Editing Time travel (section)

== Time dilation ==
{{main|Time dilation}}
[[File:Time dilation02.gif|thumb|Transversal time dilation. The blue dots represent a pulse of light. Each pair of dots with light "bouncing" between them is a clock. For each group of clocks, the other group appears to be ticking more slowly, because the moving clock's light pulse has to travel a larger distance than the stationary clock's light pulse. That is so, even though the clocks are identical and their relative motion is perfectly reciprocal.]]
There is a great deal of observable evidence for time dilation in special relativity<ref name="tomroberts">{{cite web|last=Roberts|first=Tom|title=What is the experimental basis of Special Relativity?|date=October 2007|url=http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#Tests_of_time_dilation|access-date=April 26, 2017|archive-date=May 1, 2013|archive-url=https://web.archive.org/web/20130501002220/http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#Tests_of_time_dilation|url-status=live}}</ref> and gravitational time dilation in general relativity,<ref name="scoutrocket">{{cite web|title=Scout Rocket Experiment|url=http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/gratim.html#c3|last1=Nave|first1=Carl Rod|work=HyperPhysics|year=2012|access-date=April 26, 2017|archive-date=April 26, 2017|archive-url=https://web.archive.org/web/20170426195700/http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/gratim.html#c3|url-status=live}}</ref><ref name="hafelekeating">{{cite web|title=Hafele-Keating Experiment|url=http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/airtim.html#c3|last1=Nave|first1=Carl Rod|work=HyperPhysics|year=2012|access-date=April 26, 2017|archive-date=April 18, 2017|archive-url=https://web.archive.org/web/20170418005731/http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/airtim.html#c3|url-status=live}}</ref><ref name="GPS">{{cite web|last=Pogge|first=Richard W.|title=GPS and Relativity|date=April 26, 2017|url=http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html|access-date=April 26, 2017|archive-date=November 14, 2015|archive-url=https://web.archive.org/web/20151114135709/http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html|url-status=live}}</ref> for example in the famous and easy-to-replicate observation of [[Experimental testing of time dilation|atmospheric muon decay]].<ref name=easwar>{{cite journal|author1=Easwar, Nalini|author2=Macintire, Douglas A.|title=Study of the effect of relativistic time dilation on cosmic ray muon flux – An undergraduate modern physics experiment|journal=American Journal of Physics|volume=59|issue=7|year=1991|pages=589–592|doi=10.1119/1.16841|bibcode=1991AmJPh..59..589E|url=https://scholarworks.smith.edu/cgi/viewcontent.cgi?article=1041&context=phy_facpubs|access-date=2020-09-08|archive-date=2020-11-04|archive-url=https://web.archive.org/web/20201104084110/https://scholarworks.smith.edu/cgi/viewcontent.cgi?article=1041&context=phy_facpubs|url-status=live}}</ref><ref>{{cite journal|author1=Coan, Thomas|author2=Liu, Tiankuan|author3=Ye, Jingbo|title=A Compact Apparatus for Muon Lifetime Measurement and Time Dilation Demonstration in the Undergraduate Laboratory|journal=American Journal of Physics|volume=74|issue=2|pages=161–164|year=2006|doi=10.1119/1.2135319|arxiv=physics/0502103|bibcode=2006AmJPh..74..161C|s2cid=30481535}}</ref><ref name="Ferraro" /> The theory of relativity states that the [[speed of light]] is [[Speed of light#Fundamental role in physics|invariant]] for all observers in any [[frame of reference]]; that is, it is always the same. Time dilation is a direct consequence of the invariance of the speed of light.<ref name="Ferraro">{{citation|title=Einstein's Space-Time: An Introduction to Special and General Relativity|first1=Rafael|last1=Ferraro|pages=52–53|publisher=Springer Science & Business Media|date=2007|isbn=9780387699462|bibcode=2007esti.book.....F}}</ref> Time dilation may be regarded in a limited sense as "time travel into the future": a person may use time dilation so that a small amount of [[proper time]] passes for them, while a large amount of proper time passes elsewhere. This can be achieved by traveling at [[relativistic speed]]s or through the effects of [[gravity]].<ref>Serway, Raymond A. (2000) ''Physics for Scientists and Engineers with Modern Physics'', Fifth Edition, Brooks/Cole, p. 1258, {{ISBN|0030226570}}.</ref>

For two identical clocks moving relative to each other without accelerating, each clock measures the other to be ticking slower. This is possible due to the [[relativity of simultaneity]]. However, the symmetry is broken if one clock accelerates, allowing for less proper time to pass for one clock than the other. The [[twin paradox]] describes this: one twin remains on Earth, while the other undergoes acceleration to [[relativistic speed]] as they travel into space, turn around, and travel back to Earth; the traveling twin ages less than the twin who stayed on Earth, because of the time dilation experienced during their acceleration. General relativity treats the effects of acceleration and the effects of gravity as [[Equivalence principle|equivalent]], and shows that time dilation also occurs in [[gravity well]]s, with a clock deeper in the well ticking more slowly; this effect is taken into account when calibrating the clocks on the satellites of the [[Global Positioning System]], and it could lead to significant differences in rates of aging for observers at different distances from a large gravity well such as a [[black hole]].<ref name="Gott"/>{{rp|33–130}}

A time machine that utilizes this principle might be, for instance, a spherical shell with a diameter of five meters and the [[Jupiter mass|mass of Jupiter]]. A person at its center will travel forward in time at a rate four times slower than that of distant observers. Squeezing the mass of a large planet into such a small structure is not expected to be within humanity's technological capabilities in the near future.<ref name="Gott" />{{rp|76–140}} With current technologies, it is only possible to cause a human traveler to age less than companions on Earth by a few milliseconds after a few hundred days of space travel.<ref>{{cite magazine|last=Mowbray|first=Scott|title=Let's Do the Time Warp Again|url=http://www.popsci.com/scitech/article/2002-02/lets-do-time-warp-again?page=2|magazine=Popular Science|access-date=8 July 2011|date=19 February 2002|quote=Spending just over two years in Mir's Earth orbit, going 17,500 miles per hour, put Sergei Avdeyev 1/50th of a second into the future{{nbsp}}... 'he's the greatest time traveler we have so far.'|archive-date=28 June 2010|archive-url=https://web.archive.org/web/20100628190931/http://www.popsci.com/scitech/article/2002-02/lets-do-time-warp-again?page=2|url-status=live}}</ref>