How to tell someone, that someone else "can"not answer a question I asked multiple times?

Question

I work at a unversity in the physics department. Since quite some time, I have a very specific question regarding some equations and how they were rearranged into some result. I have asked my Professor regarding this several times, but I have never been able to get a clear answer and at this point I am "too afraid to ask once more". Of course, I am not really afraid but I have asked this so often that at some point it becomes somehow weird, especially since this is only some detail.

Now I have been assigned with supervising an undergraduate, who will write his Master's thesis about a topic which is very similar to the one I am working on. I can already see that he will come across the same question in the near future and there are several possible things I could do to answer the question if it arises.

1. I could simply tell him that I don't know the answer. This would of course be OK but does not actually solve the problem. Especially since for his work a better answer to the question I am trying to get an answer to might be required.
2. I could tell him that I asked this several times to the professor, but he either did not understand or was unable to provide an answer. This is the more factually correct answer but it sounds a bit accusatory and could put me in the position that the student tells the professor about this since I would be the first one whom he would ask and who should be able to provide an answer.
3. I could ask the co-authors. In this case, the question would be whether I inform my professor about this. If I do so, he will ask me why and I probably end up in a position where I will have to ask the same question all over again, not to mention that he did not answer the question in a way I understand.

Answer 1

This is somewhat of a frame challenge, but I suggest you try asking the question on Physics.SE (or Math.SE, or even MathOverflow, depending on where it fits best) so that you can become the person who can answer the question.

Answer 2

Ask the professor's co-authors. Maybe they did that bit of the work.

I could ask the co-authors. In this case, the question would be whether I inform my professor about this. If I do so, he will ask me why and I probably end up in a position where I will have to ask the same question all over again, not to mention that he did not answer the question in a way I understand.

You could cc your professor, e.g., Dear X & Y, I'm Prof Y's student and I've been studying your joint work (entitled ABC). Prof Y and I have discussed blah-blah on several occasions and I still don't follow. Perhaps you can help me understand. Depending on specifics, you may like to propose a video call or (post-pandemic) visiting X & Y, or perhaps email will suffice.

I don't understand why you're concerned about informing your professor.

Answer 3

I can see a number of ways that your situation might occur.

First, it may be that there is no known answer. Possibly an error in the result itself.

Second, it may be that the answer probably exists but that the supervisor doesn't want to take the time and effort to work out the details, being busy and believing that to be your job.

Third, the answer may exist, but, again, the supervisor believes that it is yours to find and thinks it is important that you do so. You suggest that it is important to your work and they want you to work that out yourself.

As to how to obtain the answer, other than through your own efforts and how to answer the other student, I think any of your suggestions might work. I don't think that 2 sounds as bad as you fear, however. The suggestion of Dawn in a comment to ask around in your research group might also be fruitful if people are interested in the answer.

But, I think that, overall, you would come out ahead by coming up with the answer yourself. You may be lacking some insight that you can only obtain by developing the answer rather than hearing it.

Answer 4

When this happened to me in the past, I eventually realized that it's because the professor doesn't know the answer. It's common to assume they do, because to students professors seem like omniscient Gods with the answers to life, the universe, and everything, but there are certainly things they don't know. In fact one could argue that at the end of a PhD, the student is expected to become more expert at their thesis subject than their professor.

I suggest figuring it out yourself, either by thinking about it yourself or by asking someone else (Physics/Math.SE, the co-authors, or anyone). You can then explain the solution to the professor afterwards if it comes up again.