# Why is the past also represented as a light cone in the opposite direction? (layman)

I recently heard about light cones as mentioned in Stephen Hawking's A Brief History of Time. I understand light can be seen as a cone when a flash of light is spread from an event (point in spacetime) and confined to a 2D plane and then extended along a perpendicular axis representing time. Why is it that there is also a "mirror image" of this cone behind this event? What is the point of the past here if the light did not yet exist?

A snippet from Wikipedia: Light cone mentioning the past light cone:

The past light cone behaves like the future light cone in reverse, a circle which contracts in radius at the speed of light until it converges to a point at the exact position and time of the event E.

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## 1 Answer

The future light cone of an event $$A$$ contains all other events for which all observers agree that $$A$$ happened first.

The past light cone contains all other events for which all observers agree that $$A$$ happened afterwards.

For events outside of either light cone, observers in different reference frames can disagree about whether $$A$$ happened afterwards or beforehand. This is important if you are interested in whether two events are causally connected. We have a theoretical prejudice which suggests you cannot "cause" something which already happened in the past. If two events $$A$$ and $$B$$ are outside of each others' light cones, so that we cannot say which came first, then this prejudice means that neither can have been a cause of the other.

We say that such not-well-ordered events are "spacelike-separated," and that the spacetime around an event is divided into its past, its future, and "elsewhere."

• Thank you. I can now see how the past light cone includes all events that can theoretically reach the current event 𝐴 and have some effect on it. Thus, I see how an observer at the event 𝐴 can only observe those events that are in the past light cone. The future light cone contains all events 𝐴 that may be affected by 𝐴 itself. 𝐴 can only be influenced by events in its past light cone and influence events in its future light cone. May 24, 2019 at 13:56