Question 1

Once a black hole forms, the size of its event horizon is determined only by

Accepted Answer

The size of the event horizon of a black hole is solely determined by the mass inside it. The more the mass, the larger the event horizon. Therefore, choice B is correct. Choice A is incorrect because the size of the star that collapsed into the black hole does not determine the size of the event horizon. Choice C is incorrect because the time since the black hole formed does not affect the size of the event horizon. Choice D is incorrect because the composition of the material that formed the black hole does not affect the size of the event horizon. Choice E is incorrect because the size of the event horizon varies depending on the mass inside the black hole.

Question 2

The region around a black hole where everything is trapped, and nothing can get out to interact with the rest of the universe, is called

Accepted Answer

The region around a black hole where nothing can escape is called the event horizon. At this point, the gravitational pull is so intense that even light cannot escape, making it impossible for anything beyond the event horizon to communicate with the rest of the universe. The singularity is the central point of the black hole where all matter is concentrated to an infinitely small point, but it is not a region that can trap anything. The neutron star radius and gravitational redshift zone are not related to black hole event horizons. "Day-time television" is a humorous and irrelevant option.

Question 3

According to Einstein's general theory of relativity, the stronger a star's gravity,

Accepted Answer

According to Einstein's general theory of relativity, the stronger a star's gravity, the slower time runs near it. This is known as time dilation and has been experimentally verified.

Question 4

The astronomer who first worked out the mathematical description of black hole event horizons was

Accepted Answer

Karl Schwarzschild was a German astronomer and physicist who, in 1916, first worked out the mathematical description of black hole event horizons, which is now known as the Schwarzschild radius.

Question 5

Suppose each of the following objects could collapse into a black hole. Each black hole would have a sphere around it that is the limit for escape -- once you are inside this region, you cannot get away. For which object would this region be the largest in diameter?

Accepted Answer

The size of the region from which nothing can escape around a black hole, also known as its event horizon, is directly proportional to its mass. Therefore, the black hole formed by an entire galaxy of stars, with a mass of about a billion times that of our Sun, would have the largest event horizon.

Question 6

Which of the following statements about the way the mass of a white dwarf affects spacetime is correct?

Accepted Answer

According to Einstein's theory of general relativity, all masses, including white dwarfs, curve spacetime. Light will follow the curvature of spacetime caused by the white dwarf's mass.

Question 7

Deep inside a black hole (and hidden from our view) is the compressed center, where all the "stuff" of the star goes. Astronomer call this central point

Accepted Answer

The compressed center of a black hole is called a singularity, where the laws of physics as we know them break down.

Question 8

In the far future, a starship becomes trapped inside the event horizon of a black hole. Although the crew discovers that their ship cannot out, they at least want to send a message to other ships in the area to stay away from the danger zone. If they send out a message in the form of a radio wave, what will be its fate?

Accepted Answer

Once inside the event horizon, even light cannot escape the immense gravity of a black hole. Therefore, the radio wave message will never be able to emerge from the event horizon and reach other ships in the area.

Question 9

According to the general theory of relativity, light and other radiation coming from a white dwarf or a neutron star should (and experiments show that it does) exhibit

Accepted Answer

According to the general theory of relativity, light and other radiation coming from a white dwarf or a neutron star should exhibit a gravitational redshift, meaning that the radiation is stretched to longer wavelengths due to the presence of strong gravitational fields. This has been observed in experiments.

Question 10

The equivalence principle (principle of equivalence) says that

Accepted Answer

The equivalence principle states that the effects of gravity are equivalent to the effects of acceleration. This means that there is no experiment that can distinguish between being in a gravitational field and being in an accelerating reference frame. This principle was first formulated by Albert Einstein and is a cornerstone of his theory of general relativity.

Question 11

From which of the following will a wave of light show the greatest gravitational redshift:

Accepted Answer

The gravitational redshift of a wave of light increases as the gravitational field it passes through becomes stronger. A white dwarf is an extremely dense object with a very strong gravitational field, so a wave of light passing near a white dwarf will experience the greatest gravitational redshift compared to the other objects listed.

Question 12

If you are in a freely falling elevator near the top of a tall building, as the elevator falls, your weight would be:

Accepted Answer

In a freely falling elevator, you and the elevator are accelerating downward at the same rate due to gravity. This creates a condition of free fall, where you would experience weightlessness, as there is no normal force acting on you from the elevator floor.

Question 13

Einstein suggested that the regular change (advance) in the perihelion of the planet Mercury could be explained by:

Accepted Answer

Einstein's theory of general relativity predicted that the mass of the Sun would warp the fabric of spacetime, causing the planets to follow curved paths. This would lead to a gradual precession of the orbit of Mercury, which was observed and confirmed to match Einstein's prediction.

Question 14

When astronauts aboard the International Space Station (ISS) in space let go of an orange, it just floats there. Why is that?

Accepted Answer

The ISS is in orbit around the Earth, which means it is falling around the Earth. When something is in free fall, it feels weightless, so the orange just floats there. This is because the force of gravity and the force of the orbital motion are perfectly balanced, creating microgravity.

Question 15

According to the general theory of relativity, the presence of mass

Accepted Answer

According to the general theory of relativity, the presence of mass causes a curvature (or warping) of spacetime. This curvature is what causes objects to move on curved paths around massive objects like planets and stars. It also predicts phenomena like gravitational lensing, where light is bent by the gravitational field of a massive object.

Question 16

To predict whether a star will ultimately become a black hole, what is the key property of the star we should look at?

Accepted Answer

The key property to look at to predict whether a star will become a black hole is its mass. Only stars that have a mass greater than 20 times that of the Sun have the potential to become black holes. This is because a star's mass determines how much pressure and temperature build up at its core, where nuclear fusion occurs. If the core's temperature and pressure become high enough, the fusion of lighter elements into heavier ones will create an iron core. Since iron cannot undergo fusion to release energy, the core collapses, triggering a supernova explosion that can either leave behind a neutron star or a black hole, depending on the star's mass.

Question 17

When Einstein proposed his General Theory of Relativity, he suggested some pretty strange ideas about space, time, and gravity. How did scientists in 1919 show that Einstein's theory described the behavior of the real world and wasn't just a crazy hypothesis?

Accepted Answer

The observation of starlight coming close to the Sun during an eclipse, known as the bending of starlight, provided evidence for the bending of space-time predicted by Einstein's General Theory of Relativity. This observation confirmed that Einstein's theory was a valid description of the behavior of the real world.

Question 18

Which of the following can a black hole not "eat" (swallow)?

Accepted Answer

Black holes can technically "swallow" anything that comes too close to its event horizon, including planets, stars, and even other black holes. This is because the gravitational pull is so strong that it traps even light particles. Therefore, the correct answer is E, as black holes can technically "eat" anything.

Question 19

When a light wave leaves a region of strong gravity, compared to the same wave leaving a spaceship in empty space, the wave in strong gravity will have

Accepted Answer

When a light wave leaves a region of strong gravity, it experiences a gravitational redshift. This phenomenon is a result of the general theory of relativity and implies that the light's wavelength becomes longer, its frequency becomes lower, and it loses energy as it climbs out of the gravitational well. Thus, all of the statements A, B, C, and D are correct descriptions of what happens to the light wave, making E the correct choice.

Question 20

In 1959, Pound and Rebka did an experiment to test the prediction of Einstein's theory of general relativity about the relationship between the pace of time and the strength of gravity. When two identical atomic clocks, one on the ground floor and one on the top floor, were compared,

Accepted Answer

Einstein's theory of general relativity predicts that time runs slower in stronger gravitational fields. Since gravity is slightly stronger on the ground floor than on the top floor, the clock on the ground floor ran a tiny bit slower.

Quiz 24: Black Holes and Curved Space-Time