Star Clusters with Second Life Mod

Ever wanted to control the stars with your outstretched arm? An open-source virtual reality platform just made it possible.

What in 1991 was a novel physics solution now comes packaged in a virtual world for you to intuitively explore. A new simulation in OpenSim, an open-source version of the popular virtual world Second Life, shows how a handful of objects floating in space react to each others’ gravity.

In physics, this is known as the N-body problem. It’s simple if you have only two objects: they orbit their common center of mass in a circle or an ellipse. But three or more objects send the system into chaos. Physicists and mathematicians banged their head against it for centuries, with a general solution emerging less than 20 years ago.

“It is exceedingly deep,” said Will Farr, a grad student at MIT and collaborator on the project. “PhD physics researchers work on it every single day, but if you’re just a motivated high school kid who knows some physics and a little bit of programming, you can do it. It’s both simple and rich.”

Now, thanks to ever-more-powerful computers, astrophysicists can model star clusters, galaxies, and entire universes, armed only with the initial positions and velocities of a serverful of particles. The number of particles in the OpenSim program is capped at 50 — vanishingly small in astronomical terms — so it’s unlikely to make any new scientific breakthroughs. But it does let you click and drag stars just to see what would happen.

“The real advantage that we see here, potentially, is the interactivity and the collaborative aspects,” Farr said. Other astrophysics simulators are out there, and some are much more detailed, but this is the first that gives users the god-like power to control and share the simulation in real time.“The interactivity is just built into these virtual worlds,” Farr said. “We were trying to take advantage of that through the stuff that we already know how to do.”

The simulation most closely resembles the core of a globular cluster, a spherical blob of 100,000 to a million stars that orbit our galaxy. They’re too big for the simulator to model in full — because it crunches the numbers and displays the results in 3D in real-time, it can handle only about 50 objects at a time. But the core of a globular cluster only has about 100 stars, and can be one of the most active places in the galaxy.

“Stars in the normal galaxy stars don’t run into each other. Ever. It does not happen,” Farr said. But stars are packed so densely and moving so rapidly in the cores of globular clusters that collisions happen all the time. What happens in the core early in the cluster’s life determines the future of the rest of the cluster. In the OpenSim program, “you can set up a core type system and watch them evolve for a minute and two, and you can point at physical processes that are going to be important to the whole cluster,” Farr said.

And the best part: people in OpenSim or Second Life can build worlds together, and then watch them come apart.

“You can invite your friends from across the world and watch it,” Farr said. “If you have an experimental bent, you can say ‘Let’s see who can create a system that can eject a body with the highest velocity,’ or ‘Let’s see who can make a system that will hold all its objects longest,’ or ‘You build a system and I’ll build one and we’ll crash them together and see what happens.’ Watching what happens with gravitational collapse is just cool.”

The project began with two executives from the Tokyo-based consulting firm Genkii, Adam Johnson and Jeff Ames. “They just kind of did this for fun,” Farr said. They teamed up with Farr and Piet Hut, an astrophysicist at Princeton and member of the Meta Institute for Computational Astrophysics, to build a program that solve the N-body problem. Most of the work was done in a coffee shop in Tokyo one afternoon last summer.

First they tweaked the laws of gravity in OpenSim. Before, free-floating objects fell straight to the ground, just like they do on real-life Earth. Now (at least in certain parts of the virtual world), the objects exert their own gravitational forces on each other, making small but realistic starclusters.

Now they’re seeking academic recognition for the work. The paper appeared on the arXiv preprint service Friday, and has been submitted to Journal of Virtual Worlds Research.

“We hope people look at this and say ‘Oh that’s fun! Let me see the code!’ And it inspires people to write their own code and get more interested in physics,” Farr said.