Wigner rotation

Part of a series on
Spacetime
Special relativity General relativity
Spacetime concepts Spacetime manifold Equivalence principle Lorentz transformations Minkowski space
General relativity Introduction to general relativity Mathematics of general relativity Einstein field equations
Classical gravity Introduction to gravitation Newton's law of universal gravitation
Relevant mathematics Four-vector Derivations of relativity Spacetime diagrams Differential geometry Curved spacetime Mathematics of general relativity Spacetime topology
Physics portal  Category
v t e

In theoretical physics, the composition of two non-collinear Lorentz boosts results in a Lorentz transformation that is not a pure boost but is the composition of a boost and a rotation. This rotation is called Thomas rotation, Thomas–Wigner rotation or Wigner rotation. If a sequence of non-collinear boosts returns an object to its initial velocity, then the sequence of Wigner rotations can combine to produce a net rotation called the Thomas precession.^[1]

The rotation was discovered by Émile Borel in 1913,^[2][3][4] rediscovered and proved by Ludwik Silberstein in his 1914 book 'Relativity', rediscovered by Llewellyn Thomas in 1926,^[5] and rederived by Wigner in 1939.^[6] Wigner acknowledged Silberstein.

There are still ongoing discussions about the correct form of equations for the Thomas rotation in different reference systems with contradicting results.^[7] Goldstein:^[8]

The spatial rotation resulting from the successive application of two non-collinear Lorentz transformations have been declared every bit as paradoxical as the more frequently discussed apparent violations of common sense, such as the twin paradox.

Einstein's principle of velocity reciprocity (EPVR) reads^[9]

We postulate that the relation between the coordinates of the two systems is linear. Then the inverse transformation is also linear and the complete non-preference of the one or the other system demands that the transformation shall be identical with the original one, except for a change of v to −v

With less careful interpretation, the EPVR is seemingly violated in some models.^[10] There is, of course, no true paradox present.

Let it be u the velocity in which the lab reference frame moves respect an object called A and let it be v the velocity in which another object called B is moving, measured from the lab reference frame. If u and v are not aligned the relative velocities of these two bodies will not be opposite, that is ${\textstyle \mathbf {v} _{AB}\neq -\mathbf {v} _{BA}}$ since there is a rotation between them

The velocity that A will measure on B will be:

${\displaystyle \mathbf {v} _{AB}={\frac {1}{1+{\frac {\mathbf {u} \cdot \mathbf {v} }{c^{2}}}}}\left[\left(1+{\frac {1}{c^{2}}}{\frac {\gamma _{\mathbf {u} }}{1+\gamma _{\mathbf {u} }}}\mathbf {u} \cdot \mathbf {v} \right)\mathbf {u} +{\frac {1}{\gamma _{\mathbf {u} }}}\mathbf {v} \right],}$

The Lorentz factor for the velocities that either A sees on B or B sees on A:

${\displaystyle \gamma =\gamma _{\mathbf {u} \oplus \mathbf {v} }=\gamma _{\mathbf {v} \oplus \mathbf {u} }=\gamma _{\mathbf {u} }\gamma _{\mathbf {v} }\left(1+{\frac {\mathbf {u} \cdot \mathbf {v} }{c^{2}}}\right)\,,}$

The angle of rotation can be calculated in two ways:

${\displaystyle \cos \epsilon ={\frac {(1+\gamma +\gamma _{\mathbf {u} }+\gamma _{\mathbf {v} })^{2}}{(1+\gamma )(1+\gamma _{\mathbf {u} })(1+\gamma _{\mathbf {v} })}}-1~,}$

Or:

${\displaystyle \cos \epsilon =-{\frac {(\mathbf {v} _{AB}\cdot \mathbf {v} _{BA})}{(v_{AB}^{2})}}}$

And the axis of rotation is:

${\displaystyle \mathbf {e} =-{\frac {\mathbf {u} \times \mathbf {v} }{|\mathbf {u} \times \mathbf {v} |}}~.}$