Gravity (software)

Gravity is an interactive N-body simulation tool designed to model the gravitational interactions between multiple bodies. It was created by the Swinburne University of Technology's Centre for Astrophysics and Supercomputing as part of their educational outreach efforts. The software allows users to set up initial conditions—such as the number of bodies, their masses, positions, and velocities—and then observe how the system evolves over time under Newtonian gravity. It is particularly valued for its real-time visualization, which makes abstract concepts like orbital mechanics and gravitational slingshots tangible for students and enthusiasts.

Gravity supports simulations of up to thousands of bodies, with adjustable parameters for gravitational constant, softening length, and integration accuracy. It includes presets for common scenarios such as the Solar System, binary star systems, and globular clusters. The software offers a 3D view with camera controls, and users can track individual bodies to observe their trajectories. It also provides tools for analyzing energy and angular momentum conservation, which are critical for verifying the accuracy of simulations. A notable feature is the ability to pause and step through time, allowing for detailed inspection of close encounters and collisions.

Gravity is extensively used in classrooms and planetariums to teach gravitational physics. Its intuitive interface allows students to experiment with different configurations, such as adding a planet to a star system and observing the resulting orbital changes. The software is also used in university-level astrophysics courses to demonstrate the three-body problem and chaotic dynamics. Swinburne University has integrated Gravity into its online astronomy courses, and it is often recommended as a supplementary tool for interactive learning.¹

Gravity is written in C++ and uses OpenGL for rendering. It employs a direct summation algorithm for computing gravitational forces, which is simple but computationally intensive for large N. The software is optimized for real-time performance on consumer hardware, using techniques such as adaptive time-stepping and Barnes-Hut tree approximations for larger simulations. The source code is available on GitHub, and it is distributed under the GNU General Public License (GPL), allowing for modification and redistribution.²

Beyond its standard use, Gravity has been employed in research contexts, such as modeling the dynamics of dwarf galaxies and testing gravitational theories. It includes a scripting interface that allows advanced users to automate simulations and export data for further analysis. A lesser-known feature is the ability to simulate non-Newtonian forces, such as a central force that varies with distance, which can be used to explore modified gravity hypotheses. Additionally, Gravity has been used in public outreach events, including the 'Gravity Discovery Centre' in Australia, where it powers interactive exhibits. The software also supports VR headsets, providing an immersive experience for exploring gravitational systems.³

N-body simulation

A simulation of a dynamical system of particles interacting under physical forces, such as gravity.

Softening length

A parameter that prevents singularities in gravitational force calculations by smoothing the potential at short distances.

Barnes-Hut tree

An algorithm that approximates gravitational forces by grouping distant bodies, reducing computational cost from O(N^2) to O(N log N).