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Billiards are a class of dynamical systems in which a point particle moves uniformly in a domain $D\subset \mathbb{R}^d$ except for mirror-like reflections from the boundary. Varying $D$ leads to examples satisfying many ergodic properties. Billiards enhance visual explanations of dynamical concepts to students and the general public. There are many applications in physics and image processing. The free motion and/or reflection rule may be generalized.

25 votes

Light rays bouncing in twisted tubes

Here is my interpretation of Anton's idea to capture the ray. I found it almost impossible to illustrate; I could only show three sections of the tube, separated by two rotations, the first counterclo …
Joseph O'Rourke's user avatar
14 votes

Light rays bouncing in twisted tubes

Here is the phase portrait of $(\theta, x \; \mathrm{mod} \; 2)$ for the rays that Dimitri suggested in his response to Q2 (if I have interpreted his suggestion correctly), using the same data as disp …
Joseph O'Rourke's user avatar
10 votes
Accepted

Current state of Straus's illumination problem

As far as I know, the specific question you ask (entirely illuminable from at least one point) remains open. (Tokarsky showed that placing a light in some spots can leave some points dark.) But you ma …
Joseph O'Rourke's user avatar
8 votes
Accepted

Existence of periodic orbits in rational billiards

Schwartz, Richard Evan. Mostly surfaces. Vol. 60. American Mathematical Society, 2011. On p.219ff of Schwartz's book, he sketches "an elementary proof, due to Boshernitsyn, that every rational po …
Joseph O'Rourke's user avatar
6 votes

Reflection of light from function graph

Just empirically, I believe the OP's $e^{-x}$ example has the property that ray reflections quickly become increasingly vertical, and so will not reach arbitrarily large $x$:           (I did not, …
Joseph O'Rourke's user avatar
5 votes

Can every $\mathbb{Z}^2$ disk be pinball-reached?

I add this image just to illustrate that matters seem more complicated (as Douglas Zare has emphasized) when the radii of the disks approaches $\frac{1}{2}$:           
Joseph O'Rourke's user avatar
3 votes

Can every $\mathbb{Z}^2$ disk be pinball-reached?

"From rational billiards to dynamics on moduli spaces." Apr. 2015. (arXiv abstract.) …
Joseph O'Rourke's user avatar
2 votes

Optic fibers after Joseph O'Rourke

Suppose that the normal frame along $\gamma$ twists. Are you sure it still constitutes an optic fiber? I am imagining, for example, smoothly twisting an ellipse of nonzero eccentricity.           
Joseph O'Rourke's user avatar
2 votes

Which polygons have *simple* periodic billiard paths?

An example that may be difficult for Aaron's line-of-symmetry argument:          
Joseph O'Rourke's user avatar
1 vote

Well-definedness of single-particle smooth billiards flow

This is just a guess, I will admit (apologies if I'm among the misunderstandings you anticipated), but perhaps Peter Guber's paper, Convex billiards. Geometriae Dedicata. …
Joseph O'Rourke's user avatar