When Black Holes Forget: A Meditation on Memory, Entropy, and Time

Physics has always held a particular fascination for me. As a student of the subject, and of astrophysics in particular, I have long been drawn to the questions that sit at the edge of what we can observe and explain — questions about the origins of matter, the behaviour of extreme objects like black holes, and the deeper laws that seem to govern not just the cosmos, but our own everyday experience of time and change. It was with this enduring curiosity that I recently came across a wonderfully written essay by Richard Dyer, a gravitational-wave researcher at the University of Cambridge, published in Aeon under the title "Cosmic Amnesia." It left me thinking for days, and I wanted to share its essential ideas here, in the hope that they will interest readers who may not have a background in physics but who share the same sense of wonder about the universe.

Dyer begins with an image most of us know well: a bell being struck. It rings out briefly, then falls silent, leaving no outward trace that it was ever struck at all. He draws a striking parallel to black holes. When two black holes collide and merge, the resulting black hole "rings" for a fleeting moment — a few milliseconds — sending ripples through the fabric of space itself, known to physicists as gravitational waves. Then, like the bell, it falls silent.

What happens next is where the essay becomes genuinely thought-provoking. Once a black hole settles, an outside observer can determine only two things about it: its mass, and how fast it spins. Nothing else about its past survives in any observable form. It makes no difference whether it formed from a single collapsing star or a long, violent series of mergers, or whether entire planets once fell into it. This is known in physics as the no-hair theorem, and it means that black holes, despite their violent and complex histories, present themselves to the universe as remarkably simple, featureless objects.

To explain why this happens, Dyer turns to a more familiar example — perfume sprayed into an empty room. At first, its scent is concentrated near the doorway. Over time, it disperses evenly through the room, and there is no way to tell, simply by smelling the air, exactly where it began. This is the essence of entropy: not merely disorder, as it is often described, but a gradual and near-irreversible loss of information about where something started. The odds of the perfume spontaneously regathering at the door are not impossible, but so vanishingly small that we never expect to witness it.

Remarkably, black holes obey the very same principle. Physicist Jacob Bekenstein, building on earlier work by his mentor John Wheeler, showed in the 1970s that a black hole's surface area behaves exactly like entropy — it can only grow or remain the same, never shrink. This suggested that black holes are not as simple as they first appear. Beyond their event horizon — the boundary around a black hole beyond which nothing, not even light, can ever escape or return — they may conceal vast, hidden information about everything that has ever fallen into them.

Here Dyer draws a distinction I found particularly elegant. In an ordinary room, forgetting is only ever a practical limitation. Someone with complete knowledge of every particle's position and motion could, in principle, reverse time and trace the perfume back to its source. A black hole allows no such possibility. Its event horizon is a genuine, one-way boundary. Even someone who fell in and somehow reconstructed the black hole's entire history could never communicate it back to the outside world. The forgetting, in this case, is final.

What I found most compelling was Dyer's closing reflection: that this same irreversibility — the quiet drift from order into disorder, from sharp detail into vague impression — is what gives time its direction, and perhaps what gives our own experience its meaning. Bells fade. Perfume disperses. Black holes fall silent. And so do we, in our own way, forget and are shaped by forgetting. As Dyer puts it, loss and existence are inseparably bound together; something must always fade for anything to have truly happened at all.

There is something quietly reassuring in this idea, even though it speaks of loss. If forgetting and fading are woven into the very fabric of the universe, then perhaps they are not to be resisted so much as accepted as the condition on which any experience, any event, any memory becomes possible in the first place. A life, much like a black hole's brief and violent ringing, leaves only an impression behind — a handful of essential truths rather than the full, chaotic record of everything that shaped it. Seen this way, forgetting is not simply loss; it is also what allows the universe, and us within it, to move forward at all, carrying forward only what truly matters while the rest settles quietly into silence.

It is essays like this one that remind me why the study of physics remains, for me, not just an intellectual pursuit but a genuinely humbling one.