View source for Relativistic Doppler effect

{{short description|Scientific phenomenon}}
[[Image:Velocity0 70c.jpg|thumb|right|Figure 1. A source of light waves moving to the right, relative to observers, with velocity 0.7c. The frequency is higher for observers on the right, and lower for observers on the left.]]
The '''relativistic Doppler effect''' is the change in [[frequency]] (and [[wavelength]]) of light, caused by the relative motion of the source and the observer (as in the classical [[Doppler effect]]), when taking into account effects described by the [[special relativity|special theory of relativity]]. 

The relativistic Doppler effect is different from the non-relativistic [[Doppler effect]] as the equations include the [[time dilation]] effect of [[special relativity]] and do not involve the medium of propagation as a reference point. They describe the total difference in observed frequencies and possess the required [[Lorentz symmetry]].

Astronomers know of three sources of [[redshift]]/[[blueshift]]: Doppler shifts; [[gravitational redshift]]s (due to light exiting a gravitational field); and [[cosmological expansion]] (where space itself stretches). This article concerns itself only with Doppler shifts.

== Summary of major results ==
{| class="wikitable"
|-
! Scenario !! Formula !! Notes
|-
| [[#Relativistic longitudinal Doppler effect|Relativistic longitudinal<br/>Doppler effect]]
| <math>\frac{\lambda_r}{\lambda_s} = \frac{f_s}{f_r} = \sqrt{\frac{1 + \beta}{1 - \beta}}</math>
| 
|-
| [[#Source and receiver are at their points of closest approach|Transverse Doppler effect,<br/>geometric closest approach]]
| <math>f_r = \gamma f_s</math>
| Blueshift
|-
| [[#Receiver sees the source as being at its closest point|Transverse Doppler effect,<br/>visual closest approach]]
| <math>f_r = \frac{f_s}{\gamma}</math>
| Redshift
|-
| [[#One object in circular motion around the other|TDE, receiver in circular<br/>motion around source]]
| <math>f_r = \gamma f_s</math>
| Blueshift
|-
| [[#One object in circular motion around the other|TDE, source in circular<br/>motion around receiver]]
| <math>f_r = \frac{f_s}{\gamma}</math>
| Redshift
|-
|[[#Source and receiver both in circular motion around a common center|TDE, source and receiver<br/>in circular motion around<br/>common center]]
|<math> \frac{\nu '}{\nu} = \left( \frac{1 - R^2 \omega ^2 }{ 1 - R' ^2 \omega ^2 } \right) ^{1/2} </math>
|No Doppler shift<br/>when <math>R = R'</math> 
|-
| [[#Motion in an arbitrary direction|Motion in arbitrary direction<br/>measured in receiver frame]]
| <math> f_r = \frac{f_s}{\gamma\left(1 + \beta \cos\theta_r\right)}</math>
| 
|-
| [[#Einstein Doppler shift equation|Motion in arbitrary direction<br/>measured in source frame]]
| <math> f_r = \gamma \left(  1 - \beta \cos \theta_s \right)  f_s</math>
| 
|}

== Derivation ==
=== {{anchor|Motion along the line of sight}} Relativistic longitudinal Doppler effect===

Relativistic Doppler shift for the longitudinal case, with source and receiver moving directly towards or away from each other, is often derived as if it were the classical phenomenon, but modified by the addition of a [[time dilation]] term.<ref>{{cite journal |last1=Sher |first1=D. |title=The Relativistic Doppler Effect |journal=Journal of the Royal Astronomical Society of Canada |date=1968 |volume=62 |pages=105–111 |url=http://adsbit.harvard.edu//full/1968JRASC..62..105S/0000105.000.html |accessdate=11 October 2018}}</ref><ref name="Gill">{{cite book |last1=Gill |first1=T. P. |title=The Doppler Effect |date=1965 |publisher=Logos Press Limited |location=London |pages=6–9 |url=https://openlibrary.org/books/OL5947329M/The_Doppler_effect |accessdate=12 October 2018}}</ref> This is the approach employed in first-year physics or mechanics textbooks such as those by Feynman<ref name=Feynman>{{cite book| title=The Feynman Lectures on Physics: Volume 1 | publisher=[[Addison-Wesley]] | location=Reading, Massachusetts |date=February 1977 | last1=Feynman | first1=Richard P. | authorlink1=Richard Feynman | last2=Leighton | first2=Robert B. | authorlink2=Robert B. Leighton | last3=Sands | first3=Matthew | authorlink3=Matthew Sands | lccn=2010938208 | isbn=9780201021165 | pages=34–7 f |chapter-url=http://www.feynmanlectures.caltech.edu/I_34.html |chapter=Relativistic Effects in Radiation}}</ref> or Morin.<ref name=Morin/>

Following this approach towards deriving the relativistic longitudinal Doppler effect, assume the receiver and the source are moving ''away'' from each other with a relative speed <math>v\,</math> as measured by an observer on the receiver or the source (The sign convention adopted here is that <math>v\,</math> is ''negative'' if the receiver and the source are moving ''towards'' each other).

Consider the problem in the [[Frame of reference|reference frame]] of the source.

Suppose one [[wavefront]] arrives at the receiver. The next wavefront is then at a distance <math>\lambda_s = c/f_s\,</math> away from the receiver (where <math>\lambda_s\,</math> is the [[wavelength]], <math>f_s\,</math> is the [[frequency]] of the waves that the source emits, and <math>c\,</math> is the [[speed of light]]).

The wavefront moves with speed <math>c\,</math>, but at the same time the receiver moves away with speed <math>v</math> during a time <math>t_s = 1/f_s = \lambda_s/c</math>, so<math display="block">\lambda_s + vt_{r,s} = ct_{r,s} \Longleftrightarrow \lambda_s = ct_{r,s}(1-v/c) \Longleftrightarrow t_{r,s} = \frac{1}{f_s (1-\beta)},</math>where <math>\beta = v / c\,</math> is [[beta (velocity)|the speed of the receiver in terms of the speed of light]], and where <math>t_{r,s}</math> is the period of light waves impinging on the receiver, ''as observed in the frame of the source.'' The corresponding frequency <math>f_{r,s}</math>is:

:<math>f_{r,s} = 1/t_{r,s} = f_s(1-\beta).</math>

Thus far, the equations have been identical to those of the classical Doppler effect with a stationary source and a moving receiver.

However, due to relativistic effects, clocks on the receiver are [[time dilation|time dilated]] relative to clocks at the source: <math>t_r = t_{r,s}/ \gamma</math>, where <math display="inline">\gamma = 1/\sqrt{1 - \beta^2}</math> is the [[Lorentz factor]]. In order to know which time is dilated, we recall that <math>t_{r,s}</math> is the time in the frame in which the source is at rest. The receiver will measure the received frequency to be

:{{EquationRef|1|Eq. 1:}} &nbsp;&nbsp; <math>f_r = f_{r,s} \gamma </math> <math>= \frac{1 - \beta}{\sqrt{1-\beta^2}} f_s </math> <math>= \sqrt{\frac{1 - \beta}{1 + \beta}}\,f_s.</math>

The ratio
:<math>\frac{f_s}{f_r} = \sqrt{\frac{1 + \beta}{1 - \beta}}</math>
is called the '''Doppler factor''' of the source relative to the receiver. (This terminology is particularly prevalent in the subject of [[astrophysics]]: see [[relativistic beaming]].)

The corresponding [[wavelength]]s are related by
:{{EquationRef|2|Eq. 2:}} &nbsp;&nbsp; <math>\frac{\lambda_r}{\lambda_s} = \frac{f_s}{f_r} = \sqrt{\frac{1 + \beta}{1 - \beta}},</math>

Identical expressions for relativistic Doppler shift are obtained when performing the analysis in the reference frame of the ''receiver'' with a moving source. This matches up with the expectations of the [[principle of relativity]], which dictates that the result can not depend on which object is considered to be the one at rest. In contrast, the classic nonrelativistic Doppler effect ''is'' dependent on whether it is the source or the receiver that is stationary with respect to the medium.<ref name=Feynman/><ref name=Morin/>

===Transverse Doppler effect===
Suppose that a source and a receiver are both approaching each other in uniform inertial motion along paths that do not collide. The ''transverse Doppler effect'' (TDE) may refer to (a) the nominal [[blueshift]] predicted by [[special relativity]] that occurs when the emitter and receiver are at their points of closest approach; or (b) the nominal [[redshift]] predicted by special relativity when the receiver '''sees''' the emitter as being at its closest approach.<ref name=Morin>{{cite book |title=Introduction to Classical Mechanics: With Problems and Solutions |chapter=Chapter 11: Relativity (Kinematics) |chapter-url=http://www.people.fas.harvard.edu/~djmorin/chap11.pdf |first1=David |last1=Morin |publisher=[[Cambridge University Press]] |year=2008 |isbn=978-1-139-46837-4 |pages=539–543 |archiveurl=https://web.archive.org/web/20180404002006/http://www.people.fas.harvard.edu/~djmorin/chap11.pdf |archivedate=4 Apr 2018}}</ref> The transverse Doppler effect is one of the main novel predictions of the special theory of relativity.

Whether a scientific report describes TDE as being a redshift or blueshift depends on the particulars of the experimental arrangement being related. For example, Einstein's original description of the TDE in 1907 described an experimenter looking at the center (nearest point) of a beam of "[[canal rays]]" (a beam of positive ions that is created by certain types of gas-discharge tubes). According to special relativity, the moving ions' emitted frequency would be reduced by the Lorentz factor, so that the received frequency would be reduced (redshifted) by the same factor.<ref group=p name=Einstein1907>{{cite journal |last=Einstein |first=Albert |year=1907 |title= On the Possibility of a New Test of the Relativity Principle (Über die Möglichkeit einer neuen Prüfung des Relativitätsprinzips) |journal=[[Annalen der Physik]] |volume=328 |issue=6 |pages=197–198 |bibcode=1907AnP...328..197E|doi=10.1002/andp.19073280613 |url=https://einsteinpapers.press.princeton.edu/vol2-trans/246 }}</ref>{{refn|group=note|In his seminal paper of 1905 introducing special relativity, Einstein had already published an expression for the Doppler shift perceived by an observer moving at an arbitrary angle with respect to an infinitely distant source of light. Einstein's 1907 derivation of the TDE represented a trivial consequence of his earlier published general expression.<ref group=p name=Einstein1905>{{cite journal |last=Einstein |first=Albert |authorlink= |year=1905 |title=Zur Elektrodynamik bewegter Körper  |language=de |journal=[[Annalen der Physik]] |volume=322 |issue=10 |pages=891–921 |doi=10.1002/andp.19053221004 |url= |accessdate= |quote= |bibcode = 1905AnP...322..891E }} [http://www.fourmilab.ch/etexts/einstein/specrel/www/ English translation: ‘On the Electrodynamics of Moving Bodies’]</ref>}}

On the other hand, Kündig (1963) described an experiment where a [[Mössbauer effect|Mössbauer absorber]] was spun in a rapid circular path around a central Mössbauer emitter.<ref group=p name=Kundig>{{cite journal|author=Kündig, Walter|date=1963|title=Measurement of the Transverse Doppler Effect in an Accelerated System|journal=Physical Review|volume=129|issue=6|pages=2371–2375|doi=10.1103/PhysRev.129.2371|bibcode = 1963PhRv..129.2371K }}</ref> As explained below, this experimental arrangement resulted in Kündig's measurement of a blueshift.

==== Source and receiver are at their points of closest approach ====
[[File:Transverse Doppler effect scenarios 3.svg|thumb|300px|Figure 2. Source and receiver are at their points of closest approach. (a) Analysis in the frame of the receiver. (b) Analysis in the frame of the source.]]
In this scenario, the point of closest approach is frame-independent and represents the moment where there is no change in distance versus time. Figure&nbsp;2 demonstrates that the ease of analyzing this scenario depends on the frame in which it is analyzed.<ref name=Morin/> 
* Fig.&nbsp;2a. If we analyze the scenario in the frame of the receiver, we find that the analysis is more complicated than it should be. The apparent position of a celestial object is displaced from its true position (or geometric position) because of the object's motion during the time it takes its light to reach an observer. The source would be time-dilated relative to the receiver, but the redshift implied by this time dilation would be offset by a blueshift due to the longitudinal component of the relative motion between the receiver and the apparent position of the source.
* Fig.&nbsp;2b. It is much easier if, instead, we analyze the scenario from the frame of the source. An observer situated at the source knows, from the problem statement, that the receiver is at its closest point to him. That means that the receiver has no longitudinal component of motion to complicate the analysis. (i.e. dr/dt = 0 where r is the distance between receiver and source) Since the receiver's clocks are time-dilated relative to the source, the light that the receiver receives is blue-shifted by a factor of gamma. In other words,

:{{EquationRef|3|Eq. 3:}} &nbsp;&nbsp; <math>f_r = \gamma f_s</math>

==== Receiver ''sees'' the source as being at its closest point ====
[[File:Transverse Doppler effect scenarios 4.svg|thumb|Figure 3. Transverse Doppler shift for the scenario where the receiver ''sees'' the source as being at its closest point.]]
This scenario is equivalent to the receiver looking at a direct right angle to the path of the source. The analysis of this scenario is best conducted from the frame of the receiver. Figure&nbsp;3 shows the receiver being illuminated by light from when the source was closest to the receiver, even though the source has moved on.<ref name=Morin/> Because the source's clock is time dilated as measured in the frame of the receiver, and because there is no longitudinal component of its motion, the light from the source, emitted from this closest point, is redshifted with frequency

:{{EquationRef|4|Eq. 4:}} &nbsp;&nbsp; <math>f_r = \frac{f_s}{\gamma}</math>

In the literature, most reports of transverse Doppler shift analyze the effect in terms of the receiver pointed at direct right angles to the path of the source, thus ''seeing'' the source as being at its closest point and observing a redshift.

==== Point of null frequency shift ====
[[File:Transverse Doppler effect scenarios 6.svg|thumb|300px|Figure 4. Null frequency shift occurs for a pulse that travels the shortest distance from source to receiver.]]
Given that, in the case where the inertially moving source and receiver are geometrically at their nearest approach to each other, the receiver observes a blueshift, whereas in the case where the receiver ''sees'' the source as being at its closest point, the receiver observes a redshift, there obviously must exist a point where blueshift changes to a redshift. In Fig.&nbsp;2, the signal travels perpendicularly to the receiver path and is blueshifted. In Fig.&nbsp;3, the signal travels perpendicularly to the source path and is redshifted.

As seen in Fig.&nbsp;4, null frequency shift occurs for a pulse that travels the shortest distance from source to receiver. When viewed in the frame where source and receiver have the same speed, this pulse is emitted perpendicularly to the source's path and is received perpendicularly to the receiver's path. The pulse is emitted slightly before the point of closest approach, and it is received slightly after.<ref name="Brown_b">{{cite web |last1=Brown |first1=Kevin S. |title=The Doppler Effect |url=https://www.mathpages.com/home/kmath587/kmath587.htm |publisher=Mathpages |accessdate=12 October 2018}}</ref>

==== One object in circular motion around the other ====
[[File:Transverse Doppler effect scenarios 5.svg|thumb|300px|Figure 5. Transverse Doppler effect for two scenarios: (a) receiver moving in a circle around the source; (b) source moving in a circle around the receiver.]]

Fig.&nbsp;5 illustrates two variants of this scenario. Both variants can be analyzed using simple time dilation arguments.<ref name=Morin/> Figure&nbsp;5a is essentially equivalent to the scenario described in Figure&nbsp;2b, and the receiver observes light from the source as being blueshifted by a factor of <math>\gamma.</math>. Figure&nbsp;5b is essentially equivalent to the scenario described in Figure&nbsp;3, and the light is redshifted.

The only seeming complication is that the orbiting objects are in accelerated motion. An accelerated particle does not have an inertial frame in which it is always at rest. However, an inertial frame can always be found which is momentarily comoving with the particle. This frame, the [[Proper reference frame (flat spacetime)|''momentarily comoving reference frame'' (MCRF)]], enables application of special relativity to the analysis of accelerated particles. If an inertial observer looks at an accelerating clock, only the clock's instantaneous speed is important when computing time dilation.<ref name=Misner>{{Cite book|author=Misner, C. W., Thorne, K. S., and Wheeler, J. A|title=Gravitation|year=1973|page=163|publisher=Freeman|isbn=978-0716703440}}</ref>

The converse, however, is not true. The analysis of scenarios where ''both'' objects are in accelerated motion requires a somewhat more sophisticated analysis. Not understanding this point has led to confusion and misunderstanding.

==== Source and receiver both in circular motion around a common center ====
[[File:Transverse Doppler effect scenarios 7.svg|thumb|Figure 6. Source and receiver are placed on opposite ends of a rotor, equidistant from the center.]]
Suppose source and receiver are located on opposite ends of a spinning rotor, as illustrated in Fig.&nbsp;6. Kinematic arguments (special relativity) and arguments based on noting that there is no difference in potential between source and receiver in the pseudogravitational field of the rotor (general relativity) both lead to the conclusion that there should be no Doppler shift between source and receiver.

In 1961, Champeney and [[Philip Burton Moon|Moon]] conducted a [[Mössbauer effect|Mössbauer rotor experiment]]  testing exactly this scenario, and found that the Mössbauer absorption process was unaffected by rotation.<ref name="Champeney" group=p>{{cite journal |last1=Champeney |first1=D. C. |last2=Moon |first2=P. B. |title=Absence of Doppler Shift for Gamma Ray Source and Detector on Same Circular Orbit |journal=Proc. Phys. Soc. |date=1961 |volume=77 |issue=2 |pages=350–352|doi=10.1088/0370-1328/77/2/318 }}</ref> They concluded that their findings supported special relativity.

This conclusion generated some controversy. A certain persistent critic of relativity maintained that, although the experiment was consistent with general relativity, it refuted special relativity, his point being that since the emitter and absorber were in uniform relative motion, special relativity demanded that a Doppler shift be observed. The fallacy with this critic's argument was, as demonstrated in section [[#Point of null frequency shift|Point of null frequency shift]], that it is simply not true that a Doppler shift must always be observed between two frames in uniform relative motion.<ref name="Sama">{{cite journal |last1=Sama |first1=Nicholas |title=Some Comments on a Relativistic Frequency-Shift Experiment of Champeney and Moon |journal=American Journal of Physics |date=1969 |volume=37 |issue=8 |pages=832–833 |doi=10.1119/1.1975859}}</ref> Furthermore, as demonstrated in section [[#Source and receiver are at their points of closest approach|Source and receiver are at their points of closest approach]], the difficulty of analyzing a relativistic scenario often depends on the choice of reference frame. Attempting to analyze the scenario in the frame of the receiver involves much tedious algebra. It is much easier, almost trivial, to establish the lack of Doppler shift between emitter and absorber in the laboratory frame.<ref name="Sama"/>

As a matter of fact, however, Champeney and Moon's experiment said nothing either pro or con about special relativity. Because of the symmetry of the setup, it turns out that virtually ''any'' conceivable theory of the Doppler shift between frames in uniform inertial motion must yield a null result in this experiment.<ref name="Sama"/>

Rather than being equidistant from the center, suppose the emitter and absorber were at differing distances from the rotor's center. For an emitter at radius <math>R'</math> and the absorber at radius <math>R</math> ''anywhere'' on the rotor, the ratio of the emitter frequency, <math>\nu ' ,</math> and the absorber frequency, <math>\nu ,</math> is given by

:{{EquationRef|5|Eq. 5:}} &nbsp;&nbsp; <math> \frac{\nu '}{\nu} = \left( \frac{1 - R^2 \omega ^2 }{ 1 - R' ^2 \omega ^2 } \right) ^{1/2} </math>

where <math>\omega</math> is the angular velocity of the rotor. The source and emitter do not have to be 180° apart, but can be at any angle with respect to the center.<ref name="Synge" group=p>{{cite journal |last1=Synge |first1=J. L. |title=Group Motions in Space-time and Doppler Effects |journal=Nature |date=1963 |volume=198 |issue=4881 |page=679|doi=10.1038/198679a0 }}</ref><ref name="Keswani">{{cite book |last1=Keswani |first1=G. H. |title=Origin and Concept of Relativity |date=1965 |publisher=Alekh Prakashan |location=Delhi, India |pages=60–61 |url=https://books.google.com/books?id=_wKbOfv3bpQC&pg=PA60 |accessdate=13 October 2018}}</ref>

===Motion in an arbitrary direction===
[[File:Doppler shift with source and receiver moving at arbitrary angles.svg|thumb|Figure 7. Doppler shift with source moving at an arbitrary angle with respect to the line between source and receiver.]]

The analysis used in section [[#Relativistic longitudinal Doppler effect|Relativistic longitudinal Doppler effect]] can be extended in a straightforward fashion to calculate the Doppler shift for the case where the inertial motions of the source and receiver are at any specified angle.<ref name=Gill/><ref name=Brown_a>
{{cite web
| last=Brown |first=Kevin S.
| title = Doppler Shift for Sound and Light
| url = http://www.mathpages.com/rr/s2-04/2-04.htm
| publisher = Mathpages 
| accessdate = 6 August 2015
}}</ref>
Fig.&nbsp;7 presents the scenario from the frame of the receiver, with the source moving at speed <math>v</math> at an angle <math>\theta_r</math> measured in the frame of the receiver. The radial component of the source's motion along the line of sight is equal to <math>v \cos{\theta_r}.</math>

The equation below can be interpreted as the classical Doppler shift for a stationary and moving source modified by the Lorentz factor <math>\gamma :</math>

:{{EquationRef|6|Eq. 6:}} &nbsp;&nbsp; <math> f_r = \frac{f_s}{\gamma\left(1 + \beta \cos\theta_r\right)}.</math>

In the case when <math>\theta_r = 90^{\circ}</math> and <math>\cos\theta_r = 0 ,</math> one obtains the [[#Transverse Doppler effect|transverse Doppler effect]]:
:<math>f_r = \frac {f_s}  {\gamma}. \,</math>

{{anchor|Einstein Doppler shift equation}}
In his 1905 paper on special relativity,<ref group=p name=Einstein1905/> Einstein obtained a somewhat different looking equation for the Doppler shift equation. After changing the variable names in Einstein's equation to be consistent with those used here, his equation reads
:{{EquationRef|7|Eq. 7:}} &nbsp;&nbsp; <math> f_r = \gamma \left(  1 - \beta \cos \theta_s \right)  f_s.</math>

The differences stem from the fact that Einstein evaluated the angle <math>\theta_s</math> with respect to the source rest frame rather than the receiver rest frame. <math> \theta_s</math> is not equal to <math> \theta_r </math> because of the effect of [[relativistic aberration]]. The relativistic aberration equation is:

:{{EquationRef|8|Eq. 8:}} &nbsp;&nbsp; <math>\cos \theta_r=\frac{\cos \theta_s-\beta }{1-\beta \cos \theta_s} \,</math>

Substituting the relativistic aberration equation {{EquationNote|8|Equation&nbsp;8}} into  {{EquationNote|6|Equation&nbsp;6}} yields  {{EquationNote|7|Equation&nbsp;7}}, demonstrating the consistency of these alternate equations for the Doppler shift.<ref name=Brown_a/>

Setting <math>\theta_r = 0</math> in {{EquationNote|6|Equation&nbsp;6}} or <math>\theta_s = 0</math> in {{EquationNote|7|Equation&nbsp;7}} yields {{EquationNote|1|Equation&nbsp;1}}, the expression for relativistic longitudinal Doppler shift.

A four-vector approach to deriving these results may be found in Landau and Lifshitz (2005).<ref name=Landau>{{cite book| title=The Classical Theory of Fields | series=Course of Theoretical Physics: Volume 2 | last1=Landau | first1=L.D. | authorlink1=Lev Landau | last2=Lifshitz | first2=E.M. | authorlink2=Evgeny Lifshitz | others=Trans. Morton Hamermesh | publisher=Elsevier Butterworth-Heinemann | year=2005 | edition=Fourth revised English | pages=116–117 | isbn=9780750627689}}</ref>

==Visualization==
[[File:Compare01.jpg|thumb|300px|Figure 8. Comparison of the relativistic Doppler effect (top) with the non-relativistic effect (bottom).]]
Fig.&nbsp;8 helps us understand, in a rough qualitative sense, how the relativistic Doppler effect and [[relativistic aberration]] differ from the non-relativistic [[Doppler effect]] and non-relativistic [[aberration of light]]. Assume that the observer is uniformly surrounded in all directions by yellow stars emitting monochromatic light of 570&nbsp;nm. The arrows in each diagram represent the observer's velocity vector relative to its surroundings, with a magnitude of 0.89&nbsp;''c''. 
* In the non-relativistic case, the light ahead of the observer is blueshifted to a wavelength of 300&nbsp;nm in the medium ultraviolet, while light behind the observer is redshifted to 5200&nbsp;nm in the intermediate infrared. Because of the aberration of light, objects formerly at right angles to the observer appear shifted forwards by 42°. 
* In the relativistic case, the light ahead of the observer is blueshifted to a wavelength of 137&nbsp;nm in the far ultraviolet, while light behind the observer is redshifted to 2400&nbsp;nm in the short wavelength infrared. Because of the relativistic aberration of light, objects formerly at right angles to the observer appear shifted forwards by 63°.
* In both cases, the monochromatic stars ahead of and behind the observer are Doppler-shifted towards invisible wavelengths. If, however, the observer had eyes that could see into the ultraviolet and infrared, he would see the stars ahead of him as brighter and more closely clustered together than the stars behind, but the stars would be far brighter and far more concentrated in the relativistic case.<ref name="Savage1999">{{cite journal |last1=Savage |first1=C. M. |last2=Searle |first2=A. C. |title=Visualizing Special Relativity |journal=The Physicist |date=1999 |volume=36 |issue=141 |url=https://web.archive.org/web/20080803005209/http://www.anu.edu.au/Physics/Searle/paper2/paper2.pdf |accessdate=17 October 2018}}</ref>

Real stars are not monochromatic, but emit a range of wavelengths approximating a [[black body]] distribution. It is not necessarily true that stars ahead of the observer would show a bluer color. This is because the whole spectral energy distribution is shifted. At the same time that visible light is blueshifted into invisible ultraviolet wavelengths, infrared light is blueshifted into the visible range. Precisely what changes in the colors one sees depends on the physiology of the human eye and on the spectral characteristics of the light sources being observed.<ref name="Brandeker">{{cite web |last1=Brandeker |first1=Alexis |title=What would a relativistic interstellar traveller see? |url=http://www.math.ucr.edu/home/baez/physics/Relativity/SR/Spaceship/spaceship.html |website=Physics FAQ |publisher=Math Department, University of California, Riverside |accessdate=17 October 2018}}</ref><ref name="Kraus2000">{{cite journal |last1=Kraus |first1=U. |title=Brightness and color of rapidly moving objects: The visual appearance of a large sphere revisited |journal=Am. J. Phys. |date=2000 |volume=68 |pages=56–60 |url=https://www.spacetimetravel.org/sphere/sphere.pdf |accessdate=17 October 2018|doi=10.1119/1.19373 }}</ref>

==Doppler effect on intensity==
{{further|Black-body radiation#Doppler effect}}

The Doppler effect (with arbitrary direction) also modifies the perceived source intensity: this can be expressed concisely by the fact that source strength divided by the cube of the frequency is a Lorentz invariant<ref group="p">{{Cite journal| volume = 79| issue = 4| pages = 1340| last = Johnson| first = Montgomery H.| last2 = Teller| first2 = Edward| title = Intensity changes in the Doppler effect| journal = Proc. Natl. Acad. Sci. USA | date = February 1982|doi=10.1073/pnas.79.4.1340 | pmid = 16593162|bibcode = 1982PNAS...79.1340J | pmc=345964}}</ref><ref group=note>Here, "source strength" refers to [[spectral intensity]] in ''frequency'', i.e., power per unit solid angle and per unit frequency, expressed in watts per steradian per hertz; for spectral intensity in ''wavelength'', the cube should be replaced by a fifth power.</ref>  This implies that the total radiant intensity (summing over all frequencies) is multiplied by the fourth power of the Doppler factor for frequency.
<!-- It would be nice if someone could expand on this, for example by providing a derivation from the Lorentz transformation laws on the electric and magnetic fields applied to a plane wave. -->

As a consequence, since [[Planck's law]] describes the [[black-body radiation]] as having a spectral intensity in frequency proportional to <math>\nu^3/\left(e^{h\nu/kT} - 1\right)</math> (where ''T'' is the source temperature and ''ν'' the frequency), we can draw the conclusion that ''a black body spectrum seen through a Doppler shift (with arbitrary direction) is still a black body spectrum'' with a temperature multiplied by the same Doppler factor as frequency.

This result provides one of the pieces of evidence that serves to distinguish the [[Big Bang theory]] from alternative theories proposed to explain the [[cosmological redshift]].<ref name="Wright_tired_light">{{cite web |last1=Wright |first1=Edward L. ("Ned") |title=Errors in Tired Light Cosmology |url=http://www.astro.ucla.edu/~wright/tiredlit.htm |website=Ned Wright's Cosmology Tutorial |publisher=Astronomy Department, University of California, Los Angeles |accessdate=17 October 2018}}</ref>

== Experimental verification ==
{{Main|Ives–Stilwell experiment|l1=Ives–Stilwell-, Mössbauer rotor-, and Spectroscopy tests of time dilation}}

Since the transverse Doppler effect is one of the main novel predictions of the special theory of relativity, the detection and precise quantification of this effect has been an important goal of experiments attempting to validate special relativity.

=== Ives and Stilwell-type measurements ===
[[File:Ives-Stilwell rationale.svg|thumb|300px|Figure 9. Why it is difficult to measure the transverse Doppler effect accurately using a transverse beam.]]
Einstein (1907) had initially suggested that the TDE might be measured by observing a beam of "[[canal rays]]" at right angles to the beam.<ref group=p name=Einstein1907/> Attempts to measure TDE following this scheme proved it to be impractical, since the maximum speed of particle beam available at the time was only a few thousandths of the speed of light.

Fig.&nbsp;9 shows the results of attempting to measure the 4861 Angstrom line emitted by a beam of canal rays (a mixture of H1+, H2+, and H3+ ions) as they recombine with electrons stripped from the dilute hydrogen gas used to fill the Canal ray tube. Here, the predicted result of the TDE is a 4861.06 Angstrom line. On the left, longitudinal Doppler shift results in broadening the emission line to such an extent that the TDE cannot be observed. The middle figures illustrate that even if one narrows one's view to the exact center of the beam, very small deviations of the beam from an exact right angle introduce shifts comparable to the predicted effect.

Rather than attempt direct measurement of the TDE, [[Ives–Stilwell experiment|Ives and Stilwell (1938)]] used a concave mirror that allowed them to simultaneously observe a nearly longitudinal direct beam (blue) and its reflected image (red). Spectroscopically, three lines would be observed: An undisplaced emission line, and blueshifted and redshifted lines. The average of the redshifted and blueshifted lines would be compared with the wavelength of the undisplaced emission line. The difference that Ives and Stilwell measured corresponded, within experimental limits, to the effect predicted by special relativity.<ref group="p" name=Ives1938>{{cite journal |last=Ives |first=H. E. |authorlink= |author2=Stilwell, G. R.  |year=1938 |title=An experimental study of the rate of a moving atomic clock |journal=Journal of the Optical Society of America |volume=28 |issue=7 |pages=215 |bibcode=1938JOSA...28..215I |url= |accessdate= |doi=10.1364/JOSA.28.000215 }}</ref>

Various of the subsequent repetitions of the Ives and Stilwell experiment have adopted other strategies for measuring the mean of blueshifted and redshifted particle beam emissions. In some recent repetitions of the experiment, modern accelerator technology has been used to arrange for the observation of two counter-rotating particle beams. In other repetitions, the energies of gamma rays emitted by a rapidly moving particle beam have been measured at opposite angles relative to the direction of the particle beam. Since these experiments do not actually measure the wavelength of the particle beam at right angles to the beam, some authors have preferred to refer to the effect they are measuring as the "quadratic Doppler shift" rather than TDE.<ref group=p>{{cite journal |last1=Olin |first1=A. |last2=Alexander |first2=T. K. |last3=Häusser |first3=O. |last4=McDonald |first4=A. B. |last5=Ewan |first5=G. T. |title=Measurement of the Relativistic Doppler Effect Using 8.6-MeV Capture γ Rays |journal=Phys. Rev. D |date=1973 |volume=8 |issue=6 |pages=1633–1639 |doi=10.1103/PhysRevD.8.1633}}</ref><ref group=p>{{cite journal |last1=Mandelberg |first1=Hirsch I. |last2=Witten |first2=Louis |title=Experimental Verification of the Relativistic Doppler Effect |journal=Journal of the Optical Society of America |date=1962 |volume=52 |issue=5 |pages=529–535 |doi=10.1364/JOSA.52.000529}}</ref>

=== Direct measurement of transverse Doppler effect ===
The advent of [[particle accelerator]] technology has made possible the production of particle beams of considerably higher energy than was available to Ives and Stilwell. This has enabled the design of tests of the transverse Doppler effect directly along the lines of how Einstein originally envisioned them, i.e. by directly viewing a particle beam at a 90° angle. For example, Hasselkamp et al. (1979) observed the ''H''α line emitted by hydrogen atoms moving at speeds ranging from 2.53×10<sup>8</sup>&nbsp;cm/s to 9.28×10<sup>8</sup>&nbsp;cm/s, finding the coefficient of the second order term in the relativistic approximation to be 0.52±0.03, in excellent agreement with the theoretical value of 1/2.<ref group=p>{{cite journal |last1=Hasselkamp |first1=D. |last2=Mondry |first2=E. |last3=Sharmann |first3=A. |title=Direct observation of the transversal Doppler-shift |journal=Zeitschrift für Physik A |date=1979 |volume=289 |issue=2 |pages=151–155|doi=10.1007/BF01435932 }}</ref>

Other direct tests of the TDE on rotating platforms were made possible by the discovery of the [[Mössbauer effect]], which enables the production of exceedingly narrow resonance lines for nuclear gamma ray emission and absorption.<ref>{{cite book |author1=Saburo Nasu |editor1-last=Yoshida |editor1-first=Yutaka |editor2-last=Langouche |editor2-first=Guido |title=Mössbauer Spectroscopy: Tutorial Book |publisher=Springer |isbn=978-3642322198 |pages=1–22 |year=2013 |chapter=General Introduction to Mössbauer Spectroscopy}}</ref> Mössbauer effect experiments have proven themselves easily capable of detecting TDE using emitter-absorber relative velocities on the order of 2×10<sup>4</sup>&nbsp;cm/s. These experiments include ones performed by Hay ''et al.'' (1960),<ref group=p>{{cite journal|author1=Hay, H. J. |author2=Schiffer, J. P. |author3=Cranshaw, T. E. |author4=Egelstaff, P. A. |date=1960|title=Measurement of the Red Shift in an Accelerated System Using the Mössbauer Effect in <sup>57</sup>Fe|journal=Physical Review Letters|volume=4|issue=4|pages=165–166|doi=10.1103/PhysRevLett.4.165|bibcode=1960PhRvL...4..165H}}</ref> Champeney ''et al.'' (1965),<ref group=p>{{cite journal|author1=Champeney, D. C. |author2=Isaak, G. R. |author3=Khan, A. M. |date=1965|title=A time dilatation experiment based on the Mössbauer effect|journal=Proceedings of the Physical Society|volume=85|issue=3|pages=583–593|doi=10.1088/0370-1328/85/3/317|bibcode = 1965PPS....85..583C }}</ref> and Kündig (1963).<ref group=p name=Kundig/>

=== Time dilation measurements ===
The transverse Doppler effect and the kinematic time dilation of special relativity are closely related. All validations of TDE represent validations of kinematic time dilation, and most validations of kinematic time dilation have also represented validations of TDE. An online resource, "What is the experimental basis of Special Relativity?" has documented, with brief commentary, many of the tests that, over the years, have been used to validate various aspects of special relativity.<ref name="Roberts">{{cite web |last1=Roberts |first1=Tom |last2=Schleif |first2=Siegmar |title=What is the experimental basis of Special Relativity? |url=http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html |website=The Original Usenet Physics FAQ |publisher=Department of Mathematics, University of California, Riverside |accessdate=16 October 2018}}</ref> Kaivola et al. (1985)<ref group=p>{{cite journal |last1=Kaivola |first1=Matti |last2=Riis |first2=Erling |last3=Lee |first3=Siu Au |title=Measurement of the Relativistic Doppler Shift in Neon |journal=Phys. Rev. Lett. |date=1985 |volume=54 |issue=4 |page=255 |doi=10.1103/PhysRevLett.54.255|url=https://aaltodoc.aalto.fi:443/bitstream/123456789/15656/1/A1_kaivola_matti_1985.pdf }}</ref> and McGowan et al. (1993)<ref group=p>{{cite journal |last1=McGowan |first1=Roger W. |last2=Giltner |first2=David M. |last3=Sternberg |first3=Scott J. |last4=Lee |first4=Siu Au |title=New measurement of the relativistic Doppler shift in neon |journal=Phys. Rev. Lett. |date=1993 |volume=70 |issue=3 |pages=251–254 |doi=10.1103/PhysRevLett.70.251|pmid=10054065 }}</ref> are examples of experiments classified in this resource as time dilation experiments. These two also represent tests of TDE. These experiments compared the frequency of two lasers, one locked to the frequency of a  neon atom transition in a fast beam, the other locked to the same transition in thermal neon. The 1993 version of the experiment verified time dilation, and hence TDE, to an accuracy of 2.3×10<sup>−6</sup>.

== Relativistic Doppler effect for sound and light ==
[[File:Doppler spacetime diagram for sound.svg|thumb|Figure 10. The relativistic Doppler shift formula is applicable to both sound and light.]]
First-year physics textbooks almost invariably analyze Doppler shift for sound in terms of Newtonian kinematics, while analyzing Doppler shift for light and electromagnetic phenomena in terms of relativistic kinematics. This gives the false impression that acoustic phenomena requires a different analysis than light and radio waves.

The traditional analysis of the Doppler effect for sound represents a low speed approximation to the exact, relativistic analysis. The fully relativistic analysis for sound is, in fact, equally applicable to both sound and electromagnetic phenomena.

Consider the spacetime diagram in Fig.&nbsp;10. Worldlines for a tuning fork (the source) and a receiver are both illustrated on this diagram. Events ''O'' and ''A'' represent two vibrations of the tuning fork. The period of the fork is the magnitude of ''OA'', and the inverse slope of ''AB'' represents the speed of signal propagation (i.e. the speed of sound) to event ''B''. We can therefore write:<ref name=Brown_a/>

: <math>c_s = \frac{x_B - x_A}{t_B - t_A}</math> (speed of sound)
: <math>v_s = -\frac{x_A}{t_A} </math> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <math> v_r = \frac{x_B}{t_B} </math> &nbsp;&nbsp;&nbsp;&nbsp; (speeds of source and receiver)
: <math> |OA| = \sqrt{ t_A^2 - (x_A/c)^2 }</math>
: <math> |OB| = \sqrt{ t_B^2 - (x_B/c)^2 }</math>

<math>v_s</math> and <math>v_r</math> are assumed to be less than <math>c_s,</math> since otherwise their passage through the medium will set up shock waves, invalidating the calculation. Some routine algebra gives the ratio of frequencies:

:{{EquationRef|9|Eq. 9:}} &nbsp;&nbsp; <math> \frac{f_r}{f_s} = \frac{|OA|}{|OB|} </math> <math> = \frac{1 - v_r/c_s}{1+v_s/c_s} \sqrt{ \frac{1 - (v_s/c)^2 }{ 1-(v_r/c)^2 }} </math>

If <math>v_r</math> and <math>v_s</math> are small compared with <math>c</math>, the above equation reduces to the classical Doppler formula for sound.

If the speed of signal propagation <math>c_s</math> approaches <math>c</math>, it can be shown that the absolute speeds <math>v_s</math> and <math>v_r</math> of the source and receiver merge into a single relative speed independent of any reference to a fixed medium. Indeed, we obtain {{EquationNote|1|Equation&nbsp;1}}, the formula for relativistic longitudinal Doppler shift.<ref name=Brown_a/>

Analysis of the spacetime diagram in Fig.&nbsp;10 gave a general formula for source and receiver moving directly along their line of sight, i.e. in collinear motion.

[[File:Doppler shift for sound with moving source and receiver.svg|thumb|Figure 11. A source and receiver are moving in different directions and speeds in a frame where the speed of sound is independent of direction.]]
Fig.&nbsp;11 illustrates a scenario in two dimensions. The source moves with velocity <math> \mathbf{v_{s}}</math> (at the time of emission). It emits a signal which travels at velocity <math>\mathbf{C}</math> towards the receiver, which is traveling at velocity <math>\mathbf{v_r}</math> at the time of reception. The analysis is performed in a coordinate system in which the signal's speed <math>|\mathbf{C}|</math> is independent of direction.<ref name=Brown_b/>

The ratio between the proper frequencies for the source and receiver is

:{{EquationRef|10|Eq. 10:}} &nbsp;&nbsp; <math> \frac{f_r}{f_s} 
= \frac{1 - \frac{|\mathbf{v_r}|}{\mathbf{|C|}} \cos ( \theta_\mathbf{C,v_r}) } 
       {1 - \frac{|\mathbf{v_s}|}{\mathbf{|C|}} \cos ( \theta_\mathbf{C,v_s})  
       }  \sqrt{ \frac{1-(v_s/c)^2}{ 1-(v_r/c)^2 } } </math>

The leading ratio has the form of the classical Doppler effect, while the square root term represents the relativistic correction. If we consider the angles relative to the frame of the source, then <math>v_s = 0</math> and the equation reduces to {{EquationNote|7|Equation&nbsp;7}}, Einstein's 1905 formula for the Doppler effect. If we consider the angles relative to the frame of the receiver, then <math>v_r = 0</math> and the equation reduces to {{EquationNote|6|Equation&nbsp;6}}, the alternative form of the Doppler shift equation discussed previously.<ref name=Brown_b/>

== See also ==
*[[Doppler effect]]
*[[Doppler beaming]]
*[[Redshift]]
*[[Blueshift]]
*[[Time dilation]]
*[[Gravitational time dilation]]
*[[Special relativity]]

== Notes ==

{{Reflist|group="note"}}

== Primary sources ==

{{Reflist|group="p"}}

== References ==

{{Reflist}}

== Further reading ==
* {{cite journal|last1=Moriconi|first1=M|title=Special theory of relativity through the Doppler effect|journal=European Journal of Physics|date=1 November 2006|volume=27|issue=6|pages=1409–1423|doi=10.1088/0143-0807/27/6/015|arxiv=physics/0605204|bibcode=2006EJPh...27.1409M}}

== External links ==
*[http://adamauton.com/warp/ Warp Special Relativity Simulator] Computer program demonstrating the relativistic Doppler effect.
* {{cite web |last1=Kraus |first1=Ute |last2=Zahn |first2=Corvin |title=Space Time Travel: Visualization of the theory of relativity |url=https://www.spacetimetravel.org/ |website=SpacetimeTravel.org |publisher=Physics and Astronomy Education Group, Hildesheim University, Germany |accessdate=17 October 2018}}

{{Relativity}}

{{DEFAULTSORT:Relativistic Doppler Effect}}
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[[Category:Special relativity]]
[[Category:Doppler effects]]