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Under the geometrical view of spacetime of general relativity, simultaneity as we understand it is a flawed concept.

The idea that two events happening in different places can be occuring at the same time is one that does not hold up under a system that describes the relationship between different frames of reference. In order to understand why this is so, an elementary grasp of the theory of relativity is required, so follow the hardlink.

Here is an ASCII rendering of what your frame of reference looks like. The 'diagonal' line is a photon, travelling at the speed of light.


                    |             /
   y-axis: time     |            /
     (light-seconds)|           /
                    |  .a    .b/
                    |         /
                    |        /
                    |       /
                    |      /
                    |     /
                    |    /
                    |   /
                    |  /
                    | /
                    |/_____________________
                    0
                         x-axis: distance
                            (light-seconds)

This diagram shows two points, (a) and (b), which represent events that look simultaneous to you. They both have the same time-value. Now, if you superimpose (mentally - nobody's ASCII skills are that good) another, moving frame of reference over the top (using the technique I outlined in theory of relativity), you will be able to see that because the grid-lines are no longer running at the same angle, the two points (a) and (b) no longer have the same time-value. This means that they no longer appear simultaneous, and indeed shows that there is no such thing as absolute simultaneity in the Webster 1913 sense, which events are considered simultaneous are dependent entirely on your relative state of motion.

Yay for ASCII!