This passage is excerpted from material published in 1997. Is there a massive black hole at the center of our...
GMAT Reading Comprehension : (RC) Questions
This passage is excerpted from material published in 1997. Is there a massive black hole at the center of our galaxy, the Milky Way? The evidence is inconclusive.
(5) Just as the Sun's mass can be determined, given knowledge of other variables, by the velocity at which its planets orbit, the mass at the center of the Milky Way can be revealed by the velocities of stars and gas orbiting the galactic center. This dynamical
(10) evidence, based on recently confirmed assumptions about the stars' velocities, argues for an extremely compact object with a mass two to three million times the mass of our Sun. Although according to current theory this makes the mass at the center
(15) of the galaxy too dense to be anything but a black hole, the relative lack of energy radiating from the galactic center presents a serious problem. A black hole's gravity attracts surrounding matter, which swirls around the black hole, emitting some energy
(20) as it is engulfed. Scientists believe that the amount of energy that escapes the black hole should be about 10 percent of the matter's rest energy (the energy equivalent of its mass according to the equation E=mc^2). But when the energy coming from the
(25) galactic center is compared to widely held predictions based on how much matter should be falling into a theoretical central black hole, there is a discrepancy by a factor of a few thousand.
The primary purpose of the passage is to
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| This passage is excerpted from material published in 1997. Is there a massive black hole at the center of our galaxy, the Milky Way? The evidence is inconclusive. | What it says: We're investigating whether there's a huge black hole at the center of our galaxy, but the evidence doesn't give us a clear answer. What it does: Sets up the central question and immediately gives us the author's conclusion - this is unresolved. Source/Type: Author's assessment of the scientific evidence Connection to Previous Sentences: This is our opening - no previous connections yet. Visualization: Imagine scientists looking at the Milky Way's center with telescopes, seeing some evidence that could suggest a black hole, but not enough to be certain. Reading Strategy Insight: The author tells us the conclusion upfront! This isn't a mystery - we know the evidence will be mixed. What We Know So Far: There might be a black hole at our galaxy's center, but evidence is unclear What We Don't Know Yet: What is this evidence? Why is it inconclusive? |
| Just as the Sun's mass can be determined, given knowledge of other variables, by the velocity at which its planets orbit, the mass at the center of the Milky Way can be revealed by the velocities of stars and gas orbiting the galactic center. | What it says: Scientists can figure out how much mass is at the galaxy's center by measuring how fast things orbit around it, similar to how we measure the Sun's mass by watching planetary orbits. What it does: Introduces the scientific method being used, with a helpful analogy to make it understandable. Source/Type: Scientific method/factual explanation Connection to Previous Sentences: This builds on the opening by explaining HOW scientists are investigating the black hole question. This is the first piece of "evidence" mentioned. Visualization: Solar System: Planets orbiting Sun at specific speeds reveal Sun's mass = 1 sun mass. Galaxy Center: Stars and gas clouds orbiting at their speeds will reveal the central mass. Reading Strategy Insight: The author helps us with a familiar analogy (planets around the Sun) before introducing the more complex galactic concept. This is simplification, not new complexity. |
| This dynamical evidence, based on recently confirmed assumptions about the stars' velocities, argues for an extremely compact object with a mass two to three million times the mass of our Sun. | What it says: Using the orbital speed method described above, scientists found something incredibly heavy (2-3 million times heavier than our Sun) packed into a very small space. What it does: Gives us the specific results of the orbital velocity method and emphasizes two key features: massive and compact. Source/Type: Scientific findings based on observational data Connection to Previous Sentences: This directly continues the orbital velocity explanation from the previous sentence. "This dynamical evidence" refers back to the velocity measurements just described. Visualization: Imagine our Sun (1 sun mass) vs. the galactic center object (2,500,000 sun masses) squeezed into a tiny space - like comparing a marble to a bowling ball, but the "bowling ball" is compressed to marble size. Reading Strategy Insight: We're getting specific numbers now, but this is still following the logical flow: Method → Results. The evidence is pointing toward something that COULD be a black hole. |
| Although according to current theory this makes the mass at the center of the galaxy too dense to be anything but a black hole, the relative lack of energy radiating from the galactic center presents a serious problem. | What it says: Based on what we know about physics, something that massive and compact should be a black hole. But there's a problem: not enough energy is coming from that area. What it does: Introduces the conflict that makes the evidence "inconclusive" - presents both sides of the puzzle. Source/Type: Scientific theory vs. observational evidence Connection to Previous Sentences: This builds directly on the mass/density findings from the previous sentence but introduces the contradiction that makes the opening statement ("evidence is inconclusive") make sense. Visualization: Theory says: 2.5 million sun masses in tiny space = BLACK HOLE. Reality shows: Very little energy radiating from that spot (much less than expected from a black hole). Reading Strategy Insight: Now we see WHY the evidence is inconclusive! This isn't new complexity - it's explaining the contradiction mentioned in the opening. |
| A black hole's gravity attracts surrounding matter, which swirls around the black hole, emitting some energy as it is engulfed. | What it says: When stuff gets pulled into a black hole, it spins around the black hole and gives off energy before disappearing. What it does: Explains the basic physics of how black holes should produce energy - sets up the explanation for why low energy is a problem. Source/Type: Established scientific understanding about black hole behavior Connection to Previous Sentences: This expands on the "lack of energy" problem from the previous sentence. The author is helping us understand WHY we should expect energy from a black hole. Visualization: Picture water swirling down a drain, but as cosmic matter spirals into the black hole, it heats up and glows, releasing energy that we should be able to detect with telescopes. Reading Strategy Insight: This is background explanation, not new evidence. The author is making sure we understand the physics before giving us the details of the problem. |
| Scientists believe that the amount of energy that escapes the black hole should be about 10 percent of the matter's rest energy (the energy equivalent of its mass according to the equation E=mc^2). | What it says: Researchers think that black holes should release energy equal to about 10% of the mass of all the matter falling into them (using Einstein's famous mass-energy equation). What it does: Provides the specific theoretical prediction for how much energy a black hole should emit. Source/Type: Scientific theoretical prediction Connection to Previous Sentences: This continues explaining black hole energy emission from the previous sentence, now giving us the specific numbers scientists use for their predictions. Visualization: If 100 pounds of matter falls into a black hole, we should detect energy equivalent to about 10 pounds of matter being converted to pure energy (E=mc^2). Reading Strategy Insight: We're still building the "expected behavior" side of the equation. The author is systematically setting up the comparison that will show us the problem. |
| But when the energy coming from the galactic center is compared to widely held predictions based on how much matter should be falling into a theoretical central black hole, there is a discrepancy by a factor of a few thousand. | What it says: When scientists compare the actual energy they observe from the galaxy's center to what they calculate should be there (if there was a black hole), the actual amount is thousands of times less than expected. What it does: Provides the specific magnitude of the problem - quantifies exactly how big the discrepancy is. Source/Type: Observational data compared to theoretical predictions Connection to Previous Sentences: This completes the comparison set up in previous sentences. We now have: Expected energy (10% of matter's rest energy) vs. Actual observed energy (thousands of times less). Visualization: Expected energy from galactic center black hole: 10,000 units. Actually observed energy: 2-3 units. That's the "factor of a few thousand" discrepancy. Reading Strategy Insight: This is the climax of the argument! Now we fully understand why the evidence is "inconclusive" from the opening. The passage has come full circle - everything connects back to that opening statement. What We Know Now: • Evidence FOR black hole: Mass and density measurements • Evidence AGAINST black hole: Energy output thousands of times too low • Result: Inconclusive evidence (exactly what the opening told us) |
2. Passage Summary:
Author's Purpose:
To explain why scientists cannot definitively determine whether there is a massive black hole at the center of our galaxy by showing how conflicting evidence makes the question unresolvable.
Summary of Passage Structure:
In this passage, the author walks us through a scientific puzzle by presenting both sides of contradictory evidence:
- First, the author states upfront that the evidence about a galactic black hole is inconclusive
- Next, the author explains how scientists measure the mass at the galaxy's center using orbital velocities and presents evidence that suggests something massive and compact enough to be a black hole
- Then, the author introduces the major problem with this conclusion by explaining that the energy output from the galactic center is far too low for a black hole
- Finally, the author quantifies exactly how big this energy problem is, showing that observed energy is thousands of times less than what theory predicts for a black hole
Main Point:
Scientists cannot definitively prove there is a black hole at the center of our galaxy because while the mass and density measurements strongly suggest a black hole, the extremely low energy output contradicts what we would expect from such an object.
Question Analysis:
This question asks us to identify the primary purpose of the entire passage - the main reason the author wrote this piece. We need to look at the overall structure and flow, not just individual details.
Connecting to Our Passage Analysis:
- Opening Statement: The passage analysis shows the author immediately states "The evidence is inconclusive" about whether there's a black hole at the galactic center
- Evidence Pattern: The analysis reveals a systematic presentation of conflicting evidence - mass/density measurements suggest a black hole, but energy output contradicts this
- Full Circle Structure: The passage analysis demonstrates how everything connects back to the opening statement about inconclusive evidence
- No Resolution Offered: The analysis shows the author presents the problem but doesn't propose solutions or argue for reframing the question
Prethinking:
Based on our passage analysis, the author's primary purpose is to explain WHY scientists cannot definitively answer whether there's a black hole at our galaxy's center. The author systematically shows both sides of contradictory evidence (orbital velocity data vs. energy output measurements) to demonstrate why the question remains unresolved. The passage doesn't try to resolve the contradiction or present multiple theories - it explains why the existing evidence leaves scientists unable to reach a conclusion.
Why It's Wrong:
• The passage doesn't present "several theories" - it presents two conflicting pieces of evidence about one potential object (a black hole)
• The author focuses on explaining why evidence is contradictory, not on proposing different theoretical explanations
• The passage structure shows explanation of a problem, not presentation of multiple solutions
Common Student Mistakes:
1. Thinking that conflicting evidence equals multiple theories?
→ Recognize that conflicting data about one object (black hole) is different from multiple theoretical explanations
1. Confusing evidence presentation with theory presentation?
→ Focus on whether the author is explaining data or proposing different scientific models
Why It's Wrong:
• The passage doesn't argue for reframing the question - it explains why the current question can't be answered
• No "new evidence" is presented that changes how we should approach the problem
• The author accepts the current framework but shows why it leads to contradictory results
Common Student Mistakes:
1. Thinking "recently confirmed assumptions" means new evidence that changes everything?
→ Note that this refers to confirming existing measurement methods, not revolutionary new findings
1. Confusing explanation of why a question is hard with arguing to change the question?
→ Distinguish between explaining difficulty vs. proposing new approaches
Why It's Wrong:
• The passage does not resolve the inconsistency - it explains why the inconsistency exists and makes the evidence inconclusive
• The author presents the contradiction but offers no solution or resolution
• The passage ends with the problem still unsolved
Common Student Mistakes:
1. Thinking that explaining a contradiction means resolving it?
→ Distinguish between describing a problem and solving it
1. Expecting that scientific passages always provide solutions?
→ Some passages explain why problems exist rather than solving them
Why It's Right:
• The passage directly states and returns to the idea that "evidence is inconclusive"
• The author systematically explains both sides of the contradictory evidence
• The passage structure shows why scientists cannot reach a definitive conclusion
• No resolution is offered - the issue genuinely remains unresolved
Key Evidence: "Is there a massive black hole at the center of our galaxy, the Milky Way? The evidence is inconclusive." Combined with the ending: "there is a discrepancy by a factor of a few thousand" - showing the problem remains unsolved.
Why It's Wrong:
• The passage doesn't challenge the underlying theory of black holes - it shows that current evidence doesn't clearly support OR contradict the theory
• The "problem" isn't with the theory's assumptions but with determining whether those assumptions apply to this specific case
• The passage accepts current black hole theory as valid but shows why we can't definitively identify one
Common Student Mistakes:
1. Thinking that inconclusive evidence means the theory is wrong?
→ Understand that theories can be correct but difficult to apply in specific cases
1. Confusing identification problems with theoretical problems?
→ The issue is determining what exists at the galactic center, not whether black hole theory is correct