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Every once in a while, we do get questions where "that's not possible" is a valid answer to the question as asked (with perhaps further clarification in comments). Sometimes the question gives criteria that are mutually exclusive in some manner, and sometimes the asker is asking for a science-based (or maybe even hard-science!) method for doing something that science says is not possible.

In my opinion, to those questions, an answer that says effectively "it's not possible, and here's why" is useful and a perfectly valid answer. I have at least two such answers on my account on Worldbuilding, to Science-based FTL drive and to What would happen if electricity stopped working? respectively. (My answer to the former is currently accepted and highest voted, and my answer to the latter is currently the highest voted.)

However, we also have a tradition on Worldbuilding to take what's stated in the question as The Truth (tm), and to work within that when proposing answers. The idea behind that, of course, is that the poster knows their world best and is (hopefully) trying to solve some small issue within that world while maintaining consistency.

In what situations is a sufficiently explained "that's not possible" an appropriate answer to a question?

Feel free to break your answer up (or even post several answers, one for each case) for questions tagged , and respectively, but if your answer doesn't apply equally well to all cases, make sure to explain in your answer which cases you are considering.

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That's Not Possible is a sufficient answer anytime the question is too constrained.

Questions that are Too Broad (unconstrained) are a problem because it's hard to know where to start answering.

If the question is too narrow, with rules that are too tight, then the options can be limited down to zero.
In those situations a response like "sorry, that won't work, here's why. Here's how to get closer if you relax this part of the question."

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    $\begingroup$ (+1 for "...that won't work, here's why. Here's how to get closer if...".) If a question doesn't explicitly involve alternative physics (or some method to modify physics as we know it), then sometimes a nudge in that direction can indeed be invaluable (as long as it doesn't derail things). Arguably, it might more often be something as simple as a small change of detail in the design itself, but the literally-impossible (e.g.: perpetual motion, exotic matter) might require the more drastic workarounds. $\endgroup$ – Charles Rockafellor Jul 15 '16 at 23:46
  • $\begingroup$ There are many questions where the conditions or the assumptions are contradictory. Several questions on space whales, for example, include contradictions that make space whales impossible without any other consideration. Space whales as science-fantasy, fine! Scientifically based, yuk! I like @CharlesRockafellor's nudge concept. It goes some way to fixing things (partly). $\endgroup$ – a4android Jul 25 '16 at 10:59
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Reality check is the big one for "that's not possible", I think a fair number of my answers are along those lines, but again, that to me is the point of the tag. Someone has come along saying "does this work?" and as often as not the answer is "no it doesn't" but it doesn't become a good answer until you explain why.

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    $\begingroup$ When talking reality check keep in mind that science is less relevant than the premises made in the post. Reality check is a check for internal consistency which may include science or not. $\endgroup$ – James Jul 15 '16 at 20:07
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Saying "that's not possible" is only allowed (as far as I understand) if you can justify your opinion.

If your answer is "impossible" then it's just not helpful.

If your answer is "it is impossible because of (insert two or three fully justified and referenced/cited reasons)" - then that's fine.

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Never. Anything is possible in fiction(like for example a 1g planet that is much more massive than earth is possible via minute dark energy in the core).

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    $\begingroup$ I asked this question because I think it's a subject we need to discuss. This answer, though, is one I disagree with strongly. If I post a hard-science question asking how to travel to Alpha Centauri in a week of Earth time, the correct answer would almost certainly be "that's not possible". Categorically ruling out "that's not possible" as a possible answer regardless of the question or the constraints imposed therein is simply counterproductive. $\endgroup$ – a CVn Jul 20 '16 at 7:42
  • $\begingroup$ I just said that anything is possible because most questions here are either fantasy or science based. Very few(less than 5% I think) are actual hard science questions where they want absolute scientific fact. I for example only have chemistry and physics as "hard science". Even then, in my whole metaverse of multiverses of universes, there are differing physical laws between universes but there is always 1 where physical laws are the same and that is the one in which my stories take place. And this metaverse has lots of connections. White holes between universes and universe exchange mainly. $\endgroup$ – Caters Jul 20 '16 at 17:24
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    $\begingroup$ That's all well and good, but this question is almost specifically about hard science, reality check and science based questions - talking about metaverses and other unverses moves away from those tags, as metaverse theory isn't really quantifiable science. $\endgroup$ – Miller86 Jul 21 '16 at 8:56
  • $\begingroup$ This raises an interesting philosophical point. The philosopher Baruch Spinoza said nothing is true or false in fiction. Of course, there are limits to what truths can be falsified or falsehoods made true. Readers expect fiction to be sufficiently consistent (logically, emotionally, conceptually, historically) to be a pleasing and plausible counterfeit for reality. An astrophysicist said he SF because it cut loose from the constraints of lightspeed & conservation of energy, because those were his day-to-day realities & it was good to get away from them. $\endgroup$ – a4android Jul 25 '16 at 10:41
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    $\begingroup$ Michael's point about not categorically ruling things in or out is good one. Travelling to Alpha Centauri one week Earth time means it has to be FTL in that fictional world. Science in our quotidian reality says that's impossible. Answers could be that's impossible, but here's a way it might be done if certain conditions are meet. With FTL, assuming relativity isn't a complete theory etc. It can be counterproductive to simply tell an OP whatever their Q is about that it's flatly impossible. Perhaps work-arounds can be found. If not, explain why. $\endgroup$ – a4android Jul 25 '16 at 10:50

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