LSAT 93 - Section 2 - Question 07

This question is a great example of how recognizing the contrapositive argument structure can be helpful. If you spot it in this problem, then you’re more likely to anticipate what we’re looking for in the answer choices.

The question stem says:

Which one of the following most logically completes the argument?

This is a Fill in the Blank question. We can see that it’s a Main Conclusion-Most Strongly Supported type of Fill in the Blank, because the blank appears in a sentence that begins with “Apparently…”. That word suggests that the rest of the statement will be something that follows from the previous line. It’s apparent from the previous lines. So let’s get into the stimulus.

The mu mesons generated by cosmic rays just outside Earth’s atmosphere travel to Earth at speeds approaching the speed of light. Mu mesons generated in the laboratory, however, are nearly at rest.

What in the world is a “mu meson”? Don’t worry, it doesn’t matter. The LSAT is trying to confuse us by using weird scientific terms. Focus instead on the contrast the first two sentences introduce. The mu mesons generated by cosmic rays just outside the Earth’s atmosphere travel very fast to Earth – near the speed of light. The mu mesons generated in the lab are “nearly at rest” – that seems to mean they’re not moving fast. So some mu mesons are super fast, and other ones are very slow.

We do need to keep track of which mu meson is which – but the description associated with each mu meson is too long to keep in our minds easily. It’s OK to condense the description and lose some of the detail, as long as you know that’s what you’re doing. So I’ll say that the mu mesons generated outside the Earth’s atmosphere are fast. The mu mesons in the lab are slow.

Mu mesons generated in the laboratory typically decay in much less time than it takes for a mu meson to travel from just outside Earth’s atmosphere to our detection apparatus on Earth.

This is a comparative statement that we have to unpack. It’s telling us about mu mesons generated in the lab and how fast they decay. What is their decay speed? Faster than it takes for a mu meson to travel from just outside Earth to some detection thing on Earth. Faster than other mu meson’s travel speed.

To help me understand this, I’ll come up with some fake numbers. Let’s say a mu meson in a lab decays in 10 seconds. That means a mu meson traveling from just outside Earth’s atmosphere to the detection thing on Earth takes longer - more than 10 seconds.

I have absolutely no idea where this is going. But let’s press on.

If mu mesons traveling through the atmosphere at speeds approaching the speed of light typically decay as fast as they do in the laboratory, then we should detect only about one one-hundredth of the number we actually do detect.

There’s a lot going on in this sentence. First, it’s a conditional statement. If the first part is true, then the second part is true. Second, it gives us enough to trigger the contrapositive of that statement.

Let’s break it down.

If the outside-atmosphere mu mesons typically decay as fast as they do in the laboratory, then we should detect a much smaller number – say, 10 mu mesons. That’s just a number I’m making up to help me understand this.

But the statement says “we should detect only about one-hundredth of the number we actually do detect.” That bolded part is telling us a fact: we detect more than what we should. So we actually detect 1,000 mu mesons, not 10.

That fact is what triggers the contrapositive. Since we know that we detect far more than 10 mu mesons, that means that the outside-atmosphere mu mesons do not typically decay as fast as the lab mu mesons. If they did decay as fast as the lab ones, then we’d see only 10. But we see a lot more than 10.

At this point, we’ve recognized the contrapositive inference. So we can expect that the conclusion of this argument, which is in the next line, is likely to have something to do with that inference.

Apparently, mu mesons moving at speeds near the speed of light ___________.

What makes sense to say about mu mesons moving at speeds near the speed of light? Remember, those are the mu mesons from outside the Earth’s atmosphere. The sentence immediately before this line allowed us to conclude that those mu mesons do not decay as fast as the lab mu mesons. So this blank should probably be filled by that idea or something close to that idea – these mu mesons do not decay as fast as the lab mu mesons.

Answer Choice (A) take longer to reach Earth than was supposed

Nothing in the stimulus causes us to question how fast the mu mesons really take to reach Earth. We know that they travel to Earth at speeds approaching the speed of light – that’s from the first sentence. No other statement undermines that claim. Although the statement immediately before the last sentence does suggest that we see a lot more mu mesons than we would expect if the outside-atmosphere mu mesons decayed faster, that statement doesn’t imply anything about how fast the outside-atmosphere mu mesons move. It only tells us something about how fast they decay (which means to break down).

Answer Choice (B) are quite difficult to detect with available equipment

The idea of difficulty of detection seems to come out of nowhere. The stimulus does mention the idea of detecting mu mesons with some kind of apparatus. And we do know that we detect a lot more mu mesons than we should if the outside-atmosphere mu mesons decayed more slowly. But what does any of this have to do with difficulty of detection? If you picked this answer, you must be making some kind of assumption connecting something the stimulus to difficulty of detection. Ask yourself what connection you’re making and whether that’s reasonable.

Answer Choice (C) are much less numerous than previously thought

This answer almost sounds like it goes against the stimulus. We know that we are detecting more mu mesons than we should if the outside-atmosphere mu mesons decayed more slowly. So if the stimulus did suggest anything about what our prior expectations were, it would seem that we actually expected to detect far fewer mu mesons. But we actually found more. So it’s wrong to say that mu mesons are “much less numerous than previously thought.” It would make more sense to say that they are more numerous than previously thought.

In any case, we actually don’t have enough in the stimulus to say anything about prior expectations. Notice that the stimulus never suggests anything about what people previously thought or expected. Although we do have a conditional that starts:

If mu mesons traveling through the atmosphere at speeds approaching the speed of light typically decay as fast as they do in the laboratory…

This doesn’t imply that anyone actually used to think that those mu mesons decayed as fast as they do in the lab. We don’t know what anyone expected about the speed of decay. I can say, “If today were the 1980s, we wouldn’t be able to use the Internet.” That doesn’t mean anyone actually thinks that today is in the 1980s.

Correct Answer Choice (D) decay more slowly than mu mesons almost at rest

This is correct, because it matches the contrapositive inference that we can expect from the sentence immediately before the sentence with the blank. Remember, we were looking for the idea that the outside-atmosphere mu mesons do not decay as fast as the lab mu mesons. The beginning of the stimulus told us that the lab mu mesons “are nearly at rest.” This answer is trying to throw us off by using the “nearly at rest” idea instead of saying “lab mu mesons.” But based on the stimulus, the lab mu mesons are the ones that are nearly at rest. So this answer is the same as one that says “decay more slowly than the lab mu mesons.”

Answer Choice (E) are probably not generated by cosmic rays

We have no basis to say that the fast-traveling mu mesons are not created by cosmic rays. The stimulus tells us that mu mesons generated by cosmic rays outside the Earth’s atmosphere travel fast. But whether there are other fast ones that are not generated by cosmic rays is a complete unknown.