Editing Pp-wave spacetime (section)

==Geometric and physical properties==
PP-waves enjoy numerous striking properties.  Some of their more abstract mathematical properties have already been mentioned. In this section a few additional properties are presented.

Consider an inertial observer in Minkowski spacetime who encounters a sandwich plane wave.  Such an observer will experience some interesting optical effects.  If he looks into the ''oncoming'' wavefronts at distant galaxies which have already encountered the wave, he will see their images undistorted.  This must be the case, since he cannot know the wave is coming until it reaches his location, for it is traveling at the speed of light.  However, this can be confirmed by direct computation of the optical scalars of the null congruence <math>\partial_v</math>.  Now suppose that after the wave passes, our observer turns about face and looks through the ''departing'' wavefronts at distant galaxies which the wave has not yet reached.  Now he sees their optical images sheared and magnified (or demagnified) in a time-dependent manner.  If the wave happens to be a [[Polarization (waves)|polarized]] ''gravitational plane wave'', he will see circular images alternately squeezed horizontally while expanded vertically, and squeezed vertically while expanded horizontally.  This directly exhibits the characteristic effect of a gravitational wave in general relativity on light.

The effect of a passing polarized gravitational plane wave on the relative positions of a cloud of (initially static) test particles will be qualitatively very similar.  We might mention here that in general, the motion of test particles in pp-wave spacetimes can exhibit [[Chaos theory|chaos]].

The fact that Einstein's field equation is [[nonlinear]] is well known.  This implies that if you have two exact solutions, there is almost never any way to [[Superposition principle|linearly superimpose]] them.  PP waves provide a rare exception to this rule: 
if you have two PP waves sharing the same covariantly constant null vector (the same geodesic null congruence, i.e. the same wave vector field), with metric functions <math>H_1, H_2</math> respectively, then <math>H_1 + H_2</math> gives a third exact solution.

[[Roger Penrose]] has observed that near a null geodesic, ''every Lorentzian spacetime looks like a plane wave''.  To show this, he used techniques imported from algebraic geometry to "blow up" the spacetime so that the given null geodesic becomes the covariantly constant null geodesic congruence of a plane wave.  This construction is called a [[Penrose limit]].

Penrose also pointed out that in a pp-wave spacetime, all the [[polynomial scalar invariants]] of the [[Riemann curvature tensor|Riemann tensor]] ''vanish identically'', yet the curvature is almost never zero. This is because in four-dimension all pp-waves belong to the class of [[VSI spacetime]]s. Such statement does not hold in higher-dimensions since there are higher-dimensional pp-waves of algebraic type II with non-vanishing  [[polynomial scalar invariants]].  If you view the Riemann tensor as a second rank tensor acting on bivectors, the vanishing of invariants is analogous to the fact that a nonzero null vector has vanishing squared length.

Penrose was also the first to understand the strange nature of causality in pp-sandwich wave spacetimes.  He showed that some or all of the null geodesics emitted at a given event will be refocused at a later event (or string of events).  The details depend upon whether the wave is purely gravitational, purely electromagnetic, or neither.

Every pp-wave admits many different Brinkmann charts.  These are related by [[coordinate transformation]]s, which in this context may be considered to be [[gauge transformation]]s.  In the case of plane waves, these gauge transformations allow us to always regard two colliding plane waves to have ''parallel wavefronts'', and thus the waves can be said to ''collide head-on''.
This is an exact result in fully nonlinear general relativity which is analogous to a similar result concerning electromagnetic [[plane wave]]s as treated in [[special relativity]].