A
misnamed thought experiment called a "
paradox" by people in an attempt to demonstrate the inconsistency of
special relativity, in which role it, of course, fails.
In this thought experiment, there are two
identical twins. One leaves
Earth in a
spaceship, travelling away at a
relativistic speed. Eventually the spacefaring twin turns around and returns to the Earth.
Upon arriving home, he/she finds that much more
time has passed on the Earth than was experienced
subjectively on the spaceship. In some versions, the stay-at-home twin is elderly while the spacefaring twin is much younger; in others, the stay-at-home twin has died centuries before. The time interval, of course, depends only upon the speed the spacefaring twin attained and how long he/she went at that speed.
People wonder where the extra
time comes from (or goes to, depending upon your point of view). The answer is that it is swallowed up by the spacefaring twin's changes of
speed, in effect fixing the
time dilation or
length contraction caused by the relativistic speed.
But be careful. Just because changes of speed cause time to be lost or gained does not mean that
acceleration is responsible. The
Three Planets thought experiment demonstrates this. The difference in age between the two twins,
all of it, happens at the moment the first twin turns around to go home.