Quoting from Gilles's excellent answer to a related question:
The tag physics is about the physics of a fictional universe, like languages is about languages in a fictional universe, magic is about magic in a fictional universe and so on.
I don't see why there shouldn't be a physics tag. It's not about real-world physics, any more than magic is about real-world magic.
Science, like magic, aliens, spaceships, physics, astronomy, etc., is a common enough trope in SF/F literature to be worth its own tag, provided we make clear what types of question would be on-topic in this tag.
However, I definitely agree that the current tag wiki for science isn't good enough. I suggest the following tag wiki excerpt (thanks to @Mazura):
For questions about the fictitious in-universe science found in works of speculative fiction. Questions purely concerning real-world science are off-topic.
... and the following tag wiki (based heavily on @Gilles's tag wiki for physics):
Use this tag if your question is about the fictional science found in the universe of a work of science fiction or fantasy. Note that this site is about fiction, not about real-world science. Expect answers based on internal consistency or author fiat ('Word of God').
If your question is about:
- how the in-universe science relates to real-world science, ask here, but do not necessarily expect answers from experts on real-world science.
- real-world science, your question is off-topic here. You might consider asking on one of the science-based Stack Exchange sites.
Please see this discussion thread for more detail about our policy on science questions.