Why Is an Event Horizon Not a Solid Surface?

What kind of boundary can be crossed without striking a wall?

You separated material surfaces from causal boundaries, compared local proper time with distant received signals, and used future light paths to explain the one-way horizon.

An instrumented probe appears in two successive positions while freely falling across a black hole's event horizon without striking a material wall. — Vastward artistic reconstruction. The probe positions and faint horizon guide teach a one-way spacetime boundary; this generated image is not an observation, a time sequence at equal intervals, or a scale drawing.

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