I made the jump from mostly-math undergrad to materials science PhD (close to chemistry/physics, if you don't know the field). I was welcomed with open arms. I

I made the jump from mostly-math undergrad to materials science PhD (close to chemistry/physics, if you don't know the field). I was welcomed with open arms. If you've got any math-heavy STEM graduate degree, you can likely jump into a physics PhD. You might need to take some senior-level undergraduate courses to catch up, but the transition is quite doable. At some point, your overall intelligence, enthusiasm, and work ethic matter more than your specific background.