Gravity is weak because the masses of elementary particles are so small. Gravity has a natural mass unit, $m_p~=~\sqrt{\hbar c/G}$, the Planck mass, which is about $10^{-5}$ g. The proton is $22$ orders of magnitude less massive. So the stuff which makes up the world is elementary particle “styrofoam stuff” which gravity couples to.

This can be seen as well with IIA strings and their S-dual heterotic strings. Those heterotic strings just do not like to stay on our brane, which they have no end points to form Chan-Paton factors or Dirichlet boundary conditions on the brane with. They slip our brane as if nothing is there. Their S-dual strings are open strings on the brane, but with puny masses --- far less than the Planck mass or the mass corresponding to the string tension.

Gravity is weak because the masses of elementary particles are so small. Gravity has a natural mass unit, $m_p~=~\sqrt{\hbar c/G}$, the Planck mass, which is about $10^{-5}$ g. The proton is $22$ orders of magnitude less massive. So the stuff which makes up the world is elementary particle “styrofoam stuff” which gravity couples to.

This can be seen as well with IIA strings and their S-dual heterotic strings. Those heterotic strings just do not like to stay on our brane, which they have no end points to form Chan-Paton factors or Dirichlet boundary conditions on the brane with. They slip through our brane as if nothing is there. Their S-dual strings are open strings on the brane, but with puny masses --- far less than the Planck mass or the mass corresponding to the string tension.