Absolute Space in Natural Philosophy
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Related TopicsAbsolute motion Relational space Relational motion Ontology of space
Absolute space is the hypothesis that space is an entity or structure that can determine the states of motion of material bodies but which exists independently of material bodies.
Newton and Absolute Space
Among Newton’s many contributions to natural philosophy, his concept of absolute space would play an important role in the debates in the nature of space and motion over the ensuing centuries. In the Principia, Newton states that “[a]bsolute space, of its own nature without reference to anything external, always remains homogeneous and immovable,” whereas “[r]elative space is any movable measure or dimension of this absolute space; such a measure or dimension is determined by our senses from the situation of the space with respect to bodies and is popularly used for immovable space” (Newton 2004, 64). The debate between the absolutists and relationists on space, time, and motion remains one of the fundamental questions in physics and the philosophy of science.
Inspired by Descartes, some natural philosophers in the latter half of the seventeenth century defined motion as a mere relation among bodies that only allows a determination of their relative change in distance and not their individual, absolute states of motion. They also followed Descartes in not regarding space as an entity that exists independently of material bodies (Descartes 1985, 227). Newton, in contrast, conceives absolute space as an immovable geometric framework that determines the individual positions of bodies and partially determines their motions but whose existence is independent of bodies (and the same holds true of absolute time; Newton 2004, 64–70). While there were anticipations of absolute space (and absolute time) in the medieval and renaissance periods, as well as several highly developed versions of the concept that were introduced by natural philosophers in the 1660s to combat the Cartesian conception (see, e.g., Dugas 1958, 290), it was Newton who refined and elevated the idea of absolute space into the form that is most familiar to modern readers.
In his De gravitatione, an unpublished tract written prior to the Principia, Newton had argued for the existence of absolute space on the grounds that the conception of place that Descartes had endorsed – that a body’s place is defined as the boundary of the surrounding contiguous bodies (Descartes 1985, 229) – cannot adequately determine bodily motion. In short, since a body’s neighborhood of contiguous bodies will inevitably change their relative position once the body moves, Newton concludes that the initial place of the body no longer exists, and thus the motion of the body cannot be well-defined (Newton 2004, 19–21). In the scholium on space and time in the first edition of the Principia, on the other hand, a number of different arguments are presented in order to establish that “absolute and relative rest and motion are distinguished from each other by their properties, causes, and effects” (Newton 2004, 66), with the most important being the “rotating bucket” and “rotating spheres” thought experiments, which are arguments from “effects” (see, e.g., Rynasiewicz 1995). Both are intended to demonstrate that the relative motion of bodies alone cannot account for the presence or absence of the forces associated with acceleration, in particular, for rotational motion and the accompanying centrifugal force effects. Thus, absolute space is required, since it can adequately explain these different scenarios. In the rotating bucket example, a bucket of water is suspended from a twisted rope and then released. The following events will occur: (i) before the bucket is released, the bucket is still, and the water is still; (ii) after the bucket is released, the bucket rotates but the water is still, and the surface flat; (iii) eventually, the bucket and water rotate together in unison and the surface of the water exhibits a curved, concave shape; and (iv) if one now grabs the bucket and stops its rotation, the water will still rotate and it will retain its concave shape for a short while. The problem for relational motion can be summarized as follows: events (i) and (iii) both have the same relational state of motion between the water and bucket, namely, they are at rest relative to one another, but there is a non-inertial force in (iii), as manifest in the concave shape of the water, but not in (i); also, events (ii) and (iv) have the same relational state of motion between the bucket and water, namely, there is a large rotational motion between the water and bucket, but there is only a non-inertial force in effect in (iv) and not in (ii). Thus, the existence of the non-inertial force of rotation is not due to motion of the water relative to the side of the bucket (since we have cases of non-inertial forces both when it is, and is not, in motion relative to the bucket). In “the rotating spheres” thought experiment, if two spheres are connected by a cord, then a rotation about their center-of-gravity can be determined by measuring the tension of the chord, but a relationist would have to declare that there is no rotation since neither body is moving with respect to the other.
The specific nature (or ontology) of space – whether space is absolute, relational, or something else – continues to influence debates in physics. While absolute space is considered to be a unique, non-material entity in contemporary versions of that doctrine, the foundation of absolute space for Newton is God (see, Grant 1981, 240–258). He claims in the General Scholium of the Principia that “He endures always and is present everywhere, and by existing always and everywhere he constitutes duration and space” (Newton 2004, 91). In the earlier De gravitatione (Newton 2004, 21), he had claimed that space is “an emanative effect of God” but that it is neither a substance (since it cannot act) nor a bodily property (since there are void spaces without body).
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