### spacetime diagram

**(Minkowski diagram)**
(graph of simplified spacetime, with fewer space dimensions)

A **spacetime diagram** (or **Minkowski diagram**) is a graph of a
simplified spacetime which can be used to show consequences
of relativity, displaying plotted worldlines. The simplest
is a 2-dimensional graph showing only a single dimension of space,
i.e., a planar "cut" through 4-dimensional spacetime. Typically,
time is represented as the vertical axis. 3-dimensional graphs are
also used, showing two space dimensions.

It is common to scale the graph so the path light would
take along this dimension shows as a 45-degree angle, i.e.,
movement at the speed of light. The slope of the line varies
inversely with the speed of a small (point-like) object represented
by the line, steeper being slower. Lines steeper than the
speed-of-light angle can represent objects, i.e.,**time-like** lines,
which could represent the position of the object at each time.
Lines less steep than 45 degrees are impossible for a point-like
object to follow: that would be exceeding the speed of light: such
lines are termed **space-like**, to indicate in any frame of reference,
there will be some space between any two separate points on the
line: an object with enough length could sit on the line, interconnecting
the points. Variations on the graph, including rotating axes, can
be used to demonstrate the effect of differing frames of reference.

(*diagram,relativity,cosmology,black holes*)
**Further reading:**

https://en.wikipedia.org/wiki/Minkowski_diagram

**Referenced by page:**

worldline

Index