graduate admissions - What would I need to show to change fields and get into a physics PhD program?

I've been working as a software engineer for a few years now, and have a bachelor's in computer science and math. My interest and knowledge in physics has been growing since I graduated, but I never took any formal courses after high school.

Obviously, entering into a physics PhD would require a huge amount background knowledge comparable to a bachelor's in physics. My question is what would I need to do to prove that I had that knowledge, and actually get accepted into a program?

Would it be enough to score high on the GRE, and have some strong letters of recommendation? Could I do it with enough statements of accomplishment on coursera? Or am I looking at a few more years of undergrad to get a second Bachelor's? Or could I get into a master's program, and use that as a stepping stone?

Thanks.