HallsofIvy said:
What does the runner do once he is inside the barn? If he keeps running then, of course, the pole hits the back of the barn. If he stops running,then, from the point of view of an observer stationary with respect to the barn, his pole regains its full length and- hits the back of the barn.
The original problem statement said that the pole is trapped in the barn.
Hence. From the perspective of the the barn, the pole enters the barn. Both doors are slammed shut while the pole is entirely inside the barn. The pole slams into a door and comes to a screeching halt. It takes some amount of time for the compression wave from the front of the pole to propagate through to the back of the pole. Hence the back of the pole stops sometimes after the front of the pole. The pole is compressed as a result of the collision.
Let's put some numbers on this. For convenience, I'll use units in which the speed of light is equal to 1. I'll measure time in microseconds. I'll measure distance in light-microseconds, which would be about 300 meters. I'll make the pole as rigid as I possibly can, to minimize compression.
(A) From the point of view of the barn
There's a pole of length 4, velocity 0.6, moving into a barn of length 5. At some point while the pole is entirely within the barn, both doors are shut.
The pole hits the rear door at a time I'll call zero. From the collision, a compression wave propagate back up the pole. By making the pole as rigid as possible, I'll let this be the speed of light. At the same time, the rear of the pole is moving in the other direction at 0.6. The two meet at a distance of 2.5 from the collision. With maximum possible rigidity, the pole compresses to this size, simply by virtue of being squashed in the collision. The compression wave moves back up the pole at speed 1, and the rear of the pole comes to rest at 2.5 micoseconds after the collision. The length of the compressed pole is now 2.5, half the size of the barn, and the pole is at rest inside the barn.
(B) From the point of view in which the pole is initially at rest
Now what does someone running with the pole see? Assume that they remain inertial, by virtue of watching from outside.
The gamma factor is 1.25. Hence they see a barn approaching the pole, with velocity 0.6C, and with a length of 4. The pole, at rest, has a length of 5.
The barn approaches the pole at high speed. Sometime after the front of the pole is swallowed by the moving barn, the rear door of the barn slams shut, and there's a collision between the pole and the door. The barn does not slow down in the slightest, and the front of the pole is pushed at high speed (0.6C) towards the back of the pole, which is still at rest. The pole, being infinitely rigid, has a pressure wave propagating back along the pole at velocity c. It takes 5 microseconds to reach the end of the pole, because that is the length of the pole. After this, the entire pole is accelerated up to the velocity of the barn. It is also compressed. In those 5 microseconds, the front of the pole has been moving towards the back at 0.6, and so the front of the pole is pushed up a distance 3 towards the back. The pole is now of length 2; half the length of the barn. It's inside the barn, moving at the same speed of 0.6.
In the meantime, the rear of the pole has been approaching the front of the barn. At the instant of collision, the rear of the pole is still distance 1 outside the barn, and it takes times 1/0.6, or about 1.67 microseconds, for the back of the pole to pass inside the barn; still well before the compression is complete.
The spacetime distance between the closing of the two doors is a space-like distance of 5. This is an invariant for all inertial observers. From the the frame in which the pole is initially at rest, the front door closes some time t after the back door has closed. Hence the distance between the two door closing events is 4 (length of the barn) + 0.6t. We have (4+0.6t)
2 - t
2 = 25, and so 9 - 4.8t + 0.64t
2 = 0, for which t = 3.75. This is well after the whole compressing pole has passed into the barn (at t = 1.67), but before the pole has finished compressing (at t = 5).
That is, 3.75 microseconds after the rear door of the moving barn shuts, the front door closes. This is a bit before the pole is fully compressed, but it is significantly after the back of the pole has entered the barn.
Cute, heh?
Cheers -- Sylas