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 space-time

• 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 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?

The Lifecycle of a Star

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.

movie

Einstein interpreted gravity as a bending of spacetime

Time and space stretch in response to mass

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

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.