View source for Speed of light

The '''speed of light''', in any medium, which is usually denoted by <math>c</math>, is a [[physical constant]] important in many areas of physics. It is denoted by "c<sub>0</sub>" especially in vacuum medium, although the symbol 'c' can be used to refer to that in any medium. It is exactly {{convert|299792458|m/s|abbr=off|0}} by definition.<ref>{{citation|title=SI Brochure: The International System of Units (SI)|edition=9|year=2019|publisher=BIPM|url=https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9-EN.pdf|page=128|accessdate=2020-01-12}}</ref><ref>{{cite book |first1=Brian |last1=Cox |first2=Jeff |last2= Forshaw |publisher=Da Capo |date=2010 |isbn= 978-0-306-81911-7 |title=Why does E=mc<sup>2</sup>?: (and why should we care?) |page=2}}</ref> A [[photon]] (particle of light) travels at this speed in a [[vacuum]]. 

According to [[special relativity]], <math>c</math> is the maximum speed at which all energy, matter, and physical information in the universe can travel. It is the speed of all [[mass]]less particles such as photons, and associated [[field (physics)|field]]s—including [[electromagnetic radiation]] such as [[light]]—in a vacuum. 

It is predicted by the current theory to be the speed of [[gravity]] (that is, gravitational waves). Such particles and waves travel at <math>c</math> regardless of the motion of the source or the inertial [[frame of reference]] of the observer. In the [[theory of relativity]], <math>c</math> interrelates [[spacetime|space and time]], and appears in the famous equation of [[mass–energy equivalence]] {{nobr |1=''E'' = ''mc''<sup>2</sup>}}.<ref name=LeClerq>{{Cite book |last1=Uzan |first1=J-P |last2=Leclercq |first2=B |year=2008 |title= The natural laws of the universe: understanding fundamental constants |url= https://books.google.com/books?id=dSAWX8TNpScC&pg=PA43 |pages=43–4 |publisher= Springer |isbn=978-0387734545 }}</ref>

The special [[theory of relativity]] is based on the prediction, so far upheld by observations, that the measured speed of light in a vacuum is the same whether or not the source of the light and the person doing the measuring are moving relative to each other. This is sometimes expressed as "the speed of light is independent of the reference frame."

== Example explaining how speed does not depend on reference frame ==
This behavior is different from our common ideas about motion as shown by this example:

George is standing on the ground next to some train tracks (railroad). There is a train rushing by at {{cvt|30|mph}}. George throws a baseball at {{cvt|90|mph}} in the direction the train is moving. Tom, a passenger on the train, has a device (like a radar gun) to measure throwing speeds. Because he is on the train, Tom is already moving at {{cvt|30|mph}} in the direction of the throw, so Tom measures the speed of the ball as only {{cvt|60|mph}}.

In other words, the speed of the baseball, as measured by Tom on the train, depends on the speed of the train.

In the example above, the train was moving at 1/3 the speed of the ball, and the speed of the ball as measured on the train was 2/3 of the throwing speed as measured on the ground.

Now, repeat the experiment with light instead of a baseball; that is, George has a flashlight instead of throwing a baseball. George and Tom both have devices that are the same to measure the speed of light (instead of the radar gun in the baseball example).

George is standing on the ground next to some train tracks. There is a train rushing by at 1/3 the speed of light. George flashes a light beam in the direction the train is moving. George measures the speed of light as {{convert|186,282|mi/s|abbr=off}}. Tom, a passenger on the train, measures the speed of the light beam. What speed does Tom measure?

Intuitively, one may think that the speed of the light from the flashlight as measured on the train should be 2/3 the speed measured on the ground, just like the speed of the baseball was 2/3. But in fact, the speed measured on the train is the full value, {{convert|186,282|mi/s|abbr=off}}, not {{convert|124,188|mi/s|abbr=off}}.

It sounds impossible, but that is what one measures. 

A consequence of this fact, that the speed of light is the same for all (inertial) observers, is that no matter how much energy is used, nothing with mass can be accelerated to reach or go faster than the speed of light. Other key consequences of the constancy of the speed of light, that the lengths of two identical objects, and the rate at which two identical clocks tick, also depend on the reference frame of the observer. These ideas were discovered in the early 1900s by [[Albert Einstein]] in his theory of [[Special Relativity]] which completely changed our understanding of space and time.

== Relation to fundamental electric and magnetic properties of space ==


[[Maxwell's equations]] predicted the speed of light and confirmed [[Michael Faraday]]'s idea that light was an [[electromagnetic wave]] (a way that energy moves). From these equations, we find that the speed of light is related to the inverse of the [[square root]] of the permittivity of free space, ''ε''<sub>0</sub>, and the permeability of free space, ''μ''<sub>0</sub>:
:<math> c =\frac {1}{\sqrt{\varepsilon_0 \mu_0}} \ . </math>

The [[index of refraction]] of a [[transparent|clear]] material is the ratio between the speed of light in a vacuum and the speed of light in that material.

== Measurement ==
=== Rømer ===
Ole Christensen Rømer used an astronomical measurement to make the first quantitative estimate of the speed of light.<ref name=cohen>{{cite journal
 |last=Cohen |first=IB
 |year=1940
 |title=Roemya and the first determination of the velocity of light (1676)
 |journal=Isis
 |volume=31 |issue=2 |pages=327–79
 |doi=10.1086/347594
 |hdl=2027/uc1.b4375710
 |s2cid=145428377
 |ref=cohen-1940
}}</ref><ref name=roemer>
{{cite journal
 |year=1676
 |title=Touchant le mouvement de la lumiere trouvé par M. Rŏmer de l'Académie Royale des Sciences
 |language=French
 |url=http://www-obs.univ-lyon1.fr/labo/fc/ama09/pages_jdsc/pages/jdsc_1676_lumiere.pdf
 |journal=Journal des sçavans
 |volume= |issue= |pages=233–36
 |ref=roemer-1676
}}<br/>Translated in {{cite journal
 |doi=10.1098/rstl.1677.0024
 |year=1677
 |title=On the motion of light by M. Romer
 |url=https://archive.org/stream/philosophicaltra02royarich#page/397/mode/1up
 |journal=Philosophical Transactions of the Royal Society
 |volume=12 |issue=136 |pages=893–95
 |s2cid=186210345
 |ref=roemer-1676-EnglishTrans
}} (As reproduced in {{Cite book
 |last1=Hutton |first1=C
 |last2=Shaw |first2=G
 |editor=Pearson, R 
 |year=1809
 |title=The Philosophical Transactions of the Royal Society of London, from Their Commencement in 1665, in the Year 1800: Abridged
 |chapter=On the Motion of Light by M. Romer
 |location=London |publisher=C. & R. Baldwin
 |volume= 2| pages=397–98
}})<br>
The account published in ''Journal des sçavans'' was based on a report that Rømer read to the French Academy of Sciences in November 1676 [[#cohen-1940|(Cohen, 1940, p.&nbsp;346)]].</ref> When measured from Earth, the periods of moons orbiting a distant planet are shorter when the Earth is approaching the planet than when the Earth is receding from it. The distance travelled by light from the planet (or its moon) to Earth is shorter when the Earth is at the point in its orbit that is closest to its planet than when the Earth is at the farthest point in its orbit, the difference in distance being the [[diameter]] of the [[Earth's orbit]] around the Sun. The observed change in the moon's [[orbital period]] is actually the difference in the time it takes light to traverse the shorter or longer distance. Rømer observed this effect for [[Jupiter (planet)|Jupiter]]'s innermost moon [[Io (moon)|Io]], and he deduced that light takes 22 minutes to cross the diameter of the Earth's orbit.

=== Bradley ===
[[File:SoL Aberration.svg |thumb |right |Aberration of light: light from a distant source appears to be from a different location for a moving telescope due to the finite speed of light.|alt=A star emits a light ray which hits the objective of a telescope. While the light travels down the telescope to its eyepiece, the telescope moves to the right. For the light to stay inside the telescope, the telescope must be tilted to the right, causing the distant source to appear at a different location to the right.]]
Another method is to use the aberration of light, discovered and explained by [[James Bradley]] in the 18th century.<ref name="Bradley1729">{{Cite journal
 |last=Bradley |first=J
 |year=1729
 |title=Account of a new discoved Motion of the Fix'd Stars
 |url=http://gallica.bnf.fr/ark:/12148/bpt6k55840n.image.f375.langEN
 |journal=Philosophical Transactions
 |volume=35 |pages=637–660
 |doi=
}}</ref> This effect results from the vector addition of the velocity of light arriving from a distant source (such as a star) and the velocity of its observer (see diagram on the right). A moving observer thus sees the light coming from a slightly different direction and consequently sees the source at a position shifted from its original position. Since the direction of the Earth's velocity changes continuously as the Earth orbits the Sun, this effect causes the apparent position of stars to move around. From the angular difference in the position of stars,<ref>at most 20.5 [[arcsecond]]s 
{{Cite book
 |last=Duffett-Smith |first=P
 |year=1988
 |title=Practical astronomy with your calculator
 |url=https://books.google.com/books?id=DwJfCtzaVvYC&q=aberration
 |page=62
 |publisher=Cambridge University Press
 |isbn=0521356997
}}</ref> it is possible to express the speed of light in terms of the Earth's velocity around the Sun. This, with the known length of a year, can be easily converted to the time needed to travel from the Sun to the Earth. In 1729, Bradley used this method to derive that light travelled 10,210 times faster than the Earth in its orbit (the modern figure is 10,066 times faster) or, equivalently, that it would take light 8&nbsp;minutes 12&nbsp;seconds to travel from the Sun to the Earth.<ref name="Bradley1729"/>

=== Modern ===
Nowadays, the "light time for unit distance"—the inverse of&nbsp;''c'' (1/c), expressed in seconds per astronomical unit—is measured by comparing the time for radio signals to reach different spacecraft in the [[Solar System]]. The position of spacecraft is calculated from the gravitational effects of the Sun and various planets. By combining many such measurements, a [[best fit]] value for the light time per unit distance is obtained. {{As of|2009}}, the best estimate, as approved by the [[International Astronomical Union]] (IAU), is:<ref name="Pitjeva09">
{{cite journal
 |last1=Pitjeva |first1=EV
 |last2=Standish |first2=EM
 |year=2009
 |title=Proposals for the masses of the three largest asteroids, the Moon-Earth mass ratio and the Astronomical Unit
 |journal=Celestial Mechanics and Dynamical Astronomy
 |volume=103 |issue=4 |pages=365–372
 |doi=10.1007/s10569-009-9203-8
|bibcode = 2009CeMDA.103..365P |s2cid=121374703
 |url=https://zenodo.org/record/1000691
 }}</ref><ref name="IAU">{{cite web
 |author=IAU Working Group on Numerical Standards for Fundamental Astronomy
 |title=IAU WG on NSFA Current Best Estimates
 |url=http://maia.usno.navy.mil/NSFA/CBE.html
 |publisher=US Naval Observatory
 |accessdate=2009-09-25
 |archive-date=2009-12-08
 |archive-url=https://web.archive.org/web/20091208011235/http://maia.usno.navy.mil/NSFA/CBE.html
 |url-status=dead
 }}</ref>
:light time for unit distance: {{val|499.004783836|(10)|u=s}}
:''c'' = {{val|0.00200398880410|(4)|u=AU/s}}
:''c'' = {{val|173.144632674|(3)|u=AU/day.}}
The relative uncertainty in these measurements is 0.02 parts per billion (2{{e|-11}}), as equivalent to the uncertainty in Earth-based measurements of length by [[interferometry]].<ref>{{cite web
 |title=NPL's Beginner's Guide to Length
 |url=http://www.npl.co.uk/educate-explore/posters/length/length-%28poster%29
 |publisher=UK National Physical Laboratory
 |accessdate=2009-10-28
 |archive-date=2010-08-31
 |archive-url=https://web.archive.org/web/20100831214704/http://www.npl.co.uk/educate-explore/posters/length/length-%28poster%29
 |url-status=dead
 }}</ref> Since the [[metre]] is defined to be the length travelled by light in a certain time interval, the measurement of the light time for unit distance can also be interpreted as measuring the length of an AU in metres. The metre is considered to be a unit of [[proper length]], whereas the AU is often used as a unit of observed length in a given frame of reference.

== Practical effects ==
The finite speed of light is a major constraint on long-distance space travel. Supposing a journey to the other side of the [[Milky Way]], the total time for a message and its reply would be about 200,000 years. Even more seriously, no spacecraft could travel faster than light, so all galactic-scale transport would be effectively one-way, and would take much longer than than any modern [[civilisation]] has existed.

The speed of light can also be of concern over very short distances. In [[supercomputer]]s, the speed of light imposes a limit on how quickly data can be sent between [[central processing unit|processor]]s.<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=9780306459702
}} and {{cite conference
 |url=https://books.google.com/books?id=sona_r6dPyQC&dq=%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, August 31-September 4, 2009
|editor=Malyshkin, V
|book-title=Parallel Computing Technologies
|publisher=Springer
|isbn=9783642032745
|page=26
}}</ref> If a processor operates at 1 [[gigahertz]], a signal can only travel a maximum of about {{convert|30|cm|ft|0}} in a single cycle. Processors must therefore be placed close to each other to minimize communication latencies; this can cause difficulty with cooling. If clock frequencies continue to increase, the speed of light will eventually become a limiting factor for the internal design of single [[integrated circuit|chips]].<ref name="processorlimit"/>

== Related pages ==
*[[Michelson–Morley experiment]]

== References ==
{{Reflist}}

[[Category:Light]]
[[Category:Astrophysics]]
[[Category:Relativity]]