Black Holes

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Black Holes
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Objectives of this Unit

• Recognize that a gravitational force exists
  between any two objects. This force is
  proportional to the masses and inversely
  proportional to the square of the distance
  between them.
• Recognize that all bodies fall at the same rate
  in the absence of air friction.
• Describe the lifecycle of a star.

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• Know the vocabulary of black holes
• Explain Einsteins Principle of Equivalence
• Describe gravity as the bending of space-time
• Compare Newtons theory of gravity with
  Einsteins theory of general relativity
• Describe how a black hole affects the area of
  space around it

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Newtons Law of Gravitation
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• What happens to the force of gravity if the mass
  of one of the bodies is doubled?
• What happens to the force of gravity if the
  distance between the objects is doubled?
• If the force of gravity changes, why do all
  objects fall at the same rate?

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The Lifecycle of a Star
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Principle of Equivalence

• Einstein said that accelerated motion and free
  fall in a gravitational field are
  indistinguishable.
• No experiment can be done within the system to
  determine if you are accelerating or if you are
  falling.
• This implies that light must bend due to a
  gravitational field.

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movie
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Einstein interpreted gravity as a bending of
spacetime
Time and space stretch in response to mass
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Black Holes

• Black Holes are an infinite curvature of
  spacetime
• Not even light can escape the gravity of a
  black hole
• Black holes end at a singularity
• Anything that passes the event horizon of a black
  hole is doomed to end up at the singularity

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• Strange Facts About Black Holes
• Light bends so much near black holes that if you
  were near one and looking away from the hole, you
  would see multiple images of every star in the
  universe, and could actually see the back of your
  own head!
• Inside a black hole the roles of time and radius
  reverse just as now you can't avoid going into
  the future, inside a black hole you can't avoid
  going in to the central singularity.
• If you stood a safe distance from a black hole
  and saw a friend fall in, he would appear to slow
  down and almost stop just outside the event
  horizon. His image would dim very rapidly.
  Unfortunately for him, from his point of view he
  would cross the event horizon just fine, and
  would meet his doom at the singularity.
• As Hawking discovered, black holes can evaporate,
  but only very slowly. Even one the mass of a
  mountain will last for ten billion years, and one
  the mass of the Sun will only evaporate after
  1067 years.