Gravitational slip
= Gravitational slip
{title2=$\Phi-\Psi$}
A difference between the two scalar <metric> potentials in a specified cosmological convention. Nonrelativistic motions probe the potential in the time-time <metric> component, while <weak gravitational lensing> probes their sum. Joint tests can constrain <modified gravity>, but ordinary anisotropic stress can also generate slip and observational bias must be controlled.