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One of the more interesting consequences of relativity is how time behaves. Almost everyone is familiar with time dilation, where time flows more slowly for a reference frame that is moving quickly relative to another reference frame (actually, time flows more slowly for the frame that experiences the greater acceleration). However, not everyone is familiar with an even more intuition-smashing consequence: the loss of simultaneity.

Suppose you are in one inertial frame of reference (frame A), and an observer is passing by in another inertial reference frame (frame B) at a significant fraction of the speed of light. A pair of lights are stationary in your reference frame, and are positioned so that they lie along frame B's line of travel. If both lights are rigged to turn on at the same time, you will see two lights flick on simultaneously. However, to the observer in frame B, the light closer to him will turn on, and after a delay, the second light will illuminate. Events that occur on the axis of travel between two frames are NOT simultaneous: in a relativistic universe, events are only synchronized in one reference frame.

This effect appears for the same reason as distance contraction, and the two are intricately entwined. To illustrate how, (as opposed to why) they work, I will use the Barn and the Pole.

Suppose a barn is located in the middle of a spacious open field. It's a nice day, so the big doors at each end are open. Now I pick up a pole that is several feet longer than the barn, and begin to run. Soon I'm running at a significant fraction of the speed of light, and it appears to me that the barn has shrunk (this is distance contraction). As I run through the barn, which is now ridiculously short, the pole is obviously too long to ever fit inside. For an observer running with me, the pole never is contained within the barn.

Now suppose a mean spirited farmer owns the barn. He sees me pick up the pole, and begin to run. Seeing a damn fool kid running at a dangerous speed toward his barn, he decides to pull a trick on me. He waits till I am in the barn, and clicks the switch to his electric doors. They slam shut simultaneously, trapping me inside (for him, as I began to run, the pole became shorter, contracting along our mutual axis). The farmer observers me, the human pancake, contained completely within his barn . Then his son, not wanting to observe the resulting gore as I smash into their barn door doing several thousand miles per second, hits the switch again, opening both doors simultaneously.

"But wait!!" you cry, confused. "Maybe objects do contract along their axis of motion. But how in the world can both the barn be shorter than the pole, and the pole be shorter than the barn?!"

Aah.

There's the interesting bit. Time has now become slippery, and messes everything up. This paradox is possible because of the loss of simultaneity. When the farmer closes both the doors at the same time, they move in synchronization for the barn-frame of reference. However, for anyone running with me, a much stranger sight can be seen. The event order that occurs for the pole-frame are: the tip of the pole enters the barn, and then the front door closes. Then this door opens again. The tip of the pole exits the barn, and the trailing edge enters the barn. Then the rear door closes, and duitifully opens again.

Neither point of view is correct, they both are. Now you see how truly weird things can get after concept of synchronization becomes local, instead of universal.

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