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 subjective
ly 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.