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S\mathbf S

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a shift transverse to both the spin and the relative velocity [ 13 ] . This is an exact identity of special relativity, not an approximation valid only for small v ; it holds all the way to v →\to c , at which point |Δ\Deltax\mathbf x| →\to |S\mathbf S|/(Mc) = ρM\rho_M exactly, since |S\mathbf S ×\times v\mathbf v| ≤\le |S\mathbf S|\,v saturates when the spin is perpendicular to the boost. No boosted observer, however extreme, can push a body’s own centroid further from its rest-frame value than ρM\rho_M . As u ranges over every inertial observer, Xμ(u,Σ)X^\mu(u,\Sigma) traces out a disk of exactly that radius, orthogonal to S\mathbf S — the world-tube first identified by Møller and reproduced in modern language…

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SS

Symbol S

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Published contexts (2)

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S\mathbf S

Equation 45 · Evolutionary Physics

The Atlas That Refuses to Close

This mathematical expression combines the displayed quantities; its precise role follows from the surrounding article text.

a shift transverse to both the spin and the relative velocity [ 13 ] . This is an exact identity of special relativity, not an approximation valid only for small v ; it holds all the way to v →\to c , at which point |Δ\Deltax\mathbf x| →\to |S\mathbf S|/(Mc) = ρM\rho_M exactly, since |S\mathbf S ×\times v\mathbf v| ≤\le |S\mathbf S|\,v saturates when the spin is perpendicular to the boost. No boosted observer, however extreme, can push a body’s own centroid further from its rest-frame value than ρM\rho_M . As u ranges over every inertial observer, Xμ(u,Σ)X^\mu(u,\Sigma) traces out a disk of exactly that radius, orthogonal to S\mathbf S — the world-tube first identified by Møller and reproduced in modern language…

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S\mathbf S

Equation 64 · Evolutionary Physics

The Atlas That Refuses to Close

This mathematical expression combines the displayed quantities; its precise role follows from the surrounding article text.

The derivation behind this trade-off runs through the ten generators of the Poincaré algebra rather than through anything special to a particular particle. The boost generator K\mathbf K is one fixed physical object for a given one-particle state; writing it as a symmetrized combination of a position candidate and the energy only closes the algebra’s commutation relations correctly if either the position candidate absorbs a spin-momentum term, yielding commuting components and frame dependence, the Newton–Wigner choice, or that term is left attached to the boost generator itself, leaving the position candidate free of it but noncommuting, the covariant choice. Both are legitimate solutions of…

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