A Three Dimensional Gravitational Billiard in a Cone

TL;DR

This paper introduces a three-dimensional gravitational billiard system within a conical boundary, deriving a Poincaré map to analyze its dynamics, revealing integrable cases and complex behavior that varies with system parameters.

Contribution

It extends the study of gravitational billiards to three dimensions with a conical boundary, deriving a new Poincaré map and analyzing the system's stability and phase space.

Findings

Identified integrable parameter cases and computed fixed points.

Observed transition from chaotic to less chaotic dynamics as angular momentum increases.

Demonstrated qualitative similarity to wedge billiards at small angular momentum.

Abstract

Billiard systems offer a simple setting to study regular and chaotic dynamics. Gravitational billiards are generalizations of these classical billiards which are amenable to both analytical and experimental investigations. Most previous work on gravitational billiards has been concerned with two dimensional boundaries. In particular the case of linear boundaries, also known as the wedge billiard, has been widely studied. In this work, we introduce a three dimensional version of the wedge; that is, we study the nonlinear dynamics of a billiard in a constant gravitational field colliding elastically with a linear cone of half angle $\theta$. We derive a two-dimensional Poincar\'{e} map with two parameters, the half angle of the cone and $\ell$, the $z$-component of the billiard's angular momentum. Although this map is sufficient to determine the future motion of the billiard, the three-dimensional nature of the physical trajectory means that a periodic orbit of the mapping does not always correspond to a periodic trajectory in coordinate space. We demonstrate several integrable cases of the parameter values, and analytically compute the system's fixed point, analyzing the stability of this orbit as a function of the parameters as well as its relation to the physical trajectory of the billiard. Next, we explore the phase space of the system numerically. We find that for small values of $\ell$ the conic billiard exhibits behavior characteristic of two-degree-of-freedom Hamiltonian systems with a discontinuity, and the dynamics is qualitatively similar to that of the wedge billiard, although the correspondence is not exact. As we increase $\ell$ the dynamics becomes on the whole less chaotic, and the correspondence with the wedge billiard is lost.