Transcript Lec11_ch13_blackholes

Ch 13--Black Holes
16 Nov 2000
ASTR103, GMU, Dr. Correll
1
What do you think?
• Are black holes just holes in space?
• What exists at the surface of a black hole?
• What power or force enables black holes to draw
things in?
• Do black holes last forever?
16 Nov 2000
ASTR103, GMU, Dr. Correll
2
Beyond Neutron Stars
• Neutron degeneracy (a quantum
mechanical “gas”) pressure can support
neutron stars of mass up to 3 Msun
– about 8 km in diameter
• For Neutron stars with mass > 3 Msun,
neutron degeneracy is overwhelmed
and the star collapses
• No other forces known which can
impede this collapse--apparently,
collapse continues to the point of nearly
infinite density--a singularity!
16 Nov 2000
ASTR103, GMU, Dr. Correll
3
Beyond Neutron Stars
• Regardless of the final endpoint of
collapse, a region around the star is
created with a gravitational field so
strong that nothing, not even
electromagnetic radiation (light, radio
waves, x-rays, etc) can escape
– spacetime curvature is so strong that all
paths lead toward the singularity--a black
hole has formed!
16 Nov 2000
ASTR103, GMU, Dr. Correll
4
Einstein’s Special Relativity
• In 1905 Einstein proposed his
new theory of relative motion-special relativity
– Your description of reality (physics) is
the same regardless of the constant
velocity at which you move (an
important but common notion)
– Regardless of your speed and
direction, you always measure the
speed of light to be the same (an
important but paradigm breaking
assumption)
16 Nov 2000
ASTR103, GMU, Dr. Correll
5
Einstein’s Special Relativity
• Leads to profound conclusions!
– length contraction of moving objects
– time dilation of moving clocks
– mass of an object increases with speed, requiring infinite energy
to reach the speed of light
– equivalence of mass and energy: E=mc2
– led to the concept of an integrated “spacetime” which is more
fundamental than “space and time”
16 Nov 2000
ASTR103, GMU, Dr. Correll
6
Einstein’s General Relativity
• In 1915 Einstein proposed his theory of
General Relativity which combined the
concepts of special relativity with his new
idea of the force of gravity being described
as the curvature of spacetime
– massive bodies curve the fabric of
spacetime
– curved spacetime determines how the
bodies will move
16 Nov 2000
ASTR103, GMU, Dr. Correll
7
Mass Curving Spacetime
• Small mass star
produces mild
depression or
curvature of
spacetime
• Massive star
produces large
depression or
curvature of
spacetime
16 Nov 2000
Curvature of 2-dim space by small mass star
Curvature of 2-dim space by large mass star
8
ASTR103, GMU, Dr. Correll
Einstein’s General Relativity
16 Nov 2000
ASTR103, GMU, Dr. Correll
9
Einstein’s General Relativity
• Confirmation of GR
– starlight deflected by the Sun’s gravity
– perihelion shift of mercury agrees with GR
predictions (Newtonian theory was inaccurate!)
– high altitude clocks run slower than clocks on the
ground
– spectra of some starlight observed to be
gravitationally redshifted
16 Nov 2000
ASTR103, GMU, Dr. Correll
10
Einstein’s General Relativity
(figure exaggerated)
16 Nov 2000
ASTR103, GMU, Dr. Correll
11
Einstein’s General Relativity
16 Nov 2000
ASTR103, GMU, Dr. Correll
12
Black Holes in Binary Systems
• By studying the Dopplershifted spectra of binary
stars we can measure the
mass
• Black hole companions
would exhibit strong x-ray
emissions as the infalling
gas is compressed and
heated
• Rapid fluctuations in x-ray
signal indicate small
diameter of source
– light travel time
16 Nov 2000
ASTR103, GMU, Dr. Correll
13
Binary Black Holes
• Coalescence and
Merger of binary black
holes would emit
enormous amount of
gravitational radiation
• Laser Interferometry
Gravitational
Observatory (LIGO)
under construction in
Washington and
Louisiana
16 Nov 2000
ASTR103, GMU, Dr. Correll
14
Black Holes in Binary Systems
• Cygnus X-1 discovered by Uhuru xray satellite
– subsequent optical observations
revealed blue supergiant (with
no x-ray emission)
– mass of blue supergiant about
30Msun
– mass of x-ray source about
7Msun
– x-ray fluctuations in hundredths
of a second indicate source
smaller than earth
16 Nov 2000
ASTR103, GMU, Dr. Correll
15
Supermassive Black Holes
• The centers of galaxies appear to contain
supermassive black holes--millions of solar masses in
size!
16 Nov 2000
ASTR103, GMU, Dr. Correll
16
Minuscule Black Holes?
• During the earliest moments of the Big
Bang, tiny pockets of matter-energy may
have been so dense that they formed
minuscule black holes
– Theoretically possible, but not yet
observed, these small primordial black
holes are an open question
16 Nov 2000
ASTR103, GMU, Dr. Correll
17
Inside a Black Hole
• The mass of a black
hole is concentrated at
the singularity
• The size of the black
hole is defined by the
surface defining the
trapped interior region
form which nothing can
escape--the event
horizon
– The Schwarzschild
radius
16 Nov 2000
ASTR103, GMU, Dr. Correll
18
Inside a Black Hole
16 Nov 2000
ASTR103, GMU, Dr. Correll
19
Inside a Black Hole
• Spinning black holes,
those with angular
momentum have
additional structure
– The singularity is an
annular ring
– The event horizon is
spherical
– An larger, oblate
spheroid defines the
ergosphere where space
is dragged around with
the spinning black hole
16 Nov 2000
ASTR103, GMU, Dr. Correll
20
Falling into a Black Hole
• The tidal forces of gravity near a black hole distort
any matter that falls into it
16 Nov 2000
ASTR103, GMU, Dr. Correll
21
Black Holes Evaporate
• According to quantum mechanics, the vacuum is
actually a sea of particles and antiparticles
spontaneously appearing and then annihilating each
other
• Near the event horizon of a black hole, some virtual
particles are trapped while their counterparts escape.
Via this Hawking process the black hole loses mass
16 Nov 2000
ASTR103, GMU, Dr. Correll
22
What do you think?
• Are black holes just holes in space?
– Possible, in theory, but most often they contain highly
compressed matter at their centers with additional matter
continually being accreted
• What exists at the surface of a black hole?
– Empty space, no stationary matter exists their. It’s the
unmarked boundary between the trapped region and the outside
• What power or force enables black holes to draw things
in?
– Gravity!
• Do black holes last forever?
– No, they evaporate (but it takes a tens/hundreds of billions of
years)
16 Nov 2000
ASTR103, GMU, Dr. Correll
23