# Coord. system:collection of charts (local homeomorphisms to ℝ𝑛) for differentiable manifold 𝑀, frame:section of tangent vector fiber bundle over 𝑀

This answer to Are ECI and ECEF both frames and/or coordinate systems? Is there a difference? (which asks about the difference between coordinate systems and frames) says:

In the language of pure mathematics, I would say that a coordinate system is a collection of charts (the local homeomorphisms to $$\mathbb{R}^n$$) for a differentiable manifold $$M$$, and a frame is a section of the tangent vector fiber bundle over $$M$$.

Question: How in the language of those who are not mathematicians might a coordinate system be seen as a collection of charts (the local homeomorphisms to $$\mathbb{R}^n$$) for a differentiable manifold $$M$$, and a frame seen as a section of the tangent vector fiber bundle over $$M$$?

Is it possible to explain what a collection of charts, a local homeomorphism, and tangent vector fiber bundles are? I'm guessing that $$\mathbb{R}^n$$ is a coordinate space based on real numbers where $$n$$ is the number of dimensions, but I don't know what the $$n$$ being a superscript versus a subscript means.

• The distinction between coordinate systems, reference frames, and charts becomes important in general relativity and in cosmology. It is (IMHO) highly erroneous to say that a recently discovered galaxy is 13.26 billion light years away. It is even somewhat erroneous to that light from that distant galaxy took that much time to arrive at the Earth in that computing that age requires making various assumptions regarding the expansion of the universe. Multiplying that time span by the speed of light assumes a Newtonian universe in which 3D space is Euclidean and time is the independent variable. Feb 2 '21 at 22:04
• On the other hand, for the next several decades, we can get away with the assumption of a Newtonian universe with relativistic effects as a small perturbative acceleration because, except for a few slow moving probes that have left / are leaving the solar system, we're stuck in our solar system. Feb 2 '21 at 22:09
• Those assumptions of a Newtonian universe with small relativistic perturbations are quite valid in the context of things in our solar system, where velocities are small compared to the speed of light and distances are large compared to the Schwarzschild radii of gravitational bodies. The fastest spacecraft will be the Parker Space Probe at perihelion, when it will move at 0.064% of the speed of light and be at two million Schwarzschild radii from the center of the Sun. Feb 2 '21 at 22:14

I'm sorry, I should have included more explanatory links in what I wrote earlier. When reading what I put together below, please keep in mind that each of these paragraphs is normally an entire grad school course in mathematics. I have attempted to clarify my meaning, but if I haven't succeeded, please note that it took me several years to understand all this. Also, that was 20 years ago, so I no longer recall exactly how I learned it.

$$\mathbb{R}^n$$ is Euclidean $$n$$-dimensional space, which is written with $$n$$ as an exponent, because that space is the product of $$n$$ copies of the real line with itself. This is different from the raising and lowering of indices used to distinguish between vectors and one-forms, which constitutes much of the machinery of calculation in general relativity.

A "chart" $$\phi$$ is a function which maps an open subset $$U$$ of a topological manifold $$M$$ to $$\mathbb{R}^n$$. This is a precise statement of what physicists normally mean by "coordinate system", which is an assignment of the values of various measurable quantities (the real number in each of the $$n$$ dimensions) to each point of $$U \subset M$$. Homeomorphism means each chart is a continuous function with a continuous inverse, formalizing the idea that $$M$$ "looks like" $$\mathbb{R}^n$$. Local means there has to be at least one of these for every point in M, but not every point has to use the same one.

A collection of charts which together cover $$M$$ (the union of all the $$U$$ is $$M$$) constitute an "atlas" as long as they are compatible with each other, which means some condition is satisfied in those places where two charts overlap. That is, if $$\phi: U \rightarrow \mathbb{R}^n$$ and $$\psi: V \rightarrow \mathbb{R}^n$$ are two charts in the atlas, then doing first $$\phi$$ inverse and then $$\psi$$, or the other way around, is a function from $$\mathbb{R}^n$$ to $$U \cap V$$ and back to $$\mathbb{R}^n$$ again. If that function is diffeomorphic (differentiable with a differentiable inverse) for all pairs of overlapping charts in the atlas, then we call $$M$$ a differentiable manifold, and the study of such objects (differential geometry) is effectively the most general kind of vector calculus it is possible to do.

If you manage to wade through all of that and still want more, then take a look at what general relativity calls vierbeins (or tetrads), and then try to read about frame bundles. A section of a frame bundle over a manifold is a function which assigns to every point a basis for the tangent vector space at that point, which is again sort of taking what physicists normally mean by "frame" and abstracting it as much as logically possible.

Does that help?

• I'm sure it does! I'm a little slow at reading Mathematics so I'll give more feedback tomorrow. Thanks very much!
– uhoh
Feb 2 '21 at 7:11
• I've heard that tomorrow never comes and apparently in this case it didn't, so I need to be more careful how and when I invoke it! I'm going to accept first and then dig in but I think I'm at least starting to get the idea now.
– uhoh
Dec 3 '21 at 1:14