# Why does only gravity leak into extra non-compact dimensions?

K. Pardo, et. al., Limits on the number of spacetime dimensions from GW170817, Journal of Cosmology and Astroparticle Physics, Vol. 2018, 2018.

which was published recently in JCAP states that they put constraints on the number of extra non-compact dimensions by looking at the difference between the Electromagnetic luminosity distance $d_L^{EM}$ and the gravitational luminosity distance $d_L^{GW}$. The approach makes sense given the assumption that gravitational waves "leak" into the extra dimensions and electromagnetic waves do not. If this assumption is true, then a given source would have $d_L^{GW}>d_L^{EM}$ since the gravitational wave signal would be weaker than expected. By looking at the ratio of $d_L^{GW}/d_L^{EM}$ one could find constraints on "how much" of the gravitational waves is "leaking". What is the basis of this assumption though? Why can only gravitational waves leak into extra non-compact dimensions and electromagnetic waves can't?

• @enumaris They are different excitations of the string. For closed strings you find that the first excited state contains a graviton. When studying the open string you have to define boundary conditions for the ends of the string: Neumann (N) or Dirichlet (D). Choosing NN boundary conditions along coordinates $X_\mu$ for $\mu =0, \ldots, p$ and DD for the remaining defines for you a (p+1)-dim. hypersurface of spacetime on which open strings can end. The excitations of the open string along this brane can be found to contain gauge fields (like EM), i.e. the gauge theory lives on the brane. – Sparticle Sep 16 '18 at 20:40