Past and future light cones?

  • #1
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Hi,

I've been reading A Brief History of Time by Stephen Hawking and I've reached a section that discusses past and future light cones of an event. I'm finding it hard to understand what a light cone is.

What is the past light cone and what is the future light cone?

Thanks,
 

Answers and Replies

  • #2
PeterDonis
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At a given event in spacetime, the past light cone is the set of worldlines of all light rays that arrive at that event from the past, and the future light cone is the set of worldlines of all light rays that depart from that event into the future.

The name comes from the fact that, if you imagine a spacetime diagram with one space dimension suppressed, and time as the vertical axis, then if we assume that the event you're interested in is at the origin, the past light cone looks like a 45-degree cone facing down from the origin, and the future light cone looks like a 45-degree cone facing up from the origin (assuming you're using units in which the speed of light is 1).
 

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