If a ``point" in Minkowski space is an EVENT,
what is a ``line"?
Well, it is a continuous sequence of events,
which in our personal life we might call a history.
In Physics, however, we are usually focussed on the
history of an object's motion through space,
which in the is known (somewhat pretentiously, perhaps)
as a WORLDLINE.
Figure: An event A
can be located in spacetime using either of two
coordinate systems O (ct, x)
and O' (ct', x')
with a common origin at (0,0)
that differ from each other by a ``boost"
(Lorentz transformation) corresponding to O'
moving at u with respect to O along the positive x axis.
If ct and x are plotted on the same scale
(as they always are),
then the WORLDLINE of a beam of light
moving along the x axis after starting from the origin
will be a straight line making the same angle to both axes.
Unlike the rotation shown in Fig.
,
however, the Lorentz transformation is not orthogonal
(does not preserve right angles) --
so that this graphical construction cannot be used
in quite the same simple intuitive way.