D P Lonie

The principle of equivalence and projective structure in spacetimes

This paper discusses the extent to which one can determine the spacetime metric from a knowledge of a certain subset of the (unparametrized) geodesics of its Levi-Civita connection, that is, from the experimental evidence of the equivalence principle. It is shown that, if the spacetime concerned is known to be vacuum, then the Levi-Civita connection is uniquely determined and its associated metric is, generically, uniquely determined up to a choice of units of measurement, by the specification of these geodesics. A special case arises here concerning pp-waves and is dealt with. It is further demonstrated that if two spacetimes share a similar subset of unparametrized geodesics they share all geodesics (that is they are projectively related). Furthermore, if one of these spacetimes is assumed vacuum then their Levi-Civita connections are again equal (and so the other metric is also a vacuum metric) and the first result above is recovered.

Projective equivalence of Einstein spaces in general relativity

There has been some recent interest in the relation between two spacetimes which have the same geodesic paths, that is, spacetimes which are projectively equivalent (sometimes called geodesically equivalent). This paper presents a short and accessible proof of the theorem that if two spacetimes have the same geodesic paths and one of them is an Einstein space then (either each is of constant curvature or) their Levi-Civita connections are identical. It also clarifies the relationship between their associated metrics. The results are extended to include the signatures (+ + + +) and (− − + +), and some examples and discussion are given in the case of dimension n > 4. Some remarks are also made which show how these results may be useful in the study of projective symmetry.

Holonomy groups and spacetimes

A study is made of the possible holonomy group types of a spacetime for which the energy-momentum tensor corresponds to a null or non-null electromagnetic field, a perfect fluid or a massive scalar field. The case of an Einstein space is also included. The techniques developed are, in addition, applied to vacuum and conformally flat spacetimes and contrasted with already known results in these two cases. Examples are given.

The principle of equivalence and cosmological metrics

This paper is concerned with the extent to which the geodesics of space-time (that is, the experimental consequences of the principle of equivalence) determine the metric in general relativity theory and, in particular, in Friedmann–Robertson–Walker–Lemaitre (FRWL) space-times. Thus it discusses projective structure in these space-times. The approach will be from a geometrical point of view and it is shown that if two space-time metrics share the same (unparametrized) geodesics and one is a (generic) FRWL metric then so is the other and that each is a member of a well defined family of projectively related (FRWL) metrics. Similar techniques are then applied to study the existence and properties of symmetries of the Weyl projective tensor and projective symmetries in FRWL space-times.

Projective collineations in spacetimes

Projective collineations in spacetimes, i.e. vector fields generating local groups of geodesic-preserving diffeomorphisms, are studied. The situation for Einstein spaces is resolved completely and some general results are established regarding arbitrary spacetimes. Examples of proper projective collineations are constructed.

Bifurcations of magnetic topology by the creation or annihilation of null points

Linear null points of a magnetic field may come together and coalesce at a secondorder null, or vice versa a second-order null may form and split, giving birth to a pair of linear nulls. Such local bifurcations lead to global changes of magnetic topology and in some cases release of magnetic energy. In two dimensions the null points are of X or O type and the flux function is a Hamiltonian; the magnetic field may undergo addle-centre, pitchfork or degenerate resonant bifurcations. In three dimensions the null points and their creation or annihilation by bifurcations are considerably more complex. The nulls possess a skeleton consisting of a spine curve and a fan surface and are of radial-type (proper or improper) or spiral-type; the type of null and the inclination of spine and fan depend on the magnitudes of the current components along and normal to the spine. In cylindrically symmetric fields a comprehensive treatment is given of the various types of saddle-node, Hopf and saddle-node—Hopfbifurcations. In fully three-dimensional situations examples are given of saddle-node and degenerate bifurcations, in which generically two nulls are created or destroyed and are joined by a separator field line, which is the intersection of the two fans. Furthermore, global bifurcations can create chaotic field lines that could perhaps trigger energy release in, for example, solar flares.

Projective structure and holonomy in four-dimensional Lorentz manifolds

This paper studies the situation when two 4-dimensional Lorentz manifolds (that is, space-times) admit the same (unparametrised) geodesics, that is, when they are projectively related. A review of some known results is given and then the problem is considered further by treating each holonomy type in turn for the space-time connection. It transpires that all holonomy possibilities can be dealt with completely except the most general one and that the consequences of two space-times being projectively related leads, in many cases, to their associated Levi-Civita connections being identical.

Holonomy and Projective Equivalence in 4-Dimensional Lorentz Manifolds ⋆

A study is made of 4-dimensional Lorentz manifolds which are projectively related, that is, whose Levi-Civita connections give rise to the same (unparameterised) geodesics. A brief review of some relevant recent work is provided and a list of new results connecting projective relatedness and the holonomy type of the Lorentz manifold in question is given. This necessitates a review of the possible holonomy groups for such manifolds which, in turn, requires a certain convenient classification of the associated curvature tensors. These reviews are provided.

Holonomy and projective symmetry in spacetimes

It is shown using a spacetime curvature classification and decomposition that for certain holonomy types of a spacetime, proper projective vector fields cannot exist. Existence is confirmed, by example, for the remaining holonomy types. In all except the most general holonomy type, a local uniqueness theorem for proper projective symmetry is established.

Some remarks on the symmetries of the curvature and Weyl tensors

This paper considers the symmetries of the curvature tensor (curvature collineations) and of the Weyl conformal tensor (Weyl conformal collineations) in general relativity. Some general results are reviewed for later application, some new ones proved and many special cases are investigated. Particular emphasis is laid on the interrelations between these two types of symmetries. A number of instructive examples of such symmetries are given.