Paradox
The problem starts with a ladder and an accompanying garage that is too small to contain the ladder. Through the relativistic effect of length contraction, the ladder can be made to fit into the garage by running it into the garage at a high enough speed.
However, both the ladder and garage occupy their own inertial reference frames, and thus both frames are equally valid frames to view the problem. From the reference frame of the garage, it is the ladder that moves with a relative velocity and so it is the ladder that undergoes length contraction.
Conversely, through symmetry, from the reference frame of the ladder it is the garage that is moving with a relative velocity and so it is the garage that undergoes a length contraction. From this perspective, the garage is made even smaller and it is impossible to fit the ladder into the garage.
Read more about this topic: Ladder Paradox
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