REVIEW THE WIKI BEFORE USING THIS TAG! For questions that require unequivocal proof that answers are correct through the use of equations, empirical evidence, scientific papers, etc. Compare to: science-fiction, science-based and internal-consistency. This tag may not be used alone. This tag may not be used with the science-fiction, science-based, or internal-consistency tags. Flag your question for moderator attention once posted.
Do NOT use this tag to "force" science-based answers or in an effort to "guarantee" the "best" science-based answers possible. The science-based tag is more than adequate for those purposes. This tag should only be used when you either need unequivocal proof that an answer is correct or because the proof is valuable to your worldbuilding efforts. Answers that do not meet this tag's expectations are subject to deletion. Using this tag without understanding this wiki is irresponsible.
Similar to questions using the science-fiction or science-based tags, questions tagged hard-science expect answers based solely on science as understood by humanity at a specified time (and, if possible, in a specified place) in human history. However, unlike the science-fiction and science-based tags, the hard-science tag expects answers to be rigidly (to the point of inflexibly) based on science.
After posting a question with this tag, you should "flag" your question for moderator attention and point out that it is tagged hard-science. A moderator will then add the hard-science notice to your question to draw respondents' attention to the fact that you are looking for hard science.
Users are reminded that this tag does not reflect the "hard science" book genre as you might find in your local bookstore. That is not the purpose of this tag.
Questions using this tag must include all the information necessary to frame the posited problem. Questions that do not contain all the necessary details or data may be closed as needing more details. Questions will be reopened once the data or details are provided.
NOTE: All answers to this question must be backed up by equations, empirical evidence, scientific papers, other citations, etc. Speculative answers and answers without sufficient citation to meet the expectation of this tag, as well as those not supported by strong scientific theory, are not welcome. Long, comprehensive answers are desirable, but respondents should remember that length and quality aren't always correlated.
The answers should be based on current and undisputed science. (Questions that specify a year and place for historical purposes require answers that reflect the undisputed science of that year and place.) This means no subjective sciences, though fields like sociology are mostly considered acceptable (See this meta post). Ideally, answers should be backed up by equations, relevant theories, and citations where possible - arXiv can be quite good for citations, though Wikipedia is usually OK too. (See this meta post.)
NOTE: Users may be tempted to use the hard-science tag to receive highly-realistic answers to highly-fanciful questions or to ask questions they don't entirely understand (thereby seeking an education rather than an answer). These, also, are not the purpose of this tag. Please avoid using this tag solely to "make my idea as realistic as possible." If you don't have the educational background to understand the answers you receive — or don't receive any answers at all — you probably should have used the science-based tag. (Although we admit there have been legitimately asked hard-science questions that, to date, have never been answered.)
This tag frames the answer, not the question. As such, it cannot be the only tag attached to the question. One or more subject-specific tags mus be included or the question will be closed as needing more details.
If you want a fanciful or imaginative answer based on Real World science (vs. magic or the pure invention of world rules, aka, a "plausible" or suspension-of-disbelief answer), use the science-fiction tag.
If you want scientifically realistic answers (aka, a "reality check") that meet suspension-of-disbelief but aren't necessarily proven (or need to be proven) as fact, use the science-based tag.
If you want to test an idea, assertion, condition, situation, circumstance, or application of rules against the rules of your fictional or imaginary world, use the internal-consistency tag.