<p>No, no, no! Don’t start from a Bachelor’s in physics. You might want to take a M.S. in physics and make sure you make up the missing base courses. At my university, we regularly get engineering students and we work with them to make up the missing coursework. It does not take too much extra to do so. Alternatively, see if you can get into a M. Eng. program that lets you take some physics electives to get up to speed.</p>
<p>With a ME degree, you have a reasonable amount of Math and certainly enough mechanics. You are probably short on electrodynamics, quantum mechanics, and statistical physics. Since you have one more year in your B.S. studies, can you take some physics courses as electives?</p>
<p>Now, as to your desire to do theoretical physics. That is a very long and hard road if you want to make a career of it. If you are serious, you probably want to get started as soon as possible to see if you have the ability to be successful. I would say that the majority of physics majors who go to graduate school want to be theoretical physicists but many of us find out relatively quickly that our aptitude is not in that direction.</p>