sim07 — Environmental Physics Coupling

Testing the M_c phase transition for the Trace→Actor Crossing (H7)

Result: NULL — the transport field does NOT produce a phase transition in M_c. Sweeping M_c from inert to fully active monotonically decreases stability and fragments the structure (57 → 128 pillars). The crossing detector never fires for any M_c. This refines H7: structure-sourced transport alone is insufficient — venting disperses the very cue that recruits deposits.
HYPOTHESIS (H7, operational)

Below a critical mass threshold M_c, the accumulated structure is inert — diffuse scatter (sim06's null). Above M_c, the structure activates: it sources a transport field T that vents pheromone away from saturated regions (the negative feedback sim06 lacked), producing consolidation into a few large vented pillars and the crossing detector firing. The transition should be sharp in M_c.

Prediction: a phase transition in M_c — morphology (scatter → few pillars) and crossing coincide.

Simulation Grid

Step 0

The mechanism (minimal lumped transport)

The ONLY addition to sim06 is the transport field T and its coupling to pheromone P. Agents are unchanged (Grassé stigmergy). This isolates the environmental-physics-coupling variable.

Sign correction: the DESIGN sketch wrote (T_local − T_neighbor_avg), which would increase P at structure (positive feedback). The prose ("saturated pillars shed their pheromone to their flanks") describes venting, so the implemented sign is (T_neighbor_avg − T_local).

Baseline vs Transport (default run)

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Time series

n_pillars over time
structure_stability over time
mean pheromone over structure
deposit_on_structure_fraction
baseline (M_c = ∞, inert) transport (M_c = 3.0, structure sources T)

Dashed lines mark the crossing detector thresholds (stability ≥ 0.90; pheromone ≥ 0.5; deposit-on-structure ≥ 0.60). The detector needs all three for ≥4 consecutive samples.

M_c sweep — the phase transition test

Sweep M_c from ∞ (never active) to 0.5 (almost always active). If H7's phase transition exists, a sharp jump should appear in pillars (down) and stability (up) at some M_c, with the crossing detector firing only above it.

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Perturbation / self-repair (the circularity safeguard)

Damage 25% of the grid at 60% of steps; measure recovery (final / pre-perturb mass). The H7 acid test: repair should succeed when T is active (above M_c) and fail when T is suppressed — proving the crossing is emergent from the physics, not imposed by the rule.

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What the null means

The transport field fragments rather than consolidates. Venting pheromone away from saturated pillars disperses the very cue that attracts deposits. Stronger transport (lower M_c, higher coupling) → more, smaller, less-stable pillars, not fewer larger ones. Stability falls monotonically (0.876 → 0.739) as M_c drops. The crossing never fires.
Why this is informative, not a failure. The negative feedback is real but its effect is wrong sign for consolidation: it removes the spatial contrast (cue at structure) that the deposit rule needs, instead of redirecting it. A lumped linear advection of a scalar cue does not reproduce the Mahadevan mechanism, where directed flow carries the cue along channels to where building should continue. The mechanism needs geometry (directed flow), not just a venting scalar.
H7 refinement. The crossing is not produced by "structure sources a scalar transport field" alone. Two candidates remain: (1) directed transport (channel geometry that carries cue to building fronts, not away from them), or (2) an external multi-rate driver (diurnal oscillation, H4) the structure rectifies — the Mahadevan energy source sim07 omits. Candidate sim08 tests the external-oscillation path.

Lineage

Built on sim06_termite_mound (null result). Design grounded in King, Ocko & Mahadevan (PNAS 2015) and Ocko, Heyde & Mahadevan (PNAS 2019). See the environmental-physics-coupling concept.