A meson (an unstable subatomic particle), when traveling at a velocity close to the speed of light, has a clearly longer decay time than its 2.1 microseconds ‘half-time’ at lower speed – when an Earth-fixed observer does the timing. But if the observer were flying along with the meson, the half-time of 2.1 microseconds would not seem to be affected by the particle’s speed, since the observer’s watch would be subjected to the same time dilation as the meson itself.
Einstein’s theory of relativity tells us that the pace of time becomes slower and slower for an object approaching the speed of light, compared with time’s rate of passage for a stationery observer. If the speed of light can be ever achieved time would come to a complete standstill. If an object could go fast, it could cover vast distance while, for a man flying along with it, no time would elapse – neither for his watch nor for his heartbeat, which controls his life span. This effect makes it possible for a stellar astronaut to travel from the Earth to a fixed star 1,000 light-years away, in what he would think was 13.2 years. If he didn’t spend any additional time at his destination, he would thus have been away from the Earth for 26.4 years. The trouble is that, during his absence, more than 2,000 years would have elapsed on Earth. Thus, upon return, he might wind up in a zoo.
Time dilation (Wikipedia)