Einstein's theory of relativity is not easy to understand because it defies common-sense, and does not apply in obvious ways to our daily life and experiences. For this reason I have decided to use the common "twin goes on a fast space voyage" scenario without the scientific jargon. This is not a proof of the theory, just a description of how it works.
I have an animated sequence of all the images below to give an idea of what happens over the journey.
Relativity will become part of our daily life eventually. For example, NASA is designing now (1998) a craft to travel to Mars, launched from a planned space station, Alpha. This craft is intended to travel at an average speed of 1/100 the speed of light with a maximum speed of around 1/80 the speed of light. Even at 1.25% the speed of light, the effect of relativity needs to be taken into account in communicating with the craft.
Firstly I must credit Paul Davies, whose example I have expanded upon. Paul is a talented Professor of Natural Philosophy at the University of Adelaide, Australia. He has written an excellent series of books published by Penguin on the topics of time, space, the cosmos, the start and end of the universe, and so on. The 'thought experiment' here is from his book 'About Time'. I heartily recommend these books to you if you're interested in a far more authoritative account of the universe than you will find here.
I would also like to thank Paul Alan Cardinale for his help and patience in correcting a serious error with my initial time calculations, his tables to clarify what each twin sees and can calculate, and for adding some valuable 'Relitavistic Tidbits'.
