# Riemannian ℤ₂ⁿ-manifolds

In a recent preprint Riemannian Structures on ℤ₂ⁿ-manifolds, Janusz Grabowski and I initiated the study of Riemannian geometry in the setting of ‘higher graded’ supermanifolds.

ℤ₂ⁿ-manifolds are, very loosely, manifolds that have coordinates with a ℤ₂ⁿ-degree (ℤ₂ⁿ := ℤ₂ x ℤ₂ x … x ℤ₂, n-times) and are commutative up to a sign factor that is determined by the standard scalar product of the ℤ₂ⁿ-degrees of the coordinates. This is, of course, very much like the situation for supermanifolds where we have a ℤ₂-degree and the standard graded commutation rule. Generalising the theory of supermanifolds to this higher setting turns out, in general, not to be straightforward. There are often complications as one has to deal with formal coordinates that are not nilpotent. Much of the foundational theory is in place, though there are plenty of questions that remain open, see, for example [1,2,3].

The question of Riemannian structures on ℤ₂ⁿ-manifolds is a natural one given the importance of Riemannian geometry in physics, engineering, and so on. Much like supermanifolds where one has metrics that carry ℤ₂-degree or parity, so we have even and odd metrics, on ℤ₂ⁿ-manifolds one has Riemannian metrics of all possible ℤ₂ⁿ-degrees. However, for the most part, there is no real complication here. Importantly, we still have the Fundamental Theorem, that is, no matter the degree of the Riemannian metric we have a canonical symmetric and torsionless connection, i.e., the Levi-Civita connection. The Riemann curvature, Ricci tensor and Ricci scalar can all be defined.

But this is where it gets interesting. While we have Riemannian metrics of arbitrary degree, there are some differences between metrics of even and odd total degree, i.e., sum the components of the ℤ₂ⁿ-degree of the metric. For ℤ₂ⁿ-manifolds with an odd Riemannian metric, the Ricci scalar identically vanishes, i.e., all odd Riemannian ℤ₂ⁿ-manifolds have zero scalar curvature.

Another interesting result is that the connection Laplacian (acting on functions) associated with an odd Riemannian metric also vanishes. This means that one cannot, using the connection Laplacian, develop the theory of harmonic functions on an odd Riemannian ℤ₂ⁿ-manifold.

References

 Covolo, Tiffany; Grabowski, Janusz; Poncin, Norbert The category of ℤn2-supermanifolds. J. Math. Phys. 57 (2016), no. 7, 073503, 16 pp.

 Bruce, Andrew James; Poncin, Norbert Functional analytic issues in ℤn2-geometry. Rev. Un. Mat. Argentina 60 (2019), no. 2, 611–636.

 Bruce, Andrew; Poncin, Norbert Products in the category of ℤn2-manifolds. J. Nonlinear Math. Phys. 26 (2019), no. 3,

# Mrs Brown on relativity I briefly watched a few moments of Mrs Brown’s Boys Christmas special 2016; a comedy program on the BBC. Agnes Brown’s grandson Bono asked his grandmother about Einstein’s theory of relativity. The joke is that this is far to complicated for an old Irish Granny to answer.

This made me think. Einstein first published on special relativity in 1905 and the field equations for general relativity were published in 1916. So humanity has had knowledge of Einstein’s relativity for 100 years now, yet Mrs Brown was unable to say anything!

Not that we should expect everyone to have a detailed mathematical knowledge of Einstein’s relativity, but should just about everyone be able to say something?

Special relativity
Special relativity is based on two postulates – that then lead to a consistent mathematical description in terms of differential geometry, but we should ignore that for now – that can be paraphrased as follows

1.  The laws of physics are the same for all non-accelerating observers.
2.  All  non-accelerating observers measure the speed of light to be the same,  irrespective of the source of light.

Even more pedagogically, all observers that are not experiencing a net force are equally as good’ as far as determining the laws of physics are concerned, and on top of that, light waves travel at a fixed speed as measured by these privileged observers.

General relativity
This is a bit harder to paraphrase, but basically we have three key features

1. All observers (not just the non-accelerating ones) are equally as good for determining the laws of physics.
2. Space-time is curved and the force of gravity is the bending’ of space and time.
3. In `small enough’ regions, the non-gravitational laws of physics reduce to that of special relativity.

Again, all this can be made mathematically precise.

Are we expecting too much?
I think it is too much to expect any real knowledge of Einstein’s relativity from the general public. So, we should not ask for this, but only a vague idea of the ideas from most people.

From my own perspective it is all differential geometry 🙂