The canonical twin paradox is explained by making a correct use of the principle of equivalence. The role of the principle of equivalence is to provide a physical agent i.e gravity which can supply the required extra aging to the rocket-bound sibling during its acceleration phase through a gravitational time-offset effect. We follow an approach where a novel variation on the twin paradox is used to connect gravity with the desynchronization in the clocks of two spatially distant, identically accelerated observers. It is shown that this approach removes certain drawbacks of an earlier effort which claims to exploit the equivalence principle in explaining the differential aging in the paradox.

The Greisen-Zatsepin-Kuzmin (GZK) cutoff (5 × 1019eV) of special relativity in the observed ultra high energy cosmic rays (UHECR) spectrum is one of the most puzzling paradoxes in physics. Experimentally a number of cosmic ray events have been detected above this GZK limit which is known as UHECR paradox. We propose a resolution of this paradox through a modification of the relativistic kinematics keeping in mind that it should not lead to predictions different from those of special relativity in the well tested domains. It is shown that theoretical limit in UHECR spectrum can be explained in the framework of Lorentz invariance violation (LIV) theories which assume the existence of a preferred frame. The present paper proposes that the velocity of the solar system with respect to the rest frame of the universe plays a role in explaining the paradox. DOI: http://dx.doi.org/10.3126/bibechana.v11i0.10375   BIBECHANA 11(1) (2014) 17-24