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\item Real form: $\mathfrak{sl}(2, \mathbb{C}) \cong\mathfrak{so}(3,1)$ (Lorentz algebra)
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\end{enumerate}
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@@ -205,7 +212,7 @@ \subsection*{6.3 The invariant interval}
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\[ds^2 = -c^2 dt^2 + dx^2 + dy^2 + dz^2\]
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The metric signature is not chosen; it is forced by the Killing form of the complexified $A_1$ algebra. On the 6-dimensional real algebra the Killing form has signature $(3,3)$ (rotations positive, boosts negative); restricted to the 4-dimensional vector representation it has signature $(1,3)$, the Minkowski metric (M13 exp\_09). The interval $s^2 = x^\top\eta\,x$ is preserved across 135 Lorentz transforms (maximum relative error $2.9\times10^{-6}$, growing with rapidity up to $\eta=10$), and the invariant form is unique: the space of symmetric bilinear forms preserved by all $\mathfrak{so}(3,1)$ generators is one-dimensional (Schur's lemma), so the metric is a consequence of the algebra rather than a choice.
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The metric signature is not chosen; it is forced by the Killing form of the complexified $A_1$ algebra. On the 6-dimensional real algebra the Killing form has signature $(3,3)$ (rotations positive, boosts negative); restricted to the 4-dimensional vector representation it has signature $(1,3)$, the Minkowski metric (M13 exp\_09). The interval $s^2 = x^{\top}\eta\,x$ is preserved across 135 Lorentz transforms (maximum relative error $2.9\times10^{-6}$, growing with rapidity up to $\eta=10$), and the invariant form is unique: the space of symmetric bilinear forms preserved by all $\mathfrak{so}(3,1)$ generators is one-dimensional (Schur's lemma), so the metric is a consequence of the algebra rather than a choice.
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