Worldvolume fields from open-string massless states

A massless open-string vector polarization tangential to a planar D-brane becomes a worldvolume gauge field. Transverse polarizations become scalar fields describing fluctuations of its position. This supports the interpretation of a Dirichlet plane as a dynamical extended object. The unprojected bosonic open-string tachyon additionally signals that its brane ground state is unstable; the massless field interpretation does not remove that instability.