# Astronomy 2 S3

If you launch two probes in opposite directions from the Space Station, they will meet as they orbit Earth. According to general relativity, why does this happen?

They are following the straightest possible paths through space time, but those paths happen to meet.

Suppose the room in which you are sitting was magically transported off the Earth, and sent accelerating through the universe at 9.8 m/s2. Assuming your doors and windows are sealed and closed, how could you tell that you’d left the Earth?

You couldn’t — the equivalence principle tells us that you won’t be able to tell the difference.

Suppose you are in a spaceship accelerating away from Earth at 9.8 m/s2 (1 g), so that you can walk on the floor of your spaceship as though experiencing normal gravity on Earth. Is it possible for you to consider yourself to be at rest? Why or why not?

Yes, by assuming you are feeling effects of gravity and that Earth is accelerating away from you because it is in freefall.

In special relativity, we learned that people in different reference frames will measure time differently. According to general relativity, why does this happen?

They are experiencing the same spacetime reality, but from different perspectives.

Assuming you are sitting still as you take this quiz, how would you draw your own worldline on a spacetime diagram?

A straight, vertical line.

Planes traveling between Seattle and Tokyo often go near Alaska because

the distance is shorter.

If your wordline is following the straightest possible path through spacetime, then

you will be weightless.

According to general relativity, why does Earth orbit the Sun?

Earth is following the straightest path possible through spacetime, but this path happens to go around and around the Sun.

According to general relativity, what is a black hole?

A hole in the observable universe.

According to general relativity, how is time affected by gravity?

Time runs slower in places where gravity is stronger.

Einstein’s general theory of relativity suggests that gravity is

caused by curvature of spacetime

Einstein’s equivalence principle says that

the effects of gravity are exactly equivalent to the effects of acceleration

What do we mean by dimension in the context of relativity?

the number of independent directions in which movement is possible

What do we mean by hyperspace?

Any space with more than three dimensions.

What is spacetime?

The inseparable combination of space and time.

If two straight lines start out parallel but eventually cross, then they must be in a

spherical geometry

Which of the following correctly describes what we mean by a great circle on the surface of the Earth?

any circle whose center is at the center of the Earth

What do we mean by gravitational time dilation?

It is the idea that time runs slower in places where gravity is stronger.

Why do we see a gravitational redshift in the spectrum of the Sun?

Time runs slower on the Sun than on Earth, making lines in the solar spectrum have lower frequency and hence longer wavelength than normal.

Which of the following best describes what we mean by a worm hole?

A hypothesized but unproven type of “tunnel” through hyperspace that connects distant points in the real universe.