In Theory of relativity, relativity, rapidity is commonly used as a measure for relativistic velocity. Mathematically, rapidity can be defined as the hyperbolic angle that differentiates two frames of reference in relative motion, each frame being associated with distance and time coordinates.
For one-dimensional motion, rapidities are additive whereas velocities must be combined by Einstein's Velocity-addition formula#Special relativity, velocity-addition formula. For low speeds, rapidity and velocity are proportional but, for higher velocities, rapidity takes a larger value, with the rapidity of light being infinite.
Using the inverse hyperbolic function , the rapidity corresponding to velocity is where c is the velocity of light. For low speeds, is approximately . Since in relativity any velocity is constrained to the interval the ratio satisfies . The inverse hyperbolic tangent has the unit interval for its domain of a function, domain and the whole real line for its range (function), range, and so the interval maps onto .

`β`. We relate rapidities to the velocity-addition formula#Special theory of relativity, velocity-addition formula
:$u\; =\; \backslash frac$
by recognizing
:$\backslash beta\_i\; =\; \backslash frac\; =\; \backslash tanh$
and so
:$\backslash begin\; \backslash tanh\; w\; \&=\; \backslash frac\; \backslash \backslash \; \&=\; \backslash tanh(w\_1+\; w\_2)\; \backslash end$
Proper acceleration (the acceleration 'felt' by the object being accelerated) is the rate of change of rapidity with respect to proper time (time as measured by the object undergoing acceleration itself). Therefore, the rapidity of an object in a given frame can be viewed simply as the velocity of that object as would be calculated non-relativistically by an inertial guidance system on board the object itself if it accelerated from rest in that frame to its given speed.
The product of and appears frequently, and is from the above arguments
:$\backslash beta\; \backslash gamma\; =\; \backslash sinh\; w\; \backslash ,.$

"The Review of Particle Physics"

''Physics Letters B'' 667 (2008) 1, Section 38.5.2 This is the rapidity of the boost along the beam axis which takes an observer from the lab frame to a frame in which the particle moves only perpendicular to the beam. Related to this is the concept of pseudorapidity. Rapidity relative to a beam axis can also be expressed as :$y\; =\; \backslash ln\; \backslash frac\; ~.$

History

In 1908 Hermann Minkowski explained how the Lorentz transformation could be seen as simply a hyperbolic rotation of the Coordinate time, spacetime coordinates, i.e., a rotation through an imaginary angle. This angle therefore represents (in one spatial dimension) a simple additive measure of the velocity between frames. The rapidity parameter replacing velocity was introduced in 1910 by Vladimir Varićak and by E. T. Whittaker. The parameter was named ''rapidity'' by Alfred Robb (1911) and this term was adopted by many subsequent authors, such as Ludwik Silberstein, Silberstein (1914), Edward Morley, Morley (1936) and Wolfgang Rindler, Rindler (2001).Area of a hyperbolic sector

The quadrature (mathematics), quadrature of the hyperbola ''xy'' = 1 by Gregoire de Saint-Vincent established the natural logarithm as the area of a hyperbolic sector, or an equivalent area against an asymptote. In spacetime theory, the connection of events by light divides the universe into Past, Future, or Elsewhere based on a Here and Now . On any line in space, a light beam may be directed left or right. Take the x-axis as the events passed by the right beam and the y-axis as the events of the left beam. Then a resting frame has time along the diagonal ''x'' = ''y''. The rectangular hyperbola ''xy'' = 1 can be used to gauge velocities (in the first quadrant). Zero velocity corresponds to (1,1). Any point on the hyperbola has coordinates $(\; e^w\; ,\; \backslash \; e^\; )$ where w is the rapidity, and is equal to the area of the hyperbolic sector from (1,1) to these coordinates. Many authors refer instead to the unit hyperbola $x^2\; -\; y^2\; ,$ using rapidity for parameter, as in the standard spacetime diagram. There the axes are measured by clock and meter-stick, more familiar benchmarks, and the basis of spacetime theory. So the delineation of rapidity as hyperbolic parameter of beam-space is a reference to the seventeenth century origin of our precious transcendental functions, and a supplement to spacetime diagramming.In one spatial dimension

The rapidity arises in the linear representation of a Lorentz boost as a vector-matrix product :$\backslash begin\; c\; t\text{'}\; \backslash \backslash \; x\text{'}\; \backslash end\; =\; \backslash begin\; \backslash cosh\; w\; \&\; -\backslash sinh\; w\; \backslash \backslash \; -\backslash sinh\; w\; \&\; \backslash cosh\; w\; \backslash end\; \backslash begin\; ct\; \backslash \backslash \; x\; \backslash end\; =\; \backslash mathbf\; \backslash Lambda\; (w)\; \backslash begin\; ct\; \backslash \backslash \; x\; \backslash end$. The matrix is of the type $\backslash begin\; p\; \&\; q\; \backslash \backslash \; q\; \&\; p\; \backslash end$ with and satisfying , so that lies on the unit hyperbola. Such matrices form the indefinite orthogonal group, indefinite orthogonal group O(1,1) with one-dimensional Lie algebra spanned by the anti-diagonal unit matrix, showing that the rapidity is the coordinate on this Lie algebra. This action may be depicted in a spacetime diagram. In matrix exponential notation, can be expressed as $\backslash mathbf\; \backslash Lambda\; (w)\; =\; e^$, where is the negative of the anti-diagonal unit matrix :$\backslash mathbf\; Z\; =\; \backslash begin\; 0\; \&\; -1\; \backslash \backslash \; -1\; \&\; 0\; \backslash end\; .$ It is not hard to prove that :$\backslash mathbf(w\_1\; +\; w\_2)\; =\; \backslash mathbf(w\_1)\backslash mathbf(w\_2)$. This establishes the useful additive property of rapidity: if , and are frames of reference, then :$w\_=\; w\_\; +\; w\_$ where denotes the rapidity of a frame of reference relative to a frame of reference . The simplicity of this formula contrasts with the complexity of the corresponding velocity-addition formula#Special theory of relativity, velocity-addition formula. As we can see from the Lorentz transformation above, the Lorentz factor identifies with :$\backslash gamma\; =\; \backslash frac\; \backslash equiv\; \backslash cosh\; w$, so the rapidity is implicitly used as a hyperbolic angle in the Lorentz transformation expressions using andExponential and logarithmic relations

From the above expressions we have :$e^\; =\; \backslash gamma(1\; +\; \backslash beta)\; =\; \backslash gamma\; \backslash left(\; 1\; +\; \backslash frac\; \backslash right)\; =\; \backslash sqrt\; \backslash frac,$ and thus :$e^\; =\; \backslash gamma(1\; -\; \backslash beta)\; =\; \backslash gamma\; \backslash left(\; 1\; -\; \backslash frac\; \backslash right)\; =\; \backslash sqrt\; \backslash frac.$ or explicitly :$w\; =\; \backslash ln\; \backslash left[\backslash gamma(1\; +\; \backslash beta)\backslash right]\; =\; -\backslash ln\; \backslash left[\backslash gamma(1\; -\; \backslash beta)\backslash right]\; \backslash ,\; .$ The relativistic Doppler effect, Doppler-shift factor associated with rapidity is $k\; =\; e^w$.In more than one spatial dimension

The relativistic velocity $\backslash boldsymbol\; \backslash beta$ is associated to the rapidity $\backslash mathbf$ of an object via :$\backslash mathfrak(3,1)\; \backslash supset\; \backslash mathrm\backslash \; \backslash approx\; \backslash mathbb^3\; \backslash ni\; \backslash mathbf\; =\; \backslash boldsymbol\; \backslash tanh^\backslash beta,\; \backslash quad\; \backslash boldsymbol\; \backslash in\; \backslash mathbb^3,$ where the vector $\backslash mathbf\; w$ is thought of as Cartesian coordinates on the 3-dimensional subspace of the Lie algebra $\backslash mathfrak(3,\; 1)\; \backslash approx\; \backslash mathfrak(3,\; 1)$ of the Lorentz group spanned by the Representation theory of the Lorentz group#Conventions and Lie algebra bases, boost generators $K\_1,\; K\_2,\; K\_3$ – in complete analogy with the one-dimensional case $\backslash mathfrak(1,\; 1)$ discussed above – and ''velocity space'' is represented by the open ball $\backslash mathbb\; B^3$ with radius $1$ since $,\; \backslash beta,\; <\; 1$. The latter follows from that $c$ is a limiting velocity in relativity (with units in which $c\; =\; 1$). The general formula for composition of rapidities isThis is to be understood in the sense that given two velocities, the resulting rapidity is the rapidity corresponding to the two velocities ''relativistically added''. Rapidities also have the ordinary addition inherited from $\backslash mathbb\; R^3$, and context decides which operation to use. :$\backslash mathbf\; w\; =\; \backslash boldsymbol\backslash tanh^\backslash beta,\; \backslash quad\; \backslash boldsymbol\; \backslash beta\; =\; \backslash boldsymbol\; \backslash beta\_1\; \backslash oplus\; \backslash boldsymbol\; \backslash beta\_2,$ where $\backslash boldsymbol\; \backslash beta\_1\; \backslash oplus\; \backslash boldsymbol\; \backslash beta\_2$ refers to velocity addition formula, relativistic velocity addition and $\backslash boldsymbol\; \backslash hat\; \backslash beta$ is a unit vector in the direction of $\backslash boldsymbol\; \backslash beta$. This operation is not commutative nor associative. Rapidities $\backslash mathbf\; w\_1,\; \backslash mathbf\; w\_2$ with directions inclined at an angle $\backslash theta$ have a resultant norm $w\; \backslash equiv\; ,\; \backslash mathbf\; w,$ (ordinary Euclidean length) given by the hyperbolic law of cosines, :$\backslash cosh\; w\; =\; \backslash cosh\; w\_1\backslash cosh\; w\_2\; +\; \backslash sinh\; w\_1\backslash sinh\; w\_2\; \backslash cos\; \backslash theta.$ The geometry on rapidity space is inherited from the hyperbolic geometry on velocity space via the map stated. This geometry, in turn, can be inferred from the addition law of relativistic velocities. Rapidity in two dimensions can thus be usefully visualized using the Poincaré disk model, Poincaré disk. Geodesics correspond to steady accelerations. Rapidity space in three dimensions can in the same way be put in Isometry (Riemannian geometry), isometry with the hyperboloid model (isometric to the -dimensional Poincaré disk (or ''ball'')). This is detailed in Minkowski space#geometry of Minkowski space, geometry of Minkowski space. The addition of two rapidities results not ''only'' in a new rapidity; the resultant total transformation is the composition of the transformation corresponding to the rapidity given above and a ''rotation'' parametrized by the vector $\backslash boldsymbol\; \backslash theta$, :$\backslash Lambda\; =\; e^e^,$ where the physicist convention for the exponential mapping is employed. This is a consequence of the commutation rule :$[K\_i,\; K\_j]\; =\; -i\backslash epsilon\_J\_k,$ where $J\_k,\; k\; =\; 1,\; 2,\; 3,$ are the Rotation group SO(3), generators of rotation. This is related to the phenomenon of Thomas precession. For the computation of the parameter $\backslash boldsymbol\; \backslash theta$, the linked article is referred to.In experimental particle physics

The energy and scalar momentum of a particle of non-zero (rest) mass are given by: :$E\; =\; \backslash gamma\; mc^2$ :$,\; \backslash mathbf\; p\; ,\; =\; \backslash gamma\; mv.$ With the definition of :$w\; =\; \backslash operatorname\; \backslash frac,$ and thus with :$\backslash cosh\; w\; =\; \backslash cosh\; \backslash left(\; \backslash operatorname\; \backslash frac\; \backslash right)\; =\; \backslash frac\; =\; \backslash gamma$ :$\backslash sinh\; w\; =\; \backslash sinh\; \backslash left(\; \backslash operatorname\; \backslash frac\; \backslash right)\; =\; \backslash frac\; =\; \backslash beta\; \backslash gamma\; ,$ the energy and scalar momentum can be written as: :$E\; =\; m\; c^2\; \backslash cosh\; w$ :$,\; \backslash mathbf\; p\; ,\; =\; m\; c\; \backslash ,\; \backslash sinh\; w.$ So, rapidity can be calculated from measured energy and momentum by :$w\; =\; \backslash operatorname\; \backslash frac=\; \backslash frac\; \backslash ln\; \backslash frac=\; \backslash ln\; \backslash frac\; ~.$ However, experimental particle physicists often use a modified definition of rapidity relative to a beam axis :$y\; =\; \backslash frac\; \backslash ln\; \backslash frac\; ,$ where is the component of momentum along the beam axis.Amsler, C. ''et al.''"The Review of Particle Physics"

''Physics Letters B'' 667 (2008) 1, Section 38.5.2 This is the rapidity of the boost along the beam axis which takes an observer from the lab frame to a frame in which the particle moves only perpendicular to the beam. Related to this is the concept of pseudorapidity. Rapidity relative to a beam axis can also be expressed as :$y\; =\; \backslash ln\; \backslash frac\; ~.$

See also

* Bondi k-calculus * Lorentz transformation * Pseudorapidity * Proper velocity * Theory of relativityRemarks

Notes and references

* Vladimir Varićak, Varićak V (1910), (1912), (1924) See Vladimir Varićak#Publications * * * Émile Borel, Borel E (1913) La théorie de la relativité et la cinématique, Comptes Rendus Acad Sci Paris 156 215-218; 157 703-705 * * Vladimir Karapetoff (1936)"Restricted relativity in terms of hyperbolic functions of rapidities", American Mathematical Monthly 43:70. * Frank Morley (1936) "When and Where", ''The Criterion'', edited by T.S. Eliot, 15:200-2009. * Wolfgang Rindler (2001) ''Relativity: Special, General, and Cosmological'', page 53, Oxford University Press. * Shaw, Ronald (1982) ''Linear Algebra and Group Representations'', v. 1, page 229, Academic Press . * (see page 17 of e-link) * * {{Relativity Special relativity