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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）』 ■ウィキペディアで「null dust solution」の詳細全文を読む スポンサード リンク
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