What I am reading and hearing about time dilation is that it is still under widespread debate. I know I’m not sure who to trust.
Time dilation caused by extreme velocities is difficult to measure, simply because we don’t have a vehicle capable of making long trips at a substantial fraction of the speed of light. There have been some experiments that suggest the accuracy of Einstein’s general and special time-dilation predictions, like one in which muons seemed to decay, or “age” much more slowly at high velocities than standing still. There have also been experiments with clocks in aircraft that show time dilation occurs because of velocity and gravitation potential.
It’s mind-bending stuff, to be sure. An atomic clock flying at high altitude runs faster relative to clocks sitting at see level. On the plane, however, it’s running normally. I remember using the Lorentz Transformation equations to check on the the relativity of time, since it seemed so counter-intuitive. What I found was that I would have to accept light propagating slower than an olympic sprinter if time were a constant.
The experiments I’ve heard about have demonstrated time-dilation to my satisfaction, anyway. The math holds up (as far as it goes, anyway), and the effect appears to be observable and repeatable. I don’t find it shocking or anything; I don’t expect to understand God’s creative process.
Incidentally, it was stated earlier that nothing can exceed the speed of light. Not completely true, in theory. Nothing can
accelerate to the speed of light, because an object’s mass increases to a limit of infinity as velocity approaches c. If an object is
already moving faster than light (like the theoretical tachyon), there’s no reason to think that it couldn’t stay that way. What that would mean in terms of time-dilation is anybody’s guess. Above the speed of light, the passage of time requires imaginary numbers to represent. There’s no way of knowing what that means.
I’ve heard it postulated, however, that tachyons all travel to a parallel universe where the Cleveland Browns could win the Super Bowl.