Relativity of simultaneity was definitely a wild concept for me in physics class. The idea that a near-c train is longer than a tunnel to its passengers and hen

Relativity of simultaneity was definitely a wild concept for me in physics class. The idea that a near-c train is longer than a tunnel to its passengers and hence is never fully inside the tunnel to them but is, after relativistic compression, shorter than the tunnel and hence at one moment entirely inside it to a resting observer blew my mind.