The best example of an answer to this question I ever saw a “light clock”. The idea being, that you have an apparatus which basically looks like a desk lamp with the “lamp” part looking down on the desk. From the “lamp” part, a laser beam is sent down to a reflector and it bounces the beam back up to the “lamp” at intervals of 1 second. While the whole lamp is stationary, the laser beam bounces up and down every second like clockwork (Pardon the pun). When you start moving the lamp forward at faster and fast speeds, the laser will wind up having to move longer distances to reach the reflector, and then longer to bounce back to the “lamp.” I’ll make this crude chart of what I mean below, and hope that it carries through after Answerbag.com formats it. -----0 <---- Laser | | <---- Stand | |___^__ <--- Reflector -------0 | | I <--- Laser beam | I | I | |____^___ Ok, HOPEFULLY that diagram isn’t too crude, and it’s what the lamp would look like in a stationary position. Now, let’s pretend that the lamp is moving close to the speed of light. If you were moving WITH the lamp itself, to YOU, it would still look like the diagram above. However, to everyone else it would look more like this. --------0 | I I | I I | I I | I I | I I | I -----------^--- (Keep in mind that the whole lamp is moving WITH the laser, I’m just not very good at drawing ANSI!) So, as you can see, the laser is now actually taking much more then a second to go from the lamp and bounce back then it did when the lamp was stationary. Thusly, to you who is moving WITH the lamp, time is moving either normally or slowly depending on your point of view of the outside world. To those of us on the outside however, you are moving much slower then we are, or again, from your point of WE are moving much faster. So, the closer you get to the speed of light, the faster the outside world is in your prospective, and the closer you get to the speed of light, the slower you move in comparison to us. So, you move forward in time like the rest of us, just at a much slower rate, which will make it seem like, to you, that have traveled forward in time when you join the rest of us coming back from the speed of light. There are actually several Russian that are calculated to be “ahead” of us by a few microseconds because of the amount of time they have spent in space (years in some cases) traveling at about 18 000 miles per hour.