![]() So now let me move from general spacetime diagrams to an example that will indicate why FTL implies time travel. This is the set of spacetime events that can perceive the event at t=0, and so, in a Universe without FTL, all the events that can be affected by whatever happened at this event at t=0. There is also a past lightcone, which would be the 45 degree lines extending backwards in time from the event: in a Universe without FTL this defines all the events that could have effected that t=0 event, because the light (and thus things moving slower than light) from those other events had time to reach the t=0 event. The light rays define the future lightcone. Because light travels at 45 degrees, anything traveling slower than light from this t=0 event is closer to the time axis than the light rays, and anything faster than light is further away from the time axis. Remember I'm suppressing most of the space dimensions: these rays of light are really emanating out in a sphere around me. Here I'm showing the null lines of light emitted from an event at the time I'm calling t=0 (when the time and space axis cross). Light travels on lines that are called "null." We show this in a spacetime diagram by saying that every spacetime diagram has light traveling at 45 degrees relative to the time axis. The special thing about relativity is that everyone measures the speed of light to be the same.
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