On the collision of impulsive gravitational waves when coupled with null dust

Abstract

The problem of colliding impulsive gravitational waves is considered when the region of space-time, after the instant of collision, is filled with a mixture of null dusts moving in opposite directions. The extension of the resulting space-time, to regions before the instant of collision, shows that null dust follows the leading edges of the impulsive waves, and, further, that one can arrange that the space-time in these regions is identical with what prevails when a perfect fluid with ε=p fills the region after the instant of collision. This ambiguity in the space-time, after the instant of collision, must be traced to an inherent ambiguity in the nature of null dust and its relation with a perfect fluid with ε=p.