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null dust solution
In mathematical physics, a null dust solution (sometimes called a null fluid) is a Lorentzian manifold in which the Einstein tensor is null. Such a spacetime can be interpreted as an exact solution of Einstein's field equation, in which the only mass-energy present in the spacetime is due to some kind of massless radiation.
==Mathematical definition==
The Einstein tensor of a null dust must have the form
where is a null vector field. This definition makes sense in the absence of any physical interpretation, but if we place a stress–energy tensor on our spacetime which happens to have the form
then Einstein's field equation is trivially satisfied, and in addition, such a stress–energy tensor has a clear physical interpretation in terms of massless radiation. The vector field specifies the direction in which the radiation is moving; the scalar multiplier specifies its intensity.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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