Weyl tensor
In general relativity , the Weyl curvature is the only part of the curvature that exists in free space—a solution of the vacuum Einstein equation—and it governs the propagation of gravitational waves through regions of space devoid of matter . 
<triplet> general relativity <subj> vacuum Einstein equation <obj> has part <triplet> vacuum Einstein equation <subj> general relativity <obj> part of