Editing Motion (section)

== Light ==
{{Main|Speed of light}}
Light moves at a speed of 299,792,458&nbsp;m/s, or {{convert|299792.458|km/s|mi/s}}, in a vacuum. The speed of light in vacuum (or <math>c</math>) is also the speed of all [[massless particle]]s and associated [[field (physics)|fields]] in a vacuum, and it is the upper limit on the speed at which energy, matter, information or [[Causality (physics)|causation]] can travel. The speed of light in vacuum is thus the upper limit for speed for all physical systems.

In addition, the speed of light is an [[invariant (physics)|invariant]] quantity: it has the same value, irrespective of the position or speed of the observer. This property makes the speed of light ''c'' a natural measurement unit for speed and a [[physical constant|fundamental constant]] of nature.

In 2019, the speed of light was redefined alongside all seven SI base units using what it calls "the explicit-constant formulation", where each "unit is defined indirectly by specifying explicitly an exact value for a well-recognized fundamental constant", as was done for the speed of light. A new, but completely equivalent, wording of the metre's definition was proposed: "The metre, symbol m, is the unit of length; its magnitude is set by fixing the numerical value of the speed of light in vacuum to be equal to exactly {{Val|299792458}} when it is expressed in the SI unit {{Nowrap|m s<sup>−1</sup>}}."<ref>{{Cite web |date=2011 |title=The "explicit-constant" formulation |url=http://www.bipm.org/en/si/new_si/explicit_constant.html |archive-url=https://web.archive.org/web/20140811195806/http://www.bipm.org/en/si/new_si/explicit_constant.html |archive-date=11 August 2014 |website=BIPM}}</ref> This implicit change to the speed of light was one of the changes that was incorporated in the [[2019 revision of the SI]], also termed the ''New SI''.<ref>See, for example:
* {{cite web |last=Conover |first=Emily |author-link=Emily Conover |date=2 November 2016 |title=Units of measure are getting a fundamental upgrade |url=https://www.sciencenews.org/article/units-measure-are-getting-fundamental-upgrade |access-date=6 February 2022 |website=[[Science News]] |language=en-US |archive-date=6 February 2022 |archive-url=https://web.archive.org/web/20220206103439/https://www.sciencenews.org/article/units-measure-are-getting-fundamental-upgrade |url-status=live }}
* {{cite journal |last1=Knotts |first1=Sandra |last2=Mohr |first2=Peter J. |last3=Phillips |first3=William D. |date=January 2017 |title=An Introduction to the New SI |url=http://scitation.aip.org/content/aapt/journal/tpt/55/1/10.1119/1.4972491 |journal=[[The Physics Teacher]] |language=en |volume=55 |issue=1 |pages=16–21 |bibcode=2017PhTea..55...16K |doi=10.1119/1.4972491 |issn=0031-921X |s2cid=117581000 |access-date=2022-08-20 |archive-date=2023-09-25 |archive-url=https://web.archive.org/web/20230925015417/https://pubs.aip.org/aapt/pte/article-abstract/55/1/16/618986/An-Introduction-to-the-New-SI?redirectedFrom=fulltext |url-status=live }}
* {{cite journal |date=11 May 2018 |title=SI Redefinition |url=https://www.nist.gov/si-redefinition |journal=[[National Institute of Standards and Technology]] |language=en |access-date=6 February 2022 |archive-date=6 February 2022 |archive-url=https://web.archive.org/web/20220206103441/https://www.nist.gov/si-redefinition |url-status=live }}</ref>

=== Superluminal motion ===
{{See also|Superluminal motion}}
Some motion appears to an observer to exceed the speed of light. Bursts of energy moving out along the [[relativistic jet]]s emitted from these objects can have a [[proper motion]] that appears greater than the speed of light. All of these sources are thought to contain a [[black hole]], responsible for the ejection of mass at high velocities. [[Light echo]]es can also produce apparent superluminal motion.<ref>{{cite journal |last=Bond |first=H. E. |display-authors=etal |year=2003 |title=An energetic stellar outburst accompanied by circumstellar light echoes |journal=[[Nature (journal)|Nature]] |volume=422 |issue=6930 |pages=405–408 |arxiv=astro-ph/0303513 |bibcode=2003Natur.422..405B |doi=10.1038/nature01508 |pmid=12660776 |s2cid=90973}}</ref> This occurs owing to how motion is often calculated at long distances; oftentimes calculations fail to account for the fact that the speed of light is finite. When measuring the movement of distant objects across the sky, there is a large time delay between what has been observed and what has occurred, due to the large distance the light from the distant object has to travel to reach us. The error in the above naive calculation comes from the fact that when an object has a component of velocity directed towards the Earth, as the object moves closer to the Earth that time delay becomes smaller. This means that the apparent speed as calculated above is ''greater'' than the actual speed. Correspondingly, if the object is moving away from the Earth, the above calculation underestimates the actual speed.<ref>{{cite journal |last1=Meyer |first1=Eileen |date=June 2018 |title=Detection of an Optical/UV Jet/Counterjet and Multiple Spectral Components in M84 |journal=The Astrophysical Journal |volume=680 |issue=1 |page=9 |arxiv=1804.05122 |bibcode=2018ApJ...860....9M |doi=10.3847/1538-4357/aabf39 |s2cid=67822924 |doi-access=free }}</ref>