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IIT JEE 2006 Sample Question Paper

Passage 3

Gamma ray bursts are bursts of very high energy electromagnetic radiation that astronomers have observed to occur roughly once per week at random locations in the sky. These bursts present a puzzle to scientists and two astronomers, Jesse and Becky, take different positions.
Jesse:
The bursts appear to be isotropic - that is evenly spread out in the sky. But our galaxy is disk shaped, so if the sources were within our galaxy, then they would not appear uniformly across the sky. Therefore, the gamma ray bursts must originate outside our galaxy.
Becky:
The problem is that each of these bursts contains a large amount of gamma radiation. If a single light bulb appeared bright from a large distance, then it would have to be a very powerful bulb. The same thing is true of gamma ray bursts. They appear bright to us, and if they are far away and if the energy emanates from them in all directions, the source must be extraordinarily powerful. We do not know of any mechanism in the universe that could produce that much energy. Therefore, the gamma ray bursts must come from lower energy sources closer to us.

Q. 11 Which of the following is an assumption made by Becky, but not necessarily by Jesse?

1. Gamma ray bursts carry electromagnetic radiation.
2. The energy from a gamma ray burst spreads out evenly in a sphere.
3. Our galaxy is shaped like a disk.
4. The direction of a light source can be determined.

Answer: B

Description: Becky uses this assumption to argue that the source of energy must be enormously powerful. But what if the gamma ray burst were directed in a thin beam at the Earth? Then an astronmer would not have to worry that the energy we see on Earth is only one tiny tiny fraction of the energy of the source. We might see most or all of the energy of the source, as if a light beam were pointed directly at us. The assumption that the gamma radiation emanates in all directions from its source is critical to Becky's argument, but Jesse may well disagree

Q. 12 Which of the following new observations would most weaken the argument of Jesse?

1. Observations from the southern sky showing uniformly distributed gamma ray bursts.
2. Observations of bursts of x-rays coming from sources far outside our galaxy.
3. Observations showing that our galaxy is not actually disk shaped, and is in fact more like a sphere.
4. Observations showing that earlier studies had overestimated the energy coming from gamma ray bursts.

Answer: C

Description: Jesse's essential point is that we see gamma ray bursts spread out in a shape that is different from the shape of our galaxy. If it turns out that our galaxy is not disk shaped, and indeed is spherical, then her argument is considerably weakened.

Q. 13 Which of the following would most weaken Becky's argument that the bursts could not occur far outside our galaxy?

1. New measurements showed that the bursts do occur in a disk shaped pattern.
2. The discovery that astronomical events occurring very close to the Earth also appear uniformly across the sky.
3. The discovery that gamma ray bursts occur twice as frequently as previously thought.
4. The discovery that the collision of black holes could theoretically produce more energy than any other known event.

Answer: B

Description: Becky's argument is based strongly on the fact that we do not know of any astronomical events that could yield the energy required for the gamma ray bursts. If new sources of high energy events are discovered, then her argument is weakened.

Q. 14 The two astronomers disagree most as to:

1. the existence of gamma ray bursts.
2. the timing of the gamma ray bursts.
3. the location of the source of the gamma ray bursts.
4. the amount of energy reaching Earth from the gamma ray bursts.

Answer: C

Description: Jesse presents an argument to show that the gamma ray bursts must occur outside our galaxy; Becky argues that the gamma ray bursts must occur closer to home. The two astronomers differ most about the location of the source of the gamma ray bursts.

Q. 15 Light from sources far from the Earth is often redshifted, which means that it has a longer wavelength. Which of the following, if true, would most support Jesse's position?

1. Radiation from the gamma ray bursts is redshifted similarly to objects known to be within our galaxy.
2. Radiation from the gamma ray bursts is redshifted in a pattern similar to objects known to be far outside our galaxy.
3. The radiation is not redshifted at all.
4. The radiation is redshifted, but not in a way that is similar to any known objects.

Answer: B

Description: If the light from the gamma ray bursts has similar redshift to light from other sources far outside the galaxy, then it would be evidence in favor of Jesse's position that the sources of the bursts originate far outside our galaxy.