“…A null dust appears frequently in classical and quantum gravity, in Vaidya spacetimes [1,2], pp-waves [6][7][8], Robinson-Trautman geometries [9], twisting solutions of the Einstein-Maxwell equations [9][10][11], in studies of classical and quantum gravitational collapse, horizon formation, mass inflation [12][13][14][15][16][17][18][19][20][21], black hole evaporation [22][23][24], and canonical Hamiltonians [4,6,25]. Colliding scalar field-null dust solutions were studied in [26,27].…”