Null dust solution

Mathematical definition

The Einstein tensor of a null dust must have the form G a b = 8 π Φ k a k b where k → 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 T a b = Φ k a k b 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.

Physical interpretation

Physically speaking, a null dust describes either gravitational radiation, or some kind of nongravitational radiation which is described by a relativistic classical field theory (such as electromagnetic radiation), or a combination of these two. Null dusts include vacuum solutions as a special case.

Phenomena which can be modeled by null dust solutions include:

In particular, a plane wave of incoherent electromagnetic radiation is a linear superposition of plane waves, all moving in the same direction but having randomly chosen phases and frequencies. (Even though the Einstein field equation is nonlinear, a linear superposition of comoving plane waves is possible.) Here, each electromagnetic plane wave has a well defined frequency and phase, but the superposition does not. Individual electromagnetic plane waves are modeled by null electrovacuum solutions, while an incoherent mixture can be modeled by a null dust.

Einstein tensor

The components of a tensor computed with respect to a frame field rather than the coordinate basis are often called physical components, because these are the components which can (in principle) be measured by an observer.

In the case of a null dust solution, an adapted frame

(a timelike unit vector field and three spacelike unit vector fields, respectively) can always be found in which the Einstein tensor has a particularly simple appearance:

Here, e → 0 is everywhere tangent to the world lines of our adapted observers, and these observers measure the energy density of the incoherent radiation to be ϵ .

From the form of the general coordinate basis expression given above, it is apparent that the stress–energy tensor has precisely the same isotropy group as the null vector field k → . It is generated by two parabolic Lorentz transformations (pointing in the e → 3 direction) and one rotation (about the e → 3 axis), and it is isometric to the three-dimensional Lie group E ( 2 ) , the isometry group of the euclidean plane.

Examples

Null dust solutions include two large and important families of exact solutions:

The pp-waves include the gravitational plane waves and the monochromatic electromagnetic plane wave. A specific example of considerable interest is

Robinson–Trautman null dusts include the Kinnersley–Walker photon rocket solutions, which include the Vaidya null dust, which includes the Schwarzschild vacuum.