The Extreme Physics Explorer
  Some tests of fundamental physics - the equation of state at supra-nuclear densities, the metric in strong gravity, the effect of magnetic fields above the quantum critical value - can only be measured using compact astrophysical objects: neutron stars and black holes. The Extreme Physics Explorer is a modest sized (~500 kg) mission that would carry a high resolution (R ~300) X-ray spectrometer and a sensitive X-ray polarimeter, both with high time resolution (~5 ?s) capability, at the focus of a large area (~5 sq.m), low resolution (HPD~1 arcmin) X-ray mirror. This instrumentation would enable new classes of tests of fundamental physics using neutron stars and black holes as cosmic laboratories. 
Martin Elvis (Harvard-Smithsonian Center for Astrophysics)