PrepTest 107, Section 4, Question 23
Quasars—celestial objects so far away that their light(66 more words not shown)
If the statements above are(15 more words not shown)
- AInstruments in use(12 more words not shown)
- BLight from quasars(6 more words not shown)
- CAnything that from(20 more words not shown)
- DNothing that is(18 more words not shown)
- ENo quasar that(9 more words not shown)
Reading the argument
We are seeing very old light.
The word “quasar” may be unfamiliar, but the passage gives us the facts needed. Its light takes at least 500 million years to reach Earth. So seeing one does not mean seeing what it looks like right now. We are receiving light that left it at least 500 million years ago.
Now combine that delay with the lifespan. To look that bright from that far away, a quasar must produce an enormous amount of light. The passage says nothing producing that much light can exist for more than about 100 million years. Even if the light started toward us at the very beginning of the quasar's life, the quasar would be gone long before the light finished the trip.
What we’re looking for
That is my prediction for what must be true: any quasar we have seen has already ceased to exist. The 1963 observation date does not change the comparison. Five hundred million years of travel is far longer than a roughly 100-million-year maximum life. We do not need to know why people first observed quasars in 1963 to connect those two facts.
Working the answers
- A
Perhaps earlier instruments were inadequate. But perhaps observers had not looked in the right place or recognized what they were seeing. The passage gives the observation date without establishing its cause. We cannot select one possible explanation and treat it as something that must be true.
- B
First seeing something is not the same as its light first arriving. The light could have reached Earth before anyone observed it. The passage dates our observations, not the arrival of the earliest quasar light.
- C
Distance matters. The original requirement concerns something as far away as a quasar appearing that bright. A much closer object could appear equally bright while producing less light. C drops the distance condition and applies the claim to anything with that apparent brightness.
- D
The lifespan limit is attached to producing that much light, not merely being far from Earth. An object at the same distance that does not burn at the specified rate is not covered by the limit. D replaces the relevant condition with a different one.
- ECorrect
Every observed quasar had to send its light on a trip lasting at least 500 million years. Yet the passage gives such an object a maximum existence of about 100 million years. By the time its light reaches us, that maximum lifespan is already exceeded. The light can still arrive and be seen even though the object that emitted it is gone.
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