I have some questions about the relative factor: 1/'root'1-v^2/c^2 If an object moves close to the speed of light, the length contraction becomes significant. I hear this "paradox": (that was said not to be a paradox, but I didn't understand it) A train of 100 meter moves at a speed that contract it to 30 meters. It moves into a tunnel that is 40 meters long, and with open doors at both ends. Let's say the train moves into this tunnel, and it is 30 meters observed by the tunnel. When the train is well inside the tunnel, the doors close instantly, and at the same time. The train is trapped here contradicted. This was only in the tunnels point of view, in the train's point of view the front door is locked when the front of the train is in the tunnel, and when the train smashed through the front door, and the end of the train is well inside, the back door closes. That I understand, but there was something this podcast wouldn't explain, (only give the very question) What if the train is well inside by the tunnels point of view, and both doors slam down. The train is INSIDE the tunnel. The front door makes the train stop instantly, and if it was unbreakable it would smash the back door to pieces with it's end. But how can this be? From the trains point of view, the end was never really inside the tunnel, it was only the front of the train that was inside. But in the tunnels point of view the whole train is inside. The smashing through the front door never occurs, so how can the train smash the back door to pieces when it doesn't fit into the tunnel? I am sure I got it misconceptet, and I would really like to here what the correct version is!