Let's have an example. Mary, a 30 year old NASA astronaut, blasts off from Cape Canaveral in her very high speed rocket in the year 2010 on a 10 year mission to a nearby star. Her identical twin sister, Susan, is a NASA ground controller for the mission. This is called the twin paradox . Not only was her clock running slow as far as a ground based observer is concerned, but her time was running slow.
Nothing can travel faster than light.
For reasons of brevity the first will not be covered here and the latter two points only very briefly. The more something weighs the more energy is required to move it, as we know from everyday experience. To move something of infinite mass would require infinite energy and this is clearly not possible, hence nothing can travel faster than light. We live in a universe dominate d by the Special Theory of Relativity where faster than light travel is not possible. There are , however, ways of breaking out of the constraints of the special theory and travelling at speeds far in excess of that of light, at least for tiny particles.