A new geometric invariant on initial data for Einstein equations
arXiv:gr-qc/0406099 · doi:10.1103/PhysRevLett.93.231101
Abstract
For a given asymptotically flat initial data set for Einstein equations a new geometric invariant is constructed. This invariant measure the departure of the data set from the stationary regime, it vanishes if and only if the data is stationary. In vacuum, it can be interpreted as a measure of the total amount of radiation contained in the data.
5 pages. Important corrections regarding the generalization to the non-time symmetric case