Reach on the Spiral: Observation, Intervention, and Where a Body Can Act

This is a position paper, not a physics result: it argues where the reach of exploration actually lies and where scarce effort best goes. Reach decomposes into three. Epistemic reach is bounded by the past light cone — one knows only the past, and light already delivers the developmental history of matter along the arc. Physical reach is bounded by an energy wall: sending a body to interstellar distance costs E = γmc², diverging exactly where the time dilation that would spare a crew becomes large, so such travel is a one-way door in time and lies centuries off. Knowledge reach is bounded by the distinction, imported from The Two Strata, between distinguishability (φ₁: what an observer can tell apart) and intervention (φ₂: records made by changing a system). Remote sensing is a pure φ₁ engine that asymptotes against degeneracy; in-situ presence is φ₂, and at interstellar range it is walled off for humans and robots alike; the terrestrial laboratory is the φ₂ we can afford, and it sets the reference frame every remote measurement is read against. Reading the arc as two windings — a build rate λ_build of enrichment and a learning rate λ_learn of diminishing returns — the design gate is their crossover, and it favours opening a new information axis (gravitational waves, neutrinos) over deepening a saturating one. The recommended allocation follows: human presence within the solar system, where a body can act; reach for the edge with light and now gravity, because receiving is the only arrival; and skilled effort into terrestrial physics and chemistry, the reachable intervention that lifts the ceiling on everything observed. Nothing here is a lens result; the paper re-describes and prioritises, and claims no new number.