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In physics, the concept of absolute time and absolute space are hypothetical concepts closely tied to the thought of Isaac Newton:[1]

Absolute, true and mathematical time, of itself, and from its own nature flows equably without regard to anything external, and by another name is called duration: relative, apparent and common time, is some sensible and external (whether accurate or unequable) measure of duration by the means of motion, which is commonly used instead of true time …

Using this definition, time runs at the same rate for all the observers in the universe and different measures of absolute time can be scaled by multiplying by a constant.

To quote Newton again:

Absolute space, in its own nature, without regard to anything external, remains always similar and immovable. Relative space is some movable dimension or measure of the absolute spaces; which our senses determine by its position to bodies: and which is vulgarly taken for immovable space …

Absolute motion is the translation of a body from one absolute place into another: and relative motion, the translation from one relative place into another

These notions imply that absolute space and time do not depend upon physical events, but are a backdrop or stage setting within which physical phenomena occur. Thus, every object has an absolute state of motion relative to absolute space, so that an object must be either in a state of absolute rest, or moving at some absolute speed.[2] To support his views, Newton provided some empirical examples: according to Newton, a solitary rotating sphere can be inferred to rotate about its axis relative to absolute space by observing the bulging of its equator, and a solitary pair of spheres tied by a rope can be inferred to be in absolute rotation about their center of gravity (barycenter) by observing the tension in the rope.

In today's classical mechanics, the notion of absolute space is replaced by the idea of inertial frames of reference.

Two bodies orbiting around a common barycenter. Supposing the bodies tied by a rope to hold them together (rather than gravity, which is ignored in this example), the rope is under tension if the bodies are rotating relative to absolute space (according to Newton), or because they rotate relative to the Universe itself (according to Mach), or because they rotate relative to an inertial frame of reference according to modern ideas.


Historical controversy

The idea of absolute space has proved particularly controversial from Newton's time to the present. For example, Leibniz was of the opinion that space made no sense except as the relative location of bodies, and time made no sense except as the relative movement of bodies.[3] Bishop Berkeley suggested that, lacking any point of reference, a sphere in an otherwise empty Universe could not be conceived to rotate, and a pair of spheres could be conceived to rotate relative to one another, but not to rotate about their center of gravity.[4] A more recent form of these objections was made by Mach. Mach's principle proposes that mechanics is entirely about relative motion of bodies and, in particular, mass is an expression of such relative motion. So, for example, a single particle in a Universe with no other bodies would have zero mass. According to Mach, Newton's examples simply illustrate relative rotation of spheres and the bulk of the Universe.[5]

When, accordingly, we say that a body preserves unchanged its direction and velocity in space, our assertion is nothing more or less than an abbreviated reference to the entire universe.

These views opposing absolute space and time may be seen from a modern stance as an attempt to introduce operational definitions for space and time, a perspective made explicit in the special theory of relativity.

Even within the context of Newtonian mechanics, the modern view is that absolute space is unnecessary. Instead, the notion of inertial frame of reference has taken precedence, that is, a preferred set of frames of reference that move uniformly with respect to one another. The laws of physics transform from one inertial frame to another according to Galilean relativity, leading to the following objections to absolute space, as outlined by Milutin Blagojević:[6]

  • The existence of absolute space contradicts the internal logic of classical mechanics since, according to Galilean principle of relativity, none of the inertial frames can be singled out.
  • Absolute space does not explain inertial forces since they are related to acceleration with respect to any one of the inertial frames.
  • Absolute space acts on physical objects by inducing their resistance to acceleration but it cannot be acted upon.

Newton himself recognized the role of inertial frames.[7]

The motions of bodies included in a given space are the same among themselves, whether that space is at rest or moves uniformly forward in a straight line.

As a practical matter, inertial frames often are taken as frames moving uniformly with respect to the fixed stars.[8] See Inertial frame of reference for more discussion on this.

Impact of special relativity

The concepts of space and time were separate in physical theory prior to the advent of special relativity theory, which connected the two and showed both to be dependent upon the observer's state of motion. In Einstein's theories, the ideas of absolute time and space were superseded by the notion of spacetime in special relativity, and by dynamically curved spacetime in general relativity.

The theory of relativity does not allow the existence of absolute time because of nonexistence of absolute simultaneity. Absolute simultaneity refers to the experimental establishment of coincidence of two or more events in time at different locations in space in a manner agreed upon by all observers in the universe. The theory of relativity postulates a maximum rate of transmission of information as the speed of light, and one consequence is that simultaneity at separated locations always is relative to the observer.[9]

See also

References and notes

  1. ^ In Philosophiae Naturalis Principia Mathmetica See the Principia on line at Andrew Motte Translation
  2. ^ Space and Time: Inertial Frames (Stanford Encyclopedia of Philosophy)
  3. ^ Rafael Ferraro (2007). } Einstein's Space-Time: An Introduction to Special and General Relativity. Springer. p. 1. ISBN 978-0-387-69946-2. }. 
  4. ^ Paul Davies, John Gribbin (2007). The Matter Myth: Dramatic Discoveries that Challenge Our Understanding of Physical Reality. Simon & Schuster. p. 70. ISBN 0743290917.,M1. 
  5. ^ Ernst Mach; as quoted by Ignazio Ciufolini, John Archibald Wheeler (1995). Gravitation and Inertia. Princeton University Press. p. 386–387. ISBN 0691033234.,M1. 
  6. ^ Milutin Blagojević (2002). Gravitation and Gauge Symmetries. CRC Press. p. 5. ISBN 0750307676.,M1. 
  7. ^ Isaac Newton: Principia, Corollary V, p. 88 in Andrew Motte translation. See the Principia on line at Andrew Motte Translation
  8. ^ C Møller (1976). The Theory of Relativity (Second Edition ed.). Oxford UK: Oxford University Press. p. 1. ISBN 019560539X. 
  9. ^ Rafael Ferraro (2007). } op. cit.. p. 59. ISBN 978-0-387-69946-2.,M1 }. 

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