ELIF: how is time relative?
In: Physics
So, the speed of light is a constant. I’m not sure that ELI5-ing the reason for this is possible, beyond just saying that photons have a mass of zero. Anyway, say you build a clock that abuses this. It’s a tube with mirrors on the ends with a single photon inside of it, bouncing up and down. It measures an exact time based on the amount of bounces of the photon, since you know how long it is. This is all well and good.
Now, say you’ve got 2 people. One of them stands still, and the other puts the clock in his truck and speeds away. To the stationary observer, each time the light bounces, it has to move at a diagonal to reach where the other mirror will be as it moves. Diagonals are longer than straight lines, and the speed of light is constant, so the clock is now runny slow to the stationary observer. However, to the observer in the truck, the clock is relatively at rest, and the rest of the world is moving. For him, the light doesn’t have to take a diagonal, so his clock reads like it’s still correct, not slow. So, when the driving observer experiences as a second, the stationary observer will experience more than a second. In general, an observer that is moving and eventually stops will have experienced less time than someone who was still during this period.
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