system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why isn't time constant?

Why is it that when you move faster through space, you move slower through time?
gpt
Imagine that you are in a spaceship that has the capability of reaching speeds near the speed of light. Inside this spaceship, there is a beam of light that is bouncing between a mirror on the floor and a mirror on the ceiling such that the light beam is in a infinite loop of bouncing between the two mirrors. To an observer inside the ship, the light is (obviously) observed to be bouncing back and forth between the mirrors at the speed of light (C).

Now, if you recall, according to relativity, there is no relative speed of light. That is, the speed of light (C) is always the same no matter the reference point. 

Imagine now that we have an observer outside the ship who is stationary relative to the ship which is also moving at the speed of light (or nearly). According to conventional relativity, the observer would observe the light beam moving at C up and down as well as ~C in the forward direction with the ship.

Now we have two observed speeds of light:

Inside the ship: |Cj|
Outside the ship: Ci+|Cj|

Since the speed as observed by the outside observer has both i and j components (up and down), the magnitude of velocity can be calculated via the Pythagorean theorem:

C=sqrt(Ci^2 +Cj^2)

This result would be greater than C, which is impossible, so in order for a constant C to satisfied as well as the conservation of energy, time must change instead.

Therefore, time would move more slowly inside the ship compared to outside the ship.

Hope that helps :)