Transcript Foundation 1 - Discovering Astronomy

Chapter 13
Black Holes
What do you think?
• Are black holes just holes in space?
• What is at the surface of a black hole?
• What power or force enables black holes to
draw things in?
• Do black holes last forever?
Einstein’s Special Theory of Relativity
• Your description of physical reality is the same
regardless of the velocity at which you move.
• Regardless of your speed or direction, you always
measure the speed of light to be the same.
• So what?
– The length of an object decreases as its speed increases
– Clocks passing by you run more slowly than do clocks
at rest
– An object approaching the speed of light becomes
infinitely massive.
• Concept of Spacetime
Einstein’s General Theory of
Relativity predicts black holes
• Mass warps space resulting in light
traveling in curved paths
Is it General Relativity right?
• The orbit of Mercury is explained by
Relativity better than Kepler’s laws
• Light is measurably deflected by the Sun’s
gravitational curving of spacetime.
• Extremely accurate clocks run more slowly
when being flown in airplanes
• Some stars have spectra that have been
gravitationally redshifted.
If we apply General Relativity to a collapsing
stellar core, we find that it can be sufficiently
dense to trap light in its gravity.
Several binary star systems contain black
holes as evidenced by X-rays emitted
Cygnus
X-1
must have a
mass of
about 7
times that of
the Sun
Other black hole candidates include:
• LMC X-3 in the Large Magallenic Cloud orbits
its companion every 1.7 days and might be about
6 solar masses
• Monoceros A0620-00 orbits an X-ray source
every 7 hours and 45 minutes and might be more
than 9 solar masses.
• V404 Cygnus has an orbital period of 6.47 days
which causes Doppler shifts to vary more than
400 km/s. It is at least 6 solar masses.
Supermassive
black holes exist
at the centers of
most galaxies
Supermassive
black holes exist
at the centers of
most galaxies
Primordial black holes may have
formed in the early universe
• The Big Bang from which the universe
emerged might have been chaotic and powerful
enough to have compressed tiny knots of
matter into primordial black holes
• Their masses could range from a few grams to
more massive than planet Earth
• These have never been observed
• Mathematical models suggest that these might
evaporate over time.
How big is a black hole?
Matter in a black hole becomes much
simpler than elsewhere in the universe
• No electrons, protons, or neutrons
• Event horizon
– the shell from within light cannot escape
• Schwarzschild radius (RSch)
– the distance from the center to the event horizon
• gravitational waves
– ripples in spacetime which carry energy away from
the black hole
• The only three properties of a black hole
– mass, angular momentum, and electrical charge
Structure of a
Kerr (Rotating)
Black Hole
In the Erogoregion,
nothing can remain at
rest as spacetime here is
being pulled around the
black hole
Falling into a black hole is an
infinite voyage as gravitational tidal
forces pull spacetime in such a way
that time becomes infinitely long
Black holes evaporate
Virtual particles that appear in pairs near a event
horizon may not be able to mutually annihilate
each other if only one manages to survive a trip
along the event horizon.
What did you think?
• Are black holes just holes in space?
No, black holes contain highly compressed matter-they are not
empty.
• What is at the surface of a black hole?
The surface of a black hole, called the event horizon, is empty
space-there is no stationary matter there.
• What power or force enables black holes to draw things
in?
The only force that pulls things in is the gravitational attraction of
the matter in the black hole.
• Do black holes last forever?
No, black holes evaporate.
Self-Check
1: Provide brief descriptions of the special and general
theories of relativity.
2: List the principal features used to describe black holes and
explain the significance of each.
3: List the three properties that completely characterize a black
hole.
4: Describe how energy can be extracted from certain types of
black holes and indicate what property a black hole must
have to allow such extraction.
5: Discuss the evidence that suggest that black holes exist in
binary systems and briefly describe how searches for such
objects are conducted.