Light-like

Spacetime interval, 3 | How far away is tomorrow?, 2

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Any two events in a spacelike region are not causally related.

Any two events in a timelike region can be causally related.

How about two events on the boundary (light-like)?

— Me@2016-06-30 06:38:06 PM

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Their spacetime distance is zero, meaning that the communication signals/particles between the two events would have to travel at the speed of light.

In other words, the two events can only be marginally causally related.

— Me@2022-12-14 10:55:20 AM

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