 Static Spacetime
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Static Spacetime

In general relativity, a spacetime is said to be static if it does not change over time and is also irrotational. It is a special case of a stationary spacetime: the geometry of a stationary spacetime does not change in time; however, it can rotate. Thus, the Kerr solution provides an example of a stationary spacetime that is not static; the non-rotating Schwarzschild solution is an example that is static.

Formally, a spacetime is static if it admits a global, non-vanishing, timelike Killing vector field $K$ which is irrotational, i.e., whose orthogonal distribution is involutive. (Note that the leaves of the associated foliation are necessarily space-like hypersurfaces.) Thus, a static spacetime is a stationary spacetime satisfying this additional integrability condition. These spacetimes form one of the simplest classes of Lorentzian manifolds.

Locally, every static spacetime looks like a standard static spacetime which is a Lorentzian warped product R $\times$ S with a metric of the form

$g[(t,x)]=-\beta (x)dt^{2}+g_{S}[x]$ ,

where R is the real line, $g_{S}$ is a (positive definite) metric and $\beta$ is a positive function on the Riemannian manifold S.

In such a local coordinate representation the Killing field $K$ may be identified with $\partial _{t}$ and S, the manifold of $K$ -trajectories, may be regarded as the instantaneous 3-space of stationary observers. If $\lambda$ is the square of the norm of the Killing vector field, $\lambda =g(K,K)$ , both $\lambda$ and $g_{S}$ are independent of time (in fact $\lambda =-\beta (x)$ ). It is from the latter fact that a static spacetime obtains its name, as the geometry of the space-like slice S does not change over time.

Examples of non-static spacetimes

In general, "almost all" spacetimes will not be static. Some explicit examples include:

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