# 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

By definition, the Einstein tensor of a null dust solution has the form $G^{ab}=8\pi \Phi \,k^{a}\,k^{b}$ where ${\vec {k}}$ is a null vector field. This definition makes sense purely geometrically, but if we place a stress–energy tensor on our spacetime of the form $T^{ab}=\Phi \,k^{a}\,k^{b}$, then Einstein's field equation is satisfied, and such a stress–energy tensor has a clear physical interpretation in terms of massless radiation. The vector field ${\vec {k}}$ specifies the direction in which the radiation is moving; the scalar multiplier $\Phi$ 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:
* a beam of neutrinos assumed for simplicity to be massless (treated according to classical physics),
* a very high-frequency electromagnetic wave,
* a beam of incoherent electromagnetic radiation.
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
    ${\vec {e}}_{0},\;{\vec {e}}_{1},\;{\vec {e}}_{2},\;{\vec {e}}_{3}$
(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:
    $G^{{\hat {a}}{\hat {b}}}=8\pi \epsilon \,\left[{\begin{matrix}1&0&0&\pm 1\\0&0&0&0\\0&0&0&0\\\pm 1&0&0&1\end{matrix}}\right]$
Here, ${\vec {e}}_{0}$ is everywhere tangent to the world lines of our adapted observers, and these observers measure the energy density of the incoherent radiation to be $\epsilon$.
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 ${\vec {k}}$. It is generated by two parabolic Lorentz transformations (pointing in the ${\vec {e}}_{3}$ direction) and one rotation (about the ${\vec {e}}_{3}$ 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:
* pp-wave spacetimes (which model generalizations of the plane waves familiar from electromagnetism),
* Robinson–Trautman null dusts (which model radiation expanding from a radiating object).
The pp-waves include the gravitational plane waves and the monochromatic electromagnetic plane wave. A specific example of considerable interest is
* the Bonnor beam, an exact solution modeling an infinitely long beam of light surrounded by a vacuum region.
Robinson–Trautman null dusts include the Kinnersley–Walker photon rocket solutions, which include the Vaidya null dust, which includes the Schwarzschild vacuum.