Fig. 1: Bertha sends a message back in time
Here's the essential setup.
1. We have two pairs of distantly separated lovers: a) Bill and Bertha and b) Jim and Jane. Woes is them.
2. If you haven't figured it out from the graph yet, Bill and Bertha are colocal (not moving with respect to one another), as are Jim and Jane.
3. Jim and Jane are also moving at some velocity with respect to Bill and Bertha (that's why the x' and t' axes for Jim and Jane are tweezed inward with respect to the x and t axes for Bill and Bertha). This is very important, and is the second most essential geometrical intuition you must develop to understand SR.
4. Missing from the graph are lines indicating the "world (or light) cone". Simply imagine four lines intersection points A at opposing 45 degree angles from the x and t axes. That represents the motion of light in each of our four traveller's frame of reference. You'll note that the light cone lines are the same for Jim and Jane as they are for Bill and Bertha despite Jim and Jane's odd set of axes (x' and t'). This is also very important, and is the most essential geometric intuition behind special relativity.
So for the past week or so a few diarists have pointed out that FTL motion violates causality in some frame of reference. Let's see why that is.
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