Does it account for relativity?
No, the physics in Universe Sandbox ² is currently only
Newtonian.
Why?
The short answer is that you need a supercomputer to
accurately simulate general relativity.
Jenn, astrophysicist and Universe Sandbox ² developer,
explains more in a blog post: "General relativity
requires simulating the spacetime itself. That is,
taking your simulation space, discretizing it to a
hi-res 3-D grid and checking the effect that each and
every point in that grid has on all neighboring points
at every timestep. Instead of simulating N number of
bodies, you are simulating a huge number of points.
You start with some initial data of the shape of your
spacetime and then see how it evolves according to the
Einstein equations, which are 10 highly non-linear
partial differential equations."
We are, however, interested in adding in a few features
which would address some effects of relativity. One
example is setting gravity to travel at the speed of
light, instead of instantaneously taking effect as it
currently does. You can read more about these in
Jenn's blog post: Gravitational Waves & Universe
Sandbox ².
First and second order corrections are a lot more reasonable to calculate than the whole theory!
Furthermore calculating the whole theory runs into interesting challenges where the coordinate system is twisted and and distorted but underlying space-time is not. For a well-known example, a black hole can be described with both Schwarzschild coordinates and Kruskal–Szekeres coordinates. The first coordinate system blows up at the event horizon, the second doesn't. The fact that it blows up is due to a bad choice of coordinate system there, and not due to local space time being particularly bizarre at that spot.
How cool - asking the question "does gravity propagate at the speed of light and if so what impact does that have on orbital dynamics" was the event that led me down the path to my PhD, so it's awesome the answer to the question is now (or soon to be?) baked into a science toy.
