Holmgren Revisited

Gerard Holmgren's famous proof of no-planes was the plane-shaped holes in the WTC towers. His point however was NOT that it was impossible for a plane to make a silhouette-like cut-out of itself in the WTC facade! Rather the idea is that it is impossible for any object, including a Boeing 767 going 540 mph, to slice into the WTC and leave a cut-out shape of itself and at the same time disintegrate in the same collision. Although a rather simple idea, his critical insight has stood the test of time. What has been lacking however, is a good example of the concept.

Recently I found a good example of a high-speed collision involving two complex objects-- in a "Mythbusters" episode, a rocket sled is blasted into a car at 650 mph, and the car is annihilated in a huge explosion:

The rocket sled is a smaller and lighter object hitting the car, which can be very roughly analogous to a plane hitting the WTC. Unfortunately, what is not clear is exactly what happens to the rocket sled as it hits the car, since is a heavy steel plate and a concrete berm behind the car. So it seems quite possible that the rocket sled cut through the car then broke apart on the steel plate and berm. Indeed, the steel plate is massively warped from the impact. But what we can say is that the rocket sled did NOT cut a simple hole through the car (that the car (WTC) was in fact totally blown apart) that at least much of the rocket sled appears to go all the way through the car and that there is a massive explosion within milliseconds of impact.

Bottom line: I would Holmgren's proof is alive and well.