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Matterwave
#12
Feb27-12, 05:18 AM
Sci Advisor
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P: 2,660
To preserve causality (and for various other reasons), physical objects can only move along time-like curves (or null in the case of photos). The time-like curves in Minkowski space-tie are exactly those curves which are always moving at no more than 45 degrees away from the vertical axis (assuming t is your vertical axis).

From any event P in Minkowski space, one an draw 2 cones, one going forward and one going backward, the edges of which make a 45 degree angle with the vertical axis. Everything in the future directed cone are events which an object at event P can affect, everything in the past directed cone are events which could have affected event P. These are the tirival chronological future and chronological past of the event P. If you include the edges of the cones, then these define the causal future and causal past.