Why don't some questions on Physics Stack Exchange have any answers for the past many years with very high votes? Why are they not active now?

The below links are two examples of such questions which are not answered in 7-8 years. I also read on that questions that they were active some 3-4 years ago. What is the problem with these types of questions?

Do they really don't have answers like proof of Goldbach's conjecture or yet their answers are very hard and why are they not active now? Can someone answer them now if one finds an answer to such questions? Is there any difference between questions which show active some time ago and closed questions?

Linear sigma models and integrable systems

Experimental test of the non-statisticality theorem?

• Is it really that difficult to imagine that the reason why some questions have not received answers in years is that the questions are hard? – Emilio Pisanty Aug 25 '20 at 12:50
• @EmilioPisanty my point was whether they are like actual unanswered questions like Yang mills theory problem. – Vaibhav Pankhala Aug 25 '20 at 16:17
• @VaibhavPankhala "Unanswered" here just means "no answers on PSE". Not necessarily "No one has ever answered this problem in all of physics" (although one could ask such a question I suppose). – BioPhysicist Aug 25 '20 at 16:19
• @BioPhysicist what is the case in given link questions? this was my question which no one answered. – Vaibhav Pankhala Aug 25 '20 at 16:29
• @VaibhavPankhala Questions don't come in a binary with Yang-Mills on one side and everything else on the other. (And I should add it is a massive misunderstanding of how research works to think in those terms.) See my edited answer for more. – Emilio Pisanty Aug 26 '20 at 12:19

• The reason those questions have not been answered is that the questions are hard. This shouldn't be so hard to understand, I hope. Those questions "are not active now" because... why would they need to be active at all?

• If a question on Stack Exchange has stood at high score for many years without getting answered:

• It could be because the question is actually an Open Problem as far as all of physics is concerned, i.e., that the problem is widely known to the community at large (beyond SE) and no solution is known to anyone.
• But it could also be because the solution is known to someone somewhere, but they have not seen the question or they are not on SE.
• Or it could be that it is a new question which is being formulated for the first time, requiring e.g. a difficult calculation, and it has not been seen by the people who can do it, or they've seen it and they didn't realize that they could, or it's waiting for Divine Intervention to give somebody a Eureka moment, or a bunch of other reasons.

Generally speaking, distinguishing between those cases is often impossible. Sometimes, when you're lucky, it's possible to identify the first case, and indeed this is what the open-problems tag on MathOverflow is about. In physics questions tend to be much more loosely defined, so that type of identification is much trickier, and PSE is not as close to the bleeding research edge as MO, so those cases are much less frequent.

For any given case, the answer is typically.... who knows! Both of the examples you've linked to look like the third case to me, but it could turn out that they're something else, and we won't know more until we know more.

• Answers can be posted to any question on the site which is not closed or on hold, regardless of its age.

• Every question on the site has a marker, just below the title, of the form Active: [date], where [date] is some temporal indicator (which will give you a precise timestamp if you hover your mouse over it). The [date] in question is the last time that the question, or any of its answers, were posted or edited, and if you click on that time then it will take you to the post where that last activity occurred.

• Note that not all activity is visible to all users. As an example, if an answer is posted and then deleted, then the time of latest activity will be marked as the posting of that answer, but the answer itself will only be visible if you have 10k reputation. (In this case, the link will just take you to the question.) For an example of this, see this thread, at least as of this writing.

• For the two examples you've linked to, there are actually 3 and 7 "answers", respectively, which were posted over the years, do not actually answer the question, and have since been deleted. In other words, just because you can't see activity on those questions, it doesn't mean that it hasn't happened. (And, also, just because you can post answers to those threads doesn't mean you should, if you don't have an actual answer to the question.)

• What is remarkable of the two links suggested by the OP is that both questions have 100+ upvotes and both have every answer deleted. There cannot be that many questions with 100+ upvotes and no undeleted answers. – ZeroTheHero Aug 25 '20 at 20:56
• @Zero Indeed those are the only two. – Emilio Pisanty Aug 26 '20 at 4:53
• A slightly broader query turns up at least this thread with similar behaviour. I don't know what else might be out there (but I've lost interest in finding out more). – Emilio Pisanty Aug 26 '20 at 12:36

Yes, there is a large difference between questions which haven't been active for a while and closed questions. Closed questions are closed because for one reason or another they aren't a good fit for our site, and they cannot be answered unless they are reopened first. Older questions are just that - older - and can still be answered normally. If you see an old question that you can answer, go for it! It will then show up as active in the "active" questions tab.

• but why on upper side of question it's is written that 'Active 3 years, 1 month ago' in first question? – Vaibhav Pankhala Aug 25 '20 at 7:51
• @VaibhavPankhala Because the last major activity, i.e the last time someone edited the question or posted or edited an answer, happened that long ago. – ACuriousMind Aug 25 '20 at 7:58

The first question is chock-full of technical jargon and it is about a fairly esoteric subject. It is not likely that there are many people who can adequately answer this question without first putting in copious effort to research it completely. Given that, it's unsurprising that there are no good answers because not many are going to look at it and find it worthwhile to put in such large amounts of effort for an old question. The answer isn't likely to be helpful anymore and the reward in reputation isn't likely to be worth the time.

As to the second question, I honestly can't even explain why it was highly voted in the first place. From the look of it, it seems to be saying "I was reading some literature and didn't understand something. I tried doing that thing myself but don't know if I did it right. Here is what I did, did I get something wrong somewhere? What am I missing?" The only difference between this question and the run-of-the-mill questions we close every day is that this isn't basic physics from a first-year textbook. To me, this still looks like a check-my-work question that doesn't seem to be about any concepts and wouldn't really be useful to others. So again, I'm not sure why it was upvoted and I don't personally care why nor that it hasn't been answered. Probably because nobody wanted to check the OP's work.