Twin paradox

In physics, the twin paradox is a thought experiment in special relativity involving twins, one of whom takes a space voyage at relativistic speeds and returns home to find that the twin who remained on Earth has aged more. This result appears puzzling because each twin sees the other twin as moving, and so, as a consequence of an incorrect and naive application of time dilation and the principle of relativity, each should paradoxically find the other to have aged less. However, this scenario can be resolved within the standard framework of special relativity: the travelling twin's trajectory involves two different inertial frames, one for the outbound journey and one for the inbound journey. Another way to understand the paradox is to realise the travelling twin is undergoing acceleration, thus becoming a non-inertial observer. In both views there is no symmetry between the spacetime paths of the twins. Therefore, the twin paradox is not actually a paradox in the sense of a logical contradiction. Starting with Paul Langevin in 1911, there have been various explanations of this paradox. These explanations "can be grouped into those that focus on the effect of different standards of simultaneity in different frames, and those that designate the acceleration [experienced by the travelling twin] as the main reason". Max von Laue argued in 1913 that since the travelling twin must be in two separate inertial frames, one on the way out and another on the way back, this frame switch is the reason for the ageing difference. Explanations put forth by Albert Einstein and Max Born invoked gravitational time dilation to explain the ageing as a direct effect of acceleration. However, it has been proven that neither general relativity, nor even acceleration, is necessary to explain the effect, as the effect still applies if two...