Black Holes are totally collapsed objects

• gravity so strong not even light can escape
• predicted by General Relativity

Schwarzschild Radius & Event Horizon

Find them by their Gravity

• X-ray Binary Stars

Black Hole Evaporation

A star more massive than about 18 M_sun would leave behind a cores larger than 2-3 M_sun:

Neutron degeneracy pressure would fail and nothing can stop its gravitational collapse.

Core would collapse into a singularity, and object with

• zero radius
• infinite density

Close to the singularity:

• Gravity is so strong that nothing, not even light, can escape.
• Infalling matter is shredded by powerful tides and crushed to infinite density.

Becomes a Black Hole:

• "Black" because they neither emit nor reflect light.
• "Hole" because nothing entering can ever escape.

Laplace (1795) described "corps obscurs"

• objects with an escape velocity of the speed of light
• used Newtonian Gravity to compute its size

A proper treatment such an extreme gravitational field requires General Relativity:

• Einstein's theory of gravitation (1915)
• First solutions by Karl Schwarzschild (1916)
• Not taken seriously until the 1960s.

Light cannot escape from a Black Hole if it comes from a radius closer than the Schwarzschild Radius, R_S:

A black hole with a mass of 1 M_sun would have a Schwarzschild Radius of R_S=3 km.

Compare this with a typical 0.6 M_sun White Dwarf, which would have a radius of about 1 R_earth (6370km), and a 1.4 M_sun neutron star, which would have a radius of about 10km.

R_S defines the "Event Horizon" surrounding the black hole's singularity:

• Events occurring inside R_S are invisible to the outside universe.
• Anything closer to the singularity than R_S can never leave the black hole
• The Event Horizon hides the singularity from the outside universe.

The Event Horizon marks the "Point of No Return" for objects falling into a Black Hole.

Far away from a black hole:

• Gravity is the same as a star of the same mass.
• If the Sun became a Black Hole, the planets would all orbit the same as before.

Close to a black hole:

• R < 3 R_S, there are no stable orbits - all matter gets sucked in.
• At R = 1.5 R_S, photons would orbit in a circle!

Two observers: Jack & Jill

Jack, in a spacesuit, is falling into a black hole. He is carrying a low-power laser beacon that flashes a beam of blue light once a second.

Jill is orbiting the black hole in a starship at a safe distance away in a stable circular orbit. She watches Jack fall in by monitoring the incoming flashes from his laser beacon.

He Said, She Said...

From Jack's point of view:

• He sees the ship getting further away.
• He flashes his blue laser at Jill once a second by his watch.

From Jill's point of view:

• Each laser flash take longer to arrive than the last
• Each laser flash become redder and fainter than the one before it.

Near the Event Horizon...

Jack Sees:

• His blue laser flash every second by his watch
• The outside world looks oddly distorted (positions of stars have changed since he started).

Jill Sees:

• Jack's laser flashing about once every hour.
• The laser flashes are now shifted to radio wavelengths, and
• the flashes are getting fainter with each flash.

Down the hole...

Jill Sees:

• One last flash from Jack's laser after a long delay (months?)
• The last flash is very faint and at very long radio wavelengths.
• She never sees another flash from Jack...

Jack Sees:

• The universe appear to vanish as he crosses the event horizon
• He gets shredded by strong tides near the singularity and crushed to infinite density.

Moral:

The powerful gravity of a black hole warps space and time around it:

• Time appears to stand still at the event horizon as seen by a distant observer.
• Time flows as it always does as seen by an infalling astronaut.
• Light emerging from near the black hole is Gravitationally Redshifted to longer (red) wavelengths.

Take a Virtual Trip to a Black Hole or Neutron Star. Pictures & movies by relativist Robert Nemiroff at the Michigan Technical University.

Question:

If no light gets out of a black hole, how can we ever hope to find one?

Answer:

Look for the effects of their gravity on their surroundings.

• A star orbiting around an unseen massive companion.
• X-rays emitted by gas heated to extreme temperatures as it falls into the black hole.

Bright, variable X-ray sources identified by X-ray observatory satellites:

• Spectroscopic binary with only one set of spectral lines - the second object is invisible.
• Gas from the visible star is dumped on the companion, heats up, and emits X-rays.
• Estimate the mass of the unseen companion from the parameters of its orbit.

A number of X-ray binaries have been found with unseen companions with Masses > 3 M_sun, too big for a Neutron Star.

Some Candidates:

Cygnus X-1: M = 6-10 M_sun

V404 Cygni: M > 6 M_sun

LMC X-3: M = 7-10 M_sun

None are as yet iron-clad cases, but in general things are looking pretty good.

"Classical" General Relativity says:

• Black Holes are totally black
• Can only grow in mass and size
• Last forever (nothing gets out once inside)

But, General Relativity does not include the effects of Quantum Mechanics.

Stephen Hawking looked at the problem by considering quantum effects occurring near the event horizon of a Black Hole.

He showed that:

• Black holes slowly "leak" particles with a blackbody spectrum.
• Each particle carries off a little of the black hole's mass
• The smaller the mass of the black hole, the faster it leaks.

Black Holes evaporate slowly by emitting subatomic particles and photons via "Hawking Radiation".

The Smaller the mass, the faster the evaporation.

For black holes in the real universe, the evaporation rate is VERY slow:

• A 3 M_sun black hole would require about 10⁶³ years to completely evaporate.
• This is about 10⁵³ times the present age of the Universe.