I've found the following paradox, and I wonder how to resolve it.
Two discs are floating in space, call them A and B. They are at a fixed distance D, coaxial, and rotate at the same speed. Each of them has a hole near the border.
The position of the hole in disc B lags behind the position of the hole in disc A, by a small amount of time. This time is exactly equal to the time it takes light to traverse D.
This means that a laser pulse that gets through hole A is going to get through hole B, and hit a detector on the other side, but the size of the holes is such that there is very little margin for error.
Now: an observer passes along this contraption, moving in the axial direction at a sizeable fraction of the speed of light.
Due to Lorentz contraction, the distance between A and B is going to be smaller in the observer's frame of reference. Plus, the rotation of the discs is going to be slower, due to time dilation.
Either of these effects would be enough to prevent the laser pulse from passing through hole B: it's still traveling at the same speed in the observer's frame of reference, but it has less ground to cover, and on top of that the other disc won't have rotated enough to put the hole in its path. So the detector doesn't get hit!
It's illogical for the detector to be hit or not hit depending on the observer. What am I missing? How to resolve this?