Question 1

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

Accepted Answer

A wormhole is a theoretical concept in astrophysics and general relativity. According to the theory, a wormhole is a type of "tunnel" through hyperspace that connects distant points in the real universe. It is hypothesized but unproven, and it is not known whether it is possible to travel through a wormhole or even whether they exist. Therefore, option A is incorrect as there is no proven way of traveling through space faster than the speed of light. Option B is correct to some extent as a wormhole is a possible method of traveling through space faster than the speed of light, but it is not confirmed. Option C is also incorrect as a wormhole is not related to time travel.

Question 2

What is spacetime?

Accepted Answer

Spacetime is the inseparable combination of space and time as described by the theory of relativity. It is a four-dimensional continuum that includes the three dimensions of space and one dimension of time. Therefore, option D is correct. Options A, B, and C are not accurate definitions of spacetime.

Question 3

What do we mean by the event horizon of a black hole?

Accepted Answer

The event horizon is the boundary (not the center) of a black hole beyond which events cannot be observed or influenced by the outside universe. It is often referred to as the "point of no return" as anything that crosses it is assumed to be inevitably pulled toward the singularity at the center of the black hole. Time dilation does occur near the event horizon, but it is not the defining characteristic of the event horizon.

Question 4

What do we mean by hyperspace?

Accepted Answer

Hyperspace refers to any space with more than three dimensions. It is a concept commonly used in science fiction and theoretical physics.

Question 5

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?

Accepted Answer

According to the principle of equivalence in general relativity, the effects of gravity are locally indistinguishable from acceleration. Therefore, if you are in a spaceship accelerating at 1 g, it is legitimate to consider yourself at rest and to view the effects you feel as equivalent to gravity, with the Earth accelerating away from you. This principle allows for different frames of reference where one can be considered at rest while experiencing acceleration due to gravity.

Question 6

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

Accepted Answer

According to general relativity, people in different reference frames are experiencing the same spacetime reality but from different perspectives. This is because time and space are not separate entities but are part of a unified spacetime that can be experienced differently depending on one's relative motion and gravitational field. This is the basis for the concept of time dilation in general relativity, where time appears to slow down for objects in motion or under the influence of stronger gravitational fields, as observed from a different reference frame.

Question 7

What do we mean by gravitational time dilation?

Accepted Answer

Gravitational time dilation is the concept in which time runs slower in regions where the gravity is more massive. The stronger the gravitational field, the more significant the effects of time dilation, as time seems to stay fast-moving in weaker gravitational fields.

Question 8

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

Accepted Answer

In spherical geometry, lines (great circles) that start out parallel can eventually cross, such as the lines of longitude on a globe.

Question 9

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

Accepted Answer

In the context of relativity, dimension refers to the number of independent directions in which movement is possible. This is typically represented by the three dimensions of space (length, width, and height) and the dimension of time.

Question 10

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?

Accepted Answer

According to general relativity, objects follow the curved spacetime around massive objects such as Earth. The probes are simply following the straightest possible paths through this spacetime, but these paths happen to cross so that the probes meet again. This is known as geodesic deviation.

Question 11

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

Accepted Answer

A worldline in a spacetime diagram represents the history of an object's location as time passes. For an object at rest (like you, sitting still), the worldline is a straight, vertical line, indicating that your position in space does not change, though time progresses.

Question 12

Which of the following is not a major idea of Einstein's general theory of relativity?

Accepted Answer

While wormholes are a popular science fiction concept and have been discussed in the context of general relativity, they are not a major idea or prediction of Einstein's theory. The other options listed (time dilation near strong gravity, a finite but unbounded universe, and the existence of black holes) are all key concepts of general relativity that have been observed and tested in various ways.

Question 13

Suppose the room in which you are sitting was magically transported off 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 Earth?

Accepted Answer

According to the equivalence principle, there is no way to distinguish between the force of gravity and the force felt by an observer in an accelerating reference frame. Therefore, if the room is accelerating through the universe at 9.8 m/s², the observer inside would not be able to tell if they had left Earth or if they were simply experiencing a strong gravitational force.

Question 14

Planes traveling between Seattle and Tokyo often go near Alaska because

Accepted Answer

The shortest route between two points on the Earth's surface is a great circle route. Due to the spherical shape of the Earth, flying a great circle route from Seattle to Tokyo involves going over or near Alaska, making the distance shorter than flying directly over the Pacific Ocean.

Question 15

Which of the following correctly describes the relationship between Newton's theory of gravity and general relativity?

Accepted Answer

Newton's theory of gravity is an approximation that works well when gravity is relatively weak, but breaks down when gravity is strong. General relativity provides a more accurate description of gravity in these strong gravity situations. While general relativity does not apply at the sub-atomic level, neither does Newton's theory of gravity.

Question 16

In what way do observations of Mercury support Einstein's general theory of relativity?

Accepted Answer

Mercury's orbit precession has been observed to match the prediction of general relativity, which takes into account the curvature of spacetime caused by the mass of the Sun. This precession cannot be accounted for by Newton's universal law of gravitation, which assumes that gravity acts instantaneously across infinite distances. Therefore, the observed precession supports Einstein's theory of general relativity over Newton's theory.

Question 17

Einstein's general theory of relativity suggests that gravity is

Accepted Answer

In Einstein's general theory of relativity, gravity is not considered as a force that acts at a distance between two masses, but it is viewed as a result of the curvature of spacetime. This curvature is caused by the presence of massive objects which create a "depression" in spacetime that attracts other objects towards it. So, option B is the correct choice. Option A is incorrect. Option C is the equation for calculating the gravitational force between two masses, but it is not an explanation of what gravity is. Option D is partly correct, as gravity is one of the four fundamental forces in nature, but it does not provide an explanation of gravity.

Question 18

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

Accepted Answer

Gravitational redshift occurs because the strong gravitational field of the Sun causes time to run slower on its surface compared to Earth. This effect, predicted by General Relativity, leads to the observed redshift in the solar spectrum, as the frequency of light decreases (or its wavelength increases) when it climbs out of the Sun's gravitational well.

Question 19

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

Accepted Answer

A great circle on the surface of Earth is any circle whose center is at the center of Earth. Only choice B fits this definition. Choices A, C, and D do not fit the definition of a great circle on the surface of Earth.

Question 20

Einstein's equivalence principle says that

Accepted Answer

Einstein's equivalence principle states that the effects of gravity are the same as the effects of acceleration. This means that if you are in a windowless spaceship and feel a force pushing you to the back of the ship, you can't tell if you are accelerating or if you are in a gravity field. This idea forms the basis of Einstein's theory of general relativity, which describes gravity in terms of the curvature of spacetime.

Quiz 27: Spacetime and Gravity