Rapidity is a measure for
relativistic velocity. For one-dimensional motion, rapidities are additive. However, velocities must be combined by Einstein's
velocity-addition formula. For low speeds, rapidity and velocity are almost exactly proportional but, for higher velocities, rapidity takes a larger value, with the rapidity of light being infinite.
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.
Using the
inverse hyperbolic functionartanh, 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
image; that is, the interval −c < v < c maps onto −∞ < w < ∞.
The
quadrature of the hyperbola xy = 1 by
Grégoire 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 [clarification needed]. 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
light-cone coordinates 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 using rapidity for a 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 a hyperbolic parameter of beam-space is a reference[clarification needed] to the seventeenth-century origin of our precious
transcendental functions, and a supplement to spacetime diagramming.
Lorentz boost
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 p2 – q2 = 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 negative of 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
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
From the above expressions we have
and thus
or explicitly
The
Doppler-shift factor associated with rapidity w is .
In experimental particle physics
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.
Rapidity relative to a beam axis can also be expressed as
Walter, Scott (1999).
"The non-Euclidean style of Minkowskian relativity"(PDF). In Jeremy John Gray (ed.). The Symbolic Universe: Geometry and Physics. Oxford University Press. pp. 91–127. Archived from
the original(PDF) on 2013-10-16. Retrieved 2009-01-08.(see page 17 of e-link)