  Parity violating electron scattering can measure the neutron density of a
heavy nucleus accurately and model independently. This is because the weak
charge of the neutron is much larger then that of the proton. The Parity Radius
Experiment (PREX) at Jefferson Laboratory aims to measure the root mean square
neutron radius of $^{208}$Pb with an absolute accuracy of 1% ($\pm 0.05$ Fm).
This is more accurate then past measurements with hadronic probes, which all
suffer from controversial strong interaction uncertainties. PREX should clearly
resolve the neutron-rich skin. Furthermore, this benchmark value for $^{208}$Pb
will provide a calibration for hadronic probes, such as proton scattering,
which can then be used to measure neutron densities of many exotic nuclei. The
PREX result will also have many implications for neutron stars. The neutron
radius of Pb depends on the pressure of neutron-rich matter: the greater the
pressure, the larger the radius as neutrons are pushed out against surface
tension. The same pressure supports a neutron star against gravity. The Pb
radius is sensitive to the equation of state at normal densities while the
radius of a 1.4 solar mass neutron star also depends on the equation of state
at higher densities. Measurements of the radii of a number of isolated neutron
stars such as Geminga and RX J185635-3754 should soon improve significantly. By
comparing the equation of state information from the radii of both Pb and
neutron stars one can search for a softening of the high density equation of
state from a phase transition to an exotic state. Possibilities include kaon
condensates, strange quark matter or color superconductors.
