While the past light-cone is generally local, the future light cone is generally non-local.
Consider the above light cone illustrated in Einstein’s THE MEANING OF RELATIVITY.
Consider the standard light cone depicted at Wikipedia, which Einstein’s light cone inspired and informed:
https://en.wikipedia.org/wiki/Light_cone#/media/File:World_line.svg
Light cone in 2D space plus a time dimension.: CC BY-SA 3.0 SVG version: K. Aainsqatsi at en.wikipedia Original PNG version: Stib at en.wikipedia — Transferred from en.wikipedia to Commons. (Original text: self-made)
https://en.wikipedia.org/wiki/Light_cone#/media/File:World_line.svg
Let us show that while the past light-cone is generally local, the future light cone is generally non-local.
First, let us consider the future light cone:
Consider a pair of entangled photons A and B which travel in opposite directions from the origin in the x1, x2 plane.
No matter how far apart they travel, they will remain entangled, meaning that their two positions will define a nonlocality.
Now consider numerous entangled pairs of photons, wherein the two photons in each pair travel away from the origin in opposite directions.
Together, the positions of the photons in all the entangled pairs of photons will by and by define a circle of nonlocality as the number of pairs of photons approaches infinity.
Now consider a third axis which is time. Together, the positions of the photons in all the entangled pairs of photons will define a conical surface of nonlocality.
Ergo, the surface of a light cone is a nonlocality.
The surface of a future light cone is thus nonlocal.
QED
Now let us consider the past light cone. Consider events which appear at the bottom circular edge of the past light cone. These events are spatially separated, and thus, generally speaking, they will not be entangled. So it is that the past light cone is generally local, as nonlocal entanglement is not generally observed.
So it is that we have shown: While the past light-cone is generally local, the future light cone is generally non-local.
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