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Single-Particle Electrodynamics - Assassination Science

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Thus, we again seek a set of three-vector parameters that will parametrise<br />

the evolution of the spin Σ (τ) without redundancy.<br />

Clearly, the set of three-vectors c ′ n of (2.98) would serve this purpose, but,<br />

again, they are not, in the author’s opinion, the most natural or intuitive<br />

choices. Instead, we shall, following the discussion of Section 2.8.3, parametrise<br />

the three-spin σ, in terms of its lab-time evolution. The analogue of<br />

(2.87) is clearly<br />

σ(t) ≡ σ + ˙σt + 1 2 ¨σt2 + 1 ...<br />

σt 3 + 1 ....<br />

σt 4 + O(t 5 ), (2.99)<br />

6 24<br />

where we again understand the unadorned quantities σ, ˙σ, ¨σ, σ ... and .... σ to<br />

denote the spin and its derivatives evaluated at t = 0.<br />

Unlike the analysis of Section 2.8.4, the conversion of the lab-time parametrisation<br />

(2.99) into a proper-time parametrisation is simple, since we<br />

already have t(τ), equation (2.91): inserting this into (2.99), we find<br />

σ(τ) ≡ σ + ˙στ + 1 2 ¨στ 2 + 1 6<br />

+ 1 24<br />

{ ...<br />

σ + ˙v 2 ˙σ } τ 3<br />

{ ....<br />

σ + 4 ˙v 2 ¨σ + 3(v·¨v) ˙σ } τ 4 + O(ε 5 ). (2.100)<br />

The similarities between this equation for σ(τ), and (2.95) for v(τ), are<br />

obvious; they are both obtained by substituting t(τ) into the lab-time Taylor<br />

expansions (2.87) and (2.99).<br />

99

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