This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
Mass-energy should be connected with a hyphen, not an en dash, I think, per WP:HYPHEN. "Mass-energy" is a compound modifier of equivalence, and mass and energy are related terms in this context. Any objection to a move?
Actually, I guess since mass and energy are reversible (ie could be energy–mass and have the same meaning), the MOS says that en dash is marginally preferred. —Alex (ASHill | talk | contribs) 19:52, 17 February 2015 (UTC)
Yes, when the terms are linked rather than one modifying the other, usually in a balanced context, the en-dash should be used. The reversibility is a good indicator of this. Here the linkage is between alternatives, meaning that mass and energy are equivalent in the context: either would do, but the linkage emphasizes the equivalence. So I'm against a move. —Quondum 20:01, 17 February 2015 (UTC)