Allen I. Janis

Structure of Gravitational Sources

The purpose of this paper is to propose a definition of multipole structure of gravitational sources in terms of the characteristic initial data for asymptotic solutions of the field equations. This definition is based upon a detailed study of the corresponding data for the linearized equations and upon the close analogy between the Maxwell and the linearized gravitational fields.

Tail of a Gravitational Wave

A first‐order quadrupole sandwich wave of gravitational radiation exploding from a first‐order Schwarzschild mass is examined to second order. If the second‐order field preceding the sandwich wave vanishes, it is shown that the region of space‐time following the sandwich wave contains a second‐order, imploding quadrupole wave. The rest of the second‐order field in the space‐time region following the sandwich wave is also given, and it is seen to consist of monopole, quadrupole, and 16‐pole nonradiative motions.

Algebraically Special H Spaces

All diverging algebraically special solutions of the complex vacuum Einstein equations which are left (or right) conformally flat (H‐spaces) are found explicitly. These metrics contain four arbitrary functions of two variables.

APPROACH TO GRAVITATIONAL RADIATION SCATTERING.

A method is presented for studying asymptotically flat spaces possessing both incoming and outgoing gravitational radiation at infinity. The method uses multipole expansions and the invariance of general relativity under time reversal; calculations are facilitated by a small‐parameter perturbation approach. Some calculations are carried out to second order to show the practicability of the method.

Simultaneity and Conventionality

The role that conventionality plays in the standard definition of simultaneity in the special theory of relativity has been the subject of much debate. I should like to review certain aspects of that debate, and suggest an analogy between an argument thought by many to show the nonconventionality of standard synchrony and an earlier argument that was not generally thought to be damaging to the conventionalist point of view.

Gravitational Radiation: Cutting the Tail

Using the Newman‐Penrose spin‐coefficient approach to gravitational radiation, we consider necessary conditions for stationary‐radiative‐stationary transitions to occur between states in a Riemannian space‐time which satisfies the empty‐space Einstein field equations everywhere outside of a spatially bounded timelike cylinder. It is found that for axially symmetric radiation of finite multipole expansion such transitions cannot occur; however, a model has been constructed for nonaxially symmetric radiation for which these transitions can occur to a certain asymptotic order in a relevant parameter while satisfying the Newman‐Penrose conservation laws.

Simultaneity and special relativistic kinematics

Starting with a careful analysis of the thought experiment known as Einstein’s train, it is shown how some basic results of special relativistic kinematics can be understood, both qualitatively and quantitatively, in terms of the relativity of simultaneity.

Radiation in cosmological backgrounds

The purpose of this investigation is to find the conditions for characteristic propagation of multipole radiation in Friedmann backgrounds. The radiation fields studied are Klein–Gordon scalar fields, conformally invariant scalar fields, electromagnetic fields, and gravitational fields. The behavior of electromagnetic and conformally invariant scalar radiation is similar to that of the corresponding radiation in flat space‐time, since both fields satisfy conformally invariant equations and the Friedmann backgrounds are conformally flat. Thus characteristically propagating solutions are possible for both fields in any Friedmann background. For the Klein–Gordon and gravitational fields, it is found that characteristic propagation is possible only for special Friedmann backgrounds. Two physically important Friedmann backgrounds, those for which P=0 and P=ρ/3 (where P is pressure and ρ is density), are among these special backgrounds for both types of radiation. In the course of this study, all Friedmann back...

Propagation of gravitational waves in Robertson-Walker backgrounds

The electric and magnetic parts of the linearized Weyl tensor, when the stress-energy tensor is that of a perfect fluid and the background is of Robertson-Walker type, are known to satisfy wave equations that differ by the presence of a source term for the electric part. It is shown here that all of the allowed solutions of the inhomogeneous equation can be obtained by applying a differential operator to the solutions of the homogeneous equation; consequently, electric-type and magnetic-type gravitational waves have the same propagation properties. The results of a complete integration of the appropriately linearized Newman-Penrose equations are given.

Can the effect precede its cause in classical electrodynamics

In classical electrodynamics, the imposition of Dirac’s asymptotic boundary condition on the differential equation of motion for a charged particle leads to the prediction of preacceleration; that is, the particle’s acceleration at time t depends on external forces acting at times later than t. Since force is usually considered to be the cause of acceleration rather than vice versa, the existence of this preacceleration has been taken to show that an effect can precede its cause. In this paper it is argued that such a retrocausal interpretation of preacceleration is either unfounded or false.