Fritz Leiber,
Time In the Round
“ Hal cleared his throat and recited: "The bubble is the locus of an infinite number of one-way holes, all centering around two points in space-time, one now and one then. The bubble looks completely open, but if you tried to step inside, you'd be stopped—and so would an atom beam. It takes more energy than an atom beam just to maintain the bubble, let alone maneuver it.""I see, I guess," Joggy whispered. "But if the hole works for light, why can't the people inside the bubble step out of it into our world?" ”
