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{{For|history and motivation|History of special relativity}}
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{{See introduction}}
{{Special relativity}}


In [[physics]], '''special relativity''' ('''SR''', also known as the '''special theory of relativity''' or '''STR''') is the accepted [[scientific theory|physical theory]] regarding the relationship between space and time. It is based on two postulates: (1) that the laws of physics are [[Invariant (physics)|invariant]] (i.e., identical) in all [[Inertial frame of reference|inertial systems]] (non-accelerating frames of reference); and (2) that the speed of light in a vacuum is the same for all observers, regardless of the motion of the light source. It was originally proposed in 1905 by [[Albert Einstein]] in the paper "[[Annus Mirabilis Papers#Special relativity|On the Electrodynamics of Moving Bodies]]".<ref name=electro>[[Albert Einstein]] (1905) "[http://web.archive.org/web/20050220050316/http://www.pro-physik.de/Phy/pdfs/ger_890_921.pdf ''Zur Elektrodynamik bewegter Körper'']", ''Annalen der Physik'' 17: 891; English translation [http://www.fourmilab.ch/etexts/einstein/specrel/www/ On the Electrodynamics of Moving Bodies] by [[George Barker Jeffery]] and Wilfrid Perrett (1923); Another English translation [[s:On the Electrodynamics of Moving Bodies|On the Electrodynamics of Moving Bodies]] by [[Megh Nad Saha]] (1920).</ref> The inconsistency of classical mechanics with [[Maxwell’s equations]] of [[electromagnetism]] led to the development of special relativity, which corrects classical mechanics to handle situations involving motions nearing the speed of light. As of today, special relativity is the most accurate model of motion at any speed. Even so, classical mechanics is still useful (due to its simplicity and high accuracy) as an approximation at small velocities relative to the speed of light.
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Special relativity implies a wide range of consequences, which have been experimentally verified,<ref>{{cite web | url = http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html | title = What is the experimental basis of Special Relativity? | accessdate = 2008-09-17 | author = Tom Roberts and Siegmar Schleif |date=October 2007 | work = Usenet Physics FAQ}}</ref> including [[length contraction]], [[time dilation]], [[Mass in special relativity|relativistic mass]], [[mass–energy equivalence]], [[Speed of light#Upper limit on speeds|a universal speed limit]], and [[relativity of simultaneity]]. It has replaced the conventional notion of an absolute universal time with the notion of a time that is dependent on reference frame and spatial position. Rather than an invariant time interval between two events, there is an invariant [[spacetime interval]]. Combined with other laws of physics, the two postulates of special relativity predict the equivalence of [[mass]] and [[energy]], as expressed in the [[mass–energy equivalence]] formula ''E''&nbsp;=&nbsp;''mc''<sup>2</sup>, where ''c'' is the [[speed of light]] in vacuum.<ref name=relativity>{{cite book |author=Albert Einstein |title=Relativity: The Special and the General Theory |page= 48 |url=http://books.google.com/?id=idb7wJiB6SsC&pg=PA50 |isbn=0-415-25384-5 |publisher=Routledge |year=2001 |edition=Reprint of 1920 translation by Robert W. Lawson }}</ref><ref name=Feynman>{{cite book |title=Six Not-so-easy Pieces: Einstein's relativity, symmetry, and space–time |author=Richard Phillips Feynman |page= 68 |url=http://books.google.com/?id=ipY8onVQWhcC&pg=PA68 |isbn=0-201-32842-9 |publisher=Basic Books |edition=Reprint of 1995 |year=1998}}</ref>
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A defining feature of special relativity is the replacement of the [[Galilean transformation]]s of classical mechanics with the [[Lorentz transformation]]s. Time and space cannot be defined separately from one another. Rather space and time are interwoven into a single continuum known as [[spacetime]]. Events that occur at the same time for one observer could occur at different times for another.
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The theory is called "special" because it applied the [[principle of relativity]] only to the [[special case]] of [[inertial frames of reference|inertial reference frames]]. Einstein later published a paper on [[general relativity]] in 1915 to apply the principle in the general case, that is, to any frame so as to handle [[General covariance|general coordinate transformations]], and [[gravity|gravitational effects]].  
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As [[Galilean invariance|Galilean relativity]] is now considered an approximation of special relativity valid for low speeds, special relativity is considered an approximation of the theory of [[general relativity]] valid for weak gravitational fields. The presence of gravity becomes undetectable at sufficiently small-scale, free-falling conditions. General relativity incorporates [[noneuclidean geometry]], so that the gravitational effects are represented by the geometric curvature of spacetime. Contrarily, special relativity is restricted to flat spacetime. The geometry of spacetime in special relativity is called [[Minkowski space]]. A locally Lorentz invariant frame that abides by Special relativity can be defined at sufficiently small scales, even in curved spacetime.
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[[Galileo Galilei]] had already postulated that there is no absolute and well-defined state of rest (no [[Preferred frame|privileged reference frames]]), a principle now called [[Galilean invariance|Galileo's principle of relativity]]. Einstein extended this principle so that it accounted for the constant speed of light,<ref>{{cite book|title=Spacetime Physics: Introduction to Special Relativity|year=1992|publisher=W. H. Freeman|isbn=0-7167-2327-1|author=Edwin F. Taylor and John Archibald Wheeler}}</ref> a phenomenon that had been recently observed in the [[Michelson–Morley experiment]]. He also postulated that it holds for all the [[laws of physics]], including both the laws of mechanics and of [[electrodynamics]].<ref name=Rindler0>{{cite book |title=Essential Relativity |author= Wolfgang Rindler |page= §1,11 p. 7 |url=http://books.google.com/?id=0J_dwCmQThgC&pg=PT148 |isbn=3-540-07970-X |publisher=Birkhäuser |year=1977 }}</ref>
  <li>[http://www.histoirepassion.eu/spip.php?article1895/ http://www.histoirepassion.eu/spip.php?article1895/]</li>
 
 
[[File:Einstein patentoffice.jpg|thumb|250px|[[Albert Einstein]] around 1905]]
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== Postulates ==
 
{{rquote|right|Reflections of this type made it clear to me as long ago as shortly after 1900, i.e., shortly after Planck's trailblazing work, that neither mechanics nor electrodynamics could (except in limiting cases) claim exact validity. Gradually I despaired of the possibility of discovering the true laws by means of constructive efforts based on known facts. The longer and the more desperately I tried, the more I came to the conviction that only the discovery of a universal formal principle could lead us to assured results... How, then, could such a universal principle be found?|Albert Einstein: ''Autobiographical Notes''<ref name="autogenerated1">Einstein, Autobiographical Notes, 1949.</ref>}}
</ul>
 
Einstein discerned two fundamental propositions that seemed to be the most assured, regardless of the exact validity of the (then) known laws of either mechanics or electrodynamics. These propositions were the constancy of the speed of light and the independence of physical laws (especially the constancy of the speed of light) from the choice of inertial system. In his initial presentation of special relativity in 1905 he expressed these postulates as:<ref name=electro />
* The Principle of Relativity – The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems in uniform translatory motion relative to each other.<ref name=electro />
* The Principle of Invariant Light Speed – "... light is always propagated in empty space with a definite velocity [speed] ''c'' which is independent of the state of motion of the emitting body." (from the preface).<ref name=electro /> That is, light in vacuum propagates with the speed ''c'' (a fixed constant, independent of direction) in at least one system of inertial coordinates (the "stationary system"), regardless of the state of motion of the light source.
 
The derivation of special relativity depends not only on these two explicit postulates, but also on several tacit assumptions ([[Duhem–Quine thesis|made in almost all theories of physics]]), including the [[isotropy]] and [[homogeneity (physics)|homogeneity]] of space and the independence of measuring rods and clocks from their past history.<ref>Einstein, "Fundamental Ideas and Methods of the Theory of Relativity", 1920</ref>
 
Following Einstein's original presentation of special relativity in 1905, many different sets of postulates have been proposed in various alternative derivations.<ref>For a survey of such derivations, see Lucas and Hodgson, Spacetime and Electromagnetism, 1990</ref> However, the most common set of postulates remains those employed by Einstein in his original paper. A more mathematical statement of the Principle of Relativity made later by Einstein, which introduces the concept of simplicity not mentioned above is:
{{quote|''Special principle of relativity'': If a system of coordinates K is chosen so that, in relation to it, physical laws hold good in their simplest form, the ''same'' laws hold good in relation to any other system of coordinates K' moving in uniform translation relatively to K.<ref name=Einstein>{{cite book |title=The Principle of Relativity: a collection of original memoirs on the special and general theory of relativity |author=Einstein, A., Lorentz, H. A., Minkowski, H., & Weyl, H. |page=111 |url=http://books.google.com/?id=yECokhzsJYIC&pg=PA111
|isbn=0-486-60081-5 |publisher=Courier Dover Publications |year=1952 }}</ref><!--Albert Einstein: ''The foundation of the general theory of relativity'', Section A, §1 -->}}
 
[[Henri Poincaré]] provided the mathematical framework for relativity theory by proving that [[Lorentz transformations]] are a subset of his [[Poincaré group]] of symmetry transformations. Einstein later derived these transformations from his axioms.
 
Many of Einstein's papers present derivations of the Lorentz transformation based upon these two principles.<ref>Einstein, On the Relativity Principle and the Conclusions Drawn from It, 1907; "The Principle of Relativity and Its Consequences in Modern Physics", 1910; "The Theory of Relativity", 1911; Manuscript on the Special Theory of Relativity, 1912; Theory of Relativity, 1913; Einstein, Relativity, the Special and General Theory, 1916; The Principle Ideas of the Theory of Relativity, 1916; What Is The Theory of Relativity?, 1919; The Principle of Relativity (Princeton Lectures), 1921; Physics and Reality, 1936; The Theory of Relativity, 1949.</ref>
 
Einstein consistently based the derivation of Lorentz invariance (the essential core of special relativity) on just the two basic principles of relativity and light-speed invariance. He wrote:
{{quote|The insight fundamental for the special theory of relativity is this: The assumptions relativity and light speed invariance are compatible if relations of a new type ("Lorentz transformation") are postulated for the conversion of coordinates and times of events... The universal principle of the special theory of relativity is contained in the postulate: The laws of physics are invariant with respect to Lorentz transformations (for the transition from one inertial system to any other arbitrarily chosen inertial system). This is a restricting principle for natural laws...<ref name="autogenerated1" />}}
Thus many modern treatments of special relativity base it on the single postulate of universal Lorentz covariance, or, equivalently, on the single postulate of [[Minkowski spacetime]].<ref>Das, A. (1993) ''The Special Theory of Relativity, A Mathematical Exposition'', Springer, ISBN 0387940421.</ref><ref>Schutz, J. (1997) Independent Axioms for Minkowski Spacetime, Addison Wesley Longman Limited, ISBN 0582317606.</ref>
 
From the principle of relativity alone without assuming the constancy of the speed of light (i.e. using the isotropy of space and the symmetry implied by the principle of special relativity) [[Lorentz transformation#From group postulates|one can show]] that the spacetime transformations between inertial frames are either Euclidean, Galilean, or Lorentzian. In the Lorentzian case, one can then obtain relativistic interval conservation and a certain finite limiting speed. Experiments suggest that this speed is the speed of light in vacuum.<ref name=Friedman>{{cite book|author=Yaakov Friedman|title=Physical Applications of Homogeneous Balls|series=Progress in Mathematical Physics|volume=40|year=2004|pages=1–21|isbn=0817633391}}</ref><ref name=Morin>David Morin (2007) ''Introduction to Classical Mechanics'', Cambridge University Press, Cambridge, chapter 11, Appendix I, ISBN 1139468375.</ref>
 
The constancy of the speed of light was motivated by [[Maxwell's theory of electromagnetism]] and the lack of evidence for the [[luminiferous ether]]. There is conflicting evidence on the extent to which Einstein was influenced by the null result of the [[Michelson–Morley experiment]].<ref>[[Michael Polanyi]] (1974) ''Personal Knowledge: Towards a Post-Critical Philosophy'', ISBN 0-226-67288-3, footnote page 10–11: Einstein reports, via Dr N Balzas in response to Polanyi's query, that "The Michelson–Morely experiment had no role in the foundation of the theory." and "..the theory of relativity was not founded to explain its outcome at all." [http://books.google.com/books?id=0Rtu8kCpvz4C&lpg=PP1&pg=PT19]</ref><ref name="mM1905"/> In any case, the null result of the Michelson–Morley experiment helped the notion of the constancy of the speed of light gain widespread and rapid acceptance.
 
== Lack of an absolute reference frame ==
The [[principle of relativity]], which states that there is no preferred [[inertial reference frame]], dates back to [[Galileo Galilei|Galileo]], and was incorporated into Newtonian physics. However, in the late 19th century, the existence of [[electromagnetic radiation|electromagnetic waves]] led physicists to suggest that the universe was filled with a substance known as "[[Luminiferous aether|aether]]", which would act as the medium through which these waves, or vibrations travelled. The aether was thought to constitute an [[preferred frame|absolute reference frame]] against which speeds could be measured, and could be considered fixed and motionless. Aether supposedly possessed some wonderful properties: it was sufficiently elastic to support electromagnetic waves, and those waves could interact with matter, yet it offered no resistance to bodies passing through it. The results of various experiments, including the [[Michelson–Morley experiment]], indicated that the Earth was always 'stationary' relative to the aether – something that was difficult to explain, since the Earth is in orbit around the Sun. Einstein's solution was to discard the notion of an aether and the absolute state of rest. In relativity, any reference frame moving with uniform motion will observe the same laws of physics. In particular, the speed of light in vacuum is always measured to be ''c'', even when measured by multiple systems that are moving at different (but constant) velocities.
 
== Reference frames, coordinates and the Lorentz transformation ==
{{Main|Lorentz transformation}}
[[File:Frames of reference in relative motion.svg|thumb|right|300px|The primed system is in motion relative to the unprimed system with constant speed v only along the x-axis, from the perspective of an observer stationary in the unprimed system. By the [[principle of relativity]], an observer stationary in the primed system will view a likewise construction except that the speed they record will be −v. The changing of the speed of propagation of interaction from infinite in non-relativistic mechanics to a finite value will require a modification of the transformation equations mapping events in one frame to another.]]
 
Relativity theory depends on "[[Frame of reference|reference frames]]". The term reference frame as used here is an observational perspective in space which is not undergoing any change in motion (acceleration), from which a position can be measured along 3 spatial axes. In addition, a reference frame has the ability to determine measurements of the time of events using a 'clock' (any reference device with uniform periodicity).
 
An event is an occurrence that can be assigned a single unique time and location in space relative to a reference frame: it is a "point" in [[spacetime]]. Since the speed of light is constant in relativity in each and every reference frame, pulses of light can be used to unambiguously measure distances and refer back the times that events occurred to the clock, even though light takes time to reach the clock after the event has transpired.
 
For example, the explosion of a firecracker may be considered to be an "event". We can completely specify an event by its four spacetime coordinates: The time of occurrence and its 3-dimensional spatial location define a reference point. Let's call this reference frame ''S''.
 
In relativity theory we often want to calculate the position of a point from a different reference point.
 
Suppose we have a second reference frame ''S''′, whose spatial axes and clock exactly coincide with that of ''S'' at time zero, but it is moving at a constant velocity ''v'' with respect to ''S'' along the ''x''-axis.
 
Since there is no absolute reference frame in relativity theory, a concept of 'moving' doesn't strictly exist, as everything is always moving with respect to some other reference frame. Instead, any two frames that move at the same speed in the same direction are said to be ''comoving''. Therefore ''S'' and ''S''′ are not ''comoving''.
 
Define the [[Spacetime#Basic concepts|event]] to have spacetime coordinates {{nowrap|(''t'',''x'',''y'',''z'')}} in system ''S'' and {{nowrap|(''t''′,''x''′,''y''′,''z''′)}} in ''S''′. Then the [[Lorentz transformation]] specifies that these coordinates are related in the following way:
: <math>\begin{align}
t' &= \gamma \ (t - vx/c^2) \\
x' &= \gamma \ (x - v t) \\
y' &= y \\
z' &= z ,
\end{align}</math>
where
:<math>\gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}}</math>
is the [[Lorentz factor]] and ''c'' is the [[speed of light]] in vacuum, and the velocity ''v'' of ''S''′ is parallel to the ''x''-axis. The ''y'' and ''z'' coordinates are unaffected; only the ''x'' and ''t'' coordinates are transformed. These Lorentz transformations form a [[one-parameter group]] of [[linear mapping]]s, that parameter being called [[rapidity]].
 
There is nothing special about the ''x''-axis, the transformation can apply to the ''y'' or ''z'' axes, or indeed in any direction, which can be done by directions parallel to the motion (which are warped by the γ factor) and perpendicular; see main article for details.
 
A quantity invariant under [[Lorentz transformations]] is known as a [[Lorentz scalar]].
 
Writing the Lorentz transformation and its inverse in terms of coordinate differences, where for instance one event has coordinates {{nowrap|(''x''<sub>1</sub>, ''t''<sub>1</sub>)}} and {{nowrap|(''x''′<sub>1</sub>, ''t''′<sub>1</sub>)}}, another event has coordinates {{nowrap|(''x''<sub>2</sub>, ''t''<sub>2</sub>) }}and {{nowrap|(''x''′<sub>2</sub>, ''t''′<sub>2</sub>)}}, and the differences are defined as
 
:<math> \begin{array}{ll}
\Delta x' = x'_2-x'_1 \ , & \Delta x = x_2-x_1 \ , \\
\Delta t' = t'_2-t'_1 \ , & \Delta t = t_2-t_1 \ , \\
\end{array}</math>
 
we get
 
:<math> \begin{array}{ll}
\Delta x' = \gamma \ (\Delta x - v \,\Delta t) \ , & \Delta x = \gamma \ (\Delta x' + v \,\Delta t') \ , \\
\Delta t' = \gamma \ \left(\Delta t - \dfrac{v \,\Delta x}{c^{2}} \right) \ , & \Delta t = \gamma \ \left(\Delta t' + \dfrac{v \,\Delta x'}{c^{2}} \right) \ . \\
\end{array}</math>
 
These effects are not merely appearances; they are explicitly related to our way of measuring ''time intervals'' between events which occur at the same place in a given coordinate system (called "co-local" events). These time intervals will be ''different'' in another coordinate system moving with respect to the first, unless the events are also simultaneous. Similarly, these effects also relate to our measured distances between separated but simultaneous events in a given coordinate system of choice. If these events are not co-local, but are separated by distance (space), they will ''not'' occur at the same ''spatial distance'' from each other when seen from another moving coordinate system. However, the [[spacetime interval]] will be the same for all observers. The underlying reality remains the same. Only our perspective changes.
 
== Consequences derived from the Lorentz transformation ==
{{See also|Twin paradox|Relativistic mechanics}}
 
The consequences of special relativity can be derived from the [[Lorentz transformation]] equations.<ref>{{cite book |title=Introduction to special relativity |author=Robert Resnick |publisher=Wiley |year=1968|pages=62–63 |url=http://books.google.com/books?id=fsIRAQAAIAAJ}}</ref> These transformations, and hence special relativity, lead to different physical predictions than those of Newtonian mechanics when relative velocities become comparable to the speed of light. The speed of light is so much larger than anything humans encounter that some of the effects
predicted by relativity are initially [[counterintuitive]].
 
=== Relativity of simultaneity ===
{{See also|Relativity of simultaneity}}
[[Image:Relativity of Simultaneity.svg|thumb|Event B is simultaneous with A in the green reference frame, but it occurred before in the blue frame, and will occur later in the red frame.]]
Two events happening in two different locations that occur simultaneously in the reference frame of one inertial observer, may occur non-simultaneously in the reference frame of another inertial observer (lack of [[absolute simultaneity]]).
 
From the first equation of the Lorentz transformation in terms of coordinate differences
 
:<math>\Delta t' = \gamma \left(\Delta t - \frac{v \,\Delta x}{c^{2}} \right)</math>
 
it is clear that two events that are simultaneous in frame ''S'' (satisfying {{nowrap|1=Δ''t'' = 0}}), are not necessarily simultaneous in another inertial frame ''S''′ (satisfying {{nowrap|1=Δ''t''′ = 0}}). Only if these events are additionally co-local in frame ''S'' (satisfying {{nowrap|1=Δ''x'' = 0}}), will they be simultaneous in another frame ''S''′.
 
=== Time dilation ===
{{See also|Time dilation}}
The time lapse between two events is not invariant from one observer to another, but is dependent on the relative speeds of the observers' reference frames (e.g., the [[twin paradox]] which concerns a twin who flies off in a spaceship traveling near the speed of light and returns to discover that his or her twin sibling has aged much more).
 
Suppose a [[clock]] is at rest in the unprimed system S. Two different ticks of this clock are then characterized by {{nowrap|1=Δ''x'' = 0}}. To find the relation between the times between these ticks as measured in both systems, the first equation can be used to find:
:<math>\Delta t' = \gamma\, \Delta t </math>&nbsp;&nbsp;&nbsp;&nbsp;for events satisfying&nbsp;&nbsp;&nbsp;&nbsp;<math>\Delta x = 0 \ .</math>
This shows that the time (Δ''t''') between the two ticks as seen in the frame in which the clock is moving (''S''′), is ''longer'' than the time (Δ''t'') between these ticks as measured in the rest frame of the clock (''S''). Time dilation explains a number of physical phenomena; for example, the decay rate of [[muon]]s produced by cosmic rays impinging on the Earth's atmosphere.<ref>{{cite book|author=Daniel Kleppner and David Kolenkow|title=An Introduction to Mechanics|year=1973|pages=468–70|isbn=0070350485}}</ref>
 
=== Length contraction ===
{{See also|Lorentz contraction}}
The dimensions (e.g., length) of an object as measured by one observer may be smaller than the results of measurements of the same object made by another observer (e.g., the [[ladder paradox]] involves a long ladder traveling near the speed of light and being contained within a smaller garage).
 
Similarly, suppose a [[measuring rod]] is at rest and aligned along the x-axis in the unprimed system ''S''. In this system, the length of this rod is written as Δ''x''. To measure the length of this rod in the system ''S''′, in which the clock is moving, the distances ''x''′ to the end points of the rod must be measured simultaneously in that system ''S''′. In other words, the measurement is characterized by {{nowrap|1=Δ''t''′ = 0}}, which can be combined with the fourth equation to find the relation between the lengths Δ''x'' and Δ''x''′:
:<math>\Delta x' = \frac{\Delta x}{\gamma} </math>&nbsp;&nbsp;&nbsp;&nbsp;for events satisfying&nbsp;&nbsp;&nbsp;&nbsp;<math>\Delta t' = 0 \ .</math>
This shows that the length (Δ''x''′) of the rod as measured in the frame in which it is moving (''S''′), is ''shorter'' than its length (Δ''x'') in its own rest frame (''S'').
 
=== Composition of velocities ===
{{See also|Velocity-addition formula}}
 
Velocities (speeds) do not simply add. If the observer in ''S'' measures an object moving along the ''x'' axis at velocity ''u'', then the observer in the ''S''′ system, a frame of reference moving at velocity ''v'' in the ''x'' direction with respect to ''S'', will measure the object moving with velocity ''u''′ where (from the Lorentz transformations above):
 
:<math>u'=\frac{dx'}{dt'}=\frac{\gamma \ (dx-v dt)}{\gamma \ (dt-v dx/c^2)}=\frac{(dx/dt)-v}{1-(v/c^2)(dx/dt)}=\frac{u-v}{1-uv/c^2} \ . </math>
 
The other frame ''S'' will measure:
 
:<math>u=\frac{dx}{dt}=\frac{\gamma \ (dx'+v dt')}{\gamma \ (dt'+v dx'/c^2)}=\frac{(dx'/dt')+v}{1+(v/c^2)(dx'/dt')}=\frac{u'+v}{1+u'v/c^2} \ .</math>
 
Notice that if the object were moving at the speed of light in the ''S'' system (i.e. ''u'' = ''c''), then it would also be moving at the speed of light in the ''S''′ system. Also, if both ''u'' and ''v'' are small with respect to the speed of light, we will recover the intuitive Galilean transformation of velocities
:<math>u' \approx u-v \ . </math>
 
The usual example given is that of a train (frame ''S''′ above) traveling due east with a velocity ''v'' with respect to the tracks (frame ''S''). A child inside the train throws a baseball due east with a velocity ''u''′ with respect to the train. In classical physics, an observer at rest on the tracks will measure the velocity of the baseball (due east) as {{nowrap|1=''u'' = ''u''′ + ''v''}}, while in special relativity this is no longer true; instead the velocity of the baseball (due east) is given by the second equation: {{nowrap|1=''u'' = (''u''′ + ''v'')/(1 + ''u''′''v''/''c''<sup>2</sup>)}}. Again, there is nothing special about the ''x'' or east directions. This formalism applies to any direction by considering parallel and perpendicular motion to the direction of relative velocity ''v'', see main article for details.
 
Einstein's addition of colinear velocities is consistent with the [[Fizeau experiment]] which determined the speed of light in a fluid moving parallel to the light, but no experiment has ever tested the formula for the general case of non-parallel velocities.{{cn|date=September 2012}}
 
== Other consequences ==
=== Thomas rotation ===
{{See also|Thomas rotation}}
The orientation of an object (i.e. the alignment of its axes with the observer's axes) may be different for different observers. Unlike other relativistic effects, this effect becomes quite significant at fairly low velocities as can be seen in the [[Spin–orbit interaction|spin of moving particles]].
 
=== Equivalence of mass and energy ===
{{Main|Mass–energy equivalence}}
 
As an object's speed approaches the speed of light from an observer's point of view, its [[relativistic mass]] increases thereby making it more and more difficult to accelerate it from within the observer's frame of reference.
 
The energy content of an object at rest with mass ''m'' equals ''mc''<sup>2</sup>. Conservation of energy implies that, in any reaction, a decrease of the sum of the masses of particles must be accompanied by an increase in kinetic energies of the particles after the reaction. Similarly, the mass of an object can be increased by taking in kinetic energies.
 
In addition to the papers referenced above—which give derivations of the Lorentz transformation and describe the foundations of special relativity—Einstein also wrote at least four papers giving heuristic arguments for the equivalence (and transmutability) of mass and energy, for {{nowrap|1=''E'' = ''mc''<sup>2</sup>}}.
 
Mass–energy equivalence is a consequence of special relativity. The energy and momentum, which are separate in Newtonian mechanics, form a [[four-vector]] in relativity, and this relates the time component (the energy) to the space components (the momentum) in a nontrivial way. For an object at rest, the energy–momentum four-vector is {{nowrap|(''E'', 0, 0, 0)}}: it has a time component which is the energy, and three space components which are zero. By changing frames with a Lorentz transformation in the x direction with a small value of the velocity v, the energy momentum four-vector becomes {{nowrap|(''E'', ''Ev''/''c''<sup>2</sup>, 0, 0)}}. The momentum is equal to the energy multiplied by the velocity divided by ''c''<sup>2</sup>. As such, the Newtonian mass of an object, which is the ratio of the momentum to the velocity for slow velocities, is equal to ''E''/''c''<sup>2</sup>.
 
The energy and momentum are properties of matter and radiation, and it is impossible to deduce that they form a four-vector just from the two basic postulates of special relativity by themselves, because these don't talk about matter or radiation, they only talk about space and time. The derivation therefore requires some additional physical reasoning. In his 1905 paper, Einstein used the additional principles that Newtonian mechanics should hold for slow velocities, so that there is one energy scalar and one three-vector momentum at slow velocities, and that the conservation law for energy and momentum is exactly true in relativity. Furthermore, he assumed that the energy of light is transformed by the same Doppler-shift factor as its frequency, which he had previously shown to be true based on Maxwell's equations.<ref name=electro /> The first of Einstein's papers on this subject was "Does the Inertia of a Body Depend upon its Energy Content?" in 1905.<ref name=inertia>[http://www.fourmilab.ch/etexts/einstein/E_mc2/www/ Does the inertia of a body depend upon its energy content?] A. Einstein, ''Annalen der Physik.'' '''18''':639, 1905 (English translation by W. Perrett and G.B. Jeffery)</ref> Although Einstein's argument in this paper is nearly universally accepted by physicists as correct, even self-evident, many authors over the years have suggested that it is wrong.<ref name=Jammer>{{cite book |title=Concepts of Mass in Classical and Modern Physics |author=Max Jammer |pages=177–178 |url=http://books.google.com/?id=lYvz0_8aGsMC&pg=PA177 |isbn=0-486-29998-8 |publisher=Courier Dover Publications |year=1997 }}</ref> Other authors suggest that the argument was merely inconclusive because it relied on some implicit assumptions.<ref name= Stachel>{{cite book |title=Einstein from ''B'' to ''Z'' |page= 221 |author=John J. Stachel |url=http://books.google.com/?id=OAsQ_hFjhrAC&pg=PA215 |isbn=0-8176-4143-2 |publisher=Springer |year=2002}}</ref>
 
Einstein acknowledged the controversy over his derivation in his 1907 survey paper on special relativity. There he notes that it is problematic to rely on Maxwell's equations for the heuristic mass–energy argument. The argument in his 1905 paper can be carried out with the emission of any massless particles, but the Maxwell equations are implicitly used to make it obvious that the emission of light in particular can be achieved only by doing work. To emit electromagnetic waves, all you have to do is shake a charged particle, and this is clearly doing work, so that the emission is of energy.<ref name=survey>[http://www.webcitation.org/query?url=http://www.geocities.com/physics_world/abstracts/Einstein_1907A_abstract.htm&date=2009-10-26+00:34:19 ''On the Inertia of Energy Required by the Relativity Principle''], A. Einstein, Annalen der Physik 23 (1907): 371–384</ref><ref>In a letter to Carl Seelig in 1955, Einstein wrote "I had already previously found that Maxwell's theory did not account for the micro-structure of radiation and could therefore have no general validity.", Einstein letter to Carl Seelig, 1955.</ref>
 
===How far can one travel from the Earth?===
{{See also|Space travel using constant acceleration}}
Since one can not travel faster than light, one might conclude that a human can never travel further from Earth than 40 light years if the traveler is active between the age of 20 and 60. One would easily think that a traveler would never be able to reach more than the very few solar systems which exist within the limit of 20–40 light years from the earth. But that would be a mistaken conclusion. Because of time dilation, a hypothetical spaceship can travel thousands of light years during the pilot's 40 active years. If a spaceship could be built that accelerates at a constant [[Gravity of Earth|1g]], it will after a little less than a year be traveling at almost the speed of light as seen from Earth.  Time dilation will increase his life span as seen from the reference system of the Earth, but his lifespan measured by a clock traveling with him will not thereby change. During his journey, people on Earth will experience more time than he does.  A 5 year round trip for him will take 6½ Earth years and cover a distance of over 6 light-years.  A 20 year round trip for him (5 years accelerating, 5 decelerating, twice each) will land him back on Earth having traveled for 335 Earth years and a distance of 331 light years.<ref>{{cite web|author=Philip Gibbs and Don Koks|title=The Relativistic Rocket|url=http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html|accessdate=30 August 2012}}</ref>  A full 40 year trip at 1 g will appear on Earth to last 58,000 years and cover a distance of 55,000 light years.  A 40 year trip at 1.1 g will take 148,000 Earth years and cover about 140,000 light years.  A one-way 28 year (14 years accelerating, 14 decelerating as measured with the cosmonaut's clock) trip at 1 g acceleration could reach 2,000,000 light-years to the Andromeda Galaxy.<ref>{{cite web|author=Philip Gibbs and Don Koks|title=The Relativistic Rocket|url=http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html|accessdate=13 October 2013}}</ref>  This same time dilation is why a muon traveling close to ''c'' is observed to travel much further than ''c'' times its [[half-life]] (when at rest).<ref>[http://library.thinkquest.org/C0116043/specialtheorytext.htm The special theory of relativity shows that time and space are affected by motion]. Library.thinkquest.org. Retrieved on 2013-04-24.</ref>
 
==Causality and prohibition of motion faster than light==
{{See also|Causality (physics)|Tachyonic antitelephone}}
[[Image:light cone.svg|thumb|Diagram 2. Light cone]]In diagram 2 the interval AB is 'time-like'; i.e., there is a frame of reference in which events A and B occur at the same location in space, separated only by occurring at different times. If A precedes B in that frame, then A precedes B in all frames. It is hypothetically possible for matter (or information) to travel from A to B, so there can be a causal relationship (with A the cause and B the effect).
 
The interval AC in the diagram is 'space-like'; i.e., there is a frame of reference in which events A and C occur simultaneously, separated only in space. There are also frames in which A precedes C (as shown) and frames in which C precedes A. If it were possible for a cause-and-effect relationship to exist between events A and C, then paradoxes of causality would result. For example, if A was the cause, and C the effect, then there would be frames of reference in which the effect preceded the cause. Although this in itself won't give rise to a paradox, one can show<ref>R. C. Tolman, ''The theory of the Relativity of Motion'', (Berkeley 1917), p. 54</ref><ref>{{cite journal|author=G. A. Benford, D. L. Book, and W. A. Newcomb|doi=10.1103/PhysRevD.2.263|title=The Tachyonic Antitelephone|year=1970|journal=Physical Review D|volume=2|issue=2|pages=263|bibcode = 1970PhRvD...2..263B }}</ref> that faster than light signals can be sent back into one's own past. A causal paradox can then be constructed by sending the signal if and only if no signal was received previously.
 
Therefore, if [[causality]] is to be preserved, one of the consequences of special relativity is that no information signal or material object can travel [[faster than light]] in vacuum. However, some "things" can still move faster than light. For example, the location where the beam of a search light hits the bottom of a cloud can move faster than light when the search light is turned rapidly.<ref>{{cite book
|title=Four Decades of Scientific Explanation
|author=Wesley C. Salmon
|publisher=University of Pittsburgh
|year=2006
|isbn=0-8229-5926-7
|page=107
|url=http://books.google.com/books?id=FHqOXCd06e8C}}, [http://books.google.com/books?id=FHqOXCd06e8C&pg=PA107 Section 3.7 page 107]
</ref>
 
Even without considerations of causality, there are other strong reasons why faster-than-light travel is forbidden by special relativity. For example, if a constant force is applied to an object for a limitless amount of time, then integrating {{nowrap|1=''F'' = ''dp''/''dt''}} gives a momentum that grows without bound, but this is simply because <math>p = m \gamma v</math> approaches [[infinity]] as <math>v</math> approaches ''c''. To an observer who is not accelerating, it appears as though the object's inertia is increasing, so as to produce a smaller acceleration in response to the same force. This behavior is observed in [[particle accelerators]], where each charged particle is accelerated by the electromagnetic force.
<!-- a pair of diagrams, with x–t and x'–t' coordinates would help here -->
 
Theoretical and experimental tunneling studies carried out by [[Günter Nimtz]] and Petrissa Eckle claimed that under special conditions signals may travel faster than light.<ref name=Nimtz1>{{cite journal|author=F. Low and P. Mende|title=A Note on the Tunneling Time Problem|journal= Annals of Physics|volume= 210|pages= 380–387 |year=1991|doi=10.1016/0003-4916(91)90047-C|issue=2|bibcode = 1991AnPhy.210..380L }}</ref><ref name=Nimtz2>{{cite journal|author=A. Enders and G. Nimtz|title= On superluminal barrier traversal|journal=J. Phys. I France|volume= 2|pages= 1693–1698|year=1992|doi=10.1051/jp1:1992236|issue=9 |bibcode = 1992JPhy1...2.1693E }}</ref><ref name=Nimtz3>{{cite journal|author=S. Longhi et al|pmid=12006050|url=http://www.researchgate.net/publication/11365120_Measurement_of_superluminal_optical_tunneling_times_in_double-barrier_photonic_band_gaps|doi=10.1103/PhysRevE.65.046610|arxiv=physics/0201013|title=Measurement of superluminal optical tunneling times in double-barrier photonic band gaps|year=2002|journal=Physical Review E|volume=65|issue=4|pages=046610|last2=Laporta|first2=P.|last3=Belmonte|first3=M.|last4=Recami|first4=E.|bibcode = 2002PhRvE..65d6610L }}</ref><ref name=Eckle>P. Eckle et al., Attosecond Ionization and Tunneling Delay Time Measurements in Helium, Science, 322, 1525–1529 (2008)</ref> It was measured that fiber digital signals were traveling up to 5 times c and a zero-time tunneling electron carried the information that the atom is [[Ionization|ionized]], with photons, [[phonon]]s and electrons spending zero time in the tunneling barrier. According to Nimtz and Eckle, in this superluminal process only the Einstein causality and the special relativity but not the primitive causality are violated: Superluminal propagation does not result in any kind of time travel.<ref name=Nimtz4>{{cite journal|author=G. Nimtz, Do Evanescent Modes Violate Relativistic Causality?|doi=10.1007/3-540-34523-X_19|journal= Lect. Notes Phys. |volume=702|pages= 506–531 |year=2006|title=Do Evanescent Modes Violate Relativistic Causality?|series=Lecture Notes in Physics|isbn=978-3-540-34522-0}}</ref><ref name=Nimtz5>{{cite journal|author=G. Nimtz|title= Tunneling Violates Special Relativity| arxiv=1003.3944v1|year=2010}}</ref> [[Günter Nimtz#Scientific opponents and their interpretations|Several scientists]] have stated not only that Nimtz' interpretations were erroneous, but also that the experiment actually provided a trivial experimental confirmation of the special relativity theory.<ref name="winful">{{cite arxiv | title=Comment on "Macroscopic violation of special relativity" by Nimtz and Stahlhofen | author=Herbert Winful| date=2007-09-18 | eprint=0709.2736 | class=quant-ph}}</ref><ref>{{cite web | title=Latest "faster than the speed of light" claims wrong (again) | url=http://arstechnica.com/news.ars/post/20070816-faster-than-the-speed-of-light-no-i-dont-think-so.html | author=Chris Lee | date=2007-08-16}}</ref><ref name="WinfulHartman">{{Cite journal | author = Herbert G. Winful | title = Tunneling time, the Hartman effect, and superluminality: A proposed resolution of an old paradox | journal = Physics Reports | volume = 436 | issue = 1–2 | pages = 1–69 | date = December 2006 | url = http://sitemaker.umich.edu/herbert.winful/files/physics_reports_review_article__2006_.pdf | doi = 10.1016/j.physrep.2006.09.002 |bibcode = 2006PhR...436....1W }}</ref>
 
== Geometry of spacetime ==
{{Main|Minkowski space}}
 
===Comparison between flat Euclidean space and Minkowski space===
{{see also|line element}}
[[File:Orthogonality and rotation.svg|thumb|350px|Orthogonality and rotation of coordinate systems compared between '''left:''' [[Euclidean space]] through circular [[angle]] φ, '''right:''' in [[Minkowski spacetime]] through [[hyperbolic angle]] φ (red lines labelled ''c'' denote the [[worldline]]s of a light signal, a vector is orthogonal to itself if it lies on this line).<ref>{{cite book|title=Gravitation|author=J.A. Wheeler, C. Misner, K.S. Thorne|publisher=W.H. Freeman & Co|page=58|year=1973|isbn=0-7167-0344-0}}</ref>]]
 
Special relativity uses a 'flat' 4-dimensional Minkowski space&nbsp;– an example of a [[spacetime]]. Minkowski spacetime appears to be very similar to the standard 3-dimensional [[Euclidean space]], but there is a crucial difference with respect to time.
 
In 3D space, the [[differential (infinitesimal)|differential]] of distance (line element) ''ds'' is defined by
 
:<math> ds^2 = d\mathbf{x} \cdot d\mathbf{x} = dx_1^2 + dx_2^2 + dx_3^2, </math>
 
where {{nowrap|1=''d'''''x''' = (''dx''<sub>1</sub>, ''dx''<sub>2</sub>, ''dx''<sub>3</sub>)}} are the differentials of the three spatial dimensions. In Minkowski geometry, there is an extra dimension with coordinate ''X''<sup>0</sup> derived from time, such that the distance differential fulfills
 
:<math> ds^2 = -dX_0^2 + dX_1^2 + dX_2^2 + dX_3^2, </math>
 
where {{nowrap|1=''d'''''X''' = (''dX''<sub>0</sub>, ''dX''<sub>1</sub>, ''dX''<sub>2</sub>, ''dX''<sub>3</sub>)}} are the differentials of the four spacetime dimensions. This suggests a deep theoretical insight: special relativity is simply a [[rotational symmetry]] of our spacetime, analogous to the rotational symmetry of Euclidean space (see image right).<ref>{{cite book|title=Dynamics and Relativity|author=J.R. Forshaw, A.G. Smith|publisher=Wiley|page=247|year=2009|isbn=978-0-470-01460-8}}</ref> Just as Euclidean space uses a [[Euclidean metric]], so spacetime uses a [[Minkowski metric]]. {{Anchor|interval}}Basically, special relativity can be stated as the ''invariance of any spacetime interval'' (that is the 4D distance between any two events) when viewed from ''any inertial reference frame''. All equations and effects of special relativity can be derived from this rotational symmetry (the [[Poincaré group]]) of Minkowski spacetime.
 
The actual form of ''ds'' above depends on the metric and on the choices for the ''X''<sup>0</sup> coordinate.
To make the time coordinate look like the space coordinates, it can be treated as [[imaginary number|imaginary]]: {{nowrap|1=''X''<sub>0</sub> = ''ict''}} (this is called a [[Wick rotation]]).
According to [[Gravitation (book)|Misner, Thorne and Wheeler]] (1971, §2.3), ultimately the deeper understanding of both special and general relativity will come from the study of the Minkowski metric (described below) and to take {{nowrap|1=''X''<sup>0</sup> = ''ct''}}, rather than a "disguised" Euclidean metric using ''ict'' as the time coordinate.
 
Some authors use {{nowrap|1=''X''<sup>0</sup> = ''t''}}, with factors of ''c'' elsewhere to compensate; for instance, spatial coordinates are divided by ''c'' or factors of ''c''<sup>±2</sup> are included in the metric tensor.<ref>{{cite book |author=R. Penrose| title=[[The Road to Reality]]| publisher= Vintage books| year=2007 | isbn=0-679-77631-1}}</ref>
These numerous conventions can be superseded by using [[natural units]] where {{nowrap|1=''c'' = 1}}. Then space and time have equivalent units, and no factors of ''c'' appear anywhere.
 
===3D spacetime===
 
[[File:Special relativity- Three dimensional dual-cone.svg|thumb|Three dimensional dual-cone.]]
[[Image:Sr3.svg|thumb|Null spherical space.]]
 
If we reduce the spatial dimensions to 2, so that we can represent the physics in a 3D space
 
:<math> ds^2 = dx_1^2 + dx_2^2 - c^2 dt^2, </math>
 
we see that the [[null geodesic|null]] [[geodesic]]s lie along a dual-cone (see image right) defined by the equation;
 
:<math> ds^2 = 0 = dx_1^2 + dx_2^2 - c^2 dt^2 </math>
 
or simply
 
:<math> dx_1^2 + dx_2^2 = c^2 dt^2, </math>
 
 which is the equation of a circle of radius ''c&thinsp;dt''.
 
===4D spacetime===
 
If we extend this to three spatial dimensions, the null geodesics are the 4-dimensional cone:
 
:<math> ds^2 = 0 = dx_1^2 + dx_2^2 + dx_3^2 - c^2 dt^2 </math>
 
so
 
:<math> dx_1^2 + dx_2^2 + dx_3^2 = c^2 dt^2. </math>
 
This null dual-cone represents the "line of sight" of a point in space. That is, when we look at the [[star]]s and say "The light from that star which I am receiving is X years old", we are looking down this line of sight: a null geodesic. We are looking at an event a distance <math>d = \sqrt{x_1^2+x_2^2+x_3^2} </math> away and a time ''d/c'' in the past. For this reason the null dual cone is also known as the 'light cone'. (The point in the lower left of the picture below represents the star, the origin represents the observer, and the line represents the null geodesic "line of sight".)
 
The cone in the −''t'' region is the information that the point is 'receiving', while the cone in the +''t'' section is the information that the point is 'sending'.
 
The geometry of Minkowski space can be depicted using [[Minkowski diagram]]s, which are useful also in understanding many of the thought-experiments in special relativity.
 
Note that, in 4d spacetime, the concept of the [[center of mass]] becomes more complicated, see [[center of mass (relativistic)]].
 
== Physics in spacetime ==
 
===Transformations of physical quantities between reference frames===
 
Above, the Lorentz transformation for the time coordinate and three space coordinates illustrates that they are intertwined. This is true more generally: certain pairs of "timelike" and "spacelike" quantities naturally combine on equal footing under the same Lorentz transformation.
 
The Lorentz transformation in standard configuration above, i.e. for a boost in the ''x'' direction, can be recast into matrix form as follows:
 
:<math>\begin{pmatrix}
ct'\\ x'\\ y'\\ z'
\end{pmatrix} = \begin{pmatrix}
\gamma & -\beta\gamma & 0 & 0\\
-\beta\gamma & \gamma & 0 & 0\\
0 & 0 & 1 & 0\\
0 & 0 & 0 & 1
\end{pmatrix}
\begin{pmatrix}
ct\\ x\\ y\\ z
\end{pmatrix} =
\begin{pmatrix}
\gamma ct- \gamma\beta x\\
\gamma x - \beta \gamma ct \\ y\\ z
\end{pmatrix}.
</math>
In Newtonian mechanics, quantities which have magnitude and direction are mathematically described as 3d vectors in Euclidean space, and in general they are parametrized by time. In special relativity, this notion is extended by adding the appropriate timelike quantity to a spacelike vector quantity, and we have 4d vectors, or "[[four vector]]s", in Minkowski spacetime. The components of vectors are written using [[tensor index notation]], as this has numerous advantages. The notation makes it clear the equations are [[manifestly covariant]] under the [[Poincaré group]], thus bypassing the tedious calculations to check this fact. In constructing such equations, we often find that equations previously thought to be unrelated are, in fact, closely connected being part of the same tensor equation. Recognizing other physical quantities as tensors simplifies their transformation laws. Throughout, upper indices (superscripts) are contravariant indices rather than exponents except when they indicate a square (this is should be clear from the context), and lower indices (subscripts) are covariant indices. For simplicity and consistency with the earlier equations, Cartesian coordinates will be used.
 
The simplest example of a four-vector is the position of an event in spacetime, which constitutes a timelike component ''ct'' and spacelike component {{nowrap|1='''x''' = (''x'', ''y'', ''z'')}}, in a [[Covariance and contravariance of vectors|contravariant]] [[position vector|position]] [[four vector]] with components:
 
:<math>X^\nu = (X^0, X^1, X^2, X^3)= (ct, x, y, z).</math>
 
where we define {{nowrap|1=''X''<sup>0</sup> = ''ct''}} so that the time coordinate has the same dimension of distance as the other spatial dimensions; so that space and time are treated equally.<ref>Jean-Bernard Zuber & Claude Itzykson, ''Quantum Field Theory'', pg 5, ISBN 0-07-032071-3</ref><ref>[[Charles W. Misner]], [[Kip S. Thorne]] & [[John A. Wheeler]], ''Gravitation'', pg 51, ISBN 0-7167-0344-0</ref><ref>[[George Sterman]], ''An Introduction to Quantum Field Theory'', pg 4 , ISBN 0-521-31132-2</ref>  Now the transformation of the contravariant components of the position 4-vector can be compactly written as:
 
:<math>X^{\mu'}=\Lambda^{\mu'}{}_\nu X^\nu</math>
 
where there is an [[Einstein notation|implied summation]] on ''ν'' from 0 to 3, and <math>\Lambda^{\mu'}{}_{\nu}</math> is a [[matrix (mathematics)|matrix]].
 
More generally, all contravariant components of a [[four-vector]] <math>T^\nu</math> transform from one frame to another frame by a [[Lorentz transformation]]:
 
:<math>T^{\mu'} = \Lambda^{\mu'}{}_{\nu} T^\nu</math>
 
Examples of other 4-vectors include the [[four-velocity]] ''U''<sup>μ</sup>, defined as the derivative of the position 4-vector with respect to [[proper time]]:
 
:<math>U^\mu = \frac{dX^\mu}{d\tau} = \gamma(v)( c , v_x , v_y, v_z ) . </math>
 
where the Lorentz factor is:
 
:<math>\gamma(v)= \frac{1}{\sqrt{1- (v/c)^2}} \,,\quad v^2 = v_x^2 + v_y^2 + v_z^2 \,.</math>
 
The [[Mass in special relativity|relativistic energy]] <math>E = \gamma(v)mc^2</math> and [[relativistic momentum]] <math>\mathbf{p} = \gamma(v)m \mathbf{v}</math> of an object are respectively the timelike and spacelike components of a [[Covariance and contravariance of vectors|covariant]] [[four momentum]] vector:
 
:<math>P_\nu = m U_\nu = m\gamma(v)(c,v_x,v_y,v_z)= (E/c,p_x,p_y,p_z).</math>
 
where ''m'' is the [[invariant mass]].
 
The [[four-acceleration]] is the proper time derivative of 4-velocity:
 
:<math>A^\mu = \frac{d U^\mu}{d\tau} \,.</math>
 
The transformation rules for ''three''-dimensional velocities and accelerations are very awkward; even above in standard configuration the velocity equations are quite complicated owing to their non-linearity. On the other hand, the transformation of ''four''-velocity and ''four''-acceleration are simpler by means of the Lorentz transformation matrix.
 
The [[four-gradient]] of a [[scalar field]] φ transforms covariantly rather than contravariantly:
 
:<math>\begin{pmatrix} \frac{1}{c}\frac{\partial \phi}{\partial t'} & \frac{\partial \phi}{\partial x'} & \frac{\partial \phi}{\partial y'} & \frac{\partial \phi}{\partial z'}\end{pmatrix} = \begin{pmatrix} \frac{1}{c}\frac{\partial \phi}{\partial t} & \frac{\partial \phi}{\partial x} & \frac{\partial \phi}{\partial y} & \frac{\partial \phi}{\partial z}\end{pmatrix}\begin{pmatrix}
\gamma & -\beta\gamma & 0 & 0\\
-\beta\gamma & \gamma & 0 & 0\\
0 & 0 & 1 & 0\\
0 & 0 & 0 & 1
\end{pmatrix} \,.</math>
 
that is:
 
:<math>(\partial_{\mu'} \phi) = \Lambda_{\mu'}{}^{\nu} (\partial_\nu \phi)\,,\quad \partial_{\mu} \equiv \frac{\partial}{\partial x^{\mu}}\,.</math>
 
only in Cartesian coordinates. It's the [[covariant derivative]] which transforms in manifest covariance, in Cartesian coordinates this happens to reduce to the partial derivatives, but not in other coordinates.
 
More generally, the ''co''variant components of a 4-vector transform according to the ''inverse'' Lorentz transformation:
 
:<math>\Lambda_{\mu'}{}^{\nu} T^{\mu'} =  T^\nu</math>
 
where <math> \Lambda_{\mu'}{}^{\nu}</math> is the reciprocal matrix of <math>\Lambda^{\mu'}{}_{\nu}</math>.
 
The postulates of special relativity constrain the exact form the Lorentz transformation matrices take.
 
More generally, most physical quantities are best described as (components of) [[tensor]]s. So to transform from one frame to another, we use the well-known [[Tensor|tensor transformation law]]<ref>{{cite book
|title=Spacetime and Geometry: An Introduction to General Relativity |author=Sean M. Carroll |publisher=Addison Wesley |year=2004 |isbn=0-8053-8732-3 |page=22 |url=http://books.google.com/books?id=1SKFQgAACAAJ}}</ref>
 
:<math>T^{\alpha' \beta' \cdots \zeta'}_{\theta' \iota' \cdots \kappa'} =
\Lambda^{\alpha'}{}_{\mu} \Lambda^{\beta'}{}_{\nu} \cdots \Lambda^{\zeta'}{}_{\rho}
\Lambda_{\theta'}{}^{\sigma} \Lambda_{\iota'}{}^{\upsilon} \cdots \Lambda_{\kappa'}{}^{\phi}
T^{\mu \nu \cdots \rho}_{\sigma \upsilon \cdots \phi}</math>
 
where <math>\Lambda_{\chi'}{}^{\psi}</math> is the reciprocal matrix of <math>\Lambda^{\chi'}{}_{\psi}</math>. All tensors transform by this rule.
 
An example of a four dimensional second order [[antisymmetric tensor]] is the [[relativistic angular momentum]], which has six components: three are the classical [[angular momentum]], and the other three are related to the boost of the center of mass of the system. The derivative of the relativistic angular momentum with respect to proper time is the relativistic torque, also second order [[antisymmetric tensor]].
 
The [[electromagnetic field tensor]] is another second order antisymmetric [[tensor field]], with six components: three for the [[electric field]] and another three for the [[magnetic field]]. There is also the [[stress–energy tensor]] for the electromagnetic field, namely the [[electromagnetic stress–energy tensor]].
 
===Metric===
 
The [[metric tensor]] allows one to define the [[inner product]] of two vectors, which in turn allows one to assign a magnitude to the vector. Given the four-dimensional nature of spacetime the [[Minkowski metric]] ''η'' has components (valid in any [[inertial reference frame]]) which can be arranged in a {{nowrap|4 × 4}} matrix:
 
:<math>\eta_{\alpha\beta} = \begin{pmatrix}
-1 & 0 & 0 & 0\\
0 & 1 & 0 & 0\\
0 & 0 & 1 & 0\\
0 & 0 & 0 & 1
\end{pmatrix}</math>
 
which is equal to its reciprocal, <math>\eta^{\alpha\beta}</math>, in those frames. Throughout we use the signs as above, different authors use different conventions – see [[Minkowski metric]] alternative signs.
 
The [[Poincaré group]] is the most general group of transformations which preserves the Minkowski metric:
 
:<math>\eta_{\alpha\beta} = \eta_{\mu'\nu'} \Lambda^{\mu'}{}_\alpha \Lambda^{\nu'}{}_\beta \!</math>
 
and this is the physical symmetry underlying special relativity.
 
The metric can be used for [[raising and lowering indices]] on vectors and tensors. Invariants can be constructed using the metric, the inner product of a 4-vector ''T'' with another 4-vector ''S'' is:
 
:<math>T^{\alpha}S_{\alpha}=T^{\alpha}\eta_{\alpha\beta}S^{\beta} = T_{\alpha}\eta^{\alpha\beta}S_{\beta} = \text{invariant scalar}</math>
 
Invariant means that it takes the same value in all inertial frames, because it is a scalar (0 rank tensor), and so no Λ appears in its trivial transformation. The magnitude of the 4-vector ''T'' is the positive square root of the inner product with itself:
 
:<math>|\mathbf{T}| = \sqrt{T^{\alpha}T_{\alpha}}</math>
 
One can extend this idea to tensors of higher order, for a second order tensor we can form the invariants:
 
:<math>T^{\alpha}{}_{\alpha}\,,T^{\alpha}{}_{\beta}T^{\beta}{}_{\alpha}\,,T^{\alpha}{}_{\beta}T^{\beta}{}_{\gamma}T^{\gamma}{}_{\alpha} = \text{invariant scalars}\,,</math>
 
similarly for higher order tensors. Invariant expressions, particularly inner products of 4-vectors with themselves, provide equations that are useful for calculations, because one doesn't need to perform Lorentz transformations to determine the invariants.
 
===Relativistic kinematics and invariance===
 
The coordinate differentials transform also contravariantly:
 
:<math>dX^{\mu'}=\Lambda^{\mu'}{}_\nu dX^\nu</math>
 
so the squared length of the differential of the position four-vector ''dX<sup>μ</sup>'' constructed using
 
:<math>d\mathbf{X}^2 = dX^\mu \,dX_\mu = \eta_{\mu\nu}\,dX^\mu \,dX^\nu = -(c dt)^2+(dx)^2+(dy)^2+(dz)^2\,</math>
 
is an invariant. Notice that when the [[line element]] ''d'''''X'''<sup>2</sup> is negative that {{sqrt|−''d'''''X'''<sup>2</sup>}} is the differential of [[proper time]], while when ''d'''''X'''<sup>2</sup> is positive, {{sqrt|''d'''''X'''<sup>2</sup>}} is differential of the [[proper distance]].
 
The 4-velocity ''U''<sup>μ</sup> has an invariant form:
 
:<math>{\mathbf U}^2 = \eta_{\nu\mu} U^\nu U^\mu = -c^2 \,,</math>
 
which means all velocity four-vectors have a magnitude of ''c''. This is an expression of the fact that there is no such thing as being at coordinate rest in relativity: at the least, you are always moving forward through time. Differentiating the above equation by ''τ'' produces:
 
:<math>2\eta_{\mu\nu}A^\mu U^\nu = 0.</math>
 
So in special relativity, the acceleration four-vector and the velocity four-vector are orthogonal.
 
===Relativistic dynamics and invariance===
 
The invariant magnitude of the [[four-momentum|momentum 4-vector]] generates the [[energy–momentum relation]]:
 
:<math>\mathbf{P}^2 = \eta^{\mu\nu}P_\mu P_\nu = -(E/c)^2 + p^2 .</math>
 
We can work out what this invariant is by first arguing that, since it is a scalar, it doesn't matter which reference frame we calculate it, and then by transforming to a frame where the total momentum is zero.
 
:<math>\mathbf{P}^2 = - (E_\mathrm{rest}/c)^2 = - (m c)^2 .</math>
 
We see that the rest energy is an independent invariant. A rest energy can be calculated even for particles and systems in motion, by translating to a frame in which momentum is zero.
 
The rest energy is related to the mass according to the celebrated equation discussed above:
 
:<math>E_\mathrm{rest} = m c^2.</math>
 
Note that the mass of systems measured in their center of momentum frame (where total momentum is zero) is given by the total energy of the system in this frame. It may not be equal to the sum of individual system masses measured in other frames.
 
To use [[Newton's third law of motion]], both forces must be defined as the rate of change of momentum with respect to the same time coordinate. That is, it requires the 3D force defined above. Unfortunately, there is no tensor in 4D which contains the components of the 3D force vector among its components.
 
If a particle is not traveling at ''c'', one can transform the 3D force from the particle's co-moving reference frame into the observer's reference frame. This yields a 4-vector called the [[four-force]]. It is the rate of change of the above energy momentum [[four-vector]] with respect to proper time. The covariant version of the four-force is:
 
:<math>F_\nu = \frac{d P_{\nu}}{d \tau} = m A_\nu </math>
 
In the rest frame of the object, the time component of the four force is zero unless the "[[invariant mass]]" of the object is changing (this requires a non-closed system in which energy/mass is being directly added or removed from the object) in which case it is the negative of that rate of change of mass, times ''c''. In general, though, the components of the four force are not equal to the components of the three-force, because the three force is defined by the rate of change of momentum with respect to coordinate time, i.e. ''dp''/''dt'' while the four force is defined by the rate of change of momentum with respect to proper time, i.e. ''dp''/''d''τ.
 
In a continuous medium, the 3D ''density of force'' combines with the ''density of power'' to form a covariant 4-vector. The spatial part is the result of dividing the force on a small cell (in 3-space) by the volume of that cell. The time component is −1/''c'' times the power transferred to that cell divided by the volume of the cell. This will be used below in the section on electromagnetism.
 
== Relativity and unifying electromagnetism ==
{{Main|Classical electromagnetism and special relativity|Covariant formulation of classical electromagnetism}}
Theoretical investigation in [[classical electromagnetism]] led to the discovery of wave propagation. Equations generalizing the electromagnetic effects found that finite propagation speed of the '''E''' and '''B''' fields required certain behaviors on charged particles. The general study of moving charges forms the [[Liénard–Wiechert potential]], which is a step towards special relativity.
 
The Lorentz transformation of the [[electric field]] of a moving charge into a non-moving observer's reference frame results in the appearance of a mathematical term commonly called the [[magnetic field]]. Conversely, the ''magnetic'' field generated by a moving charge disappears and becomes a purely ''electrostatic'' field in a comoving frame of reference. [[Maxwell's equations]] are thus simply an empirical fit to special relativistic effects in a classical model of the Universe. As electric and magnetic fields are reference frame dependent and thus intertwined, one speaks of ''electromagnetic'' fields. Special relativity provides the transformation rules for how an electromagnetic field in one inertial frame appears in another inertial frame.
 
[[Maxwell's equations]] in the 3D form are already consistent with the physical content of special relativity, although they are easier to manipulate in a [[manifestly covariant]] form, i.e. in the language of [[tensor]] calculus.<ref>{{cite book|title=Formal Structure of Electromagnetics: General Covariance and Electromagnetics|year=1962|publisher=Dover Publications Inc.|isbn=0-486-65427-3|author=E. J. Post}}</ref> See main links for more detail.
 
== Status ==
{{Main|Tests of special relativity|Criticism of relativity theory}}
 
Special relativity in its [[Minkowski spacetime]] is accurate only when the [[absolute value]] of the [[gravitational potential]] is much less than ''c''<sup>2</sup> in the region of interest.<ref>{{cite book
|title=Einstein's general theory of relativity: with modern applications in cosmology
|author=Øyvind Grøn and Sigbjørn Hervik
|publisher=Springer
|year=2007
|isbn=0-387-69199-5
|page=195
|url=http://books.google.com/books?id=IyJhCHAryuUC}}, [http://books.google.com/books?id=IyJhCHAryuUC&pg=PA195 Extract of page 195 (with units where c=1)]
</ref> In a strong gravitational field, one must use [[general relativity]]. General relativity becomes special relativity at the limit of weak field. At very small scales, such as at the [[Planck length]] and below, quantum effects must be taken into consideration resulting in [[quantum gravity]]. However, at macroscopic scales and in the absence of strong gravitational fields, special relativity is experimentally tested to extremely high degree of accuracy (10<sup>−20</sup>)<ref>The number of works is vast, see as example:<br />
{{cite journal|author=Sidney Coleman, Sheldon L. Glashow|title=Cosmic Ray and Neutrino Tests of Special Relativity|journal= Phys. Lett. |volume=B405 |year=1997|pages= 249–252|arxiv=hep-ph/9703240|doi=10.1016/S0370-2693(97)00638-2|issue=3–4|bibcode = 1997PhLB..405..249C }}<br />
An overview can be found on [http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html this page]</ref>
and thus accepted by the physics community. Experimental results which appear to contradict it are not reproducible and are thus widely believed to be due to experimental errors.
 
Special relativity is mathematically self-consistent, and it is an organic part of all modern physical theories, most notably [[quantum field theory]], [[string theory]], and general relativity (in the limiting case of negligible gravitational fields).
 
Newtonian mechanics mathematically follows from special relativity at small velocities (compared to the speed of light) – thus Newtonian mechanics can be considered as a special relativity of slow moving bodies. See [[classical mechanics]] for a more detailed discussion.
 
Several experiments predating Einstein's 1905 paper are now interpreted as evidence for relativity. Of these it is known Einstein was aware of the Fizeau experiment before 1905,<ref>{{cite journal|author=John D. Norton|year=2004|first1=John D.|journal=Archive for History of Exact Sciences|title= Einstein's Investigations of Galilean Covariant Electrodynamics prior to 1905|pages= 45–105|volume=59|url=http://philsci-archive.pitt.edu/archive/00001743/|doi=10.1007/s00407-004-0085-6|bibcode=2004AHES...59...45N}}</ref> and historians have concluded that Einstein was at least aware of the Michelson–Morley experiment as early as 1899 despite claims he made in his later years that it played no role in his development of the theory.<ref name="mM1905">{{cite journal|author= Jeroen van Dongen|title=On the role of the Michelson–Morley experiment: Einstein in Chicago|url=http://philsci-archive.pitt.edu/4778/1/Einstein_Chicago_Web2.pdf|journal= Eprint arXiv:0908.1545|volume= 0908|page= 1545|year= 2009|bibcode= 2009arXiv0908.1545V|arxiv=0908.1545}}</ref>
* The [[Fizeau experiment]] (1851, repeated by Michelson and Morley in 1886) measured the speed of light in moving media, with results that are consistent with relativistic addition of colinear velocities.
* The famous [[Michelson–Morley experiment]] (1881, 1887) gave further support to the postulate that detecting an absolute reference velocity was not achievable. It should be stated here that, contrary to many alternative claims, it said little about the invariance of the speed of light with respect to the source and observer's velocity, as both source and observer were travelling together at the same velocity at all times.
* The [[Trouton–Noble experiment]] (1903) showed that the torque on a capacitor is independent of position and inertial reference frame.
* The [[Experiments of Rayleigh and Brace]] (1902, 1904) showed that length contraction doesn't lead to birefringence for a co-moving observer, in accordance with the relativity principle.
 
[[Particle accelerator]]s routinely accelerate and measure the properties of particles moving at near the speed of light, where their behavior is completely consistent with relativity theory and inconsistent with the earlier [[Newtonian mechanics]]. These machines would simply not work if they were not engineered according to relativistic principles. In addition, a considerable number of modern experiments have been conducted to test special relativity. Some examples:
* [[Tests of relativistic energy and momentum]] – testing the limiting speed of particles
* [[Ives–Stilwell experiment]] – testing relativistic Doppler effect and time dilation
* [[Time dilation of moving particles]] – relativistic effects on a fast-moving particle's half-life
* [[Kennedy–Thorndike experiment]] – time dilation in accordance with Lorentz transformations
* [[Hughes–Drever experiment]] – testing isotropy of space and mass
* [[Modern searches for Lorentz violation]] – various modern tests
* Experiments to test [[emission theory]] demonstrated that the speed of light is independent of the speed of the emitter.
* Experiments to test the [[aether drag hypothesis]] – no "aether flow obstruction".
 
==Theories of relativity and quantum mechanics==
''Special'' relativity can be combined with [[quantum mechanics]] to form [[relativistic quantum mechanics]]. It is an [[List of unsolved problems in physics|unsolved problem in physics]] how [[general relativity|''general'' relativity]] and quantum mechanics can be unified; [[quantum gravity]] and a "[[theory of everything]]", which require such a unification, are active and ongoing areas in theoretical research.
 
The early [[Bohr model|Bohr–Sommerfeld atomic model]] explained the [[fine structure]] of [[alkali metal]] atoms using both special relativity and the preliminary knowledge on [[quantum mechanics]] of the time.<ref>{{cite book|title=Quantum Physics of Atoms, Molecules, Solids, Nuclei and Particles|edition=2nd|pages=114–116|author=R. Resnick, R. Eisberg|publisher=John Wiley & Sons|year=1985|isbn=978-0-471-87373-0}}</ref>
 
In 1928, [[Paul Dirac]] constructed an influential [[relativistic wave equation]], now known as the [[Dirac equation]] in his honour,<ref name="Dirac">{{cite journal | author = P.A.M. Dirac | authorlink = P.A.M. Dirac | year =1930 | title = A Theory of Electrons and Protons | journal =Proceedings of the Royal Society | volume = A126 | page = 360 |bibcode=1930RSPSA.126..360D | jstor=95359 | doi=10.1098/rspa.1930.0013 | issue = 801}}</ref> that is fully compatible both with special relativity and with the final version of quantum theory existing after 1926. This equation explained not only the intrinsic angular momentum of the electrons called ''[[spin (physics)|spin]]'', it also led to the prediction of the [[antiparticle]] of the electron (the [[positron]]),<ref name="Dirac" /><ref>{{cite journal|author=C.D. Anderson|authorlink=Carl David Anderson |title=The Positive Electron|journal=Phys. Rev. |volume=43|pages=491–494 |year=1933|doi=10.1103/PhysRev.43.491|issue=6|bibcode = 1933PhRv...43..491A }}</ref> and [[fine structure]] could only be fully explained with special relativity. It was the first foundation of ''[[relativistic quantum mechanics]]''. In non-relativistic quantum mechanics, spin is [[phenomenology (science)|phenomenological]] and cannot be explained.
 
On the other hand, the existence of antiparticles leads to the conclusion that relativistic quantum mechanics is not enough for a more accurate and complete theory of particle interactions. Instead, a theory of particles interpreted as quantized fields, called ''[[quantum field theory]]'', becomes necessary; in which particles can be [[annihilation|created and destroyed]] throughout space and time.
 
==See also==
{{portal|Physics}}
:'''People''': [[Hendrik Lorentz]] | [[Henri Poincaré]] | [[Albert Einstein]] | [[Max Planck]] | [[Hermann Minkowski]] | [[Max von Laue]] | [[Arnold Sommerfeld]] | [[Max Born]] | [[Gustav Herglotz]] | [[Richard C. Tolman]]
:'''Relativity''': [[Theory of relativity]] | [[History of special relativity]] | [[Principle of relativity]] | [[General relativity]] | [[Fundamental Speed]] | [[Frame of reference]] | [[Inertial frame of reference]] | [[Lorentz transformations]] | [[Bondi k-calculus]] | [[Einstein synchronisation]] | [[Rietdijk–Putnam Argument]] | [[Special relativity (alternative formulations)]] | [[Criticism of relativity theory]] | [[Relativity priority dispute]]
:'''Physics''': [[Newtonian Mechanics]] | [[spacetime]] | [[speed of light]] | [[Relativity of simultaneity|simultaneity]] | [[center of mass (relativistic)]] | [[physical cosmology]] | [[Doppler effect]] | [[relativistic Euler equations]] | [[Aether drag hypothesis]] | [[Lorentz ether theory]] | [[Moving magnet and conductor problem]] | [[Shape waves]] | [[Relativistic heat conduction]] | [[Relativistic disk]] | [[Thomas precession]] | [[Born rigidity]] | [[Born coordinates]]
:'''Mathematics''': [[Derivations of the Lorentz transformations]] | [[Minkowski space]] | [[four-vector]] | [[world line]] | [[light cone]] | [[Lorentz group]] | [[Poincaré group]] | [[geometry]] | [[tensors]] | [[split-complex number]] | [[Algebra of physical space|Relativity in the APS formalism]]
:'''Philosophy''': [[actualism]] | [[conventionalism]] | [[Formalism (mathematics)|formalism]]
:'''Paradoxes''': [[Twin paradox]] | [[Ehrenfest paradox]] | [[Ladder paradox]] | [[Bell's spaceship paradox]] | [[Mocanu's velocity composition paradox|Velocity composition paradox]]
{{Tests of special relativity}}
{{Tensors}}
 
==References==
{{reflist|colwidth=35em}}
 
===Textbooks===
* Einstein, Albert (1920). [[s:Relativity: The Special and General Theory|Relativity: The Special and General Theory]].
* Einstein, Albert (1996). ''The Meaning of Relativity''. Fine Communications. ISBN 1-56731-136-9
* Logunov, Anatoly A. (2005) [http://arxiv.org/pdf/physics/0408077 Henri Poincaré and the Relativity Theory] (transl. from Russian by G. Pontocorvo and V. O. Soleviev, edited by V. A. Petrov) Nauka, Moscow.
* [[Charles Misner]], [[Kip Thorne]], and [[John Archibald Wheeler]] (1971) ''Gravitation''. W. H. Freeman & Co. ISBN 0-7167-0334-3
* Post, E.J., 1997 (1962) ''Formal Structure of Electromagnetics: General Covariance and Electromagnetics''. Dover Publications.
* [[Wolfgang Rindler]] (1991). Introduction to Special Relativity (2nd ed.), Oxford University Press. ISBN 978-0-19-853952-0; ISBN 0-19-853952-5
* Harvey R. Brown (2005). Physical relativity: space–time structure from a dynamical perspective, Oxford University Press, ISBN 0-19-927583-1; ISBN 978-0-19-927583-0
* {{Cite book| last =Qadir | first =Asghar | authorlink =Asghar Qadir | title =Relativity: An Introduction to the Special Theory | publisher =[[World Scientific|World Scientific Publications]] | year = 1989| location =Singapore | page =128 | url = http://books.google.com/?id=X5YofYrqFoAC&printsec=frontcover| isbn =9971-5-0612-2 }}
* Silberstein, Ludwik (1914) [[List of important publications in physics#The Theory of Relativity — Ludwik Silberstein|The Theory of Relativity]].
* {{cite book |title=Space, Time and Spacetime |author=Lawrence Sklar |url=http://books.google.com/?id=cPLXqV3QwuMC&pg=PA206 |isbn=0-520-03174-1 |publisher=University of California Press |year=1977 }}
* {{cite book |title=Philosophy of Physics |author=Lawrence Sklar |url=http://books.google.com/?id=L3b_9PGnkMwC&pg=PA74 |isbn=0-8133-0625-6 |publisher=Westview Press |year=1992 }}
* Taylor, Edwin, and [[John Archibald Wheeler]] (1992) ''Spacetime Physics'' (2nd ed.). W.H. Freeman & Co. ISBN 0-7167-2327-1
* Tipler, Paul, and Llewellyn, Ralph (2002). ''Modern Physics'' (4th ed.). W. H. Freeman & Co. ISBN 0-7167-4345-0
 
===Journal articles===
* {{Cite journal | last1 = Alvager | first1 = ''et al.'' | author2 = Farley, F. J. M. | year = 1964 | author3 = Kjellman, J. | author4 = Wallin, L. | title = Test of the Second Postulate of Special Relativity in the GeV region | journal = Physics Letters | volume = 12 | issue = 3| page = 260 | doi = 10.1016/0031-9163(64)91095-9 | bibcode = 1964PhL....12..260A }}
* {{Cite journal | doi = 10.1086/430652 | last1 = Darrigol | first1 = Olivier | year = 2004 | title = The Mystery of the Poincaré–Einstein Connection | journal = Isis | volume = 95 | issue = 4| pages = 614–26 | pmid = 16011297 }}
* {{Cite journal | doi = 10.1103/PhysRevA.56.4405 | last1 = Wolf | first1 = Peter | last2 = Petit | first2 = Gerard | year = 1997 | title = Satellite test of Special Relativity using the Global Positioning System | journal = Physical Review A | volume = 56 | issue = 6| pages = 4405–09 |bibcode = 1997PhRvA..56.4405W }}
 
==External links==
{{Wikisource portal|Relativity}}
{{Wikibooks|Special Relativity}}
{{Wikiversity|Special Relativity}}
{{Wiktionary|special relativity}}
 
===Original works===
* [http://www.physik.uni-augsburg.de/annalen/history/einstein-papers/1905_17_891-921.pdf ''Zur Elektrodynamik bewegter Körper''] Einstein's original work in German, [[Annalen der Physik]], [[Bern]] 1905
* [http://www.fourmilab.ch/etexts/einstein/specrel/specrel.pdf ''On the Electrodynamics of Moving Bodies''] English Translation as published in the 1923 book ''The Principle of Relativity''.
 
===Special relativity for a general audience (no mathematical knowledge required)===
* [http://en.wikibooks.org/wiki/Special_Relativity Wikibooks: Special Relativity]
* [http://www.phys.unsw.edu.au/einsteinlight Einstein Light] An [http://www.sciam.com/article.cfm?chanID=sa004&articleID=0005CFF9-524F-1340-924F83414B7F0000 award]-winning, non-technical introduction (film clips and demonstrations) supported by dozens of pages of further explanations and animations, at levels with or without mathematics.
* [http://www.einstein-online.info/en/elementary/index.html Einstein Online] Introduction to relativity theory, from the Max Planck Institute for Gravitational Physics.
* Audio: Cain/Gay (2006) – [http://www.astronomycast.com/astronomy/einsteins-theory-of-special-relativity/ Astronomy Cast]. Einstein's Theory of Special Relativity
 
===Special relativity explained (using simple or more advanced mathematics)===
<!--[dead link] * [http://www.geocities.com/autotheist/Bondi/intro.htm Bondi K-Calculus] – A simple introduction to the special theory of relativity. -->
* [http://gregegan.customer.netspace.net.au/FOUNDATIONS/01/found01.html Greg Egan's ''Foundations''].
* [http://cosmo.nyu.edu/hogg/sr/ The Hogg Notes on Special Relativity] A good introduction to special relativity at the undergraduate level, using calculus.
* [http://www.relativitycalculator.com/E=mc2.shtml Relativity Calculator: Special Relativity] – An algebraic and integral calculus derivation for {{nowrap|1=''E'' = ''mc''<sup>2</sup>}}.
* [http://www.mathpages.com/rr/rrtoc.htm MathPages – Reflections on Relativity] A complete online book on relativity with an extensive bibliography.
* [http://www.lightandmatter.com/html_books/0sn/ch07/ch07.html Relativity] An introduction to special relativity at the undergraduate level, without calculus.
* {{gutenberg|no=5001|name=Relativity: the Special and General Theory}}, by [[Albert Einstein]]
* [http://www.phys.vt.edu/~takeuchi/relativity/notes Special Relativity Lecture Notes] is a standard introduction to special relativity containing illustrative explanations based on drawings and spacetime diagrams from Virginia Polytechnic Institute and State University.
* [http://www.rafimoor.com/english/SRE.htm Understanding Special Relativity] The theory of special relativity in an easily understandable way.
* [http://digitalcommons.unl.edu/physicskatz/49/ An Introduction to the Special Theory of Relativity] (1964) by Robert Katz, "an introduction ... that is accessible to any student who has had an introduction to general physics and some slight acquaintance with the calculus" (130 pp; pdf format).
* [http://www.physics.mq.edu.au/~jcresser/Phys378/LectureNotes/VectorsTensorsSR.pdf Lecture Notes on Special Relativity] by J D Cresser Department of Physics Macquarie University.
 
===Visualization===
* [http://www.hakenberg.de/diffgeo/special_relativity.htm Raytracing Special Relativity] Software visualizing several scenarios under the influence of special relativity.
* [http://www.anu.edu.au/Physics/Savage/RTR/ Real Time Relativity] The Australian National University. Relativistic visual effects experienced through an interactive program.
* [http://www.spacetimetravel.org Spacetime travel] A variety of visualizations of relativistic effects, from relativistic motion to black holes.
* [http://www.anu.edu.au/Physics/Savage/TEE/ Through Einstein's Eyes] The Australian National University. Relativistic visual effects explained with movies and images.
* [http://www.adamauton.com/warp/ Warp Special Relativity Simulator] A computer program to show the effects of traveling close to the speed of light.
* [http://www.youtube.com/watch?v=C2VMO7pcWhg Animation clip] visualizing the Lorentz transformation.
* [http://math.ucr.edu/~jdp/Relativity/SpecialRelativity.html Original interactive FLASH Animations] from John de Pillis illustrating Lorentz and Galilean frames, Train and Tunnel Paradox, the Twin Paradox, Wave Propagation, Clock Synchronization, etc.
* [http://www.anu.edu.au/physics/Searle/ Relativistic Optics at the ANU]
* [http://lightspeed.sourceforge.net/ lightspeed] An OpenGL-based program developed to illustrate the effects of special relativity on the appearance of moving objects.
 
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Latest revision as of 08:32, 18 December 2014

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