Editing Hyperbolic motion (relativity)

{{short description|Motion of an object with constant proper acceleration in special relativity.}}
[[Image:HyperbolicMotion.svg|thumb|Hyperbolic motion can be visualized on a Minkowski diagram, where the motion of the accelerating particle is along the <math>X</math>-axis. Each hyperbola is defined by 
<math>x=\pm c^2/\alpha</math> and <math>\eta=\alpha\tau/c</math> (with <math>c=1, \alpha=1</math>) in equation ({{equationNote|2}}).]]

'''Hyperbolic motion''' is the motion of an object with constant [[proper acceleration]] in [[special relativity]].  It is called hyperbolic motion because the equation describing the path of the object through [[spacetime]] is a [[hyperbola]], as can be seen when graphed on a [[Minkowski diagram]] whose coordinates represent a suitable inertial (non-accelerated) frame. This motion has several interesting features, among them that it is possible to outrun a [[photon]] if given a sufficient head start, as may be concluded from the diagram.<ref>{{harvnb|Misner|Thorne|Wheeler|1973|loc=Chapter 6.}}</ref>

==History==
[[Hermann Minkowski]] (1908) showed the relation between a point on a [[worldline]] and the magnitude of [[four-acceleration]] and a "curvature hyperbola" ({{langx|de|Krümmungshyperbel}}).<ref>{{Cite journal|author=Minkowski, Hermann|year=1909|title=Raum und Zeit. Vortrag, gehalten auf der 80. Naturforscher-Versammlung zu Köln am 21. September 1908.|trans-title=Wikisource translation: [[s:Space and Time|Space and Time]]|journal=Jahresbericht der Deutschen Mathematiker-Vereinigung|location=Leipzig|title-link=s:de:Raum und Zeit (Minkowski)}}</ref> In the context of [[Born rigidity]], [[Max Born]] (1909) subsequently coined the term "hyperbolic motion" ({{langx|de|Hyperbelbewegung}}) for the case of constant magnitude of four-acceleration, then provided a detailed description for [[Electric charge|charged]] particles in hyperbolic motion, and introduced the corresponding "hyperbolically accelerated reference system" ({{langx|de|hyperbolisch beschleunigtes Bezugsystem}}).<ref name=born>{{Cite journal|author=Born, Max|year=1909|title=Die Theorie des starren Elektrons in der Kinematik des Relativitätsprinzips|trans-title=Wikisource translation: [[s:Translation:The Theory of the Rigid Electron in the Kinematics of the Principle of Relativity|The Theory of the Rigid Electron in the Kinematics of the Principle of Relativity]]|journal=Annalen der Physik|volume=335|issue=11|pages=1–56 |doi=10.1002/andp.19093351102|bibcode=1909AnP...335....1B|url=https://zenodo.org/record/1424151}}</ref> Born's formulas were simplified and extended by [[Arnold Sommerfeld]] (1910).<ref name=sommerfeld /> For early reviews see the textbooks by [[Max von Laue]] (1911, 1921)<ref name=laue /> or [[Wolfgang Pauli]] (1921).<ref name=pauli /> See also Galeriu (2015)<ref name=Galeriu /> or Gourgoulhon (2013),<ref>{{cite book |author=Gourgoulhon, E. |year=2013 |title=Special Relativity in General Frames: From Particles to Astrophysics |pages=396|publisher=Springer|isbn=978-3642372766}}</ref> and [[Acceleration (special relativity)#History]].

==Worldline==
The proper acceleration <math>\alpha</math> of a particle is defined as the [[acceleration]] that a particle "feels" as it accelerates from one [[inertial reference frame]] to another. If the proper acceleration is directed parallel to the line of motion, it is related to the ordinary  [[acceleration (special relativity)#Three-acceleration|three-acceleration in special relativity]] <math>a=du/dT</math> by

:<math>\alpha=\gamma^3 a=\frac{1}{\left(1-u^2/c^2\right)^{3/2}}\frac{du}{dT},</math>

where <math>u</math> is the instantaneous speed of the particle, <math>\gamma</math> the [[Lorentz factor]], <math>c</math> is the [[speed of light]], and <math>T</math> is the coordinate time.  Solving for the [[equation of motion]] gives the desired formulas, which can be expressed in terms of coordinate time <math>T</math> as well as [[proper time]] <math>\tau</math>. For simplification, all initial values for time, location, and velocity can be set to 0, thus:<ref name=laue>{{Cite book|author=von Laue, M.|year=1921|title=Die Relativitätstheorie, Band 1|edition=fourth edition of "Das Relativitätsprinzip"|pages=[https://archive.org/details/dierelativitts01laueuoft/page/89 89]–90, 155–166|publisher=Vieweg|url=https://archive.org/details/dierelativitts01laueuoft}}; First edition 1911, second expanded edition 1913, third expanded edition 1919.</ref><ref name=pauli>{{Citation
|author=Pauli, Wolfgang|author-link=Wolfgang Pauli|year=1921
|journal=Encyclopädie der Mathematischen Wissenschaften|title= Die Relativitätstheorie|pages=539–776|volume=5|issue=2
|url=http://resolver.sub.uni-goettingen.de/purl?PPN360709672%7CLOG_0265}} 
<br />In English: {{cite book|author=Pauli, W.|title=Theory of Relativity|series=Fundamental Theories of Physics|volume=165|publisher=Dover Publications|year=1981|orig-year=1921|isbn=0-486-64152-X}}</ref><ref>{{Cite book|author=Møller, C.|title=The theory of relativity|year=1955|pages=[https://archive.org/details/theoryofrelativi029229mbp/page/n91 74]–75|publisher=Oxford Clarendon Press|url=https://archive.org/details/theoryofrelativi029229mbp}}</ref><ref>{{Cite book|author=Rindler, W.|title=Essential Relativity|url=https://archive.org/details/essentialrelativ00rind_279|url-access=limited|year=1977|pages=[https://archive.org/details/essentialrelativ00rind_279/page/n61 49]–50|publisher=Springer|isbn=354007970X}}</ref><ref>PhysicsFAQ (2016), "Relativistic rocket", see external links</ref>

{{NumBlk|:|<math>\begin{array}{c|c}
\begin{align}u(T) & =\frac{\alpha T}{\sqrt{1+\left(\frac{\alpha T}{c}\right)^{2}}}\\
 & =c\tanh\left(\operatorname{arsinh}\frac{\alpha T}{c}\right)\\
X(T) & =\frac{c^{2}}{\alpha}\left(\sqrt{1+\left(\frac{\alpha T}{c}\right)^{2}}-1\right)\\
 & =\frac{c^{2}}{\alpha}\left(\cosh\left(\operatorname{arsinh}\frac{\alpha T}{c}\right)-1\right)\\
c\tau(T) & =\frac{c^{2}}{\alpha}\ln\left(\sqrt{1+\left(\frac{\alpha T}{c}\right)^{2}}+\frac{\alpha T}{c}\right)\\
 & =\frac{c^{2}}{\alpha}\operatorname{arsinh}\frac{\alpha T}{c}
\end{align}
 & \begin{align}u(\tau) & =c\tanh\frac{\alpha\tau}{c}\\
\\
X(\tau) & =\frac{c^{2}}{\alpha}\left(\cosh\frac{\alpha\tau}{c}-1\right)\\
\\
cT(\tau) & =\frac{c^{2}}{\alpha}\sinh\frac{\alpha\tau}{c}\\
\\
\\
\end{align}
\end{array}</math>|{{equationRef|1}}}}

This gives <math>\left(X+c^{2}/\alpha\right)^{2}-c^{2}T^{2}=c^{4}/\alpha^{2}</math>, which is a hyperbola in time T and the spatial location variable <math>X</math>. In this case, the accelerated object is located at <math>X=0</math> at time <math>T=0</math>. If instead there are initial values different from zero, the formulas for hyperbolic motion assume the form:<ref>{{Cite book|author=Gallant, J.|title=Doing Physics with Scientific Notebook: A Problem Solving Approach|year=2012|pages=437–441|publisher=John Wiley & Sons|isbn=978-0470665978}}</ref><ref>{{Cite journal|author=Müller, T., King, A., & Adis, D.|year=2006|title=A trip to the end of the universe and the twin "paradox"|journal=American Journal of Physics|volume=76|issue=4|pages=360–373|arxiv=physics/0612126|doi=10.1119/1.2830528|bibcode=2008AmJPh..76..360M|s2cid=42983285}}</ref><ref name=fraundorf>{{Cite arXiv|author=Fraundorf, P. |year=2012|title=A traveler-centered intro to kinematics|pages=IV–B|class=physics.pop-ph |eprint=1206.2877}}</ref>

:<math>{\scriptstyle \begin{array}{c|c}
\begin{align}u(T) & =\frac{u_{0}\gamma_{0}+\alpha T}{\sqrt{1+\left(\frac{u_{0}\gamma_{0}+\alpha T}{c}\right)^{2}}}\quad\\
 & =c\tanh\left\{ \operatorname{arsinh}\left(\frac{u_{0}\gamma_{0}+\alpha T}{c}\right)\right\} \\
X(T) & =X_{0}+\frac{c^{2}}{\alpha}\left(\sqrt{1+\left(\frac{u_{0}\gamma_{0}+\alpha T}{c}\right)^{2}}-\gamma_{0}\right)\\
 & =X_{0}+\frac{c^{2}}{\alpha}\left\{ \cosh\left[\operatorname{arsinh}\left(\frac{u_{0}\gamma_{0}+\alpha T}{c}\right)\right]-\gamma_{0}\right\} \\
c\tau(T) & =c\tau_{0}+\frac{c^{2}}{\alpha}\ln\left(\frac{\sqrt{c^{2}+\left(u_{0}\gamma_{0}+\alpha T\right){}^{2}}+u_{0}\gamma_{0}+\alpha T}{\left(c+u_{0}\right)\gamma_{0}}\right)\\
 & =c\tau_{0}+\frac{c^{2}}{\alpha}\left\{ \operatorname{arsinh}\left(\frac{u_{0}\gamma_{0}+\alpha T}{c}\right)-\operatorname{artanh}\left(\frac{u_{0}}{c}\right)\right\} 
\end{align}
 & \begin{align}u(\tau) & =c\tanh\left\{ \operatorname{artanh}\left(\frac{u_{0}}{c}\right)+\frac{\alpha\tau}{c}\right\} \\
\\
X(\tau) & =X_{0}+\frac{c^{2}}{\alpha}\left\{ \cosh\left[\operatorname{artanh}\left(\frac{u_{0}}{c}\right)+\frac{\alpha\tau}{c}\right]-\gamma_{0}\right\} \\
\\
cT(\tau) & =cT_{0}+\frac{c^{2}}{\alpha}\left\{ \sinh\left[\operatorname{artanh}\left(\frac{u_{0}}{c}\right)+\frac{\alpha\tau}{c}\right]-\frac{u_{0}\gamma_{0}}{c}\right\} 
\end{align}
\end{array}}</math>

==Rapidity==
The worldline for hyperbolic motion (which from now on will be written as a function of proper time) can be simplified in several ways. For instance, the expression

:<math>X=\frac{c^{2}}{\alpha}\left(\cosh\frac{\alpha\tau}{c}-1\right)</math>

can be subjected to a spatial shift of amount <math>c^2/\alpha</math>, thus

:<math>X=\frac{c^{2}}{\alpha}\cosh\frac{\alpha\tau}{c}</math>,<ref>Pauli (1921), p. 628, used the notation <math>x^{4}=a\operatorname{ch}\frac{ct}{a}</math> where <math>a=\frac{c^{2}}{b}</math></ref>

by which the observer is at position <math>X=c^2/\alpha</math> at time <math>T=0</math>. Furthermore, by setting <math>x=c^2/\alpha</math> and introducing the [[rapidity]] <math>\eta=\operatorname{artanh}\frac{u}{c}=\frac{\alpha\tau}{c}</math>,<ref name=fraundorf /> the equations for hyperbolic motion reduce to<ref name=sommerfeld>{{Cite journal |author=Sommerfeld, Arnold |year=1910 |title=Zur Relativitätstheorie II: Vierdimensionale Vektoranalysis|trans-title=Wikisource translation: [[s:Translation:On the Theory of Relativity II: Four-dimensional Vector Analysis|On the Theory of Relativity II: Four-dimensional Vector Analysis]] |journal=Annalen der Physik |volume=338 |issue=14 |pages=649–689|doi=10.1002/andp.19103381402|bibcode=1910AnP...338..649S|url=https://zenodo.org/record/1424179 }}</ref><ref name=sommerfeld2>Sommerfeld (1910), pp. 670-671 used the form <math>x=r\cos(\varphi)</math> and <math>l=r\sin(\varphi)</math> with the imaginary angle <math>i\psi</math> and imaginary time <math>l=ict</math>.</ref>

{{NumBlk|:|<math>cT=x\sinh\eta,\quad X=x\cosh\eta</math>|{{equationRef|2}}}}

with the hyperbola <math>X^{2}-c^{2}T^{2}=x^{2}</math>.

==Charged particles in hyperbolic motion==

Born (1909),<ref name=born /> Sommerfeld (1910),<ref name=sommerfeld /> von Laue (1911),<ref name=laue /> Pauli (1921)<ref name=pauli /> also formulated the equations for the [[electromagnetic field]] of [[charged particle]]s in hyperbolic motion.<ref name=Galeriu>{{Cite journal|author=Galeriu, C.|year=2017|orig-year=2015|title=Electric charge in hyperbolic motion: the early history|journal=Archive for History of Exact Sciences|volume=71|issue=4|pages=1–16|doi=10.1007/s00407-017-0191-x|arxiv=1509.02504|s2cid=118510589}}</ref> This was extended by [[Hermann Bondi]] & [[Thomas Gold]] (1955)<ref name=Bondi>{{Cite journal|author=Bondi, H., & Gold, T.|year=1955|title=The field of a uniformly accelerated charge, with special reference to the problem of gravitational acceleration|journal=Proceedings of the Royal Society of London|volume=229|issue=1178|pages=416–424|doi=10.1098/rspa.1955.0098|bibcode=1955RSPSA.229..416B|s2cid=121563673}}</ref> and Fulton & Rohrlich (1960)<ref name=Fulton>{{Cite journal|author1=Fulton, Thomas |author2=Rohrlich, Fritz |year=1960|title=Classical radiation from a uniformly accelerated charge|journal=Annals of Physics|volume=9|issue=4|pages=499–517|doi=10.1016/0003-4916(60)90105-6|bibcode=1960AnPhy...9..499F}}</ref><ref name=Rohrlich>{{Cite journal|author=Rohrlich, Fritz|year=1963|title=The principle of equivalence|journal=Annals of Physics|volume=22|issue=2|pages=169–191|doi=10.1016/0003-4916(63)90051-4|bibcode=1963AnPhy..22..169R}}</ref>

:<math>\begin{align}E_{\rho'}'= & \frac{\left(8e/\alpha^{2}\right)\rho'z'}{\xi^{\prime3}}\\
E_{z'}'= & \frac{-\left(4e/\alpha^{2}\right)1/\alpha^{2}+t^{\prime2}+\rho^{\prime2}-z^{\prime2}}{\xi^{\prime3}}\\
E_{\varphi'}'= & H_{\varphi'}'=H_{z'}'=0\\
H_{\varphi'}'= & \frac{\left(8e/\alpha^{2}\right)\rho't'}{\xi^{\prime3}}\\
\xi'= & \sqrt{\left(1/\alpha^{2}+t^{\prime2}-\rho^{\prime2}-z^{\prime2}\right)^{2}+\left(2\rho'/\alpha\right)^{2}}
\end{align}</math>

This is related to the controversially<ref name=Lyle>{{Cite book|author=Stephen Lyle|title=Uniformly Accelerating Charged Particles: A Threat to the Equivalence Principle|year=2008|publisher=Springer|isbn=978-3540684770}}</ref><ref name=Gron3>{{Cite journal|author=Øyvind Grøn|year=2012|title=Review Article: Electrodynamics of Radiating Charges|journal=Advances in Mathematical Physics|volume=2012|pages=528631|doi=10.1155/2012/528631|doi-access=free|hdl=10642/1380|hdl-access=free}}</ref> discussed question, whether charges in perpetual hyperbolic motion do radiate or not, and whether this is consistent with the [[equivalence principle]] – even though it is about an ideal situation, because perpetual hyperbolic motion is not possible. While early authors such as Born (1909) or Pauli (1921) argued that no radiation arises, later authors such as Bondi & Gold<ref name=Bondi /> and Fulton & Rohrlich<ref name=Fulton /><ref name=Rohrlich /> showed that radiation does indeed arise.

==Proper reference frame==
[[Image:Event-horizon-particle.svg|right|thumb|The light path through '''E''' marks the apparent event horizon of an observer '''P''' in hyperbolic motion.]]
{{Main|Rindler coordinates|Proper reference frame (flat spacetime)}}
In equation ({{equationNote|2}}) for hyperbolic motion, the expression <math>x</math> was constant, whereas the rapidity <math>\eta</math> was variable. However, as pointed out by Sommerfeld,<ref name=sommerfeld2 /> one can define <math>x</math> as a variable, while making <math>\eta</math> constant. This means, that the equations become transformations indicating the simultaneous rest shape of an accelerated body with hyperbolic coordinates <math>(x,y,z,\eta)</math> as seen by a comoving observer

:<math>cT=x\sinh\eta,\quad X=x\cosh\eta,\quad Y=y,\quad Z=z</math>

By means of this transformation, the proper time becomes the time of the hyperbolically accelerated frame. These coordinates, which are commonly called Rindler coordinates (similar variants are called [[Rindler coordinates#Variants of transformation formulas|Kottler-Møller coordinates or Lass coordinates]]), can be seen as a special case of Fermi coordinates or Proper coordinates, and are often used in connection with the [[Unruh effect]]. Using these coordinates, it turns out that observers in hyperbolic motion possess an apparent [[event horizon]], beyond which no signal can reach them.

==Special conformal transformation==
A lesser known method for defining a reference frame in hyperbolic motion is the employment of the [[special conformal transformation]], consisting of an [[Inversive geometry|inversion]], a [[translation (physics)|translation]], and another inversion.<ref>Galeriu, Cǎlin (2019) "Electric charge in hyperbolic motion: the special conformal solution", [[European Journal of Physics]] 40(6) {{doi| 10.1088/1361-6404/ab3df6}}</ref> It is commonly interpreted as a [[gauge transformation]] in Minkowski space, though some authors alternatively use it as an acceleration transformation (see Kastrup for a critical historical survey).<ref name=kastrup /> It has the form

:<math>X^{\mu}=\frac{x^{\mu}-a^{\mu}x^{2}}{1-2ax+a^{2}x^{2}}</math>

Using only one spatial dimension by <math>x^{\mu}=(t,x)</math>, and further simplifying by setting  <math>x=0</math>, and using the acceleration  <math>a^{\mu}=(0,-\alpha/2)</math>, it follows<ref name=Fulton2>{{Cite journal|author=Fulton, T., Rohrlich, F., & Witten, L.|year=1962|title=Physical consequences of a co-ordinate transformation to a uniformly accelerating frame|journal=Il Nuovo Cimento|volume=26|issue=4|pages=652–671|doi=10.1007/BF02781794|bibcode=1962NCim...26..652F|s2cid=121467786}}</ref>

:<math>T=\frac{t}{1-\frac{1}{4}\alpha{}^{2}t^{2}},\quad X=\frac{-\alpha t^{2}}{2\left(1-\frac{1}{4}\alpha{}^{2}t^{2}\right)}</math>

with the hyperbola <math>\left(X-1/\alpha\right)^{2}-T^{2}=1/\alpha^{2}</math>. It turns out that at <math>t=\pm(x+2/\alpha)</math> the time becomes singular, to which Fulton & Rohrlich & Witten<ref name=Fulton2 /> remark that one has to stay away from this limit, while Kastrup<ref name=kastrup>{{Cite journal|author=Kastrup, H. A.|title=On the advancements of conformal transformations and their associated symmetries in geometry and theoretical physics|journal=Annalen der Physik|volume=520|issue=9–10|year=2008|pages=631–690|arxiv=0808.2730|doi=10.1002/andp.200810324|bibcode = 2008AnP...520..631K |s2cid=12020510}}</ref> (who is very critical of the acceleration interpretation) remarks that this is one of the strange results of this interpretation.

== References ==
{{Reflist}}

== Further reading ==
* {{cite journal |author=Leigh Page |title=A New Relativity. Paper I. Fundamental Principles and Transformations Between Accelerated Systems |journal=[[Physical Review]] |volume=49 |issue=3 |pages=254–268 |date=Feb 1936 |doi=10.1103/PhysRev.49.254|bibcode = 1936PhRv...49..254P |author-link=Leigh Page }}
* {{cite journal |author=Leigh Page & Norman I. Adams |title=A New Relativity. Paper II. Transformation of the Electromagnetic Field Between Accelerated Systems and the Force Equation |journal=Physical Review |volume=49 |issue=6 |pages=466–469 |date=Mar 1936 |doi=10.1103/PhysRev.49.466|bibcode = 1936PhRv...49..466P }} 
* {{Citation|first1=Charles W.|last1=Misner|author-link=Charles W. Misner|first2=Kip. S.|last2=Thorne|author2-link=Kip Thorne|first3=John A.|last3=Wheeler|author3-link=John A. Wheeler|title=Gravitation|at=Chapter 6|publisher= W. H. Freeman|date=1973|isbn=0-7167-0344-0|title-link=Gravitation (book)}}
* {{cite journal | author = Rindler Wolfgang  | year = 1960 | title = Hyperbolic Motion in Curved Space Time | journal = [[Physical Review]] | volume = 119 | issue = 6| pages = 2082–2089 | doi = 10.1103/PhysRev.119.2082 | bibcode = 1960PhRv..119.2082R }}
* [[Ludwik Silberstein]] (1914): ''[https://archive.org/details/theoryofrelativi00silbrich The Theory of Relativity]'', page 190.
* Naber, Gregory L., ''The Geometry of Minkowski Spacetime'', Springer-Verlag, New York, 1992. {{ISBN|0-387-97848-8}} (hardcover), {{ISBN|0-486-43235-1}} (Dover paperback edition). pp 58–60.

== External links==
*Physics FAQ: [http://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/rocket.html The Relativistic Rocket]
*Mathpages: [http://www.mathpages.com/rr/s2-09/2-09.htm Accelerated Travels], [http://www.mathpages.com/home/kmath528/kmath528.htm Does A Uniformly Accelerating Charge Radiate?]

[[Category:Theory of relativity]]
[[Category:Special relativity]]
[[Category:General relativity]]
[[Category:Acceleration]]