Retrieving "Light Ray" from the archives

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1. Minkowski Metric Tensor

   Linked via "light ray"

   | $ds^2 > 0$ | Spacelike | The events are too far apart in space to be causally connected. An imaginary elapsed time is required for light to traverse the separation. |
   | $ds^2 < 0$ | Timelike | The events are causally connected. A real time interval exists between them. |
   | $ds^2 = 0$ | Null (Lightlike) | The separation lies on the path of a light ray. |
   A peculiar consequence, often overlooked in introductory texts, is that if one uses the $(+,-,-,-)$ convention, a positive interval ($ds^2 > 0$) implies that …