The event horizon is defined as the boundary of the causal past of null infinity.
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It mainly emphasizes that even if observers A and B are at a spacelike separation their choices may not be fully independent because they share a common ( region of their ) causal past.
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A new article called Causal structure has been created ( technically, it's a redirect from " causal spacetime structure " ) . " Causal past ", " causal curve " and " causal future " have been redirected to this new article . contribs ) 20 : 22, 15 November 2007 ( UTC)
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