Erasing even a single bit means releasing heat. This discovery by Rolf Landauer hinted that information is neither material nor illusory. Imagine that every event casts a shadow: that is information, the collection of all unrealized possibilities. When you pick a key for a lock, the shadow of the other keys hangs in the air of your choice. Forgetting which key goes where extinguishes that shadow, and the world warms up ever so slightly.

With black holes, it’s the same story. Falling into one, an object loses not just its substance: it’s stripped of the history of its unrealized versions. One might think the shadow vanishes forever. But Jacob Bekenstein and Stephen Hawking proved otherwise. A black hole has entropy (a measure of disorder), and therefore it has information. Perhaps it returns to the world through spacetime curvature, which itself dictates which events are fated to occur. Thus, the shadows of the past define the boundaries of the future.