In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of which makes a journey into space in a high-speed rocket 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 traveling, and so, according to a naive application of time dilation, each should paradoxically find the other to have aged more slowly. However, this scenario can be resolved within the standard framework of special relativity (because the twins are not equivalent; the space twin experienced additional, asymmetrical acceleration when switching direction to return home), and therefore is not a paradox in the sense of a logical contradiction.
Starting with Paul Langevin in 1911, there have been numerous explanations of this paradox, many based upon there being no contradiction because there is no symmetry—only one twin has undergone acceleration and deceleration, thus differentiating the two cases. Max von Laue argued in 1913 that since the traveling 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 aging difference, not the acceleration per se. Explanations put forth by Albert Einstein and Max Born invoked gravitational time dilation to explain the aging as a direct effect of acceleration.
The twin paradox has been verified experimentally by precise measurements of atomic clocks flown in aircraft and satellites. For example, gravitational time dilation and special relativity together have been used to explain the Hafele–Keating experiment.
Read more about Twin Paradox: History, Specific Example, Resolution of The Paradox in Special Relativity, Transfer of Clock Reading in A Twin Paradox Trip, The Equivalence of Biological Aging and Clock Time-keeping, What It Looks Like: The Relativistic Doppler Shift, Calculation of Elapsed Time From The Doppler Diagram, Viewpoint of The Traveling Twin, Difference in Elapsed Time As A Result of Differences in Twins' Spacetime Paths, Difference in Elapsed Times: How To Calculate It From The Ship, A Rotational Version, Explanation in Terms of Mach's Principle
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