Black Holes

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Black Holes
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.
• Know the vocabulary of black holes
• Explain Einstein’s Principle of Equivalence
• Describe gravity as the bending of spacetime
• Compare Newton’s theory of gravity with
Einstein’s theory of general relativity
• Describe how a black hole affects the area
of space around it
Newton’s Law of Gravitation
•What happens to the force of gravity
if the mass of one of the bodies is
•What happens to the force of gravity
if the distance between the objects is
•If the force of gravity changes, why
do all objects fall at the same rate?
The Lifecycle of a Star
Principle of Equivalence
• Einstein said that accelerated motion and
free fall in a gravitational field are
• 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.
Einstein interpreted gravity as a
bending of spacetime
Time and
stretch in
Black Holes
• Black Holes are an infinite curvature of
• 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
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 10^67 years.

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