### 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 effect of differing frames of reference.

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

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

**Referenced by page:**

worldline

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