Grothendieck once asked "What is a meter?" (<https://golem.ph.utexas.edu/category/2006/08/letter_from_grothendieck.html>). This innocent sounding question, made me to think about how coordinate systems are defined in physics.


How are coordinate systems in physics defined, for example in special relativity where the coordinate system is assumed to be given.


**I have tried a possibility, and was asking myself, if there are other mathematical ways to define a coordinate system.** (Thanks for your help):


Here is a possible


For the definition of a physical affine basis $B = (v\_1 = p\_1-p\_0, v\_2 = p\_2-p\_0, v\_3=p\_3-p\_0)$ of the spatial space $\mathbb{R}^3$, there
has to be an a priori defined basis $\hat{B}$, so that $B$ can refer to this basis $\hat{B}$.


Observer $A$ at point $p=p\_0$ can do the following:


* Choose nearby points $p\_1,p\_2,p\_3$ in spatial space,
without actually to give those points any a priori coordinates and define "affine vectors": $$v\_1 = p\_1 p\_0, v\_2 = p\_2 p\_0, v\_3 = p\_3 p\_0$$
* It is possible for the observer $A$ to measure the distance between two (nearby) points: $$d\_{ij} = d(p\_i,p\_j), 0\le i,j \le 3$$
* Oberser $A$ then can compute the gram matrix:
$$G = (g\_{ij}), g\_{ij} = \frac{1}{2}(d\_{0i}^2+d\_{0j}^2-d\_{ij}^2), 1\le i,j \le 3$$
we assume by the Schönberg criterion, that this matrix is a positive definite matrix, and hence especially a Gram matrix.
* Observer $A$ then can do Cholesky decomposition of the Gram matrix : $$G = C C^T, C = (x\_1,x\_2,x\_3)$$
* Observer $A$ can do the Gram-Schmidt procedure to get an orthonormal basis: $e\_1,e\_2,e\_3$ given $x\_1,x\_2,x\_3$.
* Hence a posteriori observer $A$ can choose the points $p\_i$ in such a way that $$G=\mathbf{1}$$,
since $\left< e\_i,e\_j \right> = \delta\_{ij},$ where $\delta$ is the Kronecker $\delta$.


Observer $B$ at point $q$ can do the same procedure, to get the orthonormal Basis $\mathbf{1} = G\_q = C\_q C\_q^T$.


By the [unitary freedom of square roots](https://en.wikipedia.org/wiki/Square_root_of_a_matrix#Unitary_freedom_of_square_roots), there exists an orthogonal matrix $O\_{pq}$ such that $C\_p O\_{pq} = C\_q$
hence:


$$O\_{pq}=C\_q C\_p^{-1} = C\_q C\_p^T$$


From this last equation we get:


* $O\_{pq}^T = O\_{pq}^{-1} = C\_p C\_q^{-1}= O\_{qp}$
* $O\_{pp} = \mathbf{1}$
* $O\_{pr} O\_{rq} = O\_{pq}$


Setting $w\_{pq} := \log(O\_{pq})$ we get


* $w\_{pq} = -w\_{qp}$
* $w\_{pp} = \mathbf{0}$
* $w\_{pr} + w\_{rq} = w\_{pq}$


Hence one might define the affine vectors between the points $p,q$ as $w\_{pq}$. The distance between $p,q$ might be given as:


$$d(p,q) = |w\_{pq}|\_F = |\log(O\_{pq})|\_F = |\log(C\_q C\_p^T)|\_F$$


where $|.|\_F$ denotes the Frobenius norm.


Also asked here, in case it is not appropriate for this forum:
<https://physics.stackexchange.com/questions/679409/how-are-spatial-coordinate-systems-in-physics-defined>



How are spatial coordinate systems in physics defined?
This question has been explored in the context of global positioning systems, which need to account for general relativity. The traditional Minkowski coordinates $(t,x,y,z)$ of flat space-time do not allow for an immediate positioning in an unknown gravitational field.


[Tarantola](https://en.wikipedia.org/wiki/Albert_Tarantola) and colleagues propose a symmetric coordinate system with four times, see [Gravimetry, Relativity, and the Global Navigation Satellite Systems](https://arxiv.org/abs/0905.3798) and this [talk.](http://www.ipgp.fr/~tarantola/Files/Professional/Teaching/Seminar/Lessons/Coll/Coordinates-RG.pdf) If four satellite clocks – having an arbitrary space-time trajectory – broadcast their proper time – using electromagnetic signals,– then, any observer receives, at any point along his personal space-time trajectory, four times, corresponding to the four signals arriving at that space-time point. These four times, $\tau\_1,\tau\_2,\tau\_3,\tau\_4$, are, by definition, the coordinates of the space-time point.


In [Using pulsars to define space-time coordinates](https://arxiv.org/abs/0905.4121) Coll and Tarantola propose to replace the satellite clocks by pulsars, to obtain a relativistic coordinate system valid in a domain larger than our Solar system.




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![](https://upload.wikimedia.org/wikipedia/en/2/26/AlbertTarantola2006.jpg) 
. 
"What is a meter?" Albert Tarantola in front of the original meter.


