= Phototaxis
{wiki}
Phototaxis is directed organism motion in response to light. An ideal spherical swimmer subject to light <torque> $\mathbf p\times\mathbf L$ and rotational resistance $\alpha$ aligns towards the light while ambient <vorticity> rotates it. Its deterministic orientation satisfies $\dot{\mathbf p}=\boldsymbol\omega\times\mathbf p/2+[\mathbf L-(\mathbf p\cdot\mathbf L)\mathbf p]/\alpha$. The right-hand side is perpendicular to $\mathbf p$ and preserves its unit length.
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