Imagine your twin is zipping past you at speeds that are significant when compared to the speed of light in vaccuum, with all her stuff.
Wonderful things happen. Your twin ages more slowly, the plant in her pot grows more slowly, her atomic clock and her wristwatch run more slowly, compared to your stuff. Only photons maintain their speed. You have traveled ahead of her in time.
Forget that hypothetical experiment.
Every second, really fast muons are generated ten km high up and hitting the ground. We know their half life in the lab. Half life is the time it takes for a large number of them to decay to half their previous quantity. ie the time for 10,000 muons to become 5000 muons as a result of natural particle decay. You would expect practically all the muons to have decayed en-route in non-relatavistic mechanics. But in reality about half of them actually reach the ground. They are moving so fast, that time has slowed down for them and they haven’t had enough time to decay off! If you ask them they will tell you that they have the time right, its just that the distance to the ground is 2 km and not 10km from the point they were generated.
Right here, on earth around you at this very minute, there are particles whose time is much slower than your time and whose idea of the same distance is much shorter, than you measured, simply because they are much faster than you.
Both you and the muon and your hypothetical twin agree on “Now” 4 pm at the time of their creation. After that, everyone is on their own time. You will not agree when the next microsecond is up.
Hmm. Your sister might feel she’s been gone five years and intersect your time-frame when you are 50 years old. If you were 20 when you last agreed on “Now”, she is a 25 year old and you are a 50 year old celebrating your mid-life crisis! If she left on 29th April 2007, she will think your date of reunion is 29th April 2012 while you insist that it is 29th April 2032.
Today your sister can’t take off as fast as in my example, but the day may come when she can.
And there are already very big stars and such and very tiny particles and such that have tremendous speeds relative to you. And they are all on their own time. Now revisit your thoughts about past, present and future. What do they mean?