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Absolute and Relative Motion

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Introduction

A debate in modern philosophy of physics is whether motion is absolute or relative. The debate began in the 1600s, so it deserves a close look here. Primarily, it was a controversy in metaphysics, but it had epistemic aspects too. I begin with the former and then touch upon the latter at the end.

Detailed Description

But first, some motivation. “Absolute” and “relative” were explanatory concepts in theories of motion, which were philosophical accounts of a privileged state synonymously called real, true, objective, or physical motion. The state was privileged because physical theory was about it: in early-modern science, descriptions and predictions were about the true motions of bodies. Caveat: “absolute” and “relative” were, and remain, deeply ambiguous terms; failure to clarify them well has caused much confusion and misguided controversy. It is doubtful that we can get a principled, univocal distinction (between absolute and relative) that applies both to current...

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  • Absolute space
  • Newton
  • Leibniz

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Notes

  1. 1.

    There was also Tycho’s hybrid picture, of course. The real, or true, Ptolemaic orbit of the Sun around the stationary earth is as presented in Ptolemy, Almagest, Book III. The earth’s Copernican true motion is in De revolutionibus, Book I.

  2. 2.

    Here are two examples known to the early moderns. Cannonballs shot along the local meridian deviate westward, and yet the horizontal component of their motion is inertial, so it should follow the meridian’s north-south line. Objects dropped from rest deviate eastward, even though the only genuine force on them is downward, due to gravity. Of course, these violations of the Law are just apparent; they are due to the earth (to which the above motions are referred) not being a true inertial frame, because it spins.

  3. 3.

    Bodies are extended too, but finite and bounded, whereas Absolute Space is infinite and unbounded. And, bodies are divisible in a strong sense: Physical agencies can cause any body’s parts to become separated by a finite distance. That is conceptually impossible with Absolute Space. Lastly, bodies affect our senses, causing perceptions, whereas Absolute Space is imperceptible. Newton made these points in De gravitatione, a youthful essay in metaphysics (cf. Newton 2014).

  4. 4.

    Some modern writers suggest that Absolute Space is responsible for undisturbed bodies continuing in their inertial motion, if they so move. Whatever the truth of this notion, it is anachronistic in our context, and it lacks evidential support from it. The early moderns did not think that force-free motion requires explanation (let alone by appeal to space itself); only deviations from inertial motion needed explaining (in terms of the powers of bodies, of course).

  5. 5.

    Topologically, Absolute Space was three-dimensional, unbounded, continuous everywhere, dense, and connected. Though the term “topology” arose in 1847, the key ideas behind it were already in place by 1700.

  6. 6.

    Huygens had taught how to attach test objects to a spinning system and infer the latter’s angular speed w from the tension-force between it and the test object. Newton had taught Hooke how to find evidence that the earth really spins: by dropping test objects from a height, and measuring their eastward deflection relative to the earth, not to any external frames such as the stars or the sun. For explanation, cf. Stan (2015, 2016).

  7. 7.

    r denotes a linear distance in Absolute Space: from the spinning body’s absolute place (at an instant) to the absolute place of the center of rotation. And, w denotes the rate of change with respect to Absolute Time: of the body’s angular position in Absolute Space.

  8. 8.

    Newton rated it a hypothesis not because he had no evidence for it, but because his official warrant fell short of what he required for knowledge claims to count as “deduced from phenomena,” hence as beyond hypothetical according to his epistemic standards.

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Correspondence to Marius Stan .

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Stan, M. (2021). Absolute and Relative Motion. In: Jalobeanu, D., Wolfe, C.T. (eds) Encyclopedia of Early Modern Philosophy and the Sciences. Springer, Cham. https://doi.org/10.1007/978-3-319-20791-9_22-1

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