There is no ‘rigid’ in special relativity, the best you can do is Born-rigid. Even so, its trivial to define a ball in special relativity, just define it in the frame of a corotating observer and use four vectors to move to the same collection of events in other frames You learn that a ‘ball’ in special relativity has some observer dependent properties, but thats because length and time are observer dependent in special relativity. So ‘radius’ isn’t a good concept, but ‘the radius so-and-so measures’ IS a good concept.
Why, is there a nice, platonic-ideal type definition of a rigid ball in the map (compatible with special relativity)? What happens to its radius when you spin it?
There is no ‘rigid’ in special relativity, the best you can do is Born-rigid. Even so, its trivial to define a ball in special relativity, just define it in the frame of a corotating observer and use four vectors to move to the same collection of events in other frames You learn that a ‘ball’ in special relativity has some observer dependent properties, but thats because length and time are observer dependent in special relativity. So ‘radius’ isn’t a good concept, but ‘the radius so-and-so measures’ IS a good concept.