Black holes.

[himu]

A black hole is a region of spacetime from which gravity prevents anything, including light, from escaping. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return. The hole is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics. Quantum field theory in curved spacetime predicts that event horizons emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of a stellar mass (the mass of our sun) or greater.

Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. There is a general consensus that supermassive black holes exist in the centers of most galaxies.

Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as light. Matter falling onto a black hole can form an accretion disk heated by friction, forming some of the brightest objects in the universe. If there are other stars orbiting a large black hole, their orbit can be used to determine its mass and location. These data can be used to exclude possible alternatives (such as neutron stars). In this way, astronomers have identified numerous stellar black hole candidates in binary systems, and established that the core of our Milky Way galaxy contains a supermassive black hole of about 4.3 million solar masses

--------------------------------------------------------------------------------------------------

The author discusses quantum field theory in order to ________________

Highlight the difficulty in detecting some black holes

More clearly define what constitutes a black body in thermodynamics

Better explain why black holes are "black"

Help the reader better understand the general theory of relativity

Confirm that a black body in thermodynamics is similar to a black hole in space
-----------------------------------------------------------------------------------------------------

The passage suggests that neutron stars ____________________

are more common than black holes

are found throughout the Milky Way galaxies

can be confused for black holes

are not as bright as the accretion disk in a black hole

have a visible interior
--------------------------------------------------------------------------------------------------

According to the passage, all of the following would be helpful in discovering new supermassive black holes in the universe except _______________________

Observing interactions with matter

Observing interactions with electromagnetic radiation

Observing levels of emitted radiation

Analyzing the orbits of stars

Focusing on the centers of major galaxies

[bubbliiiiiiii]

A black hole is a region of spacetime from which gravity prevents anything, including light, from escaping. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return. The hole is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics. Quantum field theory in curved spacetime predicts that event horizons emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of a stellar mass (the mass of our sun) or greater.

Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. There is a general consensus that supermassive black holes exist in the centers of most galaxies.

Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as light. Matter falling onto a black hole can form an accretion disk heated by friction, forming some of the brightest objects in the universe. If there are other stars orbiting a large black hole, their orbit can be used to determine its mass and location. These data can be used to exclude possible alternatives (such as neutron stars). In this way, astronomers have identified numerous stellar black hole candidates in binary systems, and established that the core of our Milky Way galaxy contains a supermassive black hole of about 4.3 million solar masses

--------------------------------------------------------------------------------------------------

The author discusses quantum field theory in order to ________________

Highlight the difficulty in detecting some black holes

More clearly define what constitutes a black body in thermodynamics

Better explain why black holes are "black"

Help the reader better understand the general theory of relativity

Confirm that a black body in thermodynamics is similar to a black hole in space
-----------------------------------------------------------------------------------------------------

The passage suggests that neutron stars ____________________

are more common than black holes

are found throughout the Milky Way galaxies

can be confused for black holes

are not as bright as the accretion disk in a black hole

have a visible interior
--------------------------------------------------------------------------------------------------

According to the passage, all of the following would be helpful in discovering new supermassive black holes in the universe except _______________________

Observing interactions with matter

Observing interactions with electromagnetic radiation

Observing levels of emitted radiation

Analyzing the orbits of stars

Focusing on the centers of major galaxies

[bubbliiiiiiii]

What is source of this question?

[himu]

Veritas !
[spoiler]
A C C



Solution: A

Explanation: This difficult "function" question requires that you look at the context around quantum field theory in the first paragraph. In the sentence before, you learn why a black hole is "black" through a comparison between black holes and black bodies in thermodynamics. What follows is used to introduce a NEW discussion on how the similarity of black holes with black bodies in terms of temperature (explained with the quantum field theory) makes it difficult for black holes to be detected with radiation. As a result (A) is correct. For (B) and (C), quantum field theory does nothing to help you understand more why black holes are black and it does not more clearly define a black body. For (D), its discussion has nothing to do with general relativity and for (E), it does not CONFIRM their similarity, but rather introduces temperature similarities to highlight difficulties in detection with radiation. Answer is (A).
---------------------------------------------------------------------------------------------------


Solution: C

Explanation: The key to this question lies in the sentence "These data can be used to exclude possible alternatives (such as neutron stars)," which follows data about mass and location of orbiting stars. If that data is used to EXCLUDE neutron stars then it must be true that neutron stars are sometimes confused for black holes. (C) is correct. For (A), no information is given about the frequency in comparison to black holes. Likewise for (B) and (D) you are told nothing about the frequency of neutron stars in the Milky Way or whether neutron stars are bright. Lastly, you have no idea whether neutron stars do or do not have a visible interior.

----------------------------------------------------------------------------------------------

Solution: C

Explanation: It is relatively easy to prove that (A) and (B) would be helpful in discovering new supermassive black holes because of this sentence in the third paragraph: "the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as light". (D) can be proved from the following sentence: "If there are other stars orbiting a large black hole, their orbit can be used to determine its mass and location" and (E) can be proved form this sentence: "There is a general consensus that supermassive black holes exist in the centers of most galaxies." While radiation is discussed in the first paragraph, you learn that the larger the black hole, the harder it is to measure any radiation from that black hole. Answer is (C).
[/spoiler]