Occasionally I see references in Astronomy to the speed of something as "supersonic." I'm having trouble reconciling this ...

So, I complained about MythBuster's explanation of relative velocity. How would I explain this? I would start by saying that velocity is relative. Here is the definition for velocity: I put the "avg" ...

A design is presented of a laboratory experiment that could test the suitability of relativistic gravity for propulsion of spacecraft to relativistic speeds. The first exact time-dependent solutions ...