The hemisphere model of Hyperbolic Space

geodesicgeometryhyperbolic-geometry

I am a beginner to hyperbolic geometry. I just started exploring the hyperbolic space via different model from Thruston's three dimensional geometry and topology. In chapter 2, the book mention a way to transfer the geometry of the Poincare disk(let's stick to hyperbolic plane for a visual understanding) to the hemisphere model as follows:

Place the Poincare disk $\mathbb{D}^2$ on the plane $x=0$ in $\mathbb{R}^3$, surrounded by a unit sphere $S^2$. Now with center as the southpole $(-1,0,0)$ project $\mathbb{D}^2$ to the northern hemisphere by inverse stereographic projection.

Here are my questions:

  1. The projection is not the usual projection in $\mathbb{R}^3$ (via straight line) instead its by hyperbolic geodesic rays(arc of semi circles) that intesects the disk $\mathbb{D}^2$ (perpendicularly) and then passes through the upper hemisphere and intersects the sphere $S^2$ (perpendicularly) right? I don't know if this is the correct interpretation?

  2. Also,(Thruston says) $\mathbb{H}^n$ as a totally geodesic space seats inside $\mathbb{H}^{n+1}$(I think it means just the inverse steoregraphic projection of the $n$ dimensional Poincare ball $H^n$ to the $n+1$ northern hemisphere of the $(n+1)$ Poincare ball $H^{n+1}$). If that is true I don't see how a geodesics in the $H^{n+1}$ is indeed a geodesic of $H^n$? Because in the hemisphere model the geodesics are semicircle in the upper northern hemisphere which are perpendicular to the equator.

Thanks for any comments!

Best Answer

Apparently I drew the correct thing long ago. Four of the models fit together in one diagram. I am taking the vertical direction as $z$ for this picture.

Alright, the hyperboloid model is the arc, in the diagram we have just $z^2 = x^2 + y^2 + 1, $ where I am showing just one of the directions $x,y.$ Geodesics are the intersections of the hyperboloid (of rotation) with planes through the origin.

The line segment (actually disc) $z=1, x^2 + y^2 < 1$ becomes the Beltrami-Klein model, projection around $(0,0,0).$

The line segment (actually disc) $z=0, x^2 + y^2 < 1$ becomes the Poincare disc model, projection from hyperboloid around $(-1,0,0).$

The hemisphere model maps two ways: (A) by vertical projection up to Beltrami-Klein$z=1 \; \;$ (B) by standard stereographic projection down to the Poincare disc $z=0$

Work on these for a while, the half space models are a bit different

enter image description here

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