Why is the 'physics' tag not allowed?
Oops! Your question couldn't be submitted because: The 'physics' tag is not allowed. Creating the new tag 'physics' requires at least 1000 reputation. Try something from the existing tags list instead. New tags cannot be created from the mobile site. The tag 'physics' is new.
Your first obstacle is that you need 1000 reputation points to create a new tag. If you think a tag is missing and don't have 1000 reputation points, you can suggest it in this meta post.
If you have 1000 reputation points, there might be another obstacle: Certain words are blacklisted, e.g. physics.
Returning to OP's specific question, a 'physics' tag would be pointless, because then all questions should presumably be tagged with it. The point of tagging is that the tag word should give non-trivial information about the question. A 'physics' tag would not tell us something we didn't know already: That it's about physics!
