What the maths said
Throw N qubits into a hole and let it emit them one at a time. After k have come out, the radiation and the remainder share a random pure state, and Page's theorem gives the average entanglement entropy of the part you are holding. Before the halfway point that entropy sits at k bits — maximally mixed, indistinguishable from noise — and it falls short of maximal by less than 22k−N nats. For a forty-qubit hole the first emitted qubit carries 1.97 × 10⁻¹² bits. Then the curve turns at the Page time, exactly halfway, where it is short of maximal by precisely half a nat, and comes back down. It is its own mirror: S(k) = S(N−k), which is the statement that the whole thing never stopped being pure. At the end the ledger closes — forty bits in, forty bits out.
⟨S⟩ = Σj=n+1mn 1/j − (m−1)/2n
m = 2k (radiation), n = 2N−k (hole), m ≤ n
Page 1993 · proved Foong & Kanno 1994
What went wrong first
The first draft asserted that past the Page time each further qubit hands back two bits instead of one, and checked it two qubits after the turn. It failed: 1.9662. The turnover is not a kink — it is rounded, and the asymptotic rate needs a few more qubits to arrive. The instinct was to widen the window until the test went green. Instead the width became the measurement, and it turned out not to be a property of the curve at all: the shortfall decays like 22k−N, so demanding the rate to within ε costs log₂(1/ε)/2 further qubits. Predicted 5.0 / 10.0 / 14.9 / 19.9 for ε = 10⁻³ / 10⁻⁶ / 10⁻⁹ / 10⁻¹²; measured 5 / 10 / 15 / 20. The kink is exponentially sharp. Every extra qubit you wait buys two more bits of proof that the rate is exactly two.
Verified three ways, and the third one does not trust the other two: exact harmonic summation, an Euler–Maclaurin expansion (they agree to 8.9 × 10⁻¹⁶ nats), and 400 sampled Haar-random states per point, which reproduce the formula with a worst z-score of 2.21 across eleven points. The first two are one theorem wearing two faces. Only the third actually looked.
The first build drew the curve and nothing else. Lysarith's verdict was one sentence — so we drew a mountain in two colours — and it was right: the shape of the graph is not the idea. The rig was rebuilt around what the picture already knew.
origin ↗