# What goes wrong with Stokes theorem if a surface is not orientable?

For the Möbius strip parametrized by $$\{\sigma(\theta,r)=((1+r\sin(\theta/2))\cos\theta,(1+r\sin(\theta/2))\sin\theta,r\cos(\theta/2))\ \mid \\ (\theta,r)\in A=(0,2\pi)\times(-1/2,1/2) \}$$

we get the normal vector $$\nu$$ by doing the cross product $$\nu(\theta,r)=\frac{\partial \sigma}{\partial \theta}\times\frac{\partial \sigma}{\partial r}=\begin{pmatrix}(1+r\sin{\theta\over2})\cos{\theta\over2}\cos\theta+{r\over2}\sin\theta \\(1+r\sin{\theta\over2})cos{\theta\over2}\sin\theta-{r\over2}\cos\theta \\-(1+r\sin{theta\over2})\sin{\theta\over2} \end{pmatrix}$$ which is continuous on $$A$$ but not on $$\overline A$$ because $$\nu(0,0)=(1,0,0)\ne(-1,0,0)=\nu(2\pi,0)$$

For what kind of $$F(x,y,z)=(F^1,F^2,F^3)$$ with $$F^i\in C^1(\Bbb R^3)$$ can there be a problem? I lack some intuition for the necessity of the orientability for Stokes' theorem:

$$\iint\limits_\Sigma \nabla \times F\cdot ds=\int\limits_{\partial\Sigma}F\cdot dl$$

• The problem is not with Stokes, but with defining the integral on a non-orientable manifold. – Lord Shark the Unknown Dec 29 '18 at 15:34
• This a Möbius strip which has been cut at $\theta = 0$ so it should be orientable. If the domain was $\overline{A}$, it wouldn't be orientable. – mechanodroid Dec 29 '18 at 15:49

## 1 Answer

Edit: Just noticed that your strip does not include the points $$\theta = 0, 2\pi$$ which means that it can be oriented.

However, on the full domain you cannot define the vector surface integral to begin with and so the problem is not with Stoke's Theorem per say.Remember, that by definition a vector surface integral is: $$\iint_{\mathcal{S}} \overrightarrow{F} \cdot \, \mathrm{d}\overrightarrow{S} = \iint_\mathcal{S} \left(\overrightarrow{F} \cdot \hat{n}\right) \mathrm{d}S$$

In other words you need a clear definition of $$\hat{n}$$ throughout $$\mathcal{S}$$. The Möbius strip has only one side and therefore cannot be oriented and that's why you cannot even define one side of equation for the Stoke's Theorem.

To get an intuition for why the Möbius strip cannot be oriented I suggest making one yourself. It's easy --just take a strip of paper and glue the two ends together by introducing a 180 degree twist somewhere in between. Now take a normal vector at a point of your choice anywhere on the surface. Translate this vector along the strip and arrive back at this point. Your normal vector will now be pointing in the reverse direction from when you started at that point.

This shows that you cannot define $$\hat{n}$$ consistently for this surface.

• Well, if you cut the strip at $\theta = 0$ you get an orientable surface and you lose only a segment of measure zero, which souldn't affect the integral, so why don't we define the integral that way (for this particular case of the Möbius strip)? Is it because the value will change depending on where we make the cut? That would kind of make sense – John Cataldo Dec 30 '18 at 7:47
• @JohnCataldo Yes, in this way your vector surface integral can be calculated but how would you orient $\partial \mathcal{S}$ then? – Apoorv Khurasia Dec 30 '18 at 11:17
• Does it matter? Don't we usually have to chose an orientation anyway? The result will be the same up to the sign $\pm$ – John Cataldo Dec 30 '18 at 12:31
• I am not too sure. When you define the curve you will have to exclude the start and the end points. The proof of Stoke’s theorem relies on the curve being closed. – Apoorv Khurasia Dec 30 '18 at 14:30
• @JohnCataldo. Nonetheless this is a very interesting question. I will try to work out what exactly goes wrong with Stokr’s theorem and revert back to you if I find something. – Apoorv Khurasia Dec 30 '18 at 14:32