Transcript Black Hole phenomenax

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 field
so strong
that not even light can escape its grip
BLACK HOLES are predicted….
Schwarzschild discovers that General Relativity
predicts the presence of “singularities”, regions where
gravity is so strong that not even light can escape…
Einstein refutes… “too bizarre to be true”….
… but research continues….
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/s
Black hole, @ event horizon : c
inside event horizon: >c
outside event horizon: <c
Properties of Black Holes… e.g.
From inside
this region
no information
can escape
red-shifted
red-shifted into
oblivion
Time dilation,
gravitational redshift,
“spaghettification” due to tidal forces
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 hafta
wait for it
to 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 Quasars
106-1010
X-RAY BINARIES
A 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 xray 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 BURST
GRBs 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.