A few days ago a question was asked about how close one could safely approach a small non-radiating black hole.

A comment was made that "Micro black holes do evaporate, so this question is fictional physics and off-topic".

However, there are many highly upvoted questions on the site about non-realistic physics. For example.

Does this site have a policy on questions about non-realistic physics?

I think that the accepted answer to the following meta question says that these types of questions are allowed, but it's not very clear to me where the line should be drawn.

What is the line between fiction and physics with respect to off-topic?

  • 3
    $\begingroup$ The way I see it, that black hole question has several problems. Although I don’t think “non-radiating black hole” is sufficient on its own to close a question, in this case it adds to the potential to close it. For example OP talks about a non-radiating BH then talks about high temperature. I personally voted to close as unclear. $\endgroup$
    – JMac
    Nov 25, 2019 at 14:28

1 Answer 1


This is my own opinion, but I think it depends on the role of the unrealistic physics in the question. I ultimately agree with the attitude expressed in JMac's answer on the fictional physics post you link to. If an answer to the question would rely on false physics, then the question should be closed as off topic. However, if the unrealistic physics/fiction is just being used to set up the scenario, and an actual answer to the question would then use legitimate physics, then I think the question is fine.

Of course, this is just my opinion. There are instances where just a fictional premise causes a question to get closed. One example can be found here. If you look back through the edit history, you will see that the question originally asked about a nonphysical scenario of the sun disappearing, even though the heart of the question was "how can angular momentum be conserved when traveling in a straight line?". The comments have since been deleted due to the question being edited, but there was much criticism due to the bad premise, and the question was closed as being unclear.

In general I think it is a challenging to try and determine hard and fast rules in scenarios like these. As you have shown, in the same category of "unrealistic physics" there are questions that remain open and get many up votes, and there are questions that are closed for not being about mainstream physics. I think this is an instance where the outcome greatly depends on which users view the post, how carefully the post is read, who ultimately decides to close the question, etc. Some users are more lenient on questions like these than others, and this could make the difference on some of the more "iffy" questions.

So ultimately, I think you just need to take scenarios like these on a case-by-case basis. Is the question asking us to imagine a world where there is no resemblance of physics as we know it? Or is it using an unrealistic scenario in order to explore realistic physics?

  • $\begingroup$ What is unrealistic about premordial black holes? $\endgroup$ Nov 25, 2019 at 21:15
  • 3
    $\begingroup$ @PeterMortensen Can you point out to me where I make this claim? $\endgroup$ Nov 25, 2019 at 21:57

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