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Twin paradox
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{{short description|Thought experiment in special relativity}} {{Redirect|Clock problem|mathematical problems involving the positions of the hands on a clock face|Clock angle problem}} {{For|the twin paradox in [[social choice]] and voting|No-show paradox}} {{Use dmy dates|date=December 2019}} [[File:Mark and Scott Kelly at the Johnson Space Center, Houston Texas.jpg|thumb|upright=1.15|During the [[ISS year-long mission]], astronaut [[Scott Kelly (astronaut)|Scott Kelly]] (right) aged about {{convert|8.5|ms|ms|disp=output only|abbr=off|frac=2}} less than his Earthbound twin brother [[Mark Kelly|Mark]] (left) due to relativistic effects.<ref name="Lichfield 2015">{{cite web |date=2015-03-15 |title=Astronaut Scott Kelly will return from a year in space both older and younger than his twin brother |url=https://qz.com/370729/astronaut-scott-kelly-will-return-from-a-year-in-space-both-older-and-younger-than-his-twin-brother |access-date=2024-10-15 |url-status=live | archiveurl = https://archive.today/20241023010252/https://qz.com/370729/astronaut-scott-kelly-will-return-from-a-year-in-space-both-older-and-younger-than-his-twin-brother | archivedate = 2024-10-23 }}</ref>]] {{General relativity sidebar}} {{Special relativity sidebar}} 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<ref>{{cite book |title=The Modern Revolution in Physics |edition=illustrated |first1=Benjamin |last1=Crowell |publisher=Light and Matter |year=2000 |isbn=978-0-9704670-6-5 |page=23 |url=https://books.google.com/books?id=OMs-_JK-wncC}} [https://books.google.com/books?id=OMs-_JK-wncC&pg=PA23 Extract of page 23]</ref><ref>{{cite book |title=Modern Physics |edition=3rd |first1=Raymond A. |last1=Serway |first2=Clement J. |last2=Moses |first3=Curt A. |last3=Moyer |publisher=Cengage Learning |year=2004 |isbn=978-1-111-79437-8 |page=21 |url=https://books.google.com/books?id=uTM8AAAAQBAJ}} [https://books.google.com/books?id=uTM8AAAAQBAJ&pg=PA21 Extract of page 21]</ref> and naive<!-- please do not remove this word, it appears in both cited sources: --><ref>{{cite book |title=From Special Relativity to Feynman Diagrams: A Course of Theoretical Particle Physics for Beginners |edition=illustrated |first1=Riccardo |last1=D'Auria |first2=Mario |last2=Trigiante |publisher=Springer Science & Business Media |year=2011 |isbn=978-88-470-1504-3 |page=541 |url=https://books.google.com/books?id=R-qIh6kd8d0C}} [https://books.google.com/books?id=R-qIh6kd8d0C&pg=PA541 Extract of page 541]</ref><ref>{{cite book |title=Gravitation and Spacetime |edition=3rd |first1=Hans C. |last1=Ohanian |first2=Remo |last2=Ruffini |publisher=Cambridge University Press |year=2013 |isbn=978-1-139-61954-7 |page=176 |url=https://books.google.com/books?id=JVQhAwAAQBAJ}} [https://books.google.com/books?id=JVQhAwAAQBAJ&pg=PA176 Extract of page 176]</ref> 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 frame of reference|inertial frames]], one for the outbound journey and one for the inbound journey.<ref>{{cite book |title=Foundations of Modern Cosmology |edition=illustrated |first1=John F. |last1=Hawley |first2=Katherine A. |last2=Holcomb |publisher=Oxford University Press |year=2005 |isbn=978-0-19-853096-1 |page=203 |url=https://books.google.com/books?id=s5MUDAAAQBAJ}} [https://books.google.com/books?id=s5MUDAAAQBAJ&pg=PA203 Extract of page 203]</ref> Another way to understand the paradox is to realize the travelling twin is undergoing [[acceleration]], which makes him 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".<ref name='Debs_Redhead'>{{cite journal |author1=Debs, Talal A. |author2=Redhead, Michael L.G. |title=The twin "paradox" and the conventionality of simultaneity |journal=American Journal of Physics |volume=64 |issue=4 |year=1996 |pages=384–392 |doi=10.1119/1.18252 |bibcode=1996AmJPh..64..384D}}</ref> [[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.<ref>{{Cite book |author=Miller, Arthur I. |year=1981 |title=Albert Einstein's special theory of relativity. Emergence (1905) and early interpretation (1905–1911) |location=Reading |publisher=Addison–Wesley |isbn=0-201-04679-2 |pages=[https://archive.org/details/alberteinsteinss0000mill/page/257 257–264] |url=https://archive.org/details/alberteinsteinss0000mill/page/257 }}</ref> Explanations put forth by [[Albert Einstein]] and [[Max Born]] invoked [[gravitational time dilation]] to explain the aging as a direct effect of acceleration.<ref name=Jammer>{{cite book |title=Concepts of Simultaneity: From Antiquity to Einstein and Beyond |page=165 |author=Max Jammer |author-link=Max Jammer |url=https://books.google.com/books?id=vuTXBPvswOwC&pg=PA165 |isbn=0-8018-8422-5 |publisher=The Johns Hopkins University Press |year=2006}}</ref> However, it has been proven that neither general relativity,<ref>{{cite book |title=Gravity from the Ground Up: An Introductory Guide to Gravity and General Relativity |edition=illustrated |first1=Bernard |last1=Schutz |publisher=Cambridge University Press |year=2003 |isbn=978-0-521-45506-0 |page=207 |url=https://books.google.com/books?id=P_T0xxhDcsIC}}[https://books.google.com/books?id=P_T0xxhDcsIC&pg=PA207 Extract of page 207]</ref><ref>{{cite web |url=http://math.ucr.edu/home/baez/physics/Relativity/SR/acceleration.html |title=Can Special Relativity Handle Acceleration? |first=John |last=Baez |date=1996 |access-date=30 May 2017}}</ref><ref>{{Cite web | url=https://www.scientificamerican.com/article/how-does-relativity-theor/ | title=How does relativity theory resolve the Twin Paradox?| website=[[Scientific American]]}}</ref><ref>David Halliday et al., ''The Fundamentals of Physics'', John Wiley and Sons, 1997</ref><ref>Paul Davies ''About Time'', Touchstone 1995, ppf 59.</ref> nor even acceleration, are necessary to explain the effect, as the effect still applies if two astronauts pass each other at the turnaround point and synchronize their clocks at that point. The situation at the turnaround point can be thought of as where a pair of [[observer (special relativity)|observers]], one travelling away from the starting point and another travelling toward it, pass by each other, and where the clock reading of the first observer is transferred to that of the second one, both maintaining constant speed, with both trip times being added at the end of their journey.<ref>{{cite web|url=https://www1.phys.vt.edu/~jhs/faq/twins.html|title=Frequently Asked Questions About Special Relativity - The Twin Paradox|website=Virginia Tech Physics|author=John Simonetti|access-date=2020-05-25}}</ref>
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