I have heard if twins are separated at birth and one of them is sent in a fast spaceship going near the speed of light, that when he returns he will be younger than his brother. I frankly don't believe it. How can such a stupid theory be true?
Unfortunately, if this theory were not true a lot of modern technology would not work. I'll give you one example. The Global Positioning System (GPS) is comprised of radio transmitters that send atomic-clock time to receivers on the ground. By knowing the position of the satellite transmitters and the relative lag between four of them, it is possible to infer the time and the altitude, latitude, and longitude of any position on the ground. The GPS satellites are each launched containing an accurate atomic clock. The trouble is, these satellites are all moving quickly. They are not moving near the speed of light, mind you, but fast enough to mess up the exacting lag times. Part of the specification of these things are built, the way the system was designed, is the relativistic correction. They make an artificial "twin" when they build all these precisely calibrated atomic clocks on the ground. Then they launch the clocks into orbit and they all change rate, which has to be adjusted. The twin paradox is not something that must be checked in a distant future. It has already been checked by thousands of surveyors who rely on GPS for accurate positions.
But if all things are "relative," why can't we think of the twin that stays behind as the fast twin and the twin on the rocket ship as the slow twin. What is different about them?
The twin on the rocket ship must be accelerated. It is the acceleration that breaks the symmetry. Einstein himself said that "relativity" is an unfortunate name for the theory. In retrospect, he said that he would have preferred "the theory of invariance."
Will we ever be able to travel faster than the speed of light?
The relativistic concept of a limitation on speed has been linked to the philosophical concept called "causality," or the requirement that the cause of an event comes before the effect. We have never observed an effect before its cause. Perhaps in some unimaginably high technology in the distant future, this principle will be sidestepped, but the advance will have to be made in philosophy as well as science. The invention of a faster-than-light stardrive will also mean the invention of a time machine. I am not hopeful.