system
Given the following post, respond as if you're commenting on Reddit:
human
How is this paradox involving basic relativity principles explained?

Relativity is a confusing and counter-intuitive concept to the layman, and many people only ever know that there is something called "time dilatation" and "length contration" happening, without fully comprehending (myself included). With this limited knowledge it's possible to construct paradoxes, one of which is the following:

> You sit on top of a tunnel with a length of 100m. A train of 200m (when at rest) approaches this tunnel at near light speed, experiencing a length contraction of 50%, i.e. 100m. From your perspective, the train fits into the tunnel, and as soon as it enters it completely, you close the tunnel with impenetrable doors on either side with a press of a button. However, from the perspective of the train it's the tunnel that was shortened by 50% and it should have been impossible for the train (200m) to enter the now 50m long tunnel. Explain this paradox.

Hope someone can explain it in a layman-friendly way (if in any way possible) :D

And please, for the love of god, *do not* comment on how it's impossible for a train to be this fast, to close doors immediately, for a door to be completely impenetrable, etc. It's a gedankenexperiment and not the point of this post.
gpt
While the closing and reopening of the doors was simultaneous in the ground frame, in the train's frame it's not. In relativity, different observers will disagree on the simultaneity of events happening at different places.

In the train's frame, the train enters the tunnel, then the *second* door closes, then it open backs again, then the trains' head exits the tunnel, then only after the tail has entered too, the first door closes, and then opens up again. In this frame, the train is never all inside the tunnel at any given time. But then, neither are the doors ever simultaneously closed.