  The twin paradox is the best known thought experiment associated with
Einstein's theory of relativity. An astronaut who makes a journey into space in
a high-speed rocket will return home to find he has aged less than a twin who
stayed on Earth. This result appears puzzling, since the situation seems
symmetrical, as the homebody twin can be considered to have done the travelling
with respect to the traveller. Hence it is called a "paradox". In fact, there
is no contradiction and the apparent paradox has a simple resolution in Special
Relativity with infinite flat space. In General Relativity (dealing with
gravitational fields and curved space-time), or in a compact space such as the
hypersphere or a multiply connected finite space, the paradox is more
complicated, but its resolution provides new insights about the structure of
spacetime and the limitations of the equivalence between inertial reference
frames.
