  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.
