General Relativity Part I: Gravity is Acceleration

General Relativity Part I:Gravity is Acceleration

1) You are in an elevator. Suddenly, the cable snaps and the elevator starts to fall. What happens to you while the elevator falls? You a) stay the same b) stay on the floor but feel weightless c) slowly rise to the top d) accelerate to the top When you fall, you feel weightless.

2) What will happen to the candle? The candle will a) stay at the bottom. The flame will stay lit b) stay at the bottom. The flame will go out c) rise to the top. The flame will stay lit d) rise to the top. The flame will go out

3) A bottle of water has a small hole in the top and bottom. It is thrown above volunteer A with a large parabolic arch to volunteer B. What happens? a) A gets wet as the bottle goes up. b) A gets wet as the bottle is near the top. c) A gets wet as the bottle descends. d) B gets wet as the bottle is caught.

Astronauts train for missions on the Vomit Comet. During the ½ minute parabolic flight, passengers feel weightless – while going up and while going down.

4) Why are the astronauts on the International Space Station weightless? They a)are further from the Earth b)are in freefall. c) aren’t weightless, they are in microgravity. d) aren’t weightless, it’s an illusion.

Orbital motion is another kind of freefall. Astronauts in the ISS feel weightless even though they are still very close to the Earth.

The ship orbits as a whole but the gravitational field is not constant throughout the space ship.

In the ship’s frame of reference, this causes a tidal force field.

In the ISS this microgravity is usually insignificant.

Near a Black Hole it causes spaghettification.

During blastoff, an astronaut feels up to 3 times heavier than normal so they wear G-suits.

5) A tray with a glass of water is whirled rapidly in a vertical circle. What happens to the glass at the top? The glass stays on the tray because of a) centrifugal force b) centripetal force c) inertia d) something else

Rotational motion can produce weight toward the outside. This is the basis of lettuce spinners and rotating space stations.

Pondering situations like these led Albert Einstein to what he called his ‘happiest idea’ – the Equivalence Principle. An accelerated frame is equivalent to a gravitational field.

6) The equivalence principle has some interesting consequences. A laser beam is fired horizontally in a frame on the Earth’s surface. It then a) continues horizontally b) curves up c) curves down

Einstein pointed out that it should be possible to see starlight bending if it passed near the sun. This required making observations during an eclipse so the stars near the sun could be seen.

Einstein became famous after the eclipse of 1919 ‘confirmed’ his predictions about the bending of starlight.

This shows four images of a single quasar surrounding a galaxy. The galaxy lies between us and the quasar. We see four images because some of the light was bent above, some below etc.

7) Light is bent. What happens to time? Once every second, Ali at the head of an accelerating rocket sends a signal down to Brenda at the tail. Brenda receives these signals separated in time by a) 1 s b) more than 1 s c) less than 1 s Ali Brenda

8) Brenda says that Ali’s clock is running fast. Now Brenda sends a signal up to Ali every second. Ali receives these signals separated in time by a) 1 s b) more than 1 s c) less than 1 s

The clocks on the GPS satellites are all set a bit slow so that they will ‘tick’ with the same time as the Earth.

9) Another way to understand why time is slowed is to consider what must happen to light. As light rises in a gravitational field, it gains gravitational potential energy. Therefore as it rises, the light must also a) travel slower b) travel faster c) become red shifted d) become blue shifted

In 1960 R. Pound and Rebka confirmed the red shift by comparing emitted frequencies at the top and bottom of this74-foot tower at Harvard. Photo courtesy of Lubos Motl.

General Relativity Part I: Gravity is Acceleration