system
Given the following post, respond as if you're commenting on Reddit:
human
ELIF: how is time relative?
gpt
When you move fast (and by fast we talk about significant fractions of the speed of light -- 100mph isn't "fast" here), there are 2 things that happen:

\- for you, you experience time moving at the same rate you always experience time.  The second hand on your watch would still tick once a second.

\- for someone else who is standing still watching you, they see your time as going much slower than their time.  If they could see your watch, the second hand would be moving much slower.

The faster you go, the slower your time appears to an observer looking at you.

Interestingly, when you look at the person who is standing still, you will see their time as moving much slower too -- if you could see their watch, the second hand would also be going slow.  This is because, from your perspective, you are completely still and they are moving very fast.  (This is relativity)

Time, speed, and relativity are interesting, but very strange, phenomena.

One consequence of this is that anything that travels at the speed of light (a photon, for example) basically experiences no time passing.  So a photon that leaves a star 100 light years away would take 100 years to get here, as we would observe that photon.  From the photon's perspective, no time passed at all!