A Level Physics Notes: Special and General Relativity – 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 spacetimes as
where
would
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 events
as
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.