Editing Time (section)

== Definition ==

The concept of time can be complex. Multiple notions exist, and defining time in a manner applicable to all fields without [[circular definition|circularity]] has consistently eluded scholars.<ref name=Poidevin/><ref name="Carroll2009">{{cite journal|last=Carroll |first=Sean M. |title=From Eternity to Here: The Quest for the Ultimate Theory of Time |title-link=From Eternity to Here: The Quest for the Ultimate Theory of Time |date=2009 |journal=Physics Today |publisher=Dutton |isbn=978-0-525-95133-9 |volume=63 |pages=54–55 |bibcode=2010PhT....63d..54C |doi=10.1063/1.3397046 |issue=4}}</ref><ref>{{Cite web |title=The Feynman Lectures on Physics Vol. I Ch. 5: Time and Distance |url=https://www.feynmanlectures.caltech.edu/I_05.html |access-date=2023-12-15 |website=www.feynmanlectures.caltech.edu}}</ref> Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective [[measurement|measuring systems]].<ref name="MLB">{{cite web |last= |first= |date=2011 |title=Official Baseball Rules – 8.03 and 8.04 |url=http://mlb.mlb.com/mlb/downloads/y2011/Official_Baseball_Rules.pdf |url-status=live |archive-url=https://web.archive.org/web/20170701102437/http://mlb.mlb.com/mlb/downloads/y2011/Official_Baseball_Rules.pdf |archive-date=1 July 2017 |access-date=18 May 2017 |publisher=Major League Baseball |format=Free PDF download |quote=Rule 8.03 Such preparatory pitches shall not consume more than one minute of time...Rule 8.04 When the bases are unoccupied, the pitcher shall deliver the ball to the batter within 12 seconds...The 12-second timing starts when the pitcher is in possession of the ball and the batter is in the box, alert to the pitcher. The timing stops when the pitcher releases the ball.}}</ref><ref name=Guinness>{{cite web |title=Guinness Book of Baseball World Records |publisher=Guinness World Records, Ltd. |url=http://www.baseball-almanac.com/recbooks/rb_guin.shtml |quote=The record for the fastest time for circling the bases is 13.3 seconds, set by Evar Swanson at Columbus, Ohio in 1932...The greatest reliably recorded speed at which a baseball has been pitched is 100.9 mph by Lynn Nolan Ryan (California Angels) at Anaheim Stadium in California on 20 August 1974. |access-date=7 July 2012 |url-status=live |archive-url=https://web.archive.org/web/20120606114603/http://www.baseball-almanac.com/recbooks/rb_guin.shtml |archive-date=6 June 2012  }}</ref><ref name=Zeigler>{{cite book |last=Zeigler |first=Kenneth |title=Getting organized at work : 24 lessons to set goals, establish priorities, and manage your time |publisher=McGraw-Hill |date=2008 |url=https://books.google.com/books?id=acPPD6lCCxcC |isbn=978-0-07-159138-6 |access-date=30 July 2019 |archive-date=18 August 2020 |archive-url=https://web.archive.org/web/20200818215121/https://books.google.com/books?id=acPPD6lCCxcC |url-status=live }} 108 pages.</ref> Traditional definitions of time involved the observation of periodic motion such as the apparent motion of the sun across the sky, the phases of the moon, and the passage of a free-swinging pendulum. More modern systems include the [[Global Positioning System]], other satellite systems, [[Coordinated Universal Time]] and [[mean solar time]]. Although these systems differ from one another, with careful measurements they can be synchronized.

In physics, time is a fundamental concept to define other quantities, such as [[velocity]]. To avoid a circular definition,<ref name="TrialogueP3">Duff, Okun, Veneziano, ''ibid''. p. 3. "There is no well established terminology for the fundamental constants of Nature.&nbsp;... The absence of accurately defined terms or the uses (i.e., actually misuses) of ill-defined terms lead to confusion and proliferation of wrong statements."
</ref> [[time in physics]] is [[operational definition|operationally defined]] as "what a [[clock]] reads", specifically a count of repeating events such as the [[SI second]].<ref name=DefRefs01/><ref name=Burnham/><ref>{{cite book |title=Process instruments and controls handbook |edition=3 |first1=Douglas M. |last1=Considine |first2=Glenn D. |last2=Considine |publisher=McGraw-Hill |year=1985 |isbn=978-0-07-012436-3 |pages=18–61 |url=https://books.google.com/books?id=kt1UAAAAMAAJ |bibcode=1985pich.book.....C |access-date=1 November 2016 |archive-date=31 December 2013 |archive-url=https://web.archive.org/web/20131231121535/http://books.google.com/books?id=kt1UAAAAMAAJ |url-status=live }}</ref> Although this aids in practical measurements, it does not address the essence of time. Physicists developed the concept of the [[spacetime]] continuum, where events are assigned four coordinates: three for space and one for time. Events like [[particle collision]]s, [[supernova]]s, or [[rocket launch]]es have coordinates that may vary for different observers, making concepts like "now" and "here" relative. In [[general relativity]], these coordinates do not directly correspond to the causal structure of events. Instead, the [[spacetime interval]] is calculated and classified as either space-like or time-like, depending on whether an observer exists that would say the events are separated by space or by time.<ref>{{cite book |author1=Lilley |first=Sam |url=https://books.google.com/books?id=ZS45AAAAIAAJ |title=Discovering Relativity for Yourself |publisher=Cambridge University Press Archive |year=1981 |isbn=978-0-521-23038-4 |edition=illustrated |page=125 }} [https://books.google.com/books?id=ZS45AAAAIAAJ&pg=PA125 Extract of page 125] {{Webarchive|url=https://web.archive.org/web/20240804095000/https://books.google.com/books?id=ZS45AAAAIAAJ&pg=PA125 |date=4 August 2024 }}.</ref> Since the time required for light to travel a specific distance is the same for all observers—a fact first publicly demonstrated by the [[Michelson–Morley experiment]]—all observers will consistently agree on this definition of time as a [[causal relation]].<ref>{{cite journal |last1=Surya |first1=Sumati |title=The causal set approach to quantum gravity |journal=Living Reviews in Relativity |date=December 2019 |volume=22 |issue=1 |page=5 |doi=10.1007/s41114-019-0023-1 |quote=Thus, the causal structure poset (M, ≺) of a future and past distinguishing spacetime is equivalent to its conformal geometry.|doi-access=free |arxiv=1903.11544 |bibcode=2019LRR....22....5S }}</ref>

General relativity does not address the nature of time for extremely small intervals where quantum mechanics holds. In quantum mechanics, time is treated as a universal and absolute parameter, differing from general relativity's notion of independent clocks. The [[problem of time]] consists of reconciling these two theories.<ref>{{cite journal |last1=Macías |first1=Alfredo |last2=Camacho |first2=Abel |title=On the incompatibility between quantum theory and general relativity |journal=Physics Letters B |date=May 2008 |volume=663 |issue=1–2 |pages=99–102 |doi=10.1016/j.physletb.2008.03.052 |quote=In our opinion, it is not possible to reconciliate and integrate into a common scheme the absolute and non-dynamical character of Newtonian time of canonical quantization and path integral approaches with the relativistic and dynamical character of time in general relativity.|doi-access=free |bibcode=2008PhLB..663...99M }}</ref> As of 2025, there is no generally accepted theory of quantum general relativity.<ref>{{cite news |last1=Shavit |first1=Joseph |title=Revolutionary theory finally unites quantum mechanics and Einstein's theory of general relativity |url=https://www.thebrighterside.news/post/revolutionary-theory-finally-unites-quantum-mechanics-and-einsteins-theory-of-general-relativity/ |work=The Brighter Side of News |date=18 July 2024 |language=en |quote=The prevailing consensus has been that Einstein's theory of gravity must be modified to fit within the framework of quantum theory [...] when it comes to merging these two theories into a single, comprehensive framework, the scientific community has hit a roadblock. |archive-date=4 August 2024 |access-date=4 August 2024 |archive-url=https://web.archive.org/web/20240804154452/https://www.thebrighterside.news/post/revolutionary-theory-finally-unites-quantum-mechanics-and-einsteins-theory-of-general-relativity/ |url-status=live }}</ref>