Although considerable progress has been made in generalizing the concept of angular momentum to general relativity, until now no satisfactory definition that allows for the exchange of angular momentum has been given. I here give the first such definition. It is a definition at null infinity, the place and time where gravity waves reach in the limit far from all masses. The definition applies to any isolated system of masses including those that change their angular momentum $L$ by emitting gravity waves. $\dot{L}$ is given solely in terms of parameters in principle measurable directly by Michelson interferometer gravitational wave detectors such as LIGO or LISA.

• Received 25 March 1998

DOI:http://dx.doi.org/10.1103/PhysRevLett.81.1150