Logical Reasoning

PT107 · S4 · Q23 Quasars—celestial objects so far

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Quasars—celestial objects so far away that their light takes at least 500 million years to reach Earth—have been seen since 1963.

Statements

Quasars are so far away that it takes their light 500 million years to reach Earth.

We've seen them since 1963.

conditional (for X, it would have to be Y)

Something that far → steadily burns at rate appearing as Q's do of 90 billions suns

conditional (nothing that is X is Y)

steadily burns at rate → exists less that of 90 billions suns 100 million years

Evaluate

We're looking in all Inference paragraphs for important ideas, usually telegraphed with Conditional, Causal, Comparative, Pivot, or Mathematical language.

On Must Be True, we're especially listening for Conditional and Mathematical (they are more black and white, like the task itself).

In this paragraph we have some conditional ideas in the final two sentences, which chain together.

For quasars to be appearing as they do (given how far away they are), they must be burning at the rate of 90 billion suns, which means they will burn out within 100 million years.

So, we can infer "quasars exist for less than 100 million years". That might be all the answers are testing, but does the first sentence give us anything else to work with?

Yes, actually. It takes light from a quasar 500 million years to reach Earth.

If something happens on a quasar in year 1 billion, then it doesn't reach Earth until year 1.5 billion.

If the universe is currently about 13.8 billion years old. The quasars we're seeing are a snapshot of how they looked 500 million years ago, when the universe was 13.3 billion years old.

What's the relationship between the inference we made, "quasars exist for less than 100 million years", and the fact that ?

Well, if photons left the quasar when the universe was 13.3 billion years old, they don't reach Earth until 500 million years later, when the universe is 13.8 billion years old, then they've been dead for 400 million years!

These quasars were dead by the time the universe was 13.4 billion years old. When we see them in 13.8, we're seeing how they looked when the universe was 13.3.

But we're looking at "a ghost", because even though the light is just now reaching us, we know (from the burn rate of quasars) that they could not possibly still exist, 500 million years later.

Goal

It's pretty far-fetched to proactively derive the inference that the quasars we're looking at are ghosts.

We should be content if we did the more apparent work of synthesizing the conditionals in the final two sentences and getting our brains to say, "Quasars exist for less than 100 million years".

23.

If the statements above are true, which one of the following must also be true on the basis of them?

  1. Instruments in use before 1963

    Speculating Cause

    This might be the reason we didn't see quasars until 1963. But maybe the instruments were sensitive enough and we just weren't pointing them in the right direction. Maybe we weren't looking at the right time of day/night.

    5% picked this

  2. Light from quasars first began

    Too Strong: first reached One-Claim Support

    We only know that they were first observed in 1963. It's possible that photons from quasars have been hitting Earth for millions or billions of years (not the same quasar, of course, since we learned their life spans are 100 million years tops). We should be very suspicious on Inference questions if we're picking an answer that sounds like it's being supported by one individual claim, i.e. "Quasars have been seen since 1963, thus choice (B)". The correct answer almost always involves pulling together multiple claims, so you should feel like you're doing more "legwork" to synthesize 2 or more ideas in order to support your answer.

    5% picked this

  3. Anything that from Earth appears

    Too Strong

    Too Strong: anything that bright One-Claim Support We only know that "anything that bright and that far away" must burn at the rate of 90 billions suns. This answer is saying "anything that bright" burns that hot. The Moon definitely appears from Earth at least as bright as a quasar, so according to this answer, the Moon produces more light than 90 billion suns. We should be very suspicious on Inference questions if we're picking an answer that sounds like it's being supported by one individual claim, i.e. "2nd sentence says "anything that bright produces more than 90 billion suns, thus choice (C)". The correct answer almost always involves pulling together multiple claims, so you should feel like you're doing more "legwork" to synthesize 2 or more ideas in order to support your answer. Answers supported by one-claim are almost always skewing the meaning of what was originally said. Here, what was said was "anything that far away that appears the way quasars do would have to burn like crazy".

    14% picked this

  4. Nothing that is as far

    Too Strong: nothing that far away

    We only know that quasars can't exist for more than 100 million years, because they're burning at a rate that produces more light than 90 billion suns. Other objects that burn at much slower rates could definitely exist for more than 100 million years (for example, there are presumably stars and galaxies at least as far away as quasars and they exist for billions of years).

    16% picked this

  5. Correct

    No quasar that has ever

    Why this is right

    The last two sentences combine to tell us that quasars can't exist for more than about 100 million years. Any quasar that's ever been seen from Earth was seen in the last 60 years (since 1963). Since it takes 500 million years for light from a quasar to reach us, what we've been seeing for the past 60 years is quasars, as they appeared 500 million years ago. But since they only exist for 100 million years, every quasar we've seen has been out of existence for the last 400 million years.

    Skill tested: Must be True · how this choice captures the argument's function is the move to repeat next time.

    61% picked this

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