Toward a Principle of Significance

<p>The principle of causality bounds what can influence a given point: nothing outside its past light cone. This note asks whether a complementary bound exists on what can be physically significant to a bounded observer - i.e. on the number of configurations that observer could, even in principle, operationally distinguish. We show that such a bound follows from Bousso's covariant entropy bound - a proved theorem of semiclassical general relativity - together with elementary oneshot quantum information theory, with no Banks–Fischler-type postulate anywhere in the derivation. Once the notion of operational significance (Definition 3.2) is sharpened into an explicit N-ary ensemble-discrimination task (Definition 3.2′), and a physical "configuration" is represented as a classical register tensored with the light-sheet Hilbert space, §4.6 proves 𝑁(𝑂) ≤ exp{ [ 𝐴max(𝑂) 4ℓ𝑃2 + ℎ(𝜀)] (1 − 𝜀) } directly from Bousso's theorem, the data-processing inequality, and the Holevo bound - no further conjecture is invoked. This bound is proved for a single ε-significant N-ary ensemble in the precise sense of Definition 3.2′; whether it also bounds every configuration ever operationally significant to O across its entire history, in the broader sense of Definition 3.2 that motivates the principle informally, is item (c) of §7 and is not itself established here. The zero-error bound 𝑁(𝑂) ≤ exp [ 𝐴𝑚𝑎𝑥(𝑂) 4ℓ𝑃2 ] is recovered in the ε→0 limit. §3.3 and §4.7 separately establish the finiteness of Amax(O) for arbitrary FRW spacetimes (via the apparent horizon) and for fully dynamical horizons (via the Bousso–Engelhardt holographic-screen area law), so the principle applies without restriction to Λ&gt;0 or to static horizons. We note the independent numerical cross-check offered by Lloyd's (2002) computational-capacity bound - obtained by an entirely unrelated argument and landing on the same order of magnitude, ~10¹²⁰. What remains open is narrowed to three explicitly stated technical items, given in §7.&nbsp;</p>