Proper reference frame (flat spacetime)

Short description: Coordinates system in an accelerating, "at rest" setting

A proper reference frame in the theory of relativity is a particular form of accelerated reference frame, that is, a reference frame in which an accelerated observer can be considered as being at rest. It can describe phenomena in curved spacetime, as well as in "flat" Minkowski spacetime in which the spacetime curvature caused by the energy–momentum tensor can be disregarded. Since this article considers only flat spacetime—and uses the definition that special relativity is the theory of flat spacetime while general relativity is a theory of gravitation in terms of curved spacetime—it is consequently concerned with accelerated frames in special relativity.[1][2][3] (For the representation of accelerations in inertial frames, see the article Acceleration (special relativity), where concepts such as three-acceleration, four-acceleration, proper acceleration, hyperbolic motion etc. are defined and related to each other.)

A fundamental property of such a frame is the employment of the proper time of the accelerated observer as the time of the frame itself. This is connected with the clock hypothesis (which is experimentally confirmed), according to which the proper time of an accelerated clock is unaffected by acceleration, thus the measured time dilation of the clock only depends on its momentary relative velocity. The related proper reference frames are constructed using concepts like comoving orthonormal tetrads, which can be formulated in terms of spacetime Frenet–Serret formulas, or alternatively using Fermi–Walker transport as a standard of non-rotation. If the coordinates are related to Fermi–Walker transport, the term Fermi coordinates is sometimes used, or proper coordinates in the general case when rotations are also involved. A special class of accelerated observers follow worldlines whose three curvatures are constant. These motions belong to the class of Born rigid motions, i.e., the motions at which the mutual distance of constituents of an accelerated body or congruence remains unchanged in its proper frame. Two examples are Rindler coordinates or Kottler-Møller coordinates for the proper reference frame of hyperbolic motion, and Born or Langevin coordinates in the case of uniform circular motion.

In the following, Greek indices run over 0,1,2,3, Latin indices over 1,2,3, and bracketed indices are related to tetrad vector fields. The signature of the metric tensor is (-1,1,1,1).

## Contents

* 1 History
* 2 Comoving tetrads
* 2.1 Spacetime Frenet–Serret equations
* 2.2 Fermi–Walker transport and proper transport
* 2.3 Deriving Fermi–Walker tetrads from Frenet–Serret tetrads
* 3 Proper coordinates or Fermi coordinates
* 4 Proper reference frames for timelike helices
* 4.1 Hyperbolic motion
* 4.2 Uniform circular motion
* 4.3 Catenary
* 4.4 Semicubical parabola
* 4.5 General case
* 5 Overview of historical formulas
* 5.1 Herglotz
* 5.2 Kottler
* 5.3 Møller
* 5.4 Worldlines of constant curvatures by Herglotz and Kottler
* 6 References
* 7 Bibliography
* 7.1 Textbooks
* 7.2 Journal articles
* 8 Historical sources
* 9 External links

## History

For the historical formulas of Herglotz, Kottler, and Møller in original notation, see the section #Overview of historical formulas

Some properties of Kottler-Møller or Rindler coordinates were anticipated by Albert Einstein (1907)[H 1] when he discussed the uniformly accelerated reference frame. While introducing the concept of Born rigidity, Max Born (1909)[H 2] recognized that the formulas for the worldline of hyperbolic motion can be reinterpreted as transformations into a "hyperbolically accelerated reference system". Born himself, as well as Arnold Sommerfeld (1910)[H 3] and Max von Laue (1911)[H 4] used this frame to compute the properties of charged particles and their fields (see Acceleration (special relativity) and Rindler coordinates). In addition, Gustav Herglotz (1909)[H 5] gave a classification of all Born rigid motions, including uniform rotation and the worldlines of constant curvatures. Friedrich Kottler (1912, 1914)[H 6] introduced the "generalized Lorentz transformation" for proper reference frames or proper coordinates (German: Eigensystem, Eigenkoordinaten) by using comoving Frenet–Serret tetrads, and applied this formalism to Herglotz' worldlines of constant curvatures, particularly to hyperbolic motion and uniform circular motion. Herglotz' formulas were also simplified and extended by Georges Lemaître (1924).[H 7] The worldlines of constant curvatures were rediscovered by several author, for instance, by Vladimír Petrův (1964),[4] as "timelike helices" by John Lighton Synge (1967)[5] or as "stationary worldlines" by Letaw (1981).[6] The concept of proper reference frame was later reintroduced and further developed in connection with Fermi–Walker transport in the textbooks by Christian Møller (1952)[7] or Synge (1960).[8] An overview of proper time transformations and alternatives was given by Romain (1963),[9] who cited the contributions of Kottler. In particular, Misner & Thorne & Wheeler (1973)[10] combined Fermi–Walker transport with rotation, which influenced many subsequent authors. Bahram Mashhoon (1990, 2003)[11] analyzed the hypothesis of locality and accelerated motion. The relations between the spacetime Frenet–Serret formulas and Fermi–Walker transport was discussed by Iyer & C. V. Vishveshwara (1993),[12] Johns (2005)[13] or Bini et al. (2008)[14] and others. A detailed representation of "special relativity in general frames" was given by Gourgoulhon (2013).[15]

## Comoving tetrads

### Spacetime Frenet–Serret equations

For the investigation of accelerated motions and curved worldlines, some results of differential geometry can be used. For instance, the Frenet–Serret formulas for curves in Euclidean space have already been extended to arbitrary dimensions in the 19th century, and can be adapted to Minkowski spacetime as well. They describe the transport of an orthonormal basis attached to a curved worldline, so in four dimensions this basis can be called a comoving tetrad or vierbein 𝐞(η) (also called vielbein, moving frame, frame field, local frame, repère mobile in arbitrary dimensions):[16][17][18][19]

d𝐞(0)dτ=κ1𝐞(1),d𝐞(1)dτ=κ1𝐞(0)+κ2𝐞(2),d𝐞(2)dτ=−κ2𝐞(1)+κ3𝐞(3),d𝐞(3)dτ=−κ3𝐞(2),

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(1)

Here, τ is the proper time along the worldline, the timelike field 𝐞(0) is called the tangent that corresponds to the four-velocity, the three spacelike fields are orthogonal to 𝐞(0) and are called the principal normal 𝐞(1), the binormal 𝐞(2) and the trinormal 𝐞(3). The first curvature κ1 corresponds to the magnitude of four-acceleration (i.e., proper acceleration), the other curvatures κ2 and κ3 are also called torsion and hypertorsion.

### Fermi–Walker transport and proper transport

While the Frenet–Serret tetrad can be rotating or not, it is useful to introduce another formalism in which non-rotational and rotational parts are separated. This can be done using the following equation for proper transport[20] or generalized Fermi transport[21] of tetrad 𝐞(η), namely[10][12][22][21][20][23]

d𝐞(η)dτ=−ϑ𝐞(η)

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(2)

where

ϑμν=AμUν−AνUμ⏟Fermi–Walker+Uαωβϵαβμν⏟spatial rotation

or together in simplified form:

d𝐞(η)dτ=−[(𝐔∧𝐀)𝐞(η)+𝐑⋅𝐞(η)]

with 𝐔 as four-velocity and 𝐀 as four-acceleration, and "⋅" indicates the dot product and "∧" the wedge product. The first part (𝐔∧𝐀)𝐞(η)=𝐀(𝐔⋅𝐞(η))−𝐔(𝐀⋅𝐞(η)) represents Fermi–Walker transport,[13] which is physically realized when the three spacelike tetrad fields don't change their orientation with respect to the motion of a system of three gyroscopes. Thus Fermi–Walker transport can be seen as a standard of non-rotation. The second part 𝐑 consists of an antisymmetric second rank tensor with ω as the angular velocity four-vector and ϵ as the Levi-Civita symbol. It turns out that this rotation matrix only affects the three spacelike tetrad fields, thus it can be interpreted as the spatial rotation of the spacelike fields 𝐞(i) of a rotating tetrad (such as a Frenet–Serret tetrad) with respect to the non-rotating spacelike fields 𝐟(i) of a Fermi–Walker tetrad along the same world line.

### Deriving Fermi–Walker tetrads from Frenet–Serret tetrads

Since 𝐟(i) and 𝐞(i) on the same worldline are connected by a rotation matrix, it is possible to construct non-rotating Fermi–Walker tetrads using rotating Frenet–Serret tetrads,[24][25] which not only works in flat spacetime but for arbitrary spacetimes as well, even though the practical realization can be hard to achieve.[26] For instance, the angular velocity vector between the respective spacelike tetrad fields 𝐟(i) and 𝐞(i) can be given in terms of torsions κ2 and κ3:[12][13][27][28]

ω=κ3𝐞(1)+κ2𝐞(3) and |ω|=κ22+κ32

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(3a)

Assuming that the curvatures are constant (which is the case in helical motion in flat spacetime, or in the case of stationary axisymmetric spacetimes), one then proceeds by aligning the spacelike Frenet–Serret vectors in the 𝐞(1)−𝐞(3) plane by constant counter-clockweise rotation, then the resulting intermediary spatial frame 𝐡(i) is constantly rotated around the 𝐡(3) axis by the angle Θ=|ω|τ, which finally gives the spatial Fermi–Walker frame 𝐟(i) (note that the timelike field remains the same):[25]

𝐡(1)=κ2𝐞(1)−κ3𝐞(3)|ω|𝐡(2)=𝐞(2)𝐡(3)=ω|ω|𝐟(1)=𝐡(1)cos⁡Θ−h(2)sin⁡Θ𝐟(2)=𝐡(1)sin⁡Θ+h(2)cos⁡Θ𝐟(3)=𝐡(3)𝐞(0)=𝐡(0)=𝐟(0)

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(3b)

For the special case κ3=0 and 𝐞(3)=[0,0,0,1], it follows ω=[0,0,0, κ2] and Θ=|ω|τ=κ2τ and 𝐡(i)=𝐞(i), therefore (3b) is reduced to a single constant rotation around the 𝐞(3)-axis:[29][30][31][24]

𝐟(1)=𝐞(1)cos⁡Θ−𝐞(2)sin⁡Θ𝐟(2)=𝐞(1)sin⁡Θ+𝐞(2)cos⁡Θ𝐟(3)=𝐞(3)𝐞(0)=𝐟(0)

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(3c)

## Proper coordinates or Fermi coordinates

In flat spacetime, an accelerated object is at any moment at rest in a momentary inertial frame 𝐱′=[x′0,x′1,x′2,x′3], and the sequence of such momentary frames which it traverses corresponds to a successive application of Lorentz transformations 𝐗=Λ𝐱′, where 𝐗 is an external inertial frame and Λ the Lorentz transformation matrix. This matrix can be replaced by the proper time dependent tetrads 𝐞(ν)(τ) defined above, and if 𝐪(τ) is the time track of the particle indicating its position, the transformation reads:[32]

𝐗=𝐪+𝐞(ν)𝐱′

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(4a)

Then one has to put x′0=t′=0 by which 𝐱′ is replaced by 𝐫=[x1,x2,x3] and the timelike field 𝐞(0) vanishes, therefore only the spacelike fields 𝐞(i) are present anymore. Subsequently, the time in the accelerated frame is identified with the proper time of the accelerated observer by x0=t=τ. The final transformation has the form[33][34][35][36]

𝐗=𝐪+𝐞(i)𝐫,(x0=τ)

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(4b)

These are sometimes called proper coordinates, and the corresponding frame is the proper reference frame.[20] They are also called Fermi coordinates in the case of Fermi–Walker transport[37] (even though some authors use this term also in the rotational case[38]). The corresponding metric has the form in Minkowski spacetime (without Riemannian terms):[39][40][41][42][43][44][45][46]

ds2=−[(1+𝐚⋅𝐫)2−(ω×𝐫)2]dτ2+2(ω×𝐫)dτ d𝐫+δijdxidxj

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(4c)

However, these coordinates are not globally valid, but are restricted to[43]

−(1+𝐚⋅𝐫)2+(ω×𝐫)2<0

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(4d)

## Proper reference frames for timelike helices

In case all three Frenet–Serret curvatures are constant, the corresponding worldlines are identical to those that follow from the Killing motions in flat spacetime. They are of particular interest since the corresponding proper frames and congruences satisfy the condition of Born rigidity, that is, the spacetime distance of two neighbouring worldlines is constant.[47][48] These motions correspond to "timelike helices" or "stationary worldlines", and can be classified into six principal types: two with zero torsions (uniform translation, hyperbolic motion) and four with non-zero torsions (uniform rotation, catenary, semicubical parabola, general case):[49][50][4][5][6][51][52][53][54]

Case κ1=κ2=κ3=0 produces uniform translation without acceleration. The corresponding proper reference frame is therefore given by ordinary Lorentz transformations. The other five types are:

### Hyperbolic motion

Further information: Hyperbolic motion (relativity) and Rindler coordinates

The curvatures κ1=α, κ2=κ3=0, where α is the constant proper acceleration in the direction of motion, produce hyperbolic motion because the worldline in the Minkowski diagram is a hyperbola:[55][56][57][58][59][60]

𝐗=[1αsinh⁡(ατ),1α(cosh⁡(ατ)−1),0,0]

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(5a)

The corresponding orthonormal tetrad is identical to an inverted Lorentz transformation matrix with hyperbolic functions γ=cosh⁡η as Lorentz factor and vγ=sinh⁡η as proper velocity and η=artanh⁡v=ατ as rapidity (since the torsions κ2 and κ3 are zero, the Frenet–Serret formulas and Fermi–Walker formulas produce the same tetrad):[56][61][62][63][64][65][66]

𝐞(0)=(cosh⁡(ατ), sinh⁡(ατ), 0, 0)𝐞(1)=(sinh⁡(ατ), cosh⁡(ατ), 0, 0)𝐞(2)=(0, 0, 1, 0)𝐞(3)=(0, 0, 0, 1)

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(5b)

Inserted into the transformations (4b) and using the worldline (5a) for 𝐪, the accelerated observer is always located at the origin, so the Kottler-Møller coordinates follow[67][68][62][69][70]

T=(x+1α)sinh⁡(ατ)X=(x+1α)cosh⁡(ατ)−1αY=yZ=zτ=1αartanh⁡(TX+1α)x=(X+1α)2−T2−1αy=Yz=Z

which are valid within −1/α<X<∞, with the metric

ds2=−(1+αx)2dτ2+dx2+dy2+dz2.

Alternatively, by setting 𝐪=0 the accelerated observer is located at X=1/α at time τ=T=0, thus the Rindler coordinates follow from (4b) and (5a, 5b):[71][72][73]

T=xsinh⁡(ατ)X=xcosh⁡(ατ)Y=yZ=zτ=1αartanh⁡TXx=X2−T2y=Yz=Z

which are valid within 0<X<∞, with the metric

ds2=−α2x2dτ2+dx2+dy2+dz2

### Uniform circular motion

In the following, the orthonormal tetrad and its corresponding proper reference frame are written in Cartesian coordinates. For an alternative formulation using cylindrical coordinates, see the Langevin observers in the article Born coordinates)

The curvatures κ22−κ12>0, κ3=0 produce uniform circular motion, with the worldline[74][75][76][77][78][79][80]

X=[γτ, npcos⁡(pτ), npsin⁡(pτ), 0]

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(6a)

where

κ1=−γ2hp02κ2=γ2p0p=κ22−κ12=κ2γ=γp0p0=κ22−κ12κ2=κ2γ2=pγθ=pτ=p0t=γp0τn=κ1p=vγ=γ2−1h=κ1κ22−κ12=npv=κ1κ2=hp0=nγγ=κ2p=11−v2=n2+1

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(6b)

with h as orbital radius, p0 as coordinate angular velocity, p as proper angular velocity, v as tangential velocity, n as proper velocity, γ as Lorentz factor, and θ as angle of rotation. The tetrad can be derived from the Frenet–Serret equations (1),[74][76][77][80] or more simply be obtained by a Lorentz transformation of the tetrad d(ν) of ordinary rotating coordinates:[81][82]

d(0)=(1, 0, 0, 0)d(1)=(0, cos⁡θ, sin⁡θ, 0)d(2)=(0, −sin⁡θ, cos⁡θ, 0)d(3)=(0, 0, 0, 1)𝐞(0) =γ(d(0)+vd(2)) =γ(1, −vsin⁡θ, vcos⁡θ, 0)𝐞(1) =d(1) =(0, cos⁡θ, sin⁡θ, 0)𝐞(2) =γ(d(2)+vd(0)) =γ(v, −sin⁡θ, cos⁡θ, 0)𝐞(3) =d(3) =(0, 0, 0, 1)

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(6c)

The corresponding non-rotating Fermi–Walker tetrad 𝐟(η) on the same worldline can be obtained by solving the Fermi–Walker part of equation (2).[83][84] Alternatively, one can use (6b) together with (3a), which gives

ω=[0,0,0,γ2p],|ω|=γ2p,Θ=|ω|τ=γ2p0τ=γpτ=γθ

The resulting angle of rotation Θ together with (6c) can now be inserted into (3c), by which the Fermi–Walker tetrad follows[31][24]

𝐟(0) =𝐞(0) =γ(1, −vsin⁡θ, vcos⁡θ, 0)𝐟(1) =𝐞(1)cos⁡Θ−𝐞(2)sin⁡Θ =(−γvsin⁡Θ, cos⁡θcos⁡Θ+γsin⁡θsin⁡Θ, sin⁡θcos⁡Θ−γcos⁡θsin⁡Θ, 0)𝐟(2) =𝐞(1)sin⁡Θ+𝐞(2)cos⁡Θ =(γvcos⁡Θ, cos⁡θsin⁡Θ−γsin⁡θcos⁡Θ, sin⁡θsin⁡Θ+γcos⁡θcos⁡Θ, 0)𝐟(3) =𝐞(3) =(0, 0, 0, 1)

In the following, the Frenet–Serret tetrad is used to formulate the transformation. Inserting (6c) into the transformations (4b) and using the worldline (6a) for 𝐪 gives the coordinates[74][76][85][86][87][38]

T=γ(τ+γyv)X=(x+h)cos⁡θ−yγsin⁡θY=(x+h)sin⁡θ+yγcos⁡θZ=zτ=γ−1(T−γyv)x=Xcos⁡θ+Ysin⁡θ−hy=γ−1(−Xsin⁡θ+Ycos⁡θ)z=Z

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(6d)

which are valid within (X+h)2+(γY)2≦1/p02, with the metric

ds2=−γ2[1−(x+h)2p02−γ2p02y2]dτ2+2γ2p0(x dy−y dx)dτ+dx2+dy2+dz2

If an observer resting in the center of the rotating frame is chosen with h=0, the equations reduce to the ordinary rotational transformation[88][89][90]

T=tX=xcos⁡θ−ysin⁡θY=xsin⁡θ+ycos⁡θZ=zt=Tx=Xcos⁡θ+Ysin⁡θy=−Xsin⁡θ+Ycos⁡θz=ZorT=tX+iY=(x+iy)eiθZ=z

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(6e)

which are valid within 0<X2+Y2<1/p0, and the metric

ds2=−[1−p02(x2+y2)]dt2+2p0(−y dx+x dy)dt+dx2+dy2+dz2.

The last equations can also be written in rotating cylindrical coordinates (Born coordinates):[91][92][93][94][95]

T=tX=rcos⁡(ϕ+θ)Y=rsin⁡(ϕ+θ)Z=zt=Tx=rcos⁡(Φ−θ)y=rsin⁡(Φ−θ)z=Z->T=tR=rΦ=ϕ+θZ=zt=Tr=Rϕ=Φ−θz=Z

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(6f)

which are valid within 0<r<1/p0, and the metric

ds2=−(1−p02r2)dt2+2p0r2dt dϕ+dr2+r2dϕ2+dz2

Frames (6d, 6e, 6f) can be used to describe the geometry of rotating platforms, including the Ehrenfest paradox and the Sagnac effect.

### Catenary

The curvatures κ12−κ22>0, κ3=0 produce a catenary, i.e., hyperbolic motion combined with a spacelike translation[96][97][98][99][100][101][102]

X=[γasinh⁡(aτ),γacosh⁡(aτ),nτ,0]

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(7a)

where

κ1=γaκ2=naa=κ12−κ22n=κ2a=vγ=γ2−1η=aτv=κ2κ1=nγγ=κ1a=11−v2=n2+1

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(7b)

where v is the velocity, n the proper velocity, η as rapidity, γ is the Lorentz factor. The corresponding Frenet–Serret tetrad is:[97][99]

𝐞(0)=(γcosh⁡η, γsinh⁡η, n, 0)𝐞(1)=(sinh⁡η, cosh⁡η, 0, 0)𝐞(2)=(−ncosh⁡η, −nsinh⁡η, −γ, 0)𝐞(3)=(0, 0, 0, 1)

The corresponding non-rotating Fermi–Walker tetrad 𝐟(η) on the same worldline can be obtained by solving the Fermi–Walker part of equation (2).[102] The same result follows from (3a), which gives

ω=[0,0,0,na],|ω|=na,Θ=|ω|τ=naτ

which together with (7a) can now be inserted into (3c), resulting in the Fermi–Walker tetrad

𝐟(0) =𝐞(0) =(γcosh⁡η, γsinh⁡η, n, 0)𝐟(1) =𝐞(1)cos⁡Θ−𝐞(2)sin⁡Θ =(sinh⁡ηcos⁡Θ+ncosh⁡ηsin⁡Θ, cosh⁡ηcos⁡Θ+nsinh⁡ηsin⁡Θ, γsin⁡Θ, 0)𝐟(2) =𝐞(1)sin⁡Θ+𝐞(2)cos⁡Θ =(sinh⁡ηsin⁡Θ−ncosh⁡ηcos⁡Θ, cosh⁡ηsin⁡Θ−nsinh⁡ηcos⁡Θ, −γcos⁡Θ 0)𝐟(3) =𝐞(3) =(0, 0, 0, 1)

The proper coordinates or Fermi coordinates follow by inserting 𝐞(η) or 𝐟(η) into (4b).

### Semicubical parabola

The curvatures κ12−κ22=0, κ3=0 produce a semicubical parabola or cusped motion[103][104][105][106][107][108][109]

X=[τ+16a2τ3, 12aτ2, 16a2τ3, 0] with a=κ1=κ2

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(8)

The corresponding Frenet–Serret tetrad with θ=aτ is:[104][106]

𝐞(0)=(1+12θ2, θ, 12θ2, 0)𝐞(1)=(θ, 1, θ, 0)𝐞(2)=(−12θ2, −θ, 1−12θ2, 0)𝐞(3)=(0, 0, 0, 1)

The corresponding non-rotating Fermi–Walker tetrad 𝐟(η) on the same worldline can be obtained by solving the Fermi–Walker part of equation (2).[109] The same result follows from (3a), which gives

ω=[0,0,0,a],|ω|=a,Θ=|ω|τ=aτ=θ

which together with (8) can now be inserted into (3c), resulting in the Fermi–Walker tetrad (note that Θ=θ in this case):

𝐟(0) =𝐞(0) =(1+12θ2, θ, 12θ2, 0)𝐟(1) =𝐞(1)cos⁡Θ−𝐞(2)sin⁡Θ =(θcos⁡θ+12θ2sin⁡θ, cos⁡θ+θsin⁡θ, θcos⁡θ+(12θ2−1)sin⁡θ, 0)𝐟(2) =𝐞(1)sin⁡Θ+𝐞(2)cos⁡Θ =(θsin⁡θ−12θ2cos⁡θ, sin⁡θ−θcos⁡θ, θsin⁡θ−(12θ2−1)cos⁡θ, 0)𝐟(3) =𝐞(3) =(0, 0, 0, 1)

The proper coordinates or Fermi coordinates follow by inserting 𝐞(η) or 𝐟(η) into (4b).

### General case

The curvatures κ1≠0, κ2≠0, κ3≠0 produce hyperbolic motion combined with uniform circular motion. The worldline is given by[110][111][112][113][114][115][116]

X=[γasinh⁡(aτ),γacosh⁡(aτ),npsin⁡(pτ),npcos⁡(pτ)]

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(9a)

where

κ1=n2p2+γ2a2κ2=1κ1(a2+p2)γnκ3=1κ1apa=12(κ12−κ22−κ32+r)p=12(−κ12+κ22+κ32+r)=γp0n=12(1r[κ12+κ22+κ32]−1)=κ12−a2p2+a2=vγ=γ2−1γ=12(1r[κ12+κ22+κ32]+1)=κ12+p2p2+a2=11−v2=n2+1r=(κ12−κ22−κ32)2+4κ12κ32p0=pγ,v=hp0=nγ,h=np,η=aτ,θ=pτ=p0t=γp0τ

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(9b)

with v as tangential velocity, n as proper tangential velocity, η as rapidity, h as orbital radius, p0 as coordinate angular velocity, p as proper angular velocity, θ as angle of rotation, γ is the Lorentz factor. The Frenet–Serret tetrad is[111][113]

𝐞(0)=(γcosh⁡η, γsinh⁡η, −nsin⁡θ, −ncos⁡θ)𝐞(1)=1κ1(γasinh⁡η, γacosh⁡η, −npcos⁡θ, −npsin⁡θ)𝐞(2)=(−ncosh⁡η, −nsinh⁡η, γsin⁡θ, −γcos⁡θ)𝐞(3)=1κ1(npsinh⁡η, npcosh⁡η, γacos⁡θ, γasin⁡θ)

The corresponding non-rotating Fermi–Walker tetrad 𝐟(η) on the same worldline is as follows: First inserting (9b) into (3a) gives the angular velocity, which together with (9a) can now be inserted into (3b, left), and finally inserted into (3b, right) produces the Fermi–Walker tetrad. The proper coordinates or Fermi coordinates follow by inserting 𝐞(η) or 𝐟(η) into (4b) (the resulting expressions are not indicated here because of their length).

## Overview of historical formulas

In addition to the things described in the previous #History section, the contributions of Herglotz, Kottler, and Møller are described in more detail, since these authors gave extensive classifications of accelerated motion in flat spacetime.

### Herglotz

Herglotz (1909)[H 5] argued that the metric

ds2=dσ2+1A44(dν)2

where

dν=A14dξ1+A24dξ2+A34dξ3+A44dξ4dσ2=∑13ij Aijdξidξj−1A44(A14dξ1+A24dξ2+A34dξ3)2

satisfies the condition of Born rigidity when ∂∂τdσ2=0. He pointed out that the motion of a Born rigid body is in general determined by the motion of one of its point (class A), with the exception of those worldlines whose three curvatures are constant, thus representing a helix (class B). For the latter, Herglotz gave the following coordinate transformation corresponding to the trajectories of a family of motions:

(H1) xi=ai+∑14aijxj′,i=1,2,3,4,

where ai and aij are functions of proper time ϑ. By differentiation with respect to ϑ, and assuming xi as constant, he obtained

(H2) dxi′dϑ+qi+∑14pijxj′=0

Here, qi represents the four-velocity of the origin O′ of S′, and −pij is a six-vector (i.e., an antisymmetric four-tensor of second order, or bivector, having six independent components) representing the angular velocity of S′ around O′. As any six-vector, it has two invariants:

D=p23p14+p31p24+p12p34,Δ=p232+p312+p122+p142+p242+p342,

When xj′ is constant and ϑ is variable, any family of motions described by (H1) forms a group and is equivalent to an equidistant family of curves, thus satisfying Born rigidity because they are rigidly connected with S′. To derive such a group of motion, (H2) can be integrated with arbitrary constant values of qi and pij. For rotational motions, this results in four groups depending on whether the invariants D or Δ are zero or not. These groups correspond to four one-parameter groups of Lorentz transformations, which were already derived by Herglotz in a previous section on the assumption, that Lorentz transformations (being rotations in R4) correspond to hyperbolic motions in R3. The latter have been studied in the 19th century, and were categorized by Felix Klein into loxodromic, elliptic, hyperbolic, and parabolic motions (see also Möbius group).

### Kottler

Friedrich Kottler (1912)[H 6] followed Herglotz, and derived the same worldlines of constant curvatures using the following Frenet–Serret formulas in four dimensions, with c(α) as comoving tetrad of the worldline, and 1R1, 1R2, 1R3 as the three curvatures

dc1(α)ds=dc2(α)ds=dc3(α)ds=dc4(α)ds=*c2(α)R1**−c1(α)R1*c3(α)R2**−c2(α)R2*c4(α)R3**−c3(α)R3*α=1,2,3,4}

corresponding to (1). Kottler pointed out that the tetrad can be seen as a reference frame for such worldlines. Then he gave the transformation for the trajectories

𝐲=𝐱+Γ(1)c1+Γ(2)c2+Γ(3)c3+Γ(4)c4 (with h=1,2,3,4)

in agreement with (4a). Kottler also defined a tetrad whose basis vectors are fixed in normal space and therefore do not share any rotation. This case was further differentiated into two cases: If the tangent (i.e., the timelike) tetrad field is constant, then the spacelike tetrads fields c2(h),c3(h),c4(h) can be replaced by b2(h),b3(h),b4(h) who are "rigidly" connected with the tangent, thus

𝐲=𝐱+η0(1)c1+η0(2)b2+η0(3)b3+η0(4)b4

The second case is a vector "fixed" in normal space by setting η(1)=0. Kottler pointed out that this corresponds to class B given by Herglotz (which Kottler calls "Born's body of second kind")

𝐲=𝐱+η0(2)b2+η0(3)b3+η0(4)b4,

and class (A) of Herglotz (which Kottler calls "Born's body of first kind") is given by

𝐲=𝐱+Γ(2)c2+Γ(3)c3+Γ(4)c4

which both correspond to formula (4b).

* * *

In (1914a),[H 6] Kottler showed that the transformation

X=x+Γ(1)c1+Γ(2)c2+Γ(3)c3+Γ(4)c4,

describes the non-simultaneous coordinates of the points of a body, while the transformation with Γ(1)=0

X=x+Γ(2)c2+Γ(3)c3+Γ(4)c4,

describes the simultaneous coordinates of the points of a body. These formulas become "generalized Lorentz transformations" by inserting

Γ(3)=X′,Γ(4)=Y′,Γ(2)=Z′,Γ(1)=ic(T′−τ)

thus

X−x=ic(T′−τ)c1+Z′c2+X′c3+Y′c4

in agreement with (4b). He introduced the terms "proper coordinates" and "proper frame" (German: Eigenkoordinaten, Eigensystem) for a system whose time axis coincides with the respective tangent of the worldline. He also showed that the Born rigid body of second kind, whose worldlines are defined by

𝔛=x+Δ(2)c2+Δ(3)c3+Δ(4)c4,

is particularly suitable for defining a proper frame. Using this formula, he defined the proper frames for hyperbolic motion (free fall) and for uniform circular motion:

Hyperbolic motion  | Uniform circular motion

|

1914b  | 1914a  | 1921
c1(1)=0,c1(2)=0,c1(3)=1isinh⁡u,c1(4)=cosh⁡u,c2(1)=0,c2(2)=0,c2(3)=1icosh⁡u,c2(4)=−sinh⁡u,c3(1)=1,c3(2)=0,c3(3)=0,c3(4)=0,c4(1)=0,c4(2)=1,c4(3)=0,c4(4)=0,↓X=x+Δ(2)c2+Δ(3)c3+Δ(4)c4↓X=x0+𝔛′Y=y0+𝔜′Z=(b+ℤ′)cosh⁡𝔲cT=(b+ℤ′)sinh⁡𝔲(Δ(2)=𝔛′, Δ(3)=𝔜′, Δ(4)=ℤ′)↓𝔛′=X0−x0+qxT𝔜′=Y0−y0+qyTb+ℤ′=(Z0+qxT)2−c2T2c𝔗′=bartanh⁡cTZ0+qxT(X=X0+qxT, Y=Y0+qyT, Z=Z0+qxT)↓dS2=(d𝔛′)2+(d𝔜′)2+(dℤ′)2−c2(b+ℤ′b2)2(d𝔗′)2 | c1(h)=−aωsin⁡ωtic1−a2ω2c2, aωcos⁡ωtic1−a2ω2c2, 0, 11−a2ω2c2c2(h)=cos⁡ωt, sin⁡ωt, 0 0c3(h)=−sin⁡ωt1−a2ω2c2, cos⁡ωt1−a2ω2c2, 0, iaωc1−a2ω2c2c4(h)=0, 0, 1, 0↓X(h)=x(h)+Γ(1)c1(h)+Γ(2)c2(h)+Γ(3)c3(h)+Γ(4)c4(h)↓X=acos⁡ωt−aω(T−t)+Θ′1−a2ω2c2sin⁡ωt+R′cos⁡ωtY=asin⁡ωt+aω(T−t)+Θ′1−a2ω2c2cos⁡ωt+R′sin⁡ωtZ=z0+Z′icT=ict+(T−τ)+aωc2Θ′1−a2ω2c2 | X=(a+x′)cos⁡ωt−y′1−a2ω2c2sin⁡ωtY=(a+x′)sin⁡ωt+y′1−a2ω2c2cos⁡ωtZ=b+z′T=t+aωc2y′1−a2ω2c2t′=t1−a2ω2c2↓ds2=dx′2+dy′2+dz′2−2ωy′1−a2ω2c2dx′dt+2ωx′1−a2ω2c2dy′dt+(−c2+(a+x′)2ω2+y′2ω21−a2ω2c2)dt2

In (1916a) Kottler gave the general metric for acceleration-relative motions based on the three curvatures

dS2=dξ′2+dη′2+dζ′2−2c dτ′dξ′⋅η′i/R2+2c dτ′dη′⋅(ξ′i/R2−ζ′i/R3)+c dτ′dζ′⋅η′i/R3−c2dτ′2[(1−ξ′/R1)2+η′2/R22+η′/R32+(ξ′/R2−ζ′/R3)2]

In (1916b) he gave it the form:

ds2=dx2+dy2+dz2+2g14dx dit+2g24dy dit+2g34dz dit+g44(dit)2

where g14g24g34g44 are free from t, and ∂gi4∂xk+∂gk4∂xi=0, and ∂gi4∂xk−∂gk4∂xi=const., and g linear in xyz.

### Møller

Møller (1952)[7] defined the following transport equation

deidτ=(elU˙l)Ui−U˙i(ilUl)c2

in agreement with Fermi–Walker transport by (2, without rotation). The Lorentz transformation into a momentary inertial frame was given by him as

xi=fi(τ)+xk′αki(τ)

in agreement with (4a). By setting xi=xl′, x4′=0 and t=τ, he obtained the transformation into the "relativistic analogue of a rigid reference frame"

Xi=fi(t)+x′κακi(τ)

in agreement with the Fermi coordinates (4b), and the metric

ds2=dx2+dy2+dz2−c2dt2[1+gκxκc2]2

in agreement with the Fermi metric (4c) without rotation. He obtained the Fermi–Walker tetrads and Fermi frames of hyperbolic motion and uniform circular motion (some formulas for hyperbolic motion were already derived by him in 1943):

Hyperbolic motion  | Uniform circular motion

|

1943  | 1952  | 1952
x=1g{(1+gX)2−g2T2−1}y=Yz=Zt=12gln⁡1+gX+gT1+gX−gT↓ds2=dx2+dy2+dz2−(1+gx)2dt2 | αik=(U4/ic00iU1/c01000010U1/ic00U4/ic)Ui=(csinh⁡gτc, 0,0, igcosh⁡gτc)↓Xi=𝐟i(t)+x′κακi(τ)↓X=c2g(cosh⁡gtc−1)+xcosh⁡gtcY=yZ=zT=cgsinh⁡gtc+xsinh⁡gtcc↓ds2=dx2+dy2+dz2−c2dt2(1+gx/c2)2 | αik=(cos⁡αcos⁡β+γsin⁡αsin⁡βsin⁡αcos⁡β−γcos⁡αsin⁡β0−iuγcsin⁡βcos⁡αsin⁡β−γsin⁡αcos⁡βsin⁡αsin⁡β+γcos⁡αcos⁡β0iuγccos⁡β0010iuγcsin⁡α−iuγccos⁡α0γ)α=ωγτ, β=γα=ωγ2τ.

### Worldlines of constant curvatures by Herglotz and Kottler

General case  | Uniform rotation  | Catenary  | Semicubical parabola  | Hyperbolic motion

|
|
|
|

Herglotz (1909)
loxodromic  | elliptic  | hyperbolic  | parabolic  | hyperbolic
(α=0)
D≠0p21=−p12=1p34=−p43=iqi=[0,0,0,0] | D=0, Δ>0p21=−p12=1qi=[0,0,0,δi] | D=0, Δ<0p34=−p43=iqi=[α,0,0,0] | D=0, Δ=0p31=−p13=1p41=−p14=iqi=[0,β,0,δi] |
Lorentz-Transformations
x+iy=(x′+iy′)eiλϑx−iy=(x′−iy′)e−iλϑt−z=(t′−z′)eϑt+z=(t′+z′)e−ϑ | x+iy=(x′+iy′)eiϑx−iy=(x′−iy′)e−iϑz=z′t=t′+δϑ | x=x′+αϑy=y′t−z=(t′−z′)eϑt+z=(t′+z′)e−ϑ | x=x′+ϑ(t′−z′)+12δϑ2y=y′+βϑz=z′+ϑx′+12ϑ2(t′−z′)+16δϑ3t−z=t′−z′+δϑ | x=x′y=y′t−z=(t′−z′)eϑt+z=(t′+z′)e−ϑ
Trajectories (time)
x+iy=(x0+iy0)eiλux−iy=(x0−iy0)e−iλuz=z02+t2u=lg⁡z02+t2−tz0 | x+iy=(x0+iy0)eitδx−iy=(x0−iy0)e−itδz=z0 | x=x0+αlg⁡z02+t2−tz0y=y0z=z02+t2 | x=x0+12δϑ2y=y0+βϑz=z0+x0ϑ+16δϑ3t−z=δϑ | x=x0y=y0z=z02+t2
Kottler (1912, 1914)
hyperspherical curve  | uniform rotation  | catenary  | cubic curve  | hyperbolic motion
Curvatures
(1R1)2=a2λ4+b2(b2−a2λ2)2(1R2)2=−a2b2λ2(1+λ2)(b2−a2λ2)2(a2λ4+b2)(1R3)2=−λ2a2λ4+b2 | (1R1)2=a2λ4(1−a2λ2)2(1R2)2=−λ2(1−a2λ2)2(1R3)2=0 | (1R1)2=b2(b2−α2)2(1R2)2=−α2(b2−α2)2(1R3)2=0 | (1R1)2=α2(α2+2x0(1))2(1R2)2=−α2(α2+2x0(1))2=−(1R1)2(1R3)2=0 | (1R1)2=1b2(1R2)2=0(1R3)2=0
Trajectory of S4
x(1)=acos⁡λ(u−u0)x(2)=asin⁡λ(u−u0)x(3)=bcos⁡iux(4)=bsin⁡iu | x(1)=acos⁡λ(u−u0)x(2)=asin⁡λ(u−u0)x(3)=x0(3)x(4)=iu | x(1)=x0(1)+αux(2)=x0(2)x(3)=bcos⁡iux(4)=bsin⁡iu | x(1)=x0(1)+12αu2x(2)=x0(2)x(3)=x0(3)+x0(1)u+16αu3x(4)=i(x0(3)+x0(1)u+16αu2)+iαu | x(1)=x0(1)x(2)=x0(2)x(3)=bcos⁡iux(4)=bsin⁡iu
Trajectory (time)
x=acos⁡λ(u−u0)y=asin⁡λ(u−u0)z=b2+c2t2u=ln⁡−ct+b2+c2t2b | x=acos⁡ωz(t−t0)y=asin⁡ωz(t−t0)z=z0 | x=x0+αln⁡−ct+b2+c2t2by=y0z=b2+c2t2 | x=x0+12αu2y=y0z=z0+x0u+16αu3ct=z0+x0u+16αu3+αu=z+αu | x=x0y=y0z=b2+c2t2

## References

1. ↑ Misner & Thorne & Wheeler (1973), p. 163: "Accelerated motion and accelerated observers can be analyzed using special relativity."
2. ↑ Koks (2006), p. 234. "It is sometimes said that to describe physics properly in an accelerated frame, special relativity is insufficient, and the full machinery of general relativity is necessary for the job. This is quite wrong. Special relativity is entirely sufficient to derive the physics of an accelerated frame."
3. ↑ In some textbooks the same formulas and results for flat spacetime are discussed in the framework of GR, using the historical definition that SR is restricted to inertial frames, while accelerated frames belong to the framework of GR. However, since the results are the same in terms of flat spacetime, it does not affect the content of this article. For instance, Møller (1952) discusses successive Lorentz transformations, successive inertial frames, and tetrad transport (now called Fermi–Walker transport) in §§ 46, 47 related to special relativity, while rigid references frames are discussed in section §§ 90, 96 related to general relativity.
4. ↑ 4.0 4.1 Petruv (1964)
5. ↑ 5.0 5.1 Synge (1967)
6. ↑ 6.0 6.1 Letaw (1981)
7. ↑ 7.0 7.1 Møller (1952), §§ 46, 47, 90, 96
8. ↑ Synge (1960), §§ 3, 4
9. ↑ Romain (1963), particularly section VI for the "proper time approach"
10. ↑ 10.0 10.1 Misner & Thorne & Wheeler (1973), section 6.8
11. ↑ Mashhoon (1990), (2003)
12. ↑ 12.0 12.1 12.2 Iyer and Vishveshwara (1993), section 2.2
13. ↑ 13.0 13.1 13.2 Johns (2005), section 18.18
14. ↑ Bini & Cherubini & Geralico & Jantzen (2008), section 3
15. ↑ Gourgoulhon (2013)
16. ↑ Synge (1960), § 3
17. ↑ Iyer and Vishveshwara (1993), section 2.1
18. ↑ Formiga & Romero (2006), section 2
19. ↑ Gourgoulhon (2013), section 2.7.3
20. ↑ 20.0 20.1 20.2 Kajari & Buser & Feiler & Schleich (2009), section 3
21. ↑ 21.0 21.1 Hehl & Lemke & Mielke (1990), section I.6
22. ↑ Padmanabhan (2010), section 4.9
23. ↑ Gourgoulhon (2013), section 3.5.3
24. ↑ 24.0 24.1 24.2 Johns (2005), section 18.19
25. ↑ 25.0 25.1 Bini & Cherubini & Geralico & Jantzen (2008), section 3.2
26. ↑ Maluf & Faria (2008)
27. ↑ Bini & Cherubini & Geralico & Jantzen (2008), section 3.1
28. ↑ Gourgoulhon (2013), eq. 3.58
29. ↑ Irvine (1964), section VII, eq. 41
30. ↑ Bini & Jantzen (2003), Appendix A
31. ↑ 31.0 31.1 Mashhoon (2003), section 3, eq. 1.17, 1.18
32. ↑ Møller (1952), § 46
33. ↑ Møller (1952), § 96
34. ↑ Hehl & Lemke & Mielke (1990), section I.8
35. ↑ Mashhoon & Muench (2002), section 2
36. ↑ Kopeikin & Efroimsky & Kaplan (2011), section 2.6
37. ↑ Synge (1960), § 10
38. ↑ 38.0 38.1 Bini & Lusanna & Mashhoon (2005), Appendix A
39. ↑ Ni & Zimmermann (1978), including Riemannian terms
40. ↑ Hehl & Lemke & Mielke (1990), section I.8, without Riemannian terms
41. ↑ Marzlin (1994), section 2, including Riemannian terms
42. ↑ Nikolić (1999), section 2, without Riemannian terms
43. ↑ 43.0 43.1 Mashhoon & Münch (2002), section 2, without Riemannian terms
44. ↑ Bini & Jantzen (2002), section 2, including Riemannian terms
45. ↑ Voytik (2011), section 2, without Riemannian terms
46. ↑ Misner & Thorne & Wheeler (1973), section 13.6, gave the first order approximation to this metric, without Riemannian terms
47. ↑ Bel (1995), theorem 2
48. ↑ Giulini (2008), Theorem 18
49. ↑ Herglotz (1909), sections 3-4, who focuses on the four rotational motions in addition to hyperbolic motion.
50. ↑ Kottler (1912), § 6; (1914a), table I & II
51. ↑ Letaw & Pfautsch (1982)
52. ↑ Pauri & Vallisneri (2001), Appendix A
53. ↑ Rosu (2000), section 0.2.3
54. ↑ Louko & Satz (2006), section 5.2
55. ↑ Herglotz (1909), p. 408
56. ↑ 56.0 56.1 Kottler (1914a), table I (IIIb); Kottler (1914b), pp. 488-489, 492-493
57. ↑ Petruv (1964), eq. 22
58. ↑ Synge (1967), section 9
59. ↑ Pauri & Vallisneri (2001), eq. 19
60. ↑ Rosu (2000), section 0.2.3, case 2
61. ↑ Møller (1952) eq. 160
62. ↑ 62.0 62.1 Synge (1967) p. 35, type III
63. ↑ Misner & Thorne & Wheeler (1973), section 6.4
64. ↑ Louko & Satz (2006), section 5.2.2
65. ↑ Gron (2006), section 5.5
66. ↑ Formiga (2012), section V-a
67. ↑ Kottler (1914b), pp. 488-489, 492-493
68. ↑ Møller (1952), eq. 154
69. ↑ Misner & Thorne & Wheeler (1973), section 6.6
70. ↑ Muñoz & Jones (2010), eq. 37, 38
71. ↑ Pauli (1921), section 32-y
72. ↑ Rindler (1966), p. 1177
73. ↑ Koks (2006), section 7.2
74. ↑ 74.0 74.1 74.2 Kottler (1914a), table I (IIb) and § 6 section 3
75. ↑ Petruv (1964), eq. 54
76. ↑ 76.0 76.1 76.2 Nožička (1964), example 1
77. ↑ 77.0 77.1 Synge (1967), section 8
78. ↑ Pauri & Vallisneri (2001), eq. 20
79. ↑ Rosu (2000), section 0.2.3, case 3
80. ↑ 80.0 80.1 Formiga (2012), section V-b
81. ↑ Hauck & Mashhoon (2003), section 1
82. ↑ Mashhoon (2003), section 3
83. ↑ Møller (1952), § 47, eq. 164
84. ↑ Louko & Satz (2006), section 5.2.3
85. ↑ Mashhoon (1990), eq. 10-13
86. ↑ Nikolic (1999), eq. 17 (He obtained these formulas by using the transformation of Nelson).
87. ↑ Mashhoon (2003), eq. 1.22-1.25
88. ↑ Herglotz (1909), p. 412, "elliptic group"
89. ↑ Eddington (1920), p. 22.
90. ↑ de Felice (2003), section 2
91. ↑ de Sitter (1916a), p. 178
92. ↑ von Laue (1921), p. 162
93. ↑ Gron (2006), section 5.1
94. ↑ Rizzi & Ruggiero (2002), p. section 5
95. ↑ Ashby (2003), section 2
96. ↑ Herglotz (1909), pp. 408 & 413, "hyperbolic group"
97. ↑ 97.0 97.1 Kottler (1914a), table I (IIIa)
98. ↑ Petruv (1964), eq. 67
99. ↑ 99.0 99.1 Synge (1967), section 6
100. ↑ Pauri & Vallisneri (2001), eq. 22
101. ↑ Rosu (2000), section 0.2.3, case 5
102. ↑ 102.0 102.1 Louko & Satz (2006), section 5.2.5
103. ↑ Herglotz (1909), pp. 413-414, "parabolic group"
104. ↑ 104.0 104.1 Kottler (1914a), table I (IV)
105. ↑ Petruv (1964), eq. 40
106. ↑ 106.0 106.1 Synge (1967), section 7
107. ↑ Pauri & Vallisneri (2001), eq. 21
108. ↑ Rosu (2000), section 0.2.3, case 4
109. ↑ 109.0 109.1 Louko & Satz (2006), section 5.2.4
110. ↑ Herglotz (1909), pp. 411-412, "parabolic group"
111. ↑ 111.0 111.1 Kottler (1914a), table I (case I)
112. ↑ Petruv (1964), eq. 88
113. ↑ 113.0 113.1 Synge (1967), section 4
114. ↑ Pauri & Vallisneri (2001), eq. 23, 24
115. ↑ Rosu (2000), section 0.2.3, case 6
116. ↑ Louko & Satz (2006), section 5.2.6

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## Historical sources

1. ↑ Einstein, Albert (1908), "Über das Relativitätsprinzip und die aus demselben gezogenen Folgerungen", Jahrbuch der Radioaktivität und Elektronik 4: 411–462, Bibcode: 1908JRE.....4..411E, http://www.soso.ch/wissen/hist/SRT/E-1907.pdf ; English translation On the relativity principle and the conclusions drawn from it at Einstein paper project.
2. ↑ Born, Max (1909), "Die Theorie des starren Elektrons in der Kinematik des Relativitätsprinzips", Annalen der Physik 335 (11): 1–56, doi:10.1002/andp.19093351102, Bibcode: 1909AnP...335....1B, https://zenodo.org/record/1424151
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4. ↑ Laue, Max von (1911). Das Relativitätsprinzip. Braunschweig: Vieweg. https://archive.org/details/dasrelativittsp00lauegoog.
5. ↑ 5.0 5.1 Herglotz, Gustav (1910), "Über den vom Standpunkt des Relativitätsprinzips aus als starr zu bezeichnenden Körper", Annalen der Physik 336 (2): 393–415, doi:10.1002/andp.19103360208, Bibcode: 1910AnP...336..393H, https://zenodo.org/record/1424161
6. ↑ 6.0 6.1 6.2 Kottler, Friedrich (1912). "Über die Raumzeitlinien der Minkowski'schen Welt". Wiener Sitzungsberichte 2a 121: 1659–1759. Kottler, Friedrich (1914a). "Relativitätsprinzip und beschleunigte Bewegung". Annalen der Physik 349 (13): 701–748. doi:10.1002/andp.19143491303. Bibcode: 1914AnP...349..701K. http://gallica.bnf.fr/ark:/12148/bpt6k15347v.image.f737. Kottler, Friedrich (1914b). "Fallende Bezugssysteme vom Standpunkte des Relativitätsprinzips". Annalen der Physik 350 (20): 481–516. doi:10.1002/andp.19143502003. Bibcode: 1914AnP...350..481K. http://gallica.bnf.fr/ark:/12148/bpt6k153486.image.f494. Kottler, Friedrich (1916a). "Beschleunigungsrelative Bewegungen und die konforme Gruppe der Minkowski'schen Welt". Wiener Sitzungsberichte 2a 125: 899–919. Kottler, Friedrich (1916b). "Über Einsteins Äquivalenzhypothese und die Gravitation". Annalen der Physik 355 (16): 955–972. doi:10.1002/andp.19163551605. Bibcode: 1916AnP...355..955K. http://gallica.bnf.fr/ark:/12148/bpt6k15352f.image.f964. Kottler, Friedrich (1918). "Über die physikalischen Grundlagen der Einsteinschen Relativitätstheorie". Annalen der Physik 361 (14): 401–461. doi:10.1002/andp.19183611402. Bibcode: 1918AnP...361..401K. http://gallica.bnf.fr/ark:/12148/bpt6k15358h.image.f409. Kottler, Friedrich (1921). "Rotierende Bezugssysteme in einer Minkowskischen Welt". Physikalische Zeitschrift 22: 274–280 & 480–484.
7. ↑ Lemaître, G. (1924), "The motion of a rigid solid according to the relativity principle", Philosophical Magazine, Series 6 48 (283): 164–176, doi:10.1080/14786442408634478

## External links

* Physics FAQ: Acceleration in Special Relativity
* Eric Gourgoulhon (2010): Special relativity from an accelerated observer perspective

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