Earlier we mentioned the effect of time dilation: when something is in motion, time slows down. This has been experimentally verified. Normally, particles known as muons exist about two millionths of a second before exploding into other particles called neutrinos and electrons. When muons are accelerated in a particle accelerator to 99.5% the speed of light, they "live" about ten times longer than normal. This is because time moves slower for them. This would happen to people also if they could live at such speeds. In spite of this, people would not live anymore in the sense that they would not be able to do anymore than they can with their current life span. This is due to the fact that everything they do is in slow motion. So even though they would live longer, things would take more time to do and so they would not have any kind of advantage.

Let's imagine that you could move at speeds near the speed of light like the muons. Say that you are out in the inky blackness of space and pass by someone. Now, if you were moving at a constant speed, you could claim that you are stationary and the other person is moving. If you were wearing a watch, it would appear to the other person that your watch is moving slower. Since you can claim that they are the one moving, to you their clock appears to be ticking slower. So who is really moving? Consider if this happened with Fred and Bob meeting in space. As they pass each other, they set their watches to the same time. Three years pass, Fred turns around and fires off his jet pack, travels for three years, and finally meets Bob again. Fred says six years have passed, but according to Bob's watch it has been sixty years. In this case, Fred relinquished his claim that he was stationary the whole time when he underwent accelerated motion by turning on his jet pack. He could feel that he was moving. Unfortunately, we still don't know who was moving originally.

Well, what if they called each other to compare the times on their watches? Suppose we set up this same situation again, but Fred doesn't use his jet pack. Instead he calls Bob on his cell phone and announces what time his watch reads. If they could communicate instantly, this method would work perfectly to see who is really moving because it would be whomever had the slower watch. But cell phone transmissions are radio waves, so they travel at the speed of light which is very fast but not instantaneous. In fact, communication and any kind of influence, like objects, cannot move faster than the speed of light, so immediate communication isn't possible. Getting back to Fred and Bob, even if Bob takes into account the time it takes for the transmission to reach him, he finds that Fred's clock is slow compared to his. The same thing happens when Bob calls Fred and tells him what time it is. Bob does more calculations and realizes that, even taking into account the delay because of the transmission time, Fred will conclude that Bob's clock is running slower. There is no way to find out who is truly moving.