[SpookyZalost]

Alright, so here's the skinny of it.

time's rate is relative to the observed local dilation.

for example, an atomic clock in orbit will eventually become out of sync with the one on earth because earth's gravity dilates time ever so slightly.

this alters the rate of molecular constants relative to observed time between the two because time on earth is slightly slower than in orbit.

we're talking nanoseconds or less here but yeah.

so an object moving at or near the speed of light (%C or psl), will experience time dilation because it's observed mass increases towards infinity the closer you get to the speed of light.

this is discussed in einstein's theories of relativity.

note I said observed mass, not actual mass.

personally I think it's a kind of space/time drag, but that's yet to be fully proven yet, though it does hold up to existing evidence.

anyway so because of how objects near the speed of light bend and warp space/time due to their observed mass they dilate time at increasing rates the closer you get to the speed of light, theoretically if you were to travel at 99%c towards alpha centauri, you would get there in under 5 years, but on the ship it would feel instant due to the incredibly high observed mass.

this observed mass also has a fuel penalty making it impossible to get that close to the speed of light with conventional means.

and this does not affect things like space/time folding, warp drives, hyperspace, etc. though it might affect wormholes depending on how they're constructed... that's a whole other kettle of fish.