Absolute Motion
Absolute motion refers to the change in position of a body with respect to a fixed reference frame, often conceptualized as absolute space. It stands in contrast to Relative Motion, which defines movement only in relation to other bodies. The concept was central to Isaac Newton’s formulation of classical mechanics but faced significant challenges from relational philosophers and later physical theories.
Historical Context & Newtonian Mechanics
- Newton’s Definition: In the Principia, Newton argued that true motion must be distinguished from apparent motion. He posited that absolute space remains similar and immovable, regardless of whether it is in motion or at rest.
- Distinction from Relative Motion: While relative motion is observable and measurable between objects, absolute motion was proposed to explain phenomena where no visible reference frame exists, such as the rotation of the universe.
- Critiques: Philosophers like Leibniz and later Ernst Mach argued that space is merely a relational system of coordinates between material bodies, rendering the concept of absolute space metaphysical rather than physical.
Key Thought Experiments
Newton’s Bucket Argument
Newton used the rotating bucket experiment to demonstrate the reality of absolute space. As water in a bucket rotates, the surface becomes concave. Newton argued this concavity results from rotation relative to absolute space, not merely relative to the bucket walls, as the water remains flat when rotating with the bucket (zero relative motion to the bucket) but concave when the bucket stops but water continues to rotate.
For detailed analysis of the bucket’s implications on inertia and spacetime evolution, see: Newton’s Bucket: Inertia, Absolute Motion, and Spacetime’s Evolution
Evolution into General Relativity
- Mach’s Principle: Ernst Mach criticized Newton’s absolute space, suggesting that inertia arises from the gravitational interaction with all other mass in the universe. This “Machian” view influenced Einstein.
- General Relativity: Albert Einstein’s theory of General Relativity largely abolished the need for absolute space. Instead, motion and inertia are determined by the curvature of Spacetime caused by mass and energy. Local inertial frames are defined by the geodesics of spacetime geometry, effectively making the “absolute” reference frame dependent on the distribution of matter in the universe.
- Modern Interpretation: While strict Newtonian absolute motion is discarded, the concept persists in discussions regarding the isotropy of the universe (e.g., the Cosmic Microwave Background as a preferred rest frame), though this is kinematic rather than dynamic absolute space.
Related Concepts
- inertia
- Spacetime
- General Relativity
- Mach’s Principle
- Relative Motion