Click on different observers to see their information.
The top left corner will display their velocity (1c = speed of light in a vacuum), position, rotation and how much time has elapsed on their stopwatch since the simulation started.
The red block is the reference frame.
Click on the transform button to toggle between classical physics and relativistic.
You may see some jumpiness--that's cause I wrote this pretty quick :)
You may also see that some observers have 0 time--that's a bug. Time is incrementing so slowly for them, that we're hitting float underflow.