# spacetime

A worldline shown on a graph of space against time.

Spacetime is the union of space and time into a four-dimensional whole. More precisely, it is the inseparable four-dimensional manifold, or combination, which space and time are considered to form in the special and general theories of relativity. In the absence of a gravitational field, spacetime reduces to Minkowski space.

A point in spacetime is known as an** event**.
Each event has four coordinates (*x*, *y*, *z*, *t*).
Just as the *x*, *y*, *z* coordinates of a point depend on
the axes being used, so distances and time intervals, which are invariant
in Newtonian physics, may depend, in relativistic physics, on the reference
frame of an observer; this can lead to bizarre effects such as length contraction
and time dilation. A spacetime **interval** between two events is the invariant quantity analogous to distance in Euclidean
space. The spacetime interval *s* along a curve is defined by the
quantity

*ds*^{ 2} = *dx *^{2} + *dy *^{2} + *dz *^{2} - *c *^{2}*dt*^{ 2}

where *c* is the speed of light. A basic assumption of relativity theory
is that coordinate transformations leave intervals invariant. However, note
that whereas distances are always positive, intervals may be positive, zero,
or negative. Events with a spacetime interval of zero are separated by the
propagation of a light signal. Events with a positive spacetime interval
are in each other's future or past, and the value of the interval defines
the proper time measured by an observer traveling between them.

On ultramicroscopic scales, the quantum nature of spacetime would become apparent and require a quantum theory of gravity to describe it.

## Worldline

A worldline is the history of a particle as represented on a spacetime (Minkowski) diagram (see illustration). The worldline of a particle is straight if the particle moves uniformly and is curved if it moves non-uniformly (i.e. if there is an acceleration). Rays of light can be treated as the worldlines of photons. The worldlines of particles under the influence of a gravitational field are geodesics in spacetime. The worldline of a photon near a star, such as the Sun, is slightly bent due to the light being deflected by the gravitational field of the Sun.

*My Worldline* is the title of the autobiography
of George Gamow, the Ukrainian-American physicist.

## Block universe

The block universe is a view of spacetime that affords equal (ontological) status to all points in spacetime, thus regarding temporality as an illusory human construct with no reference to reality as understood by modern physics.