  We propose to use pulsar scintillation measurements to test predictions of
alternative theories of gravity. Comparing to single-path pulsar timing
measurements, the scintillation measurements can achieve a factor of 10^5
improvement in timing accuracy, due to the effect of multi-path interference.
Previous scintillation measurements of PSR B0834+06 have data acquisition for
hours, making this approach sensitive to mHz gravitational waves. Therefore it
has unique advantages in measuring gravitational effect or other mechanisms (at
mHz and above frequencies) on light propagation. We illustrate its application
in constraining scalar gravitational-wave background, in which case the
sensitivities can be greatly improved with respect to previous limits. We
expect much broader applications in testing gravity with existing and future
pulsar scintillation observations.
