The colossal magnetoresistance effect in manganites arises from a competition between delocalisation, which favours a ferromagnetic metallic phase, and localisation of electons favouring a charge, orbital and spin ordered (COS) phase. The half-doped manganites are exemplars of the COS phase, yet the magnetic ground state is not well established, and different concepts are controversially discussed: the venerable COS model of Goodenough, quasi one-dimensional delocalisation, dimerisation of the Mn ions, or Zener polaron formation. La0.5Sr1.5MnO4 is a half doped manganite with magnetically isolated Mn-O sheets. Our spin wave data at lower energies (<40meV) favour the model of Goodenough, but the most discriminating data will come from a measurment of the high energy spin waves. We propose to measure the complete spin wave spectrum to compare with predictions for the different models.