Editing Frame of reference (section)

== Definition ==
The need to distinguish between the various meanings of "frame of reference" has led to a variety of terms. For example, sometimes the type of coordinate system is attached as a modifier, as in ''Cartesian frame of reference''. Sometimes the state of motion is emphasized, as in ''[[Rotating reference frame|rotating frame of reference]]''. Sometimes the way it transforms to frames considered as related is emphasized as in ''[[Galilean frame of reference]]''. Sometimes frames are distinguished by the scale of their observations, as in ''macroscopic'' and ''microscopic frames of reference''.<ref name=macroscopic>The distinction between macroscopic and microscopic frames shows up, for example, in electromagnetism where [[Constitutive equation|constitutive relations]] of various time and length scales are used to determine the current and charge densities entering [[Maxwell's equations]]. See, for example, {{cite book |title=Electromagnetic and Optical Pulse Propagation 1: Spectral Representations in Temporally Dispersive Media |author=Kurt Edmund Oughstun |page=165 |url=https://books.google.com/books?id=behRnNRiueAC&q=macroscopic+frame++electromagnetism&pg=PA165|isbn=0-387-34599-X |year=2006 |publisher=Springer}}. These distinctions also appear in thermodynamics. See {{cite book |title=Classical Theory |author=Paul McEvoy |page=205 |url=https://books.google.com/books?id=dj0wFIxn-PoC&q=macroscopic+frame&pg=PA206 |isbn=1-930832-02-8 |year=2002 |publisher=MicroAnalytix}}.</ref>

In this article, the term ''observational frame of reference'' is used when emphasis is upon the ''state of motion'' rather than upon the coordinate choice or the character of the observations or observational apparatus. In this sense, an observational frame of reference allows study of the effect of motion upon an entire family of coordinate systems that could be attached to this frame. On the other hand, a ''coordinate system'' may be employed for many purposes where the state of motion is not the primary concern. For example, a coordinate system may be adopted to take advantage of the symmetry of a system. In a still broader perspective, the formulation of many problems in physics employs ''[[generalized coordinates]]'', ''[[normal modes]]'' or ''[[eigenvectors]]'', which are only indirectly related to space and time. It seems useful to divorce the various aspects of a reference frame for the discussion below. We therefore take observational frames of reference, coordinate systems, and observational equipment as independent concepts, separated as below:

* An observational frame (such as an [[inertial frame]] or [[non-inertial frame of reference]]) is a physical concept related to state of motion.
* A coordinate system is a mathematical concept, amounting to a choice of language used to describe observations.<ref name =Pontriagin>

In very general terms, a coordinate system is a set of arcs ''x''<sup>i</sup> = ''x''<sup>i</sup> (''t'') in a complex [[Lie group]]; see {{cite book |author=Lev Semenovich Pontri͡agin |title=L.S. Pontryagin: Selected Works Vol. 2: Topological Groups |page= 429 |year= 1986|url=https://books.google.com/books?id=JU0DT_wXu2oC&q=algebra+%22coordinate+system%22&pg=PA429|isbn=2-88124-133-6 |publisher=Gordon and Breach|edition=3rd }}. Less abstractly, a coordinate system in a space of n-dimensions is defined in terms of a basis set of vectors {'''e'''<sub>1</sub>, '''e'''<sub>2</sub>,... '''e'''<sub>n</sub>}; see {{cite book |title=Linear Algebra: A Geometric Approach |author1=Edoardo Sernesi |author2=J. Montaldi |page=95 |url=https://books.google.com/books?id=1dZOuFo1QYMC&q=algebra+%22coordinate+system%22&pg=PA95|isbn=0-412-40680-2 |year=1993 |publisher=CRC Press}} As such, the coordinate system is a mathematical construct, a language, that may be related to motion, but has no necessary connection to motion.</ref> Consequently, an observer in an observational frame of reference can choose to employ any coordinate system (Cartesian, polar, curvilinear, generalized, ...) to describe observations made from that frame of reference. A change in the choice of this coordinate system does not change an observer's state of motion, and so does not entail a change in the observer's ''observational'' frame of reference. This viewpoint can be found elsewhere as well.<ref name=Johansson>
{{cite book |title=Unification of Classical, Quantum and Relativistic Mechanics and of the Four Forces |author1=J X Zheng-Johansson |author2=Per-Ivar Johansson |page=13 
|url=https://books.google.com/books?id=I1FU37uru6QC&q=frame+coordinate+johansson&pg=PA13|isbn=1-59454-260-0 
|publisher=Nova Publishers 
|year=2006}}</ref> Which is not to dispute that some coordinate systems may be a better choice for some observations than are others.

* Choice of what to measure and with what observational apparatus is a matter separate from the observer's state of motion and choice of coordinate system.
{{efn|Here is a quotation applicable to moving observational frames <math>\mathfrak{R}</math> and various associated Euclidean three-space coordinate systems [''R'', ''R′'', ''etc.'']:<ref name=Lyle >{{cite book |title=Handbook of Continuum Mechanics: General Concepts, Thermoelasticity |page= 9 |author1=Jean Salençon |author2=Stephen Lyle |url=https://books.google.com/books?id=H3xIED8ctfUC&q=physical+%22frame+of+reference%22&pg=PA9|isbn=3-540-41443-6 |year=2001 |publisher=Springer}}</ref>

{{Cquote|We first introduce the notion of ''reference frame'', itself related to the idea of ''observer'': the reference frame is, in some sense, the "Euclidean space carried by the observer". Let us give a more mathematical definition:… the reference frame is... the set of all points in the Euclidean space with the rigid body motion of the observer. The frame, denoted <math>\mathfrak{R}</math>, is said to move with the observer.… The spatial positions of particles are labelled relative to a frame <math>\mathfrak{R}</math> by establishing a ''coordinate system'' ''R'' with origin ''O''. The corresponding set of axes, sharing the rigid body motion of the frame <math>\mathfrak{R}</math>, can be considered to give a physical realization of <math>\mathfrak{R}</math>. In a frame <math>\mathfrak{R}</math>, coordinates are changed from ''R'' to ''R′'' by carrying out, at each instant of time, the same coordinate transformation on the components of ''intrinsic'' objects (vectors and tensors) introduced to represent physical quantities ''in this frame''.| Jean Salençon, Stephen Lyle ''Handbook of Continuum Mechanics: General Concepts, Thermoelasticity'' p. 9}}
and this on the utility of separating the notions of <math>\mathfrak{R}</math> and [''R'', ''R′'', ''etc.'']:<ref name= Lakhtakia>{{cite book |title=Essays on the Formal Aspects of Electromagnetic Theory |author=Patrick Cornille (Akhlesh Lakhtakia, editor) |page=149 |url=https://books.google.com/books?id=qsOBhKVM1qYC&q=coordinate+system+%22reference+frame%22&pg=PA149|isbn=981-02-0854-5 |year=1993 |publisher=World Scientific}}</ref>
{{Cquote|As noted by Brillouin, a distinction between mathematical sets of coordinates and physical frames of reference must be made. The ignorance of such distinction is the source of much confusion… the dependent functions such as velocity for example, are measured with respect to a physical reference frame, but one is free to choose any mathematical coordinate system in which the equations are specified.|L. Brillouin in ''Relativity Reexamined'' (as quoted by Patrick Cornille in '' Essays on the Formal Aspects of Electromagnetic Theory'' p. 149) }}
and this, also on the distinction between <math>\mathfrak{R}</math> and [''R'', ''R′'', ''etc.'']:<ref name= Nerlich>{{cite book |title=What Spacetime Explains: Metaphysical essays on space and time |author-last=Nerlich |author-first=Graham |author-link=Graham Nerlich  |page=64 |url=https://books.google.com/books?id=fKK7rKOpc7AC&q=%22idea+of+a+reference+frame%22&pg=PA64|isbn=0-521-45261-9 |year=1994 |publisher=Cambridge University Press}}</ref>

{{Cquote|The idea of a reference frame is really quite different from that of a coordinate system. Frames differ just when they define different ''spaces'' (sets of ''rest'' points) or times (sets of simultaneous events). So the ideas of a space, a time, of rest and simultaneity, go inextricably together with that of frame. However, a mere shift of origin, or a purely spatial rotation of space coordinates results in a new coordinate system. So frames correspond at best to ''classes'' of coordinate systems.|Graham Nerlich: ''What Spacetime Explains'', p. 64}}

and from J. D. Norton:<ref name=Norton>John D. Norton (1993). [http://www.pitt.edu/~jdnorton/papers/decades.pdf ''General covariance and the foundations of general relativity: eight decades of dispute''], ''Rep. Prog. Phys.'', '''56''', pp. 835-7.</ref>

{{Cquote|In traditional developments of special and general relativity it has been customary not to distinguish between two quite distinct ideas. The first is the notion of a coordinate system, understood simply as the smooth, invertible assignment of four numbers to events in spacetime neighborhoods. The second, the frame of reference, refers to an idealized system used to assign such numbers […] To avoid unnecessary restrictions, we can divorce this arrangement from metrical notions. […] Of special importance for our purposes is that each frame of reference has a definite state of motion at each event of spacetime. […] Within the context of special relativity and as long as we restrict ourselves to frames of reference in inertial motion, then little of importance depends on the difference between an inertial frame of reference and the inertial coordinate system it induces. This comfortable circumstance ceases immediately once we begin to consider frames of reference in nonuniform motion even within special relativity.…More recently, to negotiate the obvious ambiguities of Einstein’s treatment, the notion of frame of reference has reappeared as a structure distinct from a coordinate system.|John D. Norton: ''General Covariance and the Foundations of General Relativity: eight decades of dispute'', ''Rep. Prog. Phys.'', '''56''', pp. 835-7.}}
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