Editing Twin paradox (section)

===Relativity of simultaneity===
[[Image:Twin Paradox Minkowski Diagram.svg|thumb|right|333px|[[Minkowski diagram]] of the twin paradox. There is a difference between the trajectories of the twins: the trajectory of the ship is equally divided between two different inertial frames, while the Earth-based twin stays in the same inertial frame.]]
For a moment-by-moment understanding of how the time difference between the twins unfolds, one must understand that in special relativity there is no concept of ''absolute present''. For different inertial frames there are different sets of events that are simultaneous in that frame. This [[relativity of simultaneity]] means that switching from one inertial frame to another requires an adjustment in what slice through spacetime counts as the "present". In the spacetime diagram on the right, drawn for the reference frame of the Earth-based twin, that twin's world line coincides with the vertical axis (his position is constant in space, moving only in time). On the first leg of the trip, the second twin moves to the right (black sloped line); and on the second leg, back to the left. Blue lines show the ''planes of simultaneity'' for the traveling twin during the first leg of the journey; red lines, during the second leg. Just before turnaround, the traveling twin calculates the age of the Earth-based twin by measuring the interval along the vertical axis from the origin to the upper blue line. Just after turnaround, if he recalculates, he will measure the interval from the origin to the lower red line. In a sense, during the U-turn the plane of simultaneity jumps from blue to red and very quickly sweeps over a large segment of the world line of the Earth-based twin. When one transfers from the outgoing inertial frame to the incoming inertial frame there is a jump discontinuity in the age of the Earth-based twin<ref name='Ohanian'/><ref name='Harris'/><ref name='Kogut'/><ref name="Wheeler, J. 1992 pp. 38, 170">Wheeler, J., Taylor, E. (1992). ''Spacetime Physics, second edition.'' W. H. Freeman: New York, pp. 38, 170-171.</ref><ref name="Einstein et al 1923">Einstein, A., Lorentz, H.A., Minkowski, H., and Weyl, H. (1923). Arnold Sommerfeld. ed. ''The Principle of Relativity.'' Dover Publications: Mineola, NY. p. 38.</ref> (6.4 years in the [[#Specific example|example]] above).