Aluminium cannot cut steel and concrete

As it is told: the towers were structural steel. An airliner is thin aluminium skin over light ribs, a machine so fragile that a bird strike can wreck it. Something soft cannot cut something hard, so the aircraft-shaped holes in the façades must have been made another way — cut in advance, or added to the footage.

The framing emerged alongside the no-planes material and spread on its own because it sounds like common sense. Its best-known illustration is a slow-speed soda-can-versus-tree comparison, which holds the one variable that matters — velocity — constant at roughly zero.

Penetration is governed by kinetic energy and momentum, not by the hardness of the projectile. A Boeing 767 near 140 tonnes at roughly 200 metres per second carries energy in the range of a few billion joules, concentrated on impact into a fraction of a second. The exterior of each tower was not solid steel: it was a perimeter curtain wall of relatively slender box columns spaced about a metre apart, spanned by aluminium cladding and glass, with the floor slabs as light concrete on steel deck. The engines, landing gear and wing spars are the dense elements, and they are what the damage maps in the collapse investigations show punching through the perimeter columns while the softer structure disintegrated — which is exactly why the holes are ragged and aircraft-shaped rather than clean silhouettes. The same physics is routine elsewhere: water jets cut steel, and aluminium slugs are used in laboratory impact testing to breach armour plate. The tower façades were designed against wind load and against a slow, lost, low-altitude aircraft, not against a fuel-laden airliner at near-cruise speed.