Are observational tests possible?

It follows from our formulae that the scintillation effects specifically predicted by scalar-tensor theories are proportional to the amplitude of the scalar field perturbation at the observer. This local character casts a serious doubt on the detectability of these effects, since the scalar field perturbation is very small for a localized source of gravitational waves.

Indeed, one can put in most cases $\phi/\Phi_0 \sim \alpha^2 h$, where $\alpha^2$ is a dimensionless constant coupling the scalar field with the metric gravitational field (see, e.g., Damour & Esposito-Farèse 1992). Experiments in the solar system and observations of binary pulsars like PSR 1913+16 indicate that $\alpha^2<10^{-3}$. Consequently, setting $h \sim 10^{-22}$ for gravitational waves emitted by localized sources gives $\phi/\Phi_0 < 10^{-25}$ and the effect is much too weak to be detected.