I just talked to my advisor today. He highly suggested that I double major. I sequenced my classes wrong and I am going to run out of classes to take while I work on my primary degree(physics). So to fill those empty spots for classes, I need to pick up another major. I'll still finish in 3 years if I can pick up calculus this summer. Time to start calling my recruiter again.
If you're wanting to disprove Einstein, you're gonna have to go into experimental, probably. Some sort of Engineering double major would be good for that.
If you want to go into theoretical physics, just double major in math. In the course of my physics degree I've taken Vector Calc, Complex Variables, and Partial Differential Equations. The only classes I'd need to finish a math major are stupid 2000 - 3000 level classes like "Linear Algebra," but it would take longer so I'm just getting my physics degree and getting out. But, if you have the time and the inclination, go for the math degree.