# How are spatial coordinate systems in physics defined?

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, 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

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 and colleagues propose a symmetric coordinate system with four times, see Gravimetry, Relativity, and the Global Navigation Satellite Systems and this talk. 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 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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"What is a meter?" Albert Tarantola in front of the original meter.

• Thanks, I will look into the references you gave. Very interesting. Nov 27 '21 at 15:08
• Thank you for mentioning the work of Tarantola. My question aimed at physical quantities and the coordinate system was just an example. ipgp.fr/~tarantola/Files/Professional/Books/… Nov 27 '21 at 16:19
• @stackExchangeUser - I don't see where Carlo wrote that "the satellites transmit their positions ...". They don't. The positions of the satellites aren't at issue and no other given coordinate system is being referenced. Nov 28 '21 at 3:16
• @MichaelEngelhardt: Yes you are right, the satellites broadcast only their time. Suppose two observers, far apart, use two distinct sets of satellites. How do they transform from one coordinate system to the other? Nov 28 '21 at 6:14
• each observer will know its position relative to every other observer; they can then transform to, say, an earth-based reference frame, but only if they know the gravitational field. (Tarantola calls this the "attachment problem", distinct from the "positioning problem", which needs no knowledge of the gravitational field.) Nov 28 '21 at 8:15