# Transport tubes in a sphere

Let $S$ be a unit-radius sphere in $\mathbb{R}^3$.

Q0. Where should one place $3$ disjoint lines intersecting $S$ to minimize the maximum distance between any two points in $S$, where distance is measured as follows. Distance off the lines is Euclidean distance, but the distance between any two points on one line is zero.

The lines are like very fast transportation tubes. (I have in mind Star Wars' Coruscant.)

With no tubes, the maximum distance is $2$, realized by any antipodal pair. One tube, or two tubes, do not help reduce the maximum, but three tubes seem to help. In particular, I believe if one selects the three tubes to be the $x,y,z$ axes, slightly displaced to satisfy disjointness, then the maximum distance is $2 \sqrt{\frac{2}{3}} + \epsilon \approx 1.63$, illustrated below.

Q1. Is there a better arrangement of the three tubes? The red point is the origin/center of the sphere. The point $(1,1,1)/\sqrt{3}$ is $\sqrt{\frac{2}{3}}$ (green) from each axis.
The natural generalization is: $d$ tubes intersecting a unit-radius sphere in $\mathbb{R}^d$.

• (I posed a variant of this question at the 2017 Canadian Conference on Computational Geometry.) – Joseph O'Rourke Jul 31 '17 at 20:23
• I recommend an alternate formulation to clarify (or contrast): take a space geodesic path (so perpendiculars off a tube to a point on the surface or on another tube) and distance will be the min of the sum of the perpendiculars. Your mention of sphere makes me think of walking on the surface to a tube entrance, not burrowing down to the tube. Gerhard "Is Exhausted By Public Transportation" Paseman, 2017.07.31. – Gerhard Paseman Jul 31 '17 at 20:39
• @GerhardPaseman: Yes, precisely a space geodesic path: nice formulation! – Joseph O'Rourke Jul 31 '17 at 20:42
• I like this formulation even better; I recommend that you include it in your post. Given d, How small a radius r is needed to cover a unit d-ball with d cylinders of radius r and height 2 ? If this question is not equivalent to yours, it should provide a method for a nice upper bound. Gerhard "Finds Covering Problems More Intuitive" Paseman, 2017.07.31. – Gerhard Paseman Aug 1 '17 at 2:37
• @Gerhard: I do not think the last reformulation is the same, since you can switch from one tube to another one (can you?)... – Ilya Bogdanov Sep 21 '17 at 16:11