## GRE exam : Possible Problem Error

• If you want to know something about the GRE subject test in physics then chances are you will find it in here.
• If something about the physics GRE it isn't already discussed in here then please put it in here.

arturodonjuan
Posts: 2
Joined: Wed Oct 05, 2016 2:51 pm

### GRE exam : Possible Problem Error

One of the problems asked for the dominant source of elements heavier than iron in the universe.
Contrary to what many people might guess, the dominant source of elements heavier than iron is actually neutron capture in asymptotic giant branch (AGB) stars (http://physics.stackexchange.com/questi ... an-iron-fe). I didn't see that answer on the test; am I missing something?

If it's a mistake, who could/should I contact regarding this?

TakeruK
Posts: 941
Joined: Mon Jan 02, 2012 3:05 pm

### Re: October 29th 2016 exam : Possible Problem Error

I am not sure if it's allowed to discuss specific questions from the exam on this board. I know in the past, the staff have removed such posts.

But while this is here, I will answer your concern without specific details to the question.

I do not think there is a mistake in the question. In the linked post, the process that you mentioned is responsible for "about 50%" of subject of the question. The other answer, one that is given as a potential choice, is probably responsible for the other ~50%.

This leads to two possibilities where the exam's choices are correct:

1. When both pathways are 50%, then either of them can be dominant. Since only one of the two pathways are given as choices, then the best answer is clearly the one that is an allowed choice.

2. Maybe there were other qualifiers that you misremembered? For example, is the question asking for the pathway that creates more mass of the subject of the question or more number of atoms of the subject of the question? Is it asking for a specific type of object, or a specific region, or the entire Universe? etc.

If you still feel like there was a potential problem in the exam, then you could contact ETS. Check their website for the contact information. If you look at old PGRE practice books, you will see that in some previous exams, questions have been removed from the test (and thus not counted) because of mistakes or ambiguities like this one.

arturodonjuan
Posts: 2
Joined: Wed Oct 05, 2016 2:51 pm

### Re: GRE exam : Possible Problem Error

I emailed ETS, and they responded. Here's a receipt of the correspondence.

My email:

"Hello ETS,

Here is the pertinent information:
• Name: [-]
Test Center: [-]
Test Date: October 29th, 2016
Test Name: GRE Physics Test
Question Number: I don't remember exactly - somewhere in the middle
Question Content: The question was the following:
• About half of all elements heavier than iron in the universe originated from:
(A) The big bang
(B) Nuclear fusion in main sequence stars
(C) Nuclear fission in main sequence stars
(D) Nuclear fission in asymptotic giant branch stars
(E) Neutron capture in supernovae explosions
I don't think any of these answer choices are correct. About half of elements heavier than iron come from the s-process in asymptotic giant branch (AGB) stars [1], and half come from the r-process. But the dominant site for the r-process is unknown! It was believed for a long time that the dominant site was core-collapse supernovae [2], but with recent observations of black-hole mergers [3] and intensive neutron star calculations [4] this belief has been skewed.

So, in summary, I believe the intended answer was (E), neutron capture in supernovae explosions, but anybody who works in nuclear astrophysics knows that that is not so simply true. In fact, it may be completely false. I've conducted research on nuclear reactions pertinent to the s-process in AGB stars, so I was aware of the complications of this question. I was quite confused when I came across this question, and ended up choosing choice (D) because I figured that maybe it was intended to say "Neutron capture in asymptotic giant branch stars".

References:
[1] http://iopscience.iop.org/article/10.10 ... 012021/pdf
[2] http://iopscience.iop.org/article/10.10 ... .text.html
[3] http://journals.aps.org/prl/pdf/10.1103 ... 116.241103
[4] http://journals.aps.org/prc/pdf/10.1103 ... .92.055805"

ETS's response:

"Your inquiry regarding a question on the Graduate Record Examinations (GRE) Physics Test that was administered on October 29, 2016 was reviewed by the ETS assessment specialists responsible for the test, as well as by the external GRE Physics Committee of Examiners.