Relativistic Rocket-Men and Foreshortened Chicken Feathers

This problem was first related to me in 1997 by Hu Zhou. I don’t know where he got it. I may have elaborated on it somewhat.

The set-up

A rocket moving at great speed, and a chicken barn with doors on either end. A pilot with insecurities that bring on annoying and dangerous displays, and a chicken farmer losing his patience just as dangerously.

The resting length of the rocket is equal to that of the barn, but from the rocket pilot’s perspective, the barn is foreshortened by speed to be much shorter than the rocket, and similarly, to the chicken farmer, the rocket is so foreshortened by speed to be much shorter than the barn.

The pilot decides to buzz the chickens by flying through the barn, but as he does so, the farmer teaches him a lesson. The farmer closes both barn doors while the rocket is in the barn (foreshortened as the rocket is, from his perspective), briefly trapping the pilot, then opens the doors just in time to let the pilot out.

The punch line

But from the point of view of the pilot, the barn is shorter than his rocket. The rocket simply would not fit inside the barn.

What is the solution?

In relativistic discussions, one should always be on the lookout for hidden assumptions of simultaneity. Here, one lurks in the closure of the barn doors: it seems they are closed at the same time, although they are separated spatially.

While from the chicken farmer’s perspective, the barn doors may have closed simultaneously, from the pilots perspective, first the back doors close, then open in front of him (he is suitably alarmed by this), but then behind him the front doors close and then open.

The chicken farmer achieves his aim, but not quite the way he may have imagined. The rocket ship escapes the barn unscathed, chicken feathers fluttering behind.