code: public double[] getElipsisRadii() {
  		if (modified) {
  			diagonalizeTensor();
  			modified = false;
  		}
  		double m = 0;
  		for (int i = 0, n = points.size(); i < n; i++) {
  			m += masses.get(i);
  		}
  		double[] r = new double[3];
  		for (int i = 0; i < 3; i++) {
  			r[i] = Math.sqrt(principalMomentsOfInertia[i] / m);
  		}
  		return r;
  	}
docs: The effective value of this distance for a certain body is known as its
  radius of / gyration with respect to the given axis. The radius of
  gyration corresponding to Ijj / is defined as /
  http://www.eng.auburn.edu/~marghitu/MECH2110/C_4.pdf / radius of gyration
  k(j) = sqrt(I(j)/m)