How To Slow Down Time (Time Dilation)

The speed of light (299,792,458 m/s or c) is an invariant. 

This means that if an object emitting light is moving; the light comes out at c. This goes against what we see in our day to day lives where velocities are additive, e.g, a javelin thrower runs at 5 m/s and throws the javelin at 15 m/s so it travels at 20 m/s. This also means if you see a light shining at you, no matter how fast you run towards the light, you will always measure it as moving at c. 

In the example below you will see a consequence of this. 

Speed = Distance/Time. In the example below the light rays travel different distances at speed c yet arrive at their destinations at the same time. This means that time taken for the light rays to travel was different (despite them starting and finishing at the same time). 

Time must have slowed or sped up.

This equation tells us that the dilated time is less than the proper time. This explains why the light could travel a longer distance yet arrive at the same time as the other ray. Time was moving faster in that frame of reference (the stationary one).

A clock moving relative to an observer will be measured to tick slower than one that is stationary.