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Is there a meaningful way to write a line element $ds^2=....$ for a spacetime with a non-integer number of dimensions?

I would imagine that a line element in a 1+2.n dimensional spacetime might look something like $ds^2=-dt^2+a^2 (dx^2 + dy^2) + b^2 dz^2$ where $dx$ and $dy$ are "regular" spacial dimensions, $dz$ is another direction with fractional dimensionality, and a and b are some functions (which I don't want to specify further since I am working with an unusual line element). I chose to square a and b to ensure that x and y remain space-like.

I will admit, though, that I am guessing at how to describe the line element. (Or, are there any resources or fields of physics/math to look into that can be useful to addressing this question.)

Edit #1: I'd like to add that if I can think of a function (let's call it b(t)) that smoothly transitions from 2-->3 and describes a change in dimensionality, maybe I can write $ds^2=-dt^2+ (dx^2 + dy^2) + (b-2)^2 dz^2$, which would in effect "turn the z dimension on or off". In this case, when b=2, the z dimension is turned off, and when b=3, we have regular Minkowski spacetime.

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  • $\begingroup$ Would Mathematics be a better home for this question? $\endgroup$
    – Qmechanic
    Feb 9, 2018 at 16:26
  • $\begingroup$ Quite possibly, I will ask there too. $\endgroup$
    – Bob
    Feb 9, 2018 at 16:31
  • $\begingroup$ For any value of $b$ other than 2 in your second expression, you still have 3+1-dimensional Minkowski space: you can define a new coordinate $\bar{z} = (b-2) z$, and then the metric becomes $ds^2 = -dt^2 + dx^2 + dy^2 + d\bar{z}^2$. So that method won't work. $\endgroup$ Feb 9, 2018 at 17:26
  • $\begingroup$ @MichaelSeifert: Thanks for the comment. I intend on having b as a function with 2 and 3 as it's limits. When $b=2$ or $b=3$, we would have 2+1 or 3+1 Minkowski space, but when $2<b<3$, we would probably have not have Minkowski space (we would have some other non-isotropic spacetime). If I write b(t) (for example), than I would explicitly have $\overline{z}$ as a function of time also. In changing the original metric $g_{\mu\nu}$ to a new metric $\overline{g}_{\mu\nu}$, wouldn't that still lead to noticeable effects from $T_{\mu\nu}=\frac{-2}{\sqrt{g}}\frac{\delta S_{mat}}{\delta g^{\mu\nu}}$? $\endgroup$
    – Bob
    Feb 9, 2018 at 20:52
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    $\begingroup$ Related: Can a fractal be a manifold? The answer appears to be "no", at least not in the usual sense. And if you don't have a manifold structure on your set, it's difficult to see how you could generalize it to get a metric. $\endgroup$ Feb 9, 2018 at 21:44

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When you say 'non-integer number of dimensions', it actually sounds like you are asking about spacetime with a fractal dimension.

I know very little about this subject, but it's a fun question to think about. It goes without saying that I welcome someone with more specialist knowledge to elaborate, but here are some highly speculative remarks for now.

To my knowledge, non-integer dimensions arise from a sort of universal roughness at arbitrarily small scales. Keep in mind that on arbitrary rough surfaces, it is possible that the curvature varies so wildly that the distance integral (defined at each scale by simultaneously approximating the geodesic path at that scale and computing the associated integral bounds) fails to converge. In this case, you need a more general concept of distance, likely tailored to the geometry at hand.

The simplest approach is to define distance functions on a family of smooth (or almost always smooth) manifolds that approaches the manifold you have in mind. This family of smooth manifolds can also be used to define a rough manifold, though the criteria for whether two families of manifolds define the same limit is probably more subtle than it is for Cauchy sequences. Work along these lines has been done for Brownian paths, for example, which exhibit a fractal structure (c.f. metric properties of mean wiggly continua.) The geodesic distance between any two points could diverge in the rough limit, but there may be some structure to its divergence that you could use to define a 'fractal distance'. In the simplest example, rescaling the distance by a power of the 'amount of roughness' could lead to a well-defined distance between points. This corresponds to measuring distance in units of a fixed 'rough meter stick'.

Exhibiting a 'rough meter stick' can be difficult, however. If you can define a non-trivial Brownian motion on the rough manifold, then such a metric could potentially be furnished by considering the distribution of times taken to travel from one extended region to another (likely defined in a limiting sense), and taking the limit as the regions shrink to two points. The shape of the distribution function may change significantly depending on the location of the points, but could have universal characteristics as the 'distance' between the points is made arbitrarily small.

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