Back

Parallel transport on a sphere

What you are seeing: a "Beltrami" spherical triangle on the unit sphere with one vertex at the north pole and the other two on a colatitude α\alpha circle, separated by longitude β\beta. Parallel transport of any vector around the triangle returns it rotated by the enclosed solid angle Ω=(1cosα)β\Omega = (1 - \cos\alpha) \beta. This is Gauss-Bonnet on the sphere of constant curvature K=1K = 1: the angle excess A+B+CπA + B + C - \pi equals the integrated curvature, which is just the area on the unit sphere.

For the full hemisphere (α=π/2\alpha = \pi/2, β=2π\beta = 2\pi) the holonomy is 2π2\pi: a full rotation. For a quarter octant (α=β=π/2\alpha = \beta = \pi/2) the holonomy is π/2\pi/2. For tiny triangles the holonomy is just the area, vanishing as α0\alpha \to 0.

Figure 1. Parallel transport of a tangent vector around a closed loop on a selectable surface. The holonomy is the spherical excess on the sphere, the apex deficit on the cone, and zero on the developable cylinder.
Surface
alpha (deg)60
beta (deg)90

WHAT TO TRY

  • Switch between the sphere, cone, and cylinder and compare the holonomy each loop accumulates.
  • Drag the canvas to rotate; on the cone, narrow the half-angle to widen the apex deficit.