In Newtonian space-time there exist four, and only four, causal classes of frames. Natural frames allow to extend this result to coordinate systems, so that coordinate systems may be also locally classified in four causal classes. These causal classes admit simple geometric descriptions and physical interpretations. For example, one can generate representatives of the four causal classes by means of the {\em linear synchronization group}. Of particular interest is the {\em local Solar time synchronization}, which reveals the limits of the frequent use of the concept of `causally oriented oordinate', such as that of `time-like coordinate'. Classical {\em positioning systems}, based in sound or light signals, are, by themselves, interesting examples of location systems, i.e. of physically constructible coordinate systems. They show that one can locate events in Newtonian space-time {\em without} any use of the concept of synchronization. In fact, the coordinate systems associated to positioning systems, belong to all the classes but the standard one, i.e. the one based in the simultaneity synchronization. The relativistic analogs of these examples, emphasize the contrast between the four Newtonian and the one hundred and ninety nine Lorentzian causal classes of frames of classical and relativistic space-times, respectively.
Asymptotic frame selection for binary black hole spacetimes II: Post-Newtonian limit
从引力辐射中选择一个优选坐标系的一种方法是< L L>通过旋转群生成元在渐近无穷远处对外尔张量作用的平均值的主轴。在本文中，我们评估这个时域平均准圆二进制使用近似（后牛顿）波形。对于非旋进不等质量双星，我们表明这个张量的主导特征向量位于沿轨道角动量。对于旋进双星，这个坐标系通常与轨道或总角动量不一致，在自旋中起主导作用。这两个量之间的差异随着时间的推移而增长，因为二进制接近螺旋的末端，岁差和更高的谐波变得更加重要。
