The only way to protect your intellectual property (in a legal versus a moral sense) is to write up your work and submit it for publication to a physics journal. It also helps, even for purely theoretical work, to have kept notebooks with dates and signatures... see Kanare's Writing the Laboratory Notebook. Journals keep careful records of when manuscripts are submitted, and the submission date is typically published at the head of the article. This submission date is how precedence of credit is preserved.

If you discuss your idea with a professional physicist s/he will be morally bound to preserve your credit, and typically this will occur. If the physicist(s) with whom you consult contribute substantially, of course they would be co-authors of the paper; and if the work is done under her/his direction, s/he will be "lead author" of the paper.

I will caution you, especially if you are not working in physics or a closely-related discipline, that the more revolutionary your idea may seem, the more likely a reputable theoretical physicist is to have seen it and recognized it for a crackpot theory. Well-known physicists often receive manuscripts which claim to be revolutionary (but typically contain subtle yet ruinous errors); often they don't bother to answer such unsolicited submissions.