1. Carl_Pham

The twin paradox is a relativity paradox, and goes like this: Jack and Jill are twins, both age 30. Jack gets in his spaceship and flies at 0.999c to Alpha Centauri and back, a distance of 4.4 light years. The trip therefore takes him 8.8 years as far as Jill is concerned, since Jack travels at essentially the speed of light, so that Jill is 38.8 years old when he returns. Since time dilates for Jack, however, he ages $dt^{(\rm Jack} = {dt^{\rm Jill}}{\sqrt{1 - v^2/c^2}} = 0.0447 dt^{\rm Jill}$ So Jack ends up only 0.39 years older. So far, so good: we all agree traveling at the speed of light, time slows down. But wait a minute -- isn't motion relative? Couldn't we just as well say that it was Jill who left Jack, traveling at nearly the speed of light, and then returned to his location, and Jack who was motionless? What right do we have to say it was Jack who was in motion and Jill stationary? The resolution is to realize that if we want to compare Jack and Jill in the same reference frame, then one of them must be severely accelerated to depart that frame, and then return to it. If Jack is the one that gets slammed back into his seat by powerful rockets, to accelerate him to nearly the speed of light, and then slow him down again on his return to Earth, then clearly Jack's frame is very different from Jill's, since Jill experiences no such force. So Jack and Jill's situation is quite different, because their reference frames are quite distinguishable.

2. imron07

@Carl_Pham Thanks, you're good at explaining :D

3. UnkleRhaukus

it makes sense now