Peter Bergmann (a student of Einsteins) in THE RIDDLE OF GRAVITATION, PAGE 60 notes And here I had been thinking only about the speed of light as invarient. Is the above coincidence?? I can't tell.... and the space time interval, is also invarient, right?? Are there other entities constant for all observers? Yes, I think four vectors such as "four vector momentum" which in simple terms is E2 - p2 that is (energy)2 - (momentum)2... I realize these transform (from one reference frame to another) as they do because of the mathematics of the formulations themselves, and the way they fit into relativity, but I can't help wondering if there is any deeper explanation or understanding. Any ideas?