Part 5/16:
Black holes, with their event horizons, introduced a puzzle: since nothing escapes a black hole once crossed, what happens to the information contained within? Initially, physicists thought that the event horizon's one-way nature shielded us from observing any loss, leading to a relaxed attitude toward the issue. The information was presumed to be hidden behind the horizon, forever inaccessible, thus avoiding any immediate contradiction with physics' laws.