New pulsar limit on local Lorentz invariance violation of gravity in the standard-model extension

In the pure-gravity sector of the minimal standard-model extension, nine Lorentz-violating coefficients of a vacuum-condensed tensor field describe dominant observable deviations from general relativity, out of which eight were already severely constrained by precision experiments with lunar laser ranging, atom interferometry, and pulsars. However, the time-time component of the tensor field, $\bar s^{\rm TT}$, dose not enter into these experiments, and was only very recently constrained by Gravity Probe B. Here we propose a novel idea of using the Lorentz boost between different frames to mix different components of the tensor field, and thereby obtain a stringent limit of $\bar s^{\rm TT}$ from binary pulsars. We perform various tests with the state-of-the-art white dwarf optical spectroscopy and pulsar radio timing observations, in order to get new robust limits of $\bar s^{\rm TT}$. With the isotropic cosmic microwave background as a preferred frame, we get $|\bar s^{\rm TT}| < 1.6 \times 10^{-5}$ (95\% CL), and without assuming the existence of a preferred frame, we get $|\bar s^{\rm TT}| < 2.8 \times 10^{-4}$ (95\% CL). These two limits are respectively about 500 times and 30 times better than the current best limit.