Q23 - Quasars - celestial objects so far

[mlee.cortez]

I'm having a hard time understanding or distinguising between answer choices letter D and letter E. Could you please explain the main reasons/factors as to why E is *the* answer and why D is incorrect.

Thanks!

[aileenann]

This question is asking us what must logically follow from the premises listed in the LR argument. You ask what distinguishes answer choices (D) and (E), listed below:

(D) Nothing that is as far from Earth as quasars are can continue to exist for more than about 100 million years.
(E) No quasar that has ever been seen from Earth exists any longer.

Generally, I think it’s easier to work from wrong to write on LR questions. That way, if a question is difficult to parse we can just skip over it in favor of getting rid of the wrong answers. For that reason, our first approach should be to figure out whether either (D) or (E) is manifestly wrong.
Let’s look at (D) first. (D) scares me a little bit because of the word "nothing" _ that is a very broad word, meaning that in choosing (D) I would be making statements not only about quasars but literally about every conceivable thing. True, there is some basis in the premises to think we can talk about more than quasars ("for anything that far away to appear from Earth the way quasars do..." and "nothing that burns at a rate that produces..."). So it might not necessarily be wrong to talk about every conceivable thing, but do we have a basis for the statement in (D)?
Notice that the two sentences we have that go beyond the scope of quasars still don’t actually speak of all objects _ rather they speak only of objects that appear as quasars do or that burn for a sufficient period of time. In contrast, answer choice (D) just talks about any object (not just burning objects) that are sufficiently far from Earth _ still exceeding the scope of the premises. For this reason, we cannot necessarily conclude the statement in (D) because it is simply unsupported. This is subtle and something you might have passed by in a first run through the answers to find obvious wrong answers, but on a second run through this should stand out.
Additionally, with respect to (D), think about what the statement is saying. It is limiting the age of any object very far away from Earth _ which doesn’t make sense at all, again because all the premises discuss is burning objects. The premises don’t tell us, for example, how old a black hole could be or even some cosmic speck of dust that just happens to be very far from Earth.
Now let’s look at (E) and think about why it’s a good answer. First, as far as thinking about scope, I like something strictly about quasars because it should be easier to evaluate and seems more likely to have support in the premises, since the premises do discuss quasars. Additionally, looking at (E), I realize I can actually check it based on my premises. I know that it takes 500 million years for light from a quasar to reach Earth, and I also know that nothing _ not even a quasar _ can burn for more than about 100 million years (the third sentence). Therefore, I know that by the time quasar light reaches Earth, the quasar might have been gone for at least 400 million years, even if I am actually seeing the very first light it produced. Therefore (E) must be true by the very arithmetic I can do based on the premises.
If you are like many LSAT students out there, one reason this is a challenging problem may be that it is about a technical scientific field. However, I would really urge you not to let this trump your logical reasoning abilities. Whenever you feel intimidated by a science passage, just try making analogies to another more concrete thing. For example, this argument doesn’t work too differently from something like the following:
I have been receiving care packages from my parents ever since I went to college. For any object in a care package to arrive from my parents, it would have to travel 2,000 mails by the United States post office in an airplane and over bumpy roads for 5 days. But no ice cream that I would be willing to eat could possibly last such a long time and throughout an arduous journey.
Therefore, any ice cream I receive from my parents in a care package is no longer really ice cream or worth eating. (this conclusion is analogous to answer choice E _ do you see why).
Thanks for your question, and please let me know if you would like any further clarification!

[mlee.cortez]

Thank you. Very clear explanation!

[backupbecool]

Hi, I actually got this question right, because I knew that E was more sure than my other option C. But for I still can't figure out why C is not right. It seems like according to the presises that C could also be right as well. Is it the "anything" wording? I think that might be it.

[ManhattanPrepLSAT1]

Really good question backupbecool!

Answer choice (C) is really close and in fact would be correct if it read, "Anything as far away as a quasar and as bright as a quasar must produce more light than would be produced by 90 billion suns."

The problem is that the stimulus qualifies the statement with information about the object being as far away as quasars, whereas answer choice (C) does not.

Theoretically, a quasar so far away would appear very faint. So an object that was closer, would not necessarily need to be so bright as 90 billion billion suns worth of light. For example, the moon might be brighter and be produced with less light, because it is so near in our sky.

Does that help clear things up?

[ptewarie]

these are some solid explanation.

The one thing to quickly point out that there is a subtle scope shift that might have gone unnoticed:

Stimulus: For anything that far away to APPEAR from earth the way Quasars do it would have to burn steadily at a rate....

D says: Nothing that is as far away as quasars can continue to exist for more than about 100 million years.

The stimulus talks about things that appear whereas D talks about generalizations.

[esnanees]

I seem to not be so clear why E is 100% correct. I understand why the rest of the ans are wrong - my qt is, if Quasars existence is only 100mil yrs, why will the stimulus mention that, its light takes about 500+mil years to reach the earth. so say we are referring to a particular quasar, it will take 500 years for the light to reach the earth, why on earth will the existence be 100mil, wouldn't it more than that? Is there something i'm missing, please help.

[mjacob0511]

Quasars are so far away that their light takes at least 500 million years to reach earth. In order for it to be possible to even see the quasars light, the quasar would have to burn at a rate that produces more light than 90 billion suns. But nothing that burns at this rate could last for more than 100 million years.

Well this is clearly saying that if we see light from a quasar (which takes 500 million years to reach us), it must have burned at the 90B X the sun which means the quasar didn't survive for more than 100 million years.

So any quasar that is visible must have burned out at least 400 million years before we can see it.