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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.

In relativity, rapidity denoted by w is commonly used as a measure for relativistic velocity. For one-dimensional motion, rapidities are additive whereas velocities must be combined by Einstein's Velocity-addition formula. For low speeds, rapidity and velocity are proportional, but for higher velocities, rapidity takes a larger value, the rapidity of light being infinite.

Using the inverse hyperbolic function artanh, the rapidity w corresponding to velocity v is w = artanh(v / c) where c is the velocity of light. For low speeds, w is approximately v / c. Since in relativity any velocity v is constrained to the interval c < v < c the ratio v / c satisfies −1 < v / c < 1. The inverse hyperbolic tangent has the unit interval (−1, 1) for its domain and the whole real line for its range, and so the interval c < v < c maps onto −∞ < w < ∞.


In 1908 Hermann Minkowski explained how the Lorentz transformation could be seen as simply a hyperbolic rotation of the spacetime coordinates, i.e., a rotation through an imaginary angle.[1] This angle therefore represents (in one spatial dimension) a simple additive measure of the velocity between frames.[2] It was used by Varićak 1910 by Whittaker 1910 [3] It was named "rapidity" by Alfred Robb 1911 [4] and this term was adopted by many subsequent authors, such as Varićak 1912, Silberstein (1914), Morley (1936) and Rindler (2001). The development of the theory of rapidity is mainly due to Varićak in writings from 1910 to 1924.[5]

In one spatial dimension[edit]

The rapidity w arises in the linear representation of a Lorentz boost as a vector-matrix product


The matrix Λ(w) is of the type with p and q satisfying p2q2 = 1, so that (p, q) lies on the unit hyperbola. Such matrices form the 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, Λ(w) can be expressed as , where Z is the anti-diagonal unit matrix

It is not hard to prove that


This establishes the useful additive property of rapidity: if A, B and C are frames of reference, then

where wPQ denotes the rapidity of a frame of reference Q relative to a frame of reference P. The simplicity of this formula contrasts with the complexity of the corresponding velocity-addition formula.

As we can see from the Lorentz transformation above, the Lorentz factor identifies with cosh w


so the rapidity w is implicitly used as a hyperbolic angle in the Lorentz transformation expressions using γ and β. We relate rapidities to the velocity-addition formula

by recognizing

and so

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 β and γ appears frequently, and is from the above arguments

Exponential and logarithmic relations[edit]

From the above expressions we have

and thus

or explicitly

The Doppler-shift factor associated with rapidity w is .

In experimental particle physics[edit]

The energy E and scalar momentum |p| of a particle of non-zero (rest) mass m are given by:

With the definition of w

and thus with

the energy and scalar momentum can be written as:

So rapidity can be calculated from measured energy and momentum by

However, experimental particle physicists often use a modified definition of rapidity relative to a beam axis

where pz is the component of momentum along the beam axis.[6] 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.

See also[edit]

Notes and references[edit]

  1. ^ Minkowski, H., Fundamental Equations for Electromagnetic Processes in Moving Bodies" 1908
  2. ^ Sommerfeld, Phys. Z 1909
  3. ^ "A History of the Theories of the Aether and Electricity" In a later 1953 edition of this book it was used consistently for the theory
  4. ^ "Optical Geometry of Motion" p.9
  5. ^ See his papers, available in translation in Wikisource
  6. ^ Amsler, C. et al., "The Review of Particle Physics", Physics Letters B 667 (2008) 1, Section 38.5.2