For the following discussion we refer to the rescaled metric

which is the metric of the Einstein cylinder. The standard conformal diagram for Minkowski space [109] is shown in Figure 2 .

Each point in the interior of the triangle corresponds to a
2-sphere. The long side of the triangle consists of all the
points in the centre,
*r*
=0 (i.e.
*U*
=
*V*). The other two sides of the triangle correspond to
null-infinity
. The points
are points in the centre with
, while
is a point with
. The lines meeting at
are lines of constant
*t*, while the lines emanating from
and converging into
are lines of constant
*r*
. In the four-dimensional space-time the lines of constant
*t*
correspond to three-dimensional space-like hypersurfaces which
are asymptotically Euclidean.

In the case of Minkowski space-time, the metric can be extended in a regular way to three points representing future and past time-like infinity and space-like infinity, but this is not generally so. Already in the case of the Schwarzschild metric, which is also an asymptotically flat metric, there are, strictly speaking, no such points because any attempt to extend the metric yields a singularity. However, it is common language to refer to this behaviour by saying that ``the points and/or are singular''. The reason for this is related to the presence of mass. For any space-time which has a non-vanishing ADM-mass, the point is necessarily singular while the singularity of the time-like infinities is, in general, related to the fall-off properties of the energy-momentum tensor in time-like directions. In the case of the Schwarzschild solution (like in any stationary black-hole solution) it is the presence of the static (stationary) black hole which is responsible for the singularity of .

Let us now assume that there is a particle which moves along
the central world-line
*r*
=0, emitting radiation. For the sake of simplicity we assume that
it emits electro-magnetic radiation which travels along the
outgoing null-cones to null-infinity. The null-cones are
symbolized in Figure
2
by the straight lines going off the particle's world-line at
. We are now interested in the behaviour of the signal along
various space-like hypersurfaces. In Figure
3
we show again the conformal diagram of Minkowski space. The
generic features discussed below will be the same for any
asymptotically flat space-time as long as we stay away from the
corners of the diagram. The reason for choosing Minkowski space
is simply a matter of convenience.

The vertical dashed line is the world-line of the particle which defines the time axis. We have displayed two asymptotically Euclidean space-like hypersurfaces intercepting the time axis at two different points and reaching out to space-like infinity. Furthermore, there are two hypersurfaces which intersect the time axis in the same two points as the asymptotically flat ones. They reach null-infinity, intersecting in a two-dimensional space-like surface. This geometric statement about the behaviour of the hypersurfaces in the unphysical space-time translates back to the physical space-time as a statement about asymptotic fall-off conditions of the induced (physical) metric on the hypersurfaces, namely that asymptotically the metric has constant negative curvature. This is, in particular, a property of the space-like hyperboloids in Minkowski space. Thus, such hypersurfaces are called hyperboloidal hypersurfaces. An important point to keep in mind is that the conformal space-time does not ``stop'' at but that it can be extended smoothly beyond. The extension is not uniquely determined as we have already discussed in connection with the embedding of Minkowski space into the Einstein cylinder (cf. Figure 1). Thus, the extension plays no role for the concept of null-infinity but it can be very helpful for technical reasons, in particular when numerical issues are discussed.

We now imagine that the central particle radiates
electro-magnetic waves of uniform frequency, i.e. proportional to
, where
is the particle's proper time. This gives rise to a retarded
electro-magnetic field on the entire space-time which has the
form
, where
*u*
is a retarded time coordinate on Minkowski space with
on the central world-line. We ignore the fall-off of the field
because it is irrelevant for our present purposes. Let us now
look at the waves on the various hypersurfaces.

In the physical space-time, the hypersurfaces extend to infinity and we can follow the waves only up to an arbitrary but finite distance along the hypersurfaces. The end-points are indicated in Figure 3 as little crosses. The resulting wave-forms are shown in Figure 4 .

The first diagram shows the situation on the asymptotically Euclidean surfaces. These are surfaces of constant Minkowski time which implies that the signal is again a pure sine. Note, however, that this is only true for these special hypersurfaces. Even in Minkowski space-time we could choose space-like hypersurfaces which are not surfaces of constant Minkowski time but which nonetheless are asymptotically Euclidean. On such surfaces the wave would look completely different.

On the hyperboloidal surfaces the waves seem to ``flatten out''. The reason for the decrease in frequency is the fact that these surfaces tend to become more ``characteristic'' as they extend to infinity, thus approaching surfaces of constant phase of the retarded field.

The final diagram shows the signal obtained by an idealized
observer which moves along the piece of
between the two intersection points with the hyperboloidal
surfaces. The signal is recorded with respect to the retarded
time
*u*
which, in the present case, is a so called Bondi parameter (see
Section
4.3). Therefore, the observer measures a signal at a single
frequency for a certain interval of this time parameter. A
different Bondi time would result in a signal during a different
time interval but with a single, appropriately scaled, frequency.
Using an arbitrary time parameter would destroy the feature that
only one frequency is present in the signal. This is, in fact,
the only information that can be transmitted from the emitter to
the receiver under the given circumstances.

The wave-forms of the signal as they appear in the conformal
space-time, i.e. with respect to a coordinate system which covers
a neighbourhood of
, are shown in Figure
5
. In the specific case of Minkowski space-time we use the
coordinates
*T*
and
*R*
on the Einstein cylinder. The signal on the asymptotically
Euclidean surfaces shows the ``piling up'' of the waves as they
approach space-like infinity. The signal on the hyperboloidal
surfaces looks very similar to the physical case. Since the field
and the surfaces are both smooth across
, the signal can continue on across null-infinity without even
noticing its presence. The points where
is crossed are indicated in the diagram as two little crosses.
The values of the field at these points are the same as the
boundary values of the signal in the third diagram. Here the
signal on the same region of
as in Figure
4
is displayed, but with respect to the coordinate
which is not a Bondi parameter. Accordingly, we see that the
wavelength of the signal is not constant.

What these diagrams teach us is the following: It has been convenient in relativity to decompose space-time into space and time by slicing it with a family of space-like hypersurfaces. In most of the work on existence theorems of the Einstein equations it has been convenient to choose them to be Cauchy surfaces and thus asymptotically Euclidean. Also, in most numerical treatments of Einstein's equations the same method is used to evolve space-times from one space-like hypersurface to the next (see Section 4). Here the hypersurfaces used are finite because the numerical grids are necessarily finite. In the approaches based on the standard Einstein equations it makes no difference whether the grid is based on a finite portion of an asymptotically Euclidean or a hyperboloidal hypersurface. The fact that the space-time should be asymptotically flat has to be conveyed entirely by a suitable boundary condition which has to be imposed at the boundary of the finite portion of the hypersurface (i.e. at the little crosses in Figure 3). However, this implies that the accuracy of the wave-form templates obtained with such approaches depends to a large extent on the quality of that boundary condition. So far there exists no suitable boundary condition which would be physically reasonable and lead to stable codes.

In the conformal approach one has the option to ``include infinity'' by using the conformal field equations (see Section 3). Then the type of the space-like hypersurfaces becomes an issue. The diagrams show that the hyperboloidal surfaces are very well suited to deal with the radiation problems. They provide a foliation of the conformal space-time on which one can base the evolution with the conformal field equations. The solution obtained will be smooth near and we ``only'' need to locate on each hypersurface to read off the value of the radiation data (as indicated in the second diagram of Figure 5).

Conformal Infinity
Jörg Frauendiener
http://www.livingreviews.org/lrr-2000-4
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