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We consider the motion of small bodies in general relativity. The key result captures a sense in which such bodies follow timelike geodesics (or, in the case of charged bodies, Lorentz-force curves). This result clarifies the relationship between approaches that model such bodies as distributions supported on a curve, and those that employ smooth fields supported in small neighborhoods of a curve. This result also applies to "bodies" constructed from wave packets of Maxwell or Klein-Gordon fields. There follows a simple and precise formulation of the optical limit for Maxwell fields., 30 pages, forthcoming in Communications in Mathematical Physics

The familiar theories of physics have the feature that the application of the theory to make predictions in specific circumstances can be done by means of an algorithm. We propose a more precise formulation of this feature --- one based on the issue of whether or not the physically measurable numbers predicted by the theory are computable in the mathematical sense. Applying this formulation to one approach to a quantum theory of gravity, there are found indications that there may exist no such algorithms in this case. Finally, we discuss the issue of whether the existence of an algorithm to implement a theory should be adopted as a criterion for acceptable physical theories., Comment: 19 pages, a pre arXiv paper posted for accessibility

There is proven a theorem, to the effect that a material body in general relativity, in a certain limit of sufficiently small size and mass, moves along a geodesic., 7 pages

For an asymptotically flat initial-data set in general relativity, the total mass-momentum may be interpreted as a Hermitian quadratic form on the complex, two-dimensional vector space of ``asymptotic spinors''. We obtain a generalization to an arbitrary initial-data set. The mass-momentum is retained as a Hermitian quadratic form, but the space of ``asymptotic spinors'' on which it is a function is modified. Indeed, the dimension of this space may range from zero to infinity, depending on the initial data. There is given a variety of examples and general properties of this generalized mass-momentum., 25 pages, LaTeX

Computation is the process of applying a procedure or algorithm to the solution of a mathematical problem. Mathematicians and physicists have been occupied for many decades pondering which problems can be solved by which procedures, and, for those that can be solved, how this can most efficiently be done. In recent years, quantum mechanics has augmented our understanding of the process of computation and of its limitations. Perspectives in Computation covers three broad topics: the computation process and its limitations, the search for computational efficiency, and the role of quantum mechani

It is well-known that the equations for a simple fluid can be cast into what is called their Lagrange formulation. We introduce a notion of a generalized Lagrange formulation, which is applicable to a wide variety of systems of partial differential equations. These include numerous systems of physical interest, in particular, those for various material media in general relativity. There is proved a key theorem, to the effect that, if the original (Euler) system admits an initial-value formulation, then so does its generalized Lagrange formulation., 34 pages, no figures, accepted in J. Math. Phys

We prove that an m-dimensional unit ball D^m in the Euclidean space {\mathbb R}^m cannot be isometrically embedded into a higher-dimensional Euclidean ball B_r^d \subset {\mathbb R}^d of radius r < 1/2 unless one of two conditions is met -- (1)The embedding manifold has dimension d >= 2m. (2) The embedding is not smooth. The proof uses differential geometry to show that if d, 20 Pages, 3 Figures

An example is given, in dimension six, of a curvature tensor having positive sectional curvatures and negative Gauss-Bonnet integrand. A large class of questions in differential geometry involves the relationship between the topology and the geometry of a compact Riemannian manifold. One of these is the Hopf conjecture: If, in even dimensions, the sectional curvatures of such a manifold are positive, then so is the Euler number. The Hopf conjecture is known to be true in dimensions two and four by the following argument (Milnor, unpublished; [2]). One first writes down the Gauss-Bonnet formula, which, in every even dimension, equates the Euler number of the manifold to a certain integral over the manifold, where the integrand involves only the curvature tensor, and that only algebraically. One then shows (in dimensions two and four) that, at each point, positivity of the sectional curvatures implies positivity of this integrand. Most attempts to prove the full Hopf conjecture have been attempts to generalize this argument [1], [3], [4], [5], [6]. Thus, there arises the following, purely algebraic, question: Over a vector space of any even dimension, does a tensor having the symmetries of a curvature tensor and having positive sectional curvatures necessarily have positive Gauss-Bonnet integrand? We here answer this question in the negative. Fix a real, six-dimensional vector space V. A wedge denotes the wedge product, and a star a Hodge star operator.2 Denote by V2 the vector space of second-rank, antisymmetric tensors over V, by V2 its dual (the space of 2-forms over V), and by V22 the vector space of symmetric linear mappings from V2 to V2. We shall make use of the following fact: For any element A of V2, (1) ((A A A)* A (A A A)*)* 9(A A A A A)*A. For A any element of V2, denote by TA the element of V22 with action Received by the editors September 3, 1974. AMS (MOS) subject classifications (1970). Primary 53B20. i Supported in part by the National Science Foundation under contract GP-34721Xi, and by the Sloan Foundation. 2 Our conventions for the star operation are these: For any form A, A* = A; for B a 2-form and C a 4-form, B(C*) = C(B*) = (B A C)* = (B* A C*)*. Note that we introduce no metric on V. ? American Mathematical Society 1976 267 This content downloaded from 157.55.39.17 on Wed, 31 Aug 2016 04:16:18 UTC All use subject to http://about.jstor.org/terms

Let H be a closed subgroup of compact Lie group G. A necessary and sufficient condition is obtained for the existence of a left-invariant Riemannian metric on G such that the subduced metric on the quotient H G has strictly positive sectional curvatures.

It is shown that, given any set of initial data for Einstein's equations which satisfy the constraint conditions, there exists a development of that data which is maximal in the sense that it is an extension of every other development. These maximal developments form a well-defined class of solutions of Einstein's equations. Any solution of Einstein's equations which has a Cauchy surface may be embedded in exactly one such maximal development.

A prescription is given for attaching to a space-time M , subject only to a causality condition, a collection of additional ‘ideal points’. Some of these represent ‘points at infinity’, others ‘singular points’. In particular, for asymptotically simple space-times, the ideal points can be interpreted as the boundary at conformal infinity. The construction is based entirely on the causal structure of M , and so leads to the introduction of ideal points also in a broad class of causal spaces. It is shown that domains of dependence can be characterized in terms of ideal points, and this makes possible an extension of the domain-of-dependence concept to causal spaces. A suggestion is made for assigning a topology to M together with its ideal points. This specifies some singular-point structure for a wide range of possible space-times.

Do there exist topologies U \mathcal {U} and V \mathcal {V} for the set R R of real numbers such that a function f f from R R to R R is smooth in some specified sense (e.g., differentiable, C n {C^n} , or C ∞ {C^\infty } ) with respect to the usual structure of the real line if and only if f f is continuous from U \mathcal {U} to V \mathcal {V} ? We show that the answer is no.

We begin with an essentially technical notion that will be used repeatedly in what follows. Indeed, it will serve as our conduit between physical observations and the mathematical formalism of quantum mechanics. Fix a quantum system. To make the discussion concrete, we suppose it to be nonrelativistic, and we describe it in the Schr6dinger representation. Thus, we have a configuration manifold C for the system, complex-valued wave functions on C (representing states of the system), a Hamiltonian operator H on such wave functions, and the Schr6dinger equation (giving the dynamics of the system). We say that a region R of configuration space is precluded if the wave function 1, as a consequence of its dynamical evolution from an initial state, becomes at some time "small" in the region R. This is intended, not as a precise definition, but rather as a summary of the meaning we have in mind, a meaning to be clarified below. We begin with an example.3 Let the system consist of, say, 101 one-dimensional particles, i.e., let the configuration manifold C be [IO'1 (coordinates x, ...., x,oo and y). Then a typical Schrodinger wave function is b (xi, . . ., x,oo, y). Let the Hamiltonian be given by