Editing Twin paradox (section)

===Role of acceleration===
Although some solutions attribute a crucial role to the acceleration of the travelling twin at the time of the turnaround,<ref name='Ohanian'/><ref name='Harris'/><ref name='Rindler'/><ref name='Weidner'>{{cite book|last=Weidner|first=Richard|title=Physics|url=https://archive.org/details/physics0000weid|url-access=registration|date=1985|publisher=Allyn and Bacon|location=Needham Heights, MA|isbn=0205111556}}</ref> others note that the effect also arises if one imagines two separate travellers, one outward-going and one inward-coming, who pass each other and synchronize their clocks at the point corresponding to "turnaround" of a single traveller. In this version, physical acceleration of the travelling clock plays no direct role;<ref name="Einstein, A. 1923 pp. 38">Einstein, A., Lorentz, H.A., Minkowski, H., and Weyl, H. (1923). [[Arnold Sommerfeld]]. ed. ''The Principle of Relativity.'' Dover Publications: Mineola, NY. pp. 38–49.</ref><ref name='Kogut'>{{cite book |title=Introduction to Relativity: For Physicists and Astronomers |first1=John B. |last1=Kogut |publisher=Academic Press |year=2012 |isbn=978-0-08-092408-3 |page=35 |url=https://books.google.com/books?id=9AKPpSxiN4IC}} [https://books.google.com/books?id=9AKPpSxiN4IC&pg=PA35 Extract of page 35]</ref><ref name='Minguzzi'/> "the issue is how long the world-lines are, not how bent".<ref name='Maudlin'>{{cite book|last=Maudlin|first=Tim|title=Philosophy of physics : space and time|date=2012|publisher=Princeton University Press|location=Princeton|isbn=9780691143095|pages=77–83}}</ref> The length referred to here is the Lorentz-invariant length or "proper time interval" of a trajectory which corresponds to the elapsed time measured by a clock following that trajectory (see Section [[#Difference_in_elapsed_time as_a_result_of_differences_in_twins'_spacetime_paths|Difference in elapsed time as a result of differences in twins' spacetime paths]] below). In Minkowski spacetime, the travelling twin must feel a different history of accelerations from the earthbound twin, even if this just means accelerations of the same size separated by different amounts of time,<ref name='Maudlin'/> however "even this role for acceleration can be eliminated in formulations of the twin paradox in curved spacetime, where the twins can fall freely along space-time geodesics between meetings".<ref name='Debs_Redhead'/>