<prompt> Umphumela, ukuhlolwa kwazo zonke izinhlaka zomhlaba zokukhanya ama-zero geodesics kunikeza ulwazi olubalulekile mayelana nesakhiwo sesikhala nesikhathi. Lesi sakhiwo singakhonjiswa kusetshenziswa amagrafu kaPenrose-Carter lapho izifunda ezinkulu ezingenamkhawulo zesikhala nezikhawu zesikhathi ezingenamkhawulo ziyancipha zihlanganiswa ukuze zilingane ebalazweni eliphelele, ngenkathi ukukhanya kusahamba ngokuxhumana njengoba kunjalo emagrafu wesikhala nesikhathi. Beqaphela ukubaluleka kwesakhiwo sembangela, uRoger Penrose nabanye basungula lokho okwaziwa ngokuthi yi-geometry yomhlaba.  <response>: A kuya ku-X 