A white hole is the exact temporal mirror of a black hole: where a black hole only swallows and lets nothing escape, a white hole only expels and lets nothing enter. Because the fundamental equations of gravity are symmetric under time reversal, running a black hole backwards yields a physically allowed object.
Loop quantum gravity proposes that matter collapsing in a black hole never reaches an infinite-density singularity; instead space quantizes and bounces at an extremely small but finite scale. The collapse rebounds, converting the black hole into a white hole rather than terminating in a mathematical point.
The rebound from black to white hole is nearly instantaneous for an observer falling in, but gravitational time dilation makes it appear to take billions of years from the outside. A process brief in its own frame can therefore look astronomically long to a distant witness, so old white holes could still be emerging now.
A horizon is defined not by a physical surface but by the direction causal influence can cross it: a black hole horizon lets things in but not out, a white hole horizon the reverse. Treating the boundary as a rule about permitted crossings, rather than a wall, reframes these objects as asymmetries in access rather than places.
The past-future asymmetry we experience arises from low-entropy initial conditions and our particular coarse-grained viewpoint, not from any time-direction built into fundamental law. Because the microscopic equations run equally well both ways, the felt difference between past and future is a feature of our perspective on the universe.
Rovelli models theoretical reasoning as committing to a tentative premise (the quantum bounce) and tracking rigorously where the mathematics leads, even into counterintuitive territory like remnant white holes. The method treats imagination disciplined by equations as a way to probe reality that observation has not yet reached.