Acceleration (special relativity)

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Accelerations in special relativity (SR) follow, as in Newtonian Mechanics, by differentiation of velocity with respect to time. Because of the Lorentz transformation and time dilation, the concepts of time and distance become more complex, which also leads to more complex definitions of "acceleration". SR as the theory of flat Minkowski spacetime remains valid in the presence of accelerations, because general relativity (GR) is only required when there is curvature of spacetime caused by the energy-momentum tensor (which is mainly determined by mass). However, since the amount of spacetime curvature is not particularly high on Earth or its vicinity, SR remains valid for most practical purposes, such as experiments in particle accelerators.[1]

One can derive transformation formulas for ordinary accelerations in three spatial dimensions (three-acceleration or coordinate acceleration) as measured in an external inertial frame of reference, as well as for the special case of proper acceleration measured by a comoving accelerometer. Another useful formalism is four-acceleration, as its components can be connected in different inertial frames by a Lorentz transformation. Also equations of motion can be formulated which connect acceleration and force. Equations for several forms of acceleration of bodies and their curved world lines follow from these formulas by integration. Well known special cases are hyperbolic motion for constant longitudinal proper acceleration or uniform circular motion. Eventually, it is also possible to describe these phenomena in accelerated frames in the context of special relativity, see Proper reference frame (flat spacetime). In such frames, effects arise which are analogous to homogeneous gravitational fields, which have some formal similarities to the real, inhomogeneous gravitational fields of curved spacetime in general relativity. In the case of hyperbolic motion one can use Rindler coordinates, in the case of uniform circular motion one can use Born coordinates.

Concerning the historical development, relativistic equations containing accelerations can already be found in the early years of relativity, as summarized in early textbooks by Max von Laue (1911, 1921)[2] or Wolfgang Pauli (1921).[3] For instance, equations of motion and acceleration transformations were developed in the papers of Hendrik Antoon Lorentz (1899, 1904),[H 1][H 2] Henri Poincaré (1905),[H 3][H 4] Albert Einstein (1905),[H 5] Max Planck (1906),[H 6] and four-acceleration, proper acceleration, hyperbolic motion, accelerating reference frames, Born rigidity, have been analyzed by Einstein (1907),[H 7] Hermann Minkowski (1907, 1908),[H 8][H 9] Max Born (1909),[H 10] Gustav Herglotz (1909),[H 11][H 12] Arnold Sommerfeld (1910),[H 13][H 14] von Laue (1911),[H 15][H 16] Friedrich Kottler (1912, 1914),[H 17] see section on history.


In accordance with both Newtonian mechanics and SR, three-acceleration or coordinate acceleration is the first derivative of velocity with respect to coordinate time or the second derivative of the location with respect to coordinate time:


However, the theories sharply differ in their predictions in terms of the relation between three-accelerations measured in different inertial frames. In Newtonian mechanics, time is absolute by in accordance with the Galilean transformation, therefore the three-acceleration derived from it is equal too in all inertial frames:[4]


On the contrary in SR, both and depend on the Lorentz transformation, therefore also three-acceleration and its components vary in different inertial frames. When the relative velocity between the frames is directed in the x-direction by with as Lorentz factor, the Lorentz transformation has the form






or for arbitrary velocities of magnitude :[5]






In order to find out the transformation of three-acceleration, one has to differentiate the spatial coordinates and of the Lorentz transformation with respect to and , from which the transformation of three-velocity (also called velocity-addition formula) between and follows, and eventually by another differentiation with respect to and the transformation of three-acceleration between and follows. Starting from (1a), this procedure gives the transformation where the accelerations are parallel (x-direction) or perpendicular (y-, z-direction) to the velocity:[6][7][8][9][H 4][H 15]






or starting from (1b) this procedure gives the result for the general case of arbitrary directions of velocities and accelerations:[10][11]






This means, if there are two inertial frames and with relative velocity , then in the acceleration of an object with momentary velocity is measured, while in the same object is accelerated by and has the momentary velocity . As with the velocity addition formulas, also these acceleration transformations guarantee that the resultant speed of the accelerated object can never reach or surpass the speed of light.


If four-vectors are used instead of three-vectors, namely as four-position and as four-velocity, then the four-acceleration of an object is obtained by differentiation with respect to proper time instead of coordinate time:[12][13][14]






where is the object's three-acceleration and its momentary three-velocity of magnitude with the corresponding Lorentz factor . If only the spatial part is considered, and when the velocity is directed in the x-direction by and only accelerations parallel (x-direction) or perpendicular (y-, z-direction) to the velocity are considered, the expression is reduced to:[15][16]

Unlike the three-acceleration previously discussed, it is not necessary to derive a new transformation for four-acceleration, because as with all four-vectors, the components of in two inertial frames with relative speed are connected by a Lorentz transformation. Thus replacing by in (1a) gives:[17]

or replacing by in (1b) gives the transformation at arbitrary relative velocity :


On the other hand, the inner product with metric signature (−,+,+,+) and consequently its magnitude is invariant, thus:[16][13][18]







Proper acceleration[edit]

In infinitesimal small durations there is always one inertial frame, which momentarily has the same velocity as the accelerated body, and in which the Lorentz transformation holds. The corresponding three-acceleration in these frames can be directly measured by an accelerometer, and is called proper acceleration[19][H 14] or rest acceleration.[20][H 12] The relation of in a momentary inertial frame and measured in an external inertial frame follows from (1c, 1d) with , , and . So in terms of (1c), when the velocity is directed in the x-direction by and when only accelerations parallel (x-direction) or perpendicular (y-, z-direction) to the velocity are considered, it follows:[12][20][19][H 1][H 2][H 14][H 12]






Generalized by (1d) for arbitrary directions of of magnitude :[21][22][18]

There is also a close relationship to the magnitude of four-acceleration: As it is invariant, it can be determined in the momentary inertial frame , in which and by it follows :[20][12][23][H 16]







Thus the magnitude of four-acceleration corresponds to the magnitude of proper acceleration. By combining this with (3), an alternative method for the determination of the connection between in and in is given, namely[13][18]

from which (4a) follows again when the velocity is directed in the x-direction by and only accelerations parallel (x-direction) or perpendicular (y-, z-direction) to the velocity are considered.

Acceleration and force[edit]

The four-force as a function of three-force is given by . Four-force and four-acceleration (2) and the invariant mass are furthermore related by ,[24] thus[25]


The relation between three-force and three-acceleration for arbitrary directions of the velocity is thus[26][27][24]






When the velocity is directed in the x-direction by and only accelerations parallel (x-direction) or perpendicular (y-, z-direction) to the velocity are considered[28][27][24][H 2][H 6]






Therefore, the Newtonian definition of mass as the ratio of three-force and three-acceleration is disadvantageous in SR, because such a mass would depend both on velocity and direction. Consequently, the following mass definitions used in older textbooks are not used anymore:[28][29][H 2]

as "longitudinal mass",
as "transverse mass".

The relation (5a) between three-acceleration and three-force can also be obtained from the equation of motion[30][26][H 2][H 6]






where is the three-momentum. The corresponding transformation of three-force between in and in (when the relative velocity between the frames is directed in the x-direction by and only accelerations parallel (x-direction) or perpendicular (y-, z-direction) to the velocity are considered) follows by substitution of the relevant transformation formulas for , , , , or from the Lorentz transformed components of four-force, with the result:[30][31][25][H 3][H 15]






Or generalized for arbitrary directions of , as well as with magnitude :[32][33]






Proper acceleration and proper force[edit]

The force in a momentary inertial frame measured by a comoving spring balance can be called proper force.[34][35] It follows from (6a, 6b) by setting and as well as and . Thus by (6a) where only accelerations parallel (x-direction) or perpendicular (y-, z-direction) to the velocity are considered:[36][34][35]






Generalized by (6b) for arbitrary directions of of magnitude :[36][37]

Since in momentary inertial frames one has , the Newtonian relation holds (which also follows from the above relation , since in the momentary rest frame one has and ), therefore (4a, 5b, 7a) can be summarized[38]






By that, the apparent contradiction in the historical definitions of transverse mass can be explained.[39] Einstein (1905) described the relation between three-acceleration and proper force[H 5]


while Lorentz (1899, 1904) and Planck (1906) described the relation between three-acceleration and three-force[H 2]


Curved world lines[edit]

By integration of the equations of motion one obtains the curved world lines of accelerated bodies corresponding to a sequence of momentary inertial frames (here, the expression "curved" is related to the form of the worldlines in Minkowski diagrams, which should not be confused with "curved" spacetime of general relativity). In connection with this, the so-called clock hypothesis of clock postulate has to be considered:[40][41] The proper time of comoving clocks is independent of acceleration, that is, the time dilation of these clocks as seen in an external inertial frame only depends on its relative velocity with respect to that frame. Two simple cases of curved world lines are now provided by integration of equation (4a) for proper acceleration:

a) Hyperbolic motion: The constant, longitudinal proper acceleration by (4a) leads to the world line[12][19][20][26][42][43][H 10][H 15]






The worldline corresponds to the hyperbolic equation , from which the name hyperbolic motion is derived. These equations are often used for the calculation of various scenarios of the twin paradox or Bell's spaceship paradox, or in relation to space travel using constant acceleration.

b) The constant, transverse proper acceleration by (4a) can be seen as a centripetal acceleration,[13] leading to the worldline of a body in uniform rotation[44][45]

where is the tangential speed, is the orbital radius, is the angular velocity as a function of coordinate time, and as the proper angular velocity.

A classification of curved worldlines can be obtained by using the differential geometry of triple curves, which can be expressed by spacetime Frenet-Serret formulas.[46] In particular, it can be shown that hyperbolic motion and uniform circular motion are special cases of motions having constant curvatures and torsions,[47] satisfying the condition of Born rigidity.[H 11][H 17] A body is called Born rigid if the spacetime distance between its infinitesimally separated worldlines or points remains constant during acceleration.

Accelerated reference frames[edit]

Instead of inertial frames, these accelerated motions and curved worldlines can also be described using accelerated or curvilinear coordinates. The proper reference frame established that way is closely related to Fermi coordinates.[48][49] For instance, the coordinates for an hyperbolically accelerated reference frame are sometimes called Rindler coordinates, or those of a uniformly rotating reference frame are called rotating cylindrical coordinates (or sometimes Born coordinates). In terms of the equivalence principle, the effects arising in these accelerated frames are analogous to effects in a homogeneous, fictitious gravitational field. In this way it can be seen, that the employment of accelerating frames in SR produces important mathematical relations, which (when further developed) play a fundamental role in the description of real, inhomogeneous gravitational fields in terms of curved spacetime in general relativity.


For further information see von Laue,[2] Pauli,[3] Miller,[50] Zahar,[51] Gourgoulhon,[49] and the historical sources in history of special relativity.

1899: Hendrik Lorentz[H 1] derived the correct (up to a certain factor ) relations for accelerations, forces and masses between a resting electrostatic systems of particles (in a stationary aether), and a system emerging from it by adding a translation, with as the Lorentz factor:

, , for by (7a);
, , for by (4a);
, , for , thus longitudinal and transverse mass by (5b);

Lorentz explained that he has no means of determining the value of . If he had set , his expressions would have assumed the exact relativistic form.

1904: Lorentz[H 2] derived the previous relations in a more detailed way, namely with respect to the properties of particles resting in the system and the moving system , with the new auxiliary variable equal to compared to the one in 1899, thus:

for as a function of by (7a);
for as a function of by (7b);
for as a function of by (4a);
for longitudinal and transverse mass as a function of the rest mass by (5b, 7b).

This time, Lorentz could show that , by which his formulas assume the exact relativistic form. He also formulated the equation of motion


which corresponds to (5c) with , with , , , , , and as electromagnetic rest mass. Furthermore, he argued, that these formulas should not only hold for forces and masses of electrically charged particles, but for other processes as well so that the earth's motion through the aether remains undetectable.

1905: Henri Poincaré[H 3] introduced the transformation of three-force (6a):

with , and as the Lorentz factor, the charge density. Or in modern notation: , , , and . As Lorentz, he set .

1905: Albert Einstein[H 5] derived the equations of motions on the basis of his special theory of relativity, which represent the relation between equally valid inertial frames without the action of a mechanical aether. Einstein concluded, that in a momentary inertial frame the equations of motion retain their Newtonian form:


This corresponds to , because and and . By transformation into a relatively moving system he obtained the equations for the electrical and magnetic components observed in that frame:


This corresponds to (5b) with , because and and and . Consequently, Einstein determined the longitudinal and transverse mass, even though he related it to the force in the momentary rest frame measured by a comoving spring balance, and to the three-acceleration in system :[39]

This corresponds to (7b) with .

1905: Poincaré[H 4] introduces the transformation of three-acceleration (1c):

where as well as and and .

Furthermore, he introduced the four-force in the form:

where and and .

1906: Max Planck[H 6] derived the equation of motion




The equations correspond to (5c) with , with and and , in agreement with those given by Lorentz (1904).

1907: Einstein[H 7] analyzed a uniformly accelerated reference frame and obtained formulas for coordinate dependent time dilation and speed of light, analogous to those given by Kottler-Møller-Rindler coordinates.

1907: Hermann Minkowski[H 9] defined the relation between the four-force (which he called the moving force) and the four acceleration

corresponding to .

1908: Minkowski[H 8] denotes the second derivative with respect to proper time as "acceleration vector" (four-acceleration). He showed, that its magnitude at an arbitrary point of the worldline is , where is the magnitude of a vector directed from the center of the corresponding "curvature hyperbola" (German: Krümmungshyperbel) to .

1909: Max Born[H 10] denotes the motion with constant magnitude of Minkowski's acceleration vector as "hyperbolic motion" (German: Hyperbelbewegung), in the course of his study of rigidly accelerated motion. He set (now called proper velocity) and as Lorentz factor and as proper time, with the transformation equations


which corresponds to (8) with and . Eliminating Born derived the hyperbolic equation , and defined the magnitude of acceleration as . He also noticed that his transformation can be used to transform into a "hyperbolically accelerated reference system" (German: hyperbolisch beschleunigtes Bezugsystem).

1909: Gustav Herglotz[H 11] extends Born's investigation to all possible cases of rigidly accelerated motion, including uniform rotation.

1910: Arnold Sommerfeld[H 13] brought Born's formulas for hyperbolic motion in a more concise form with as the imaginary time variable and as an imaginary angle:

He noted that when are variable and is constant, they describe the worldline of a charged body in hyperbolic motion. But if are constant and is variable, they denote the transformation into its rest frame.

1911: Sommerfeld[H 14] explicitly used the expression "proper acceleration" (German: Eigenbeschleunigung) for the quantity in , which corresponds to (4a), as the acceleration in the momentary inertial frame.

1911: Herglotz[H 12] explicitly used the expression "rest acceleration" (German: Ruhbeschleunigung) instead of proper acceleration. He wrote it in the form and which corresponds to (4a), where is the Lorentz factor and or are the longitudinal and transverse components of rest acceleration.

1911: Max von Laue[H 15] derived in the first edition of his monograph "Das Relativitätsprinzip" the transformation for three-acceleration by differentiation of the velocity addition

equivalent to (1c) as well as to Poincaré (1905/6). From that he derived the transformation of rest acceleration (equivalent to 4a), and eventually the formulas for hyperbolic motion which corresponds to (8):



and the transformation into a hyperbolic reference system with imaginary angle :


He also wrote the transformation of three-force as

equivalent to (6a) as well as to Poincaré (1905).

1912-1914: Friedrich Kottler[H 17] obtained general covariance of Maxwell's equations, and used four-dimensional Frenet-Serret formulas to analyze the Born rigid motions given by Herglotz (1909). He also obtained the proper reference frames for hyperbolic motion and uniform circular motion.

1913: von Laue[H 16] replaced in the second edition of his book the transformation of three-acceleration by Minkowski's acceleration vector for which he coined the name "four-acceleration" (German: Viererbeschleunigung), defined by with as four-velocity. He showed, that the magnitude of four-acceleration corresponds to the rest acceleration by


which corresponds to (4b). Subsequently, he derived the same formulas as in 1911 for the transformation of rest acceleration and hyperbolic motion, and the hyperbolic reference frame.


  1. ^ Misner & Thorne & Wheeler (1973), p. 163: "Accelerated motion and accelerated observers can be analyzed using special relativity."
  2. ^ a b von Laue (1921)
  3. ^ a b Pauli (1921)
  4. ^ Sexl & Schmidt (1979), p. 116
  5. ^ Møller (1955), p. 41
  6. ^ Tolman (1917), p. 48
  7. ^ French (1968), p. 148
  8. ^ Zahar (1989), p. 232
  9. ^ Freund (2008), p. 96
  10. ^ Kopeikin & Efroimsky & Kaplan (2011), p. 141
  11. ^ Rahaman (2014), p. 77
  12. ^ a b c d Pauli (1921), p. 627
  13. ^ a b c d Freund (2008), pp. 267-268
  14. ^ Ashtekar & Petkov (2014), p. 53
  15. ^ Sexl & Schmidt (1979), p. 198, Solution to example 16.1
  16. ^ a b Ferraro (2007), p. 178
  17. ^ Sexl & Schmidt (1979), p. 121
  18. ^ a b c Kopeikin & Efroimsky & Kaplan (2011), p. 137
  19. ^ a b c Rindler (1977), pp. 49-50
  20. ^ a b c d von Laue (1921), pp. 88-89
  21. ^ Rebhan (1999), p. 775
  22. ^ Nikolić (2000), eq. 10
  23. ^ Rindler (1977), p. 67
  24. ^ a b c Sexl & Schmidt (1979), solution of example 16.2, p. 198
  25. ^ a b Freund (2008), p. 276
  26. ^ a b c Møller (1955), pp. 74-75
  27. ^ a b Rindler (1977), pp. 89-90
  28. ^ a b von Laue (1921), p. 210
  29. ^ Pauli (1921), p. 635
  30. ^ a b Tolman (1917), pp. 73-74
  31. ^ von Laue (1921), p. 113
  32. ^ Møller (1955), p. 73
  33. ^ Kopeikin & Efroimsky & Kaplan (2011), p. 173
  34. ^ a b Shadowitz (1968), p. 101
  35. ^ a b Pfeffer & Nir (2012), p. 115, “In the special case in which the particle is momentarily at rest relative to the observer S, the force he measures will be the proper force”.
  36. ^ a b Møller (1955), p. 74
  37. ^ Rebhan (1999), p. 818
  38. ^ see Lorentz's 1904-equations and Einstein's 1905-equations in section on history
  39. ^ a b Mathpages (see external links), "Transverse Mass in Einstein's Electrodynamics", eq. 2,3
  40. ^ Rindler (1977), p. 43
  41. ^ Koks (2006), section 7.1
  42. ^ Fraundorf (2012), section IV-B
  43. ^ PhysicsFAQ (2016), see external links.
  44. ^ Pauri & Vallisneri (2000), eq. 13
  45. ^ Bini & Lusanna & Mashhoon (2005), eq. 28,29
  46. ^ Synge (1966)
  47. ^ Pauri & Vallisneri (2000), Appendix A
  48. ^ Misner & Thorne & Wheeler (1973), Section 6
  49. ^ a b Gourgoulhon (2013), entire book
  50. ^ Miller (1981)
  51. ^ Zahar (1989)


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Historical papers[edit]

  1. ^ a b c Lorentz, Hendrik Antoon (1899). "Simplified Theory of Electrical and Optical Phenomena in Moving Systems". Proceedings of the Royal Netherlands Academy of Arts and Sciences. 1: 427–442.
  2. ^ a b c d e f g Lorentz, Hendrik Antoon (1904). "Electromagnetic phenomena in a system moving with any velocity smaller than that of light". Proceedings of the Royal Netherlands Academy of Arts and Sciences. 6: 809–831.
  3. ^ a b c Poincaré, Henri (1905). "Sur la dynamique de l'électron" [Wikisource translation: On the Dynamics of the Electron]. Comptes rendus hebdomadaires des séances de l'Académie des sciences. 140: 1504–1508.
  4. ^ a b c Poincaré, Henri (1906) [1905]. "Sur la dynamique de l'électron" [Wikisource translation: On the Dynamics of the Electron]. Rendiconti del Circolo matematico di Palermo. 21: 129–176.
  5. ^ a b c Einstein, Albert (1905). "Zur Elektrodynamik bewegter Körper". Annalen der Physik. 322 (10): 891–921.; See also: English translation.
  6. ^ a b c d Planck, Max (1906). "Das Prinzip der Relativität und die Grundgleichungen der Mechanik" [Wikisource translation: The Principle of Relativity and the Fundamental Equations of Mechanics]. Verhandlungen Deutsche Physikalische Gesellschaft. 8: 136–141.
  7. ^ a b Einstein, Albert (1908) [1907], "Über das Relativitätsprinzip und die aus demselben gezogenen Folgerungen" (PDF), Jahrbuch der Radioaktivität und Elektronik, 4: 411–462, Bibcode:1908JRE.....4..411E; English translation On the relativity principle and the conclusions drawn from it at Einstein paper project.
  8. ^ a b Minkowski, Hermann (1909) [1908]. "Raum und Zeit" [Wikisource translation: Space and Time]. Jahresberichte der Deutschen Mathematiker-Vereinigung. Leipzig.
  9. ^ a b Minkowski, Hermann (1908) [1907], "Die Grundgleichungen für die elektromagnetischen Vorgänge in bewegten Körpern" [Wikisource translation: The Fundamental Equations for Electromagnetic Processes in Moving Bodies], Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-Physikalische Klasse: 53–111
  10. ^ a b c Born, Max (1909). "Die Theorie des starren Elektrons in der Kinematik des Relativitätsprinzips" [Wikisource translation: The Theory of the Rigid Electron in the Kinematics of the Principle of Relativity]. Annalen der Physik. 335 (11): 1–56. Bibcode:1909AnP...335....1B. doi:10.1002/andp.19093351102.
  11. ^ a b c Herglotz, G (1910) [1909]. "Über den vom Standpunkt des Relativitätsprinzips aus als starr zu bezeichnenden Körper" [Wikisource translation: On bodies that are to be designated as "rigid" from the standpoint of the relativity principle]. Annalen der Physik. 336 (2): 393–415. Bibcode:1910AnP...336..393H. doi:10.1002/andp.19103360208.
  12. ^ a b c d Herglotz, G. (1911). "Über die Mechanik des deformierbaren Körpers vom Standpunkte der Relativitätstheorie". Annalen der Physik. 341 (13): 493–533. Bibcode:1911AnP...341..493H. doi:10.1002/andp.19113411303.
  13. ^ a b Sommerfeld, Arnold (1910). "Zur Relativitätstheorie II: Vierdimensionale Vektoranalysis" [Wikisource translation: On the Theory of Relativity II: Four-dimensional Vector Analysis]. Annalen der Physik. 338 (14): 649–689. Bibcode:1910AnP...338..649S. doi:10.1002/andp.19103381402.
  14. ^ a b c d Sommerfeld, Arnold (1911). "Über die Struktur der gamma-Strahlen". Sitzungsberichte der mathematematisch-physikalischen Klasse der K. B. Akademie der Wissenschaften zu München (1): 1–60.
  15. ^ a b c d e Laue, Max von (1911). Das Relativitätsprinzip. Braunschweig: Vieweg.
  16. ^ a b c Laue, Max von (1913). Das Relativitätsprinzip (2. Ausgabe ed.). Braunschweig: Vieweg.
  17. ^ a b c Kottler, Friedrich (1912). "Über die Raumzeitlinien der Minkowski'schen Welt" [Wikisource translation: On the spacetime lines of a Minkowski world]. Wiener Sitzungsberichte 2a. 121: 1659–1759. Kottler, Friedrich (1914a). "Relativitätsprinzip und beschleunigte Bewegung". Annalen der Physik. 349 (13): 701–748. Bibcode:1914AnP...349..701K. doi:10.1002/andp.19143491303. Kottler, Friedrich (1914b). "Fallende Bezugssysteme vom Standpunkte des Relativitätsprinzips". Annalen der Physik. 350 (20): 481–516. Bibcode:1914AnP...350..481K. doi:10.1002/andp.19143502003.

External links[edit]