Einstein derived his equation set from an 'event' generated system based on the limitation of the finite velocity of light demonstrated by the 'Lorentz Transformation'.

Using Einstein's approach and the closed system hypothesis, each event will have the same spatial system as prior to the event but the spacetime after the event will be closed. (In relation to the mass/energy condition resulting from the event).

Each new event being closed relative to any prior event Einstein's relativity equations make allowance for the relative closure of the separate events not allowed for in classical mechanics.

My closed spacetime model using a 3 space and 1 time metric for spacetime in which the time element has a positive eigenvalue related to the negative space elements, creates an image which I found easier to visualize than the dual cone 2 space and 1 time metric.