How can I get into Harvard or MIT?

There's really nothing you can do on any of those topics as an undergraduate, so it really doesn't matter where you go for your first degree. Those are grad school topics - MIT is one of the few schools that even offers an undergrad cl in GR. Seeing as how those schools accept less than 5-10% of US applicants, and fewer foreign applicants, your chances aren't good no matter what your credentials are - but that's ok, because you'll need a PhD either way, so you can do your undergrad at pretty much any school that offers physics.