John Byron Manchak

What Is a Physically Reasonable Space-Time?

Cosmologists often use certain global properties to exclude “physically unreasonable” cosmological models from serious consideration. But, on what grounds should these properties be regarded as physically unreasonable if we cannot rule out, even with a robust type of inductive reasoning, the possibility of the properties obtaining in our own universe?

On the Existence of “Time Machines” in General Relativity

Within the context of general relativity, we consider one definition of a “time machine” proposed by Earman, Smeenk, and Wüthrich. They conjecture that, under their definition, the class of time machine spacetimes is not empty. Here, we prove this conjecture.

The Geometry of Conventionality

There is a venerable position in the philosophy of space and time that holds that the geometry of spacetime is conventional, provided one is willing to postulate a “universal force field.” Here we ask a more focused question, inspired by this literature: in the context of our best classical theories of space and time, if one understands “force” in the standard way, can one accommodate different geometries by postulating a new force field? We argue that the answer depends on one’s theory. In Newtonian gravitation the answer is yes; in relativity theory, it is no.

On Space-Time Singularities, Holes, and Extensions

Here, we clarify the relationship among three space-time conditions of interest: geodesic completeness, hole-freeness, and inextendibility. In addition, we introduce a related fourth condition: effective completeness.

Epistemic "Holes" in Spacetime

A number of models of general relativity seem to contain “holes” that are thought to be “physically unreasonable.” One seeks a condition to rule out these models. We examine a number of possibilities already in use. We then introduce a new condition: epistemic hole-freeness. Epistemic hole-freeness is not just a new condition—it is new in kind. In particular, it does not presuppose a distinction between space-times that are “physically reasonable” and those that are not.

On Gödel and the Ideality of Time

Gödel’s remarks concerning the ideality of time are examined. In the literature, some of these remarks have been somewhat neglected while others have been heavily criticized. In this article, I propose a clear and defensible sense in which Gödel’s work bears on the question whether there is an objective lapse of time in our world.

Essay Review: Topics in the Foundations of General Relativity and Newtonian Gravitation Theory

The first portion of this book amounts to a formal introduction to differential geometry and the general theory of relativity (complete with problem sets and solutions). The second amounts to an investigation of special topics pursued by David Malament over the last 4 decades or so. In what follows, I intend to summarize the material. Along the way, I hope to offer some comments on the significance of the work. Before I begin the survey of chapters, it might be helpful to say something about what the book is not. Although there is quite a bit of overlap between the first portion of the book and a standard graduate-level physics text, the two do not fully intersect (more below). In addition, the book does not consider many common topics in the philosophy of space-time physics. For example, there is no discussion of whether space and time are absolute or relational in character (cf. Sklar 1974; Friedman 1983). Finally, one does not find any precisely formulated conjectures of physical or philosophical interest that gesture toward future work (cf. Wald 1984; Earman 1995). It simply is not a book of that kind. Nonetheless, a number of topics developed by Malament in later chapters have generated a significant number of interesting results. (Indeed, I think the previous sentence is wildly understated.) Because Malament does not always do so, I hope to draw attention to some of these lines of inquiry below (a portion of which is still currently active).

Time Travel: Why It May Not Pay to Work out All the Kinks

Here, we hypothesize that a smooth nongeodesic closed timelike curve is never most efficient with respect to total acceleration if a kink is permitted at the initial (terminal) point. We support our hypothesis in a variety of ways. Most notably, we show Malament’s opposing conjecture concerning Gödel space-time to be false.

On Force in Cartesian Physics

There does not seem to be a consistent way to ground the concept of “force” in Cartesian first principles. In this article, I first review the literature on the subject. Then, I offer an alternative interpretation of force—one that seems to be coherent and consistent with Descartes’ project. Not only does the new position avoid the problems of previous interpretations, but it does so in such a way as to support and justify those previous interpretations.

Malament-Hogarth Machines

This article shows a clear sense in which general relativity allows for a type of ‘machine’ that can bring about a spacetime structure suitable for the implementation of ‘supertasks’. 1. Introduction2. Preliminaries3. Malament–Hogarth Spacetimes4. Machines5. Malament–Hogarth Machines6. Conclusion Introduction Preliminaries Malament–Hogarth Spacetimes Machines Malament–Hogarth Machines Conclusion