#16 - Comets do not give off their own light but reflect

[eyeofthetiger]

Understood! Thanks very much for the prompt and clear response!

[yrivers]

Hi, I see a lot of explanations here. Sorry to tag additional questions, but just need clarification on my C is incorrect. Adam (PowerScore Staffer) generously explained above, but to me, C still is valid. Halley's total amount of light reflected is less than scientists previously thought.

Is it incorrect simply because it's not the main point (although still accurate) of the stimulus? Thank you.

[Adam Tyson]

Actually, the amount of light reflected hasn't changed one bit! It's what that amount of light tells us about the mass that has changed.

Imagine that we previously thought that for every unit of light reflected, the comet had one unit of mass. Let's say that we measured 1 unit of light reflected. The old estimate, then, was that the comet had 1 unit of mass. But, now we have discovered something - the stuff the comet is made of is less reflective than we thought by a factor of 60. We still have 1 unit of light reflected, but our new info about the mass:light ratio tells us that the comet actually has 60 units of mass!

There's nothing here to suggest that we are wrong about the total amount of reflected light. Rather, it's the ratio of that light to mass that has changed, affecting our estimate of the mass based on the same amount of light.

I hope that sheds a little light on things for you!

[PositiveThinker]

Ive read this thread multiple times and this makes no sense to me.


Premise: the greater the mass, the greater the brightness/reflection...


"oh wow our amazing new probe shows that Haleys comet is giving of 60times LESS per unit of mass than we had previously thought.." so that must mean the comet is smaller than we estimated.


If its a one to one ratio and we find out our ration is smaller, how on earth does the comet get bigger in mass?

Also, I see the explanation and i see the assumption that the mass didn't change when the reflective light became smaller.. How do you know that? How do you know from the stimulus that the brightness didn't go down and that the size of comet had stayed the same?

[Jonathan Evans]

Hi, Positive Thinker,

Good question! Let's see whether we can break this stimulus down to determine exactly what we could prove on its basis:

1. Comets reflect light from other sources.
2. The mass of comets can be estimated from the amount of light they reflect, i.e. the more light a comet reflects, the greater its mass.
3. Halley's comet is composed of material that is much less reflective than had been previously thought.

Now, what could we prove? Ask yourself how we could combine this information to come up with a possible inference. How is it significant that Halley's comet is less reflective than what had been thought?

Think about it this way: let's say Halley's comet has a reflectivity of "100." In the past, scientists estimated the mass of Halley's comet based on this reflectivity of 100. They supposed that a certain amount of mass, say 10 kg, would indicate a reflectivity of 100. However, now we've discovered that based on the composition of Halley's comet, it would take far more mass to have this same reflectivity of 100, say 600 kg instead (since the material that makes up Halley's comet is less reflective than predicted). Thus, Halley's comet must be larger than expected to have this same reflectivity of 100. Instead of taking only 10 kg to have a reflectivity of 100, based on the materials in Halley's comet, it would take 600 kg to have this reflectivity.

I hope this helps!

[lsnewbie]

Hey eyeofthetiger,

Thanks for the question! Here's what you might be missing: we still perceive the same amount of light being reflected as we did before the new estimate (the comet didn't suddenly become 60x dimmer). So, if each unit of mass reflects 60 times less light, but the total amount of light reflected remains the same, then we obviously need a lot more units of mass! Hence, the comet must be much larger than previously thought.

Does that clear things up?

Hi PS,
I think I finally understand the reasoning for why answer choice B is correct, but I want to know how we're supposed to figure out that the total amount of light reflected remains the same. For one, it's not explicitly stated in the stimulus. When I read the stimulus, I thought less light reflected means less mass and selected answer choice C. I know that during the exam, I wouldn't even consider the fact that the total amount of light reflected remains the same. This is so frustrating! I can't imagine reading between the lines when you have less than a minute thirty for each question and then bringing in unstated inferences/assumptions. How do I learn the trick of this trade? Thanks in advance for any guidance.

[Malila Robinson]

Hi lsnewbie,
The 'light that is reflected,' refers to the light that has already been seen/observed/recorded by some method. Whatever exists in the record about the recorded brightness of the reflected light is not going to 'suddenly' change simply because the method for calculating the mass of whatever is reflecting that light, changes. It will still be the same brightness, but whatever is causing the brightness may have changed, and in this case it would have changed in relation to the hypothesized ratio of higher brightness reflection being positively related to a comet's larger mass.
Hope that helps!
-Malila

[lanereuden]

The way i think about it:

Believed to be: 1:60 ratio (1 mass units: 60 light units)
(Also, you say, it was believed to be 2:120)

Actuality it is: 1:1 ratio
Therefore,
It could be the case that the previous estimates were too low, I.e.
1:60 and not 2:120
BUT Is this an accurate explanation?

[lanereuden]

Hi, Positive Thinker,

Good question! Let's see whether we can break this stimulus down to determine exactly what we could prove on its basis:

1. Comets reflect light from other sources.
2. The mass of comets can be estimated from the amount of light they reflect, i.e. the more light a comet reflects, the greater its mass.
3. Halley's comet is composed of material that is much less reflective than had been previously thought.

Now, what could we prove? Ask yourself how we could combine this information to come up with a possible inference. How is it significant that Halley's comet is less reflective than what had been thought?

Think about it this way: let's say Halley's comet has a reflectivity of "100." In the past, scientists estimated the mass of Halley's comet based on this reflectivity of 100. They supposed that a certain amount of mass, say 10 kg, would indicate a reflectivity of 100. However, now we've discovered that based on the composition of Halley's comet, it would take far more mass to have this same reflectivity of 100, say 600 kg instead (since the material that makes up Halley's comet is less reflective than predicted). Thus, Halley's comet must be larger than expected to have this same reflectivity of 100. Instead of taking only 10 kg to have a reflectivity of 100, based on the materials in Halley's comet, it would take 600 kg to have this reflectivity.

I hope this helps!

So ratio-wise we have 1/10 and 6. What do these mean?

[Adam Tyson]

I think you may have the ratio reversed, lanereuden. If we previously thought it was 1 unit of mass to 1 unit of light, we now have discovered that it is 60 units of mass to 1 unit of light. In other words, to produce the same amount of reflected light, the comet has to be 60 times the mass that we originally believed it was. It's not 1:1 changing to 6 - it's 1:1 changing to 60:1.

If the ratio of mass to light is 60 times larger than we thought, and if the amount of light remains constant (we still see what we see, after all), then there must be more mass.

Again, going back to your earlier post, and correcting your numbers:

Believed to be: 1:1 ratio (1 mass units: 1 light units)

Actuality it is: 60:1 ratio

Here's an analogy, because I love those so much:

I hired a painter to paint my barn. The job was completed in one hour. I thought a single painter could do the whole job in an hour, so I concluded that he worked alone. Later, I discovered that an individual painter is actually 60 times slower than I thought. What can I conclude? I know the barn was painted in an hour, but now I know one guy could not have done it, not even close. There must have been more people doing the painting. Looks like it required a crew of 60 painters to finish my barn in that one hour.

I hope this clears things up!