BLACK HOLES

WHAT IS A BLACK HOLE?

A few definitions from the dictionary:

Region of space resulting from the collapse of a star

Region of space with strong gravitational field

Dungeon or dark cell in a prison

Project that requires large effort with no return

Concentration of mass with a gravitational fieldso strong that not even light can escape its grip

Schwarzschild discovers that General Relativitypredicts the presence of “singularities”, regions wheregravity is so strong that not even light can escape…

Einstein refutes… “too bizarre to be true”….… but research continues….

BLACK HOLES are predicted….

Escape velocity• Earth : 11.2 km/s• Mars: 5 km/s• Asteroid Ida (radius 50 km) : 10

m/s– You can jump maybe 4 m/s

Sun : 620 km/sBlack hole, @ event horizon : c

inside event horizon: >c outside event horizon: <c

Properties of Black Holes… e.g.

Time dilation, gravitational redshift,

“spaghettification” due to tidal forces

red-shifted

red-shifted intooblivion

From inside this region

no informationcan escape

It was thought to be a ridiculous idea, at first.But, yes.Oh, yes.

Theoretically: Black holes believed to be associated with the “death” of massive stars

The Sun will not turn into a black hole.

• It’s not massive enough for its force of gravity to overcome the natural pushback of atomic structure.

• But if it did, its event horizon would have a radius of 3 km (compared to the 700000 km it is now)

• The EH of the Earth as a BH would have a 9mm radius!

• The actual mass of the sun would be at the center of this EH sphere, compressed to a point.

What do we see when we look at a black hole ?Not much.We kinda haftawait for itto interact with something.

Observationally: several pieces of evidence

(astrophysicists like to classify by the mass of our Sun) )

Small-mass BHs in X-ray binaries

a few

Supermassive BHs in galaxy centers and Quasars106-1010

X-RAY BINARIESA Star “feeding” a Black Hole One star has reached the end of its life, and turned into a black hole. It’s close enough so it can pull material off its companion. As this material spirals in, it speeds up, heats up and gives off ultra violet, x-rays, and even gamma rays. We think this is what makes Cygnus X-1 the brightest x-ray source in the sky.

QUASARS

L~1046erg/s~ 10000000000000 Lsun!!

Quasars (quasi-stellar objects) are superenergetic distant objects. We now think they are very distant galaxies with Active Galactic Nuclei (AGN). Material is falling into a supermassive black hole at the center, and jets form at the axes of spin that radiate away some escaping energy.

Do we ever “witness” the birth of a NEW Black Hole?

We probably do when we “see” a…. GAMMA-RAY BURSTGRBs radiate more energy than the rest of the universe Combined, for a few moments.

These are the speeds of the stars closest to the center of the Milky Way.

They’re orbiting something.

Something massive.

Something

Invisible. A

supermassive black hole, methinks.

How much does light bend near a star?

• A bit, near the Sun• A lot more, near the event horizon of a

BH.• In a circle, @ the EH !• So, wow, you could see the back of your

head at the EH. If you could stand there. Which you can’t.

• If you think about it, this is a bit like a wormhole. It’s going to do some awful things to time.

These stars are actually in different places.

They could even be behind us !

A Black Hole is a hole from all directions

• Not just from the front, like holes you’ve seen before

And there’s tidal forces.

• Long before you get to the EH, the difference on the force of gravity between your head and your feet will be so extreme, you get stretched.

• Even your atoms get pulled apart.