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BrillWavesBrill Waves are pure gravitational waves, i.e. they are constructured by gravitational energy only, with no matter involved. Energy is `located' within the curvature of the spacetime. The strange thing is that gravitational energy cannot be `located' in a classic sense. At each point in space there is only vacuum, and at each point there can be an observer who locally sees minkowski space. There is no `absolute', no `invariant' quantity which could be used as a measurement for the gravitational energy.Only in the asymptotic flat space at large distances from the coordinate origin one can find similarities beetween the spacetime of pure gravitational energy and spacetime curved by the presence of matter. This leads to the interpretation that even gravitational energy has an graviational field, which is indistinguishable from the gravitational field generated by some matter distribution. This behaviour was first found by Dieter Brill in 1959. For weak energy concentrations, gravitational energy will just `flow away', dissolving into outer space, leaving minkowski space in place at later times. However, the energy concentration of a Brill Wave may be large enough such that it forms a Black Hole. Some very strong Brill Waves are Black Holes yet from the beginning, but others are not yet Black Holes from the initial data, but form one during time evolution. Such Brill Waves forming a Black hole are called `super-critical', whereas the weaker once which dissolve into Minkowski space are calles `subcritical'.
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