
• We should be able to directly image Sgr A* within 10 years
Where can you use / introduce these ideas in your teaching?
• Discuss some ideas in your groups, then we’ll do a brief ‘whole class’ discussion!

END(optional journey to the edge of event horizon if there’s time!)

• What the event horizon of a black hole looks like depends on your frame!
• Falling in Accelerating

Horizon is a conductive membrane with ‘atmosphere’ of emitted particles!

No clear boundary for event horizon No emitted particles (just virtual ones)

Lensing affects the accelerated view:

Sky is compressed into a conical region

A 10 solar mass black hole: 100 × rhorizon

Front

Side

Rear

http://www.spacetimetravel.org/

20 × rhorizon

Front

Side

Rear

4.5 × rhorizon

Front

Side

Rear

2.5 × rhorizon

Front

Side

Rear

1.5 × rhorizon

Front

Side

Rear

1.2 × rhorizon

Front

Side

Rear

1.05 × rhorizon

Front

Side

Rear

Inside?
• Tidal effects become infinite (and vary):SpaghettificationMix-master physics
• Inevitable encounter with the singularity
• And then??
• Quantum Gravity(the great unknown!)