I have some questions and refer you to the image below the caption of which states, "D-branes may be of exceptional importance in the explanation of elementary particles of the Standard Model. However, it still remains to find a solid connection between the M-theory and the Standard Model." From, http://www.diffusion.ens.fr/vip/pageJ03.html The string which is identified with a photon above is shown with the ends of the string at different positions on the brane, is there significance to this? The string ends of an on mass-shell photon must move through space at the speed of light while attached to the brane? We are told the photon string ends have opposite charge, can we make a connection to this idea and the oscillating nature of an electromagnetic wave? An electromagnetic wave in free space has equal energy in the electric and magnetic fields. Are there quantities associated with the photon string which represent two forms of energy that are also equally divided? Can a photon which is off mass shell be included in the image above, is there a representation of it? Electrons and photons have different spin, does this fact follow from the image above somehow? Thanks for any help!