The definition we previously introduced to describe the velocity of a test particle in a Schwarzschild field, as measured by a distant observer, is shown to be complete. As we desired, this definition reflects the fact that round-trip times of flight of fast test particles will be longer when governed by general relativity than when governed by a Minkowskian or Newtonian theory. In this sense, particles obeying general relativity appear to slow down. The alternate definition of particle velocity proposed by Baierlein does not have this property and is shown to be more appropriate for a local observer than for a distant observer.

• Received 15 October 1973

DOI:https://doi.org/10.1103/PhysRevD.8.4642