I'm a mathematical physics student and learning mathematics(algebraic geometry, complex geometry etc.) and theoretical physics (CFT, string theory, supersymmetry etc.) in the meanwhile.
I found an interesting phenomenon: students learning math are much more active than those laerning physics on line. Take myself as an example, I discuss math problems with schoolmates in online groups everyday, but I even don't know an online group about physics.
[edit: If you think it's a bias, ask yourself: do you discuss with your classmates about physics problems everyday online? And the frequency?
To math problems, my answer is yes, I do. Believe it or not. Maybe it's related to the difference between math and physics. ]
Is that because the difference between different physics branches is so large that people studying different areas of physics don't understand what others are doing?
[Edit: Maybe I shoule restrict it to pure math and physics. For a math student, no matter what field he's going to study, he' about to learn analysis(real, complex, fuctional analysis at least), algebra(linear algebra and modern algebra at least), geometry(analytic, differential geometry at least), and different backgrounds are more and more needed in math studies. But as far as I'm concerned, for a physics student, after learning classical and analytical mechanics, electrodynamics, quantum dynamics and statistical dynamics, they're doing different things. ]
And is that why Physics SE is more inactive than MSE?
In addition, the first question in MSE is asked in Mar 27 '10, for PSE is Aug 24 '10, but now MSE has $1040$k questions while PSE has $132$k.
Also, math sutdies maybe require more multi-subject backgrounds. For example, if you major in PDE, you may require lots of knowledge ahout manifold since we're more concerned about PDEs on Riemann manifolds. And algebraic geometry is gradually becoming the basic demand for all pure math students. But for physics, there's a huge gap between theoretical and experimental physics, and theoretical physics studies a lot of different things (although they might be the same thing or have some ingerent corrspondence at a very high level, like mirror symmetry, AdS/CFT corrspondence, M-theory etc.)