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.