  Some theories of gravity predict the existence of preferred-frame effects and
violations of conservation of energy and momentum. General relativity predicts
no such effects. In the parameterised post-Newtonian (PPN) formalism, the
parameter, \al3 $\equiv 0$ if these effects do not exist. The period
derivatives ($\dot{P}$) of millisecond pulsars (MSPs) are used to more tightly
constrain these effects by showing that $|$\al3$|$ $< 5 \times 10^{-16}$.
