Black Holes

							Black Holes
Escape Velocity
          The speed
          needed to escape
          from the
          gravitational
          field of a planet
          (or other object)
Escape Velocity
              2GM
  v
               R
 Depends on:
  – Mass of the planet/object
  – Radius of the planet/object
  What if the planet/object is
        really heavy?
 The
    escape velocity increases with the
 mass of the planet/object.

 Can an planet/object be so heavy that
 the escape velocity is greater than c?
The Speed of Light

   1675: Olaus Roemer
    (Danish astronomer)
    measures by timing
    “transits” of
    Jupiter’s moons


                          Kaufmann, Universe, 4th ed., p.80



        c  300,000 km
                               s
The Idea of a Black Hole
John Michell (1783) , Pierre-Simon Laplace (1796):

 Question: What happens if the escape speed
  from an object is greater than the speed of
  light?
 Answer: If light consists of particles of matter,
  they would not be able to escape

                 “dark star” ?
 If light cannot escape from an
 object it would appear black!
 Rearrange the escape
                               2GM
 velocity equation to     Rs     2
 determine how big a            c
 black hole would be.

 Schwarzchild   Radius
1915: General Relativity, Einstein’s Theory of Gravity



                                 Gravity is the bending
                                  of spacetime
                                 Planets and stars cause
                                  the “fabric” of the
                                  universe to stretch


  Albert Einstein
  1916: Schwarzschild shows that Black Holes can be
          produced in General Relativity (GR)




I had not expected that one could     Karl Schwarzschild
formulate the exact solution of the
problem in such a simple way.
                   —Albert Einstein
   BHs only understood & accepted in the 1960s
(Term “Black Hole” coined by John Wheeler in 1967)




     John Wheeler            Stephen Hawking
How small would our Sun have
to be to act like a Black Hole?

   M = 2.0 x 1030 kg

   RS  3 km
How small would our Earth have
 to be to act like a Black Hole?

   M = 6.0 x 1024 kg

   RS  1 cm
    How do Black Holes form?
 Stable stars have two
  forces in equilibrium
 GRAVITY pulling in
 PRESSURE pushing out
 A Black Hole forms
  when gravity crushes the
  mass into a tiny volume
Will all stars form Black Holes?
         Low mass stars (<1.4 Msun)
          will form White Dwarf stars
         Medium mass stars (<3 Msun)
          will form Neutron stars
         Large mass stars (> 3 Msun)
          will become a Black Hole.
             Low Mass Stars
   If the mass of the star is < 1.4 Msun, it will
    become a White Dwarf star.
        Medium Mass Stars
 If the mass of the
  star is < 3 Msun, it
  will become a
  Neutron star.
 One teaspoon of
  this star would
  weigh 100 million
  tons!!
          Large Mass Stars
 If the mass of the star is > 3 Msun, it will
  become a Black Hole.
 The gravity is so
  strong that it will
  crush all the mass
  into a single point!
       Do we have any proof?
   YES, but since they don’t emit light we
cannot see Black Holes we must rely on
indirect observations.
            X-ray sources
 As material gets sucked into a Black Hole it
  will ‘scream’ and emit x-rays.
 Astronomers have located some very strong
  x-ray sources.
 Computers can be used to visualize what
  happens, using real data in the non-visible
  spectrum.
           http://hubblesite.org/newscenter/archive/releases/2001/03/video/a/

Computer visualization of data from the Hubble Space Telescope
            Gravitational effects
   Black holes are heavy, so they will cause
    other visible objects to act strange




http://www.eso.org/public/outreach/press-rel/pr-2002/video/vid-02-02.mpg

     Images taken by European Space Observatory
http://www.stsci.edu/~marel/m32anim.html
         Gravitational lensing
   Black holes are heavy, so they will make
    light bend when it passes nearby.
Animation of black hole passing in front of deep space image
   Other Black Hole Info




Time stops inside a black hole
        Other Black Hole Info
At the centre of a black hole is a SINGULARITY,
much like the one that would have occurred during
BIG BANG
    Other Black Hole Info




Black holes actually ‘glow’ and evaporate
      Dispelling the Myths …
BHs are not cosmic vacuum
cleaners: only inside the horizon
is matter pulled inexorably inward

Far away from a BH, gravity
is no different than for any
other object with the same mass

If a BH were to replace the sun, the orbits of planets, asteroids,
moons, etc., would be unchanged
(though it would get really really cold).
              For more information
Check out the following sites!!
http://hubblesite.org/explore_astronomy/black_holes/
http://www.thinktechnologies.com/portfolio/demos/Blackhole.swf
http://casa.colorado.edu/~ajsh/schw.shtml