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Thomas precession normally occurs when a spinning particle such as an electron orbits around an atomic nucleus (figure 2); this contributes to its effective spin-orbit coupling, which in turn affects spectral lines (see e.g. [41, section 11.8]).https://arxiv.org/abs/1711.05753
Abstractly, the Thomas precession is due to an additional rotation the spin of the particle pics up as a result of of the orbit movement. In this sense, the phenomenon is due to a geometric phase/Berry phase.
From a group theoretic perspective, Thomas precession occurs because Lorentz boost do not commute. For example, we have [Ky,Kx]=iJz. The rotation that we get by combining boosts is known as a Wigner rotation.
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[T]he revolution of the particle around the nucleus can be interpreted as a sequence of Lorentz boosts.[…] Abstractly, the effect is due to Wigner rotations [3] that appear in representations of the Poincaré group, so it is purely kinematical—it follows solely from space-time symmetries. https://arxiv.org/abs/1711.05753
To get the Lorentz transformation mapping from frame 1 to 3 directly, we compose boosts:
exp(aycδtKy+axcδtKx)exp(vxcKx)=exp(vxcKx+axcδtKx+aycδtKy+ayvx2c2[Ky,Kx]δt+⋯)
by the Dynkin formula version of the Baker-Campbell-Hausdorff theorem. From the commutation relationships for the Lorentz group we get [Ky,Kx]=iJz, so that ayvx2c2[Ky,Kx]δt corresponds to a rotation about the z axis of ayvxδt/(2c2) radians (here, somewhat obviously Jz,Jy,Jz are "generators of rotations", *i.e.* tangents to rotations about the x,y,z axes at the identity, respectively, whilst Jz,Jy,Jz are "generators" of boosts).https://physics.stackexchange.com/a/99459/37286
[T]he phenomenon of Thomas precession is one of the best established kinematical consequences of Poincaré symmetryhttps://cqgplus.com/2018/02/15/can-you-see-asymptotic-symmetries/