Editing Speed of light (section)

== Practical effects of finiteness ==
The speed of light is of relevance to [[telecommunications]]: the one-way and  [[round-trip delay time]] are greater than zero. This applies from small to astronomical scales. On the other hand, some techniques depend on the finite speed of light, for example in distance measurements.

=== Small scales ===
In [[computer]]s, the speed of light imposes a limit on how quickly data can be sent between [[central processing unit|processors]]. If a processor operates at 1{{nbsp}}[[gigahertz]], a signal can travel only a maximum of about {{convert|30|cm|ft|0}} in a single clock cycle – in practice, this distance is even shorter since the [[printed circuit board]] refracts and slows down signals. Processors must therefore be placed close to each other, as well as [[Computer memory|memory]] chips, to minimize communication latencies, and care must be exercised when routing wires between them to ensure [[signal integrity]]. If clock frequencies continue to increase, the speed of light may eventually become a limiting factor for the internal design of single [[integrated circuit|chips]].<ref name="processorlimit">
{{Cite book
 |last=Parhami |first=B.
 |year=1999
 |title=Introduction to parallel processing: algorithms and architectures
 |url=https://books.google.com/books?id=ekBsZkIYfUgC
 |page=5
 |publisher=[[Plenum Press]]
 |isbn=978-0-306-45970-2
}}</ref><ref name="processorlimit2">
{{Cite conference
 |url=https://books.google.com/books?id=sona_r6dPyQC&q=%22speed+of+light%22+processor+limit&pg=PA26
 |title=Software Transactional Memories: An Approach for Multicore Programming
 |first1=D. |last1=Imbs
 |first2=Michel |last2=Raynal
 |year=2009
 |conference=10th International Conference, PaCT 2009, Novosibirsk, Russia, 31 August – 4 September 2009
 |editor=Malyshkin, V.
 |publisher=Springer
 |isbn=978-3-642-03274-5
 |page=26
}}</ref>

=== Large distances on Earth ===
[[File:Light world trip.ogg|thumb|Acoustic representation of the speed of light: in the period between beeps, light travels the circumference of Earth at the equator.]]
Given that the equatorial circumference of the Earth is about {{val|40075|u=km}} and that ''c'' is about {{val|300000|u=km/s}}, the theoretical shortest time for a piece of information to travel half the globe along the surface is about 67 milliseconds. When light is traveling in [[optical fibre]] (a [[Transparency and translucency|transparent material]]) the actual transit time is longer, in part because the speed of light is slower by about 35% in optical fibre with an refractive index ''n'' around 1.52.<ref name=Midwinter>
{{Cite book
 | last = Midwinter |first=J. E.
 | year = 1991
 | title = Optical Fibers for Transmission
 | edition = 2
 | publisher = Krieger
 | isbn = 978-0-89464-595-2
}}</ref> Straight lines are rare in global communications and the travel time increases when signals pass through electronic switches or signal regenerators.<ref>
{{Cite web
 |date=June 2007
 |title=Theoretical vs real-world speed limit of Ping
 |url=http://royal.pingdom.com/2007/06/01/theoretical-vs-real-world-speed-limit-of-ping/
 |website=Pingdom
 |access-date=5 May 2010
 |archive-date=2 September 2010
 |archive-url=https://web.archive.org/web/20100902224536/http://royal.pingdom.com/2007/06/01/theoretical-vs-real-world-speed-limit-of-ping/
 |url-status=dead
}}</ref>

Although this distance is largely irrelevant for most applications, latency becomes important in fields such as [[high-frequency trading]], where traders seek to gain minute advantages by delivering their trades to exchanges fractions of a second ahead of other traders. For example, traders have been switching to [[microwave]] communications between trading hubs, because of the advantage which radio waves travelling at near to the speed of light through air have over comparatively slower [[fibre optic]] signals.<ref>{{Cite journal |last1=Buchanan |first1=Mark |date=11 February 2015 |title=Physics in finance: Trading at the speed of light |journal=Nature |volume=518 |issue=7538 |pages=161–163 |bibcode=2015Natur.518..161B |doi=10.1038/518161a |pmid=25673397 |doi-access=free}}</ref><ref>{{Cite news |date=10 May 2013 |title=Time is money when it comes to microwaves |newspaper=Financial Times |url=http://www.ft.com/cms/s/2/2bf37898-b775-11e2-841e-00144feabdc0.html |archive-url=https://ghostarchive.org/archive/20221210211258/https://www.ft.com/content/2bf37898-b775-11e2-841e-00144feabdc0 |archive-date=10 December 2022 |url-access=subscription |access-date=25 April 2014 |url-status=live }}</ref>

=== Spaceflight and astronomy ===
[[File:Earth and Moon speed of light by James O'Donoghue.gif|thumb|upright=1.8|alt=The diameter of the moon is about one quarter of that of Earth, and their distance is about thirty times the diameter of Earth. A beam of light starts from the Earth and reaches the Moon in about a second and a quarter.|A beam of light is depicted travelling between the Earth and the Moon in the time it takes a light pulse to move between them: 1.255 seconds at their mean orbital (surface-to-surface) distance. The relative sizes and separation of the Earth–Moon system are shown to scale.]]
Similarly, communications between the Earth and spacecraft are not instantaneous. There is a brief delay from the source to the receiver, which becomes more noticeable as distances increase. This delay was significant for communications between [[Mission Control Center|ground control]] and [[Apollo 8]] when it became the first crewed spacecraft to orbit the [[Moon]]: for every question, the ground control station had to wait at least three&nbsp;seconds for the answer to arrive.<ref>
{{Cite web
 |url         = https://history.nasa.gov/ap08fj/15day4_orbits789.htm
 |title       = Day 4: Lunar Orbits 7, 8 and 9
 |work        = The Apollo 8 Flight Journal
 |publisher   = NASA
 |access-date = 16 December 2010
 |url-status  = dead
 |archive-url  = https://web.archive.org/web/20110104032114/http://history.nasa.gov/ap08fj/15day4_orbits789.htm
 |archive-date = 4 January 2011
}}</ref>

The communications delay between Earth and [[Mars]] can vary between five and twenty minutes depending upon the relative positions of the two planets. As a consequence of this, if a robot on the surface of Mars were to encounter a problem, its human controllers would not be aware of it until approximately {{nowrap|4–24 minutes}} later. It would then take a further {{nowrap|4–24 minutes}} for commands to travel from Earth to Mars.<ref>{{Cite web |last=Ormston |first=Thomas |date=2012-05-08 |title=Time delay between Mars and Earth – Mars Express |url=https://blogs.esa.int/mex/2012/08/05/time-delay-between-mars-and-earth/ |access-date=2024-07-16 |website=MARS EXPRESS ESA’s mission to the Red Planet |language=en-US}}</ref><ref>{{Cite journal |last1=Parisi |first1=Megan |last2=Panontin |first2=Tina |last3=Wu |first3=Shu-Chieh |last4=Mctigue |first4=Kaitlin |last5=Vera |first5=Alonso |date=2023 |title=Effects of Communication Delay on Human Spaceflight Missions |url=https://openaccess.cms-conferences.org/publications/book/978-1-958651-74-2/article/978-1-958651-74-2_6 |journal=Human-Centered Aerospace Systems and Sustainability Applications |publisher=AHFE Open Acces |volume=98 |doi=10.54941/ahfe1003920 |isbn=978-1-958651-74-2|url-access=subscription }}</ref>

Receiving light and other signals from distant astronomical sources takes much longer. For example, it takes 13&nbsp;billion (13{{e|9}}) years for light to travel to Earth from the faraway galaxies viewed in the [[Hubble Ultra-Deep Field]] images.<ref name=Hubble>
{{Cite press release
 |date=5 January 2010
 |title=Hubble Reaches the "Undiscovered Country" of Primeval Galaxies
 |url=https://www.nasa.gov/mission_pages/hubble/science/undiscovered-country.html
 |publisher=[[Space Telescope Science Institute]]
}}</ref><ref>
{{Cite web
 |title=The Hubble Ultra Deep Field Lithograph
 |url=http://www.nasa.gov/pdf/283957main_Hubble_Deep_Field_Lithograph.pdf
 |publisher=NASA
 |access-date=4 February 2010
}}</ref> Those photographs, taken today, capture images of the galaxies as they appeared 13&nbsp;billion years ago, when the universe was less than a billion years old.<ref name=Hubble/> The fact that more distant objects appear to be younger, due to the finite speed of light, allows astronomers to infer the [[evolution of stars]], [[Galaxy formation and evolution|of galaxies]], and [[history of the universe|of the universe]] itself.<ref>{{Cite book|last=Mack|first=Katie|url=https://www.worldcat.org/oclc/1180972461|title=The End of Everything (Astrophysically Speaking)|date=2021|publisher=Penguin Books|isbn=978-0-141-98958-7|location=London|pages=18–19|oclc=1180972461|author-link=Katie Mack (astrophysicist)}}</ref>

Astronomical distances are sometimes expressed in [[light-year]]s, especially in [[popular science]] publications and media.<ref>{{Cite web
 |title=The IAU and astronomical units
 |url=http://www.iau.org/public/measuring/
 |publisher=[[International Astronomical Union]]
 |access-date=11 October 2010
 |archive-date=5 June 2013
 |archive-url=https://web.archive.org/web/20130605024231/http://www.iau.org/public/measuring/
 |url-status=dead
 }}</ref> A light-year is the distance light travels in one [[Julian year (astronomy)|Julian year]], around 9461&nbsp;billion kilometres, 5879&nbsp;billion miles, or 0.3066 [[parsec]]s. In round figures, a light year is nearly 10&nbsp;trillion kilometres or nearly 6&nbsp;trillion miles. [[Proxima Centauri]], the closest star to Earth after the Sun, is around 4.2 light-years away.<ref name=starchild>Further discussion can be found at 
{{Cite web
 |year=2000
 |title=StarChild Question of the Month for March 2000
 |url=http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question19.html
 |work=StarChild
 |publisher=NASA
 |access-date=22 August 2009
}}</ref>

=== Distance measurement ===
{{Main|Distance measurement}}
[[Radar]] systems measure the distance to a target by the time it takes a radio-wave pulse to return to the radar antenna after being reflected by the target: the distance to the target is half the round-trip [[Radar#Transit time|transit time]] multiplied by the speed of light. A [[Global Positioning System]] (GPS) receiver measures its distance to [[GPS satellites]] based on how long it takes for a radio signal to arrive from each satellite, and from these distances calculates the receiver's position. Because light travels about {{val|300000|u=kilometres}} ({{val|186000|u=miles}}) in one second, these measurements of small fractions of a second must be very precise. The [[Lunar Laser Ranging experiment]], [[radar astronomy]] and the [[Deep Space Network]] determine distances to the Moon,<ref name=science265_5171_482>
{{Cite journal
 |last=Dickey |first=J. O.
 |title=Lunar Laser Ranging: A Continuing Legacy of the Apollo Program
 |journal=Science | volume=265 | issue=5171
 |pages=482–490 |date=July 1994
 |doi=10.1126/science.265.5171.482
 |bibcode=1994Sci...265..482D | pmid=17781305|s2cid=10157934
 |display-authors=etal|url=https://trs.jpl.nasa.gov/bitstream/2014/32452/1/94-0193.pdf}}</ref> planets<ref name=cm26_181>{{Cite journal
 |last=Standish |first=E. M.
 |title=The JPL planetary ephemerides
 |journal=Celestial Mechanics |volume=26 |date=February 1982
 |issue=2 |pages=181–186 |doi=10.1007/BF01230883
 |bibcode=1982CeMec..26..181S |s2cid=121966516
}}</ref> and spacecraft,<ref name=pieee95_11_2202>
{{Cite journal
 |last1=Berner |first1=J. B.
 |last2=Bryant |first2=S. H.
 |last3=Kinman |first3=P. W.
 |title=Range Measurement as Practiced in the Deep Space Network
 |journal=Proceedings of the IEEE |date=November 2007 |volume=95 |issue=11 |pages=2202–2214
 |doi=10.1109/JPROC.2007.905128 |s2cid=12149700
 |url=https://trs.jpl.nasa.gov/bitstream/2014/40972/1/07-0166.pdf}}
</ref> respectively, by measuring round-trip transit times.