Spacetime Cones - The Order of Events
A real object always travels slower than light, following worldlines like the red curve shown below.
We can write the distance ds between two events in spacetime as wherewould be the time interval between the two events as recorded by a clock that travelled between the two events.
For an object to be able to travel between the two eventsas for any two events on the worldline of the particle shown in red. For these twp events every observer will agree on the order of events. In the region labelled 'Elsewhere'and observers will not necessarily agree on the order of events.