File: 11 October 2026 · Source date: 11 October 2026
Past and future cone
The boundary drawn by the light rays emanating from an event in the space-time diagram indicates events that may be affected in the future. The history cone, on the other hand, contains regions that may affect that event. Establishing a direct causal link to events outside the cone would require faster-than-light transmission.
Why can synchronicity change?
Observers moving at different speeds may disagree about whether distant events are occurring at the same time. Nevertheless, the internal and external separation of the light cone is maintained; the sequence of causally connected events cannot be reversed.
Curved space-time cones
In general relativity, matter and energy bend geometry, and the paths followed by light bend accordingly. Yet in every small region no object or information can cross the light signal next to it. This local rule is the basis of the global causal structure.
Related reading
discussion question
What does it mean physically to say that an event that we cannot reach takes place “now”?
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