  In the standard formulation of the twin paradox an accelerated twin considers
himself as at rest and his brother as moving. Hence, when formulating the twin
paradox, one uses the general principle of relativity, i.e. that accelerated
and rotational motion is relative. The significance of perfect inertial
dragging for the validity of the principle of relativity is made clear. Three
new results are reviewed in the discussion. A cosmic time effect which cannot
be reduced to the gravitational or the kinematical time dilation. Perfect
dragging in an exact solution of Einsteins field equations describing flat
spacetime inside a shell with Kerr spacetime outside it. An extended model of
Minkowski spacetime in order to avoid introducing absolute acceleration and
rotation through the asymptotic emptiness of the Kerr spacetime.
