I just asked this question, and realized this could also be asked on the physics.SE site. Is it considered a faux pas on SE to do so?
You generally shouldn't copy and paste the same question on multiple sites. If you realize that your question would fit better on another site, just flag it for moderator attention and explain it, moderators can migrate questions to any SE site.
This specific question was already migrated to Physics by a moderator.
There are somewhat rare cases where you may benefit by asking different groups of users a similar question, because the context of their expertise will provide a different perspective.
But if you are asking the same question of two different groups simply to reach a larger audience, I would frown upon that. Ask the question first on the site you feel will most likely provide the best answer. If you do not receive an adequate answer, then it might be okay to ask your question again to another group of users — as long as the question is on topic and appropriate for that second site.
But cutting-and-pasting between two sites is never okay. If you want a different perspective, you should phrase the question specifically for that group.