List members , I am sure you will find this subject interesting due to it's myriad implications for the "reality" we experience . @Echo_on , I am keen to know your views on this , given your research in 2D for "squaring the circle" , whereas this article is about something similar in 3D...

However , anyhow - even those who don't have interest in mathematics or geometry , should try reading this :-

# The Tortuous Geometry of the Flat Torus

In 2012, mathematics has given birth to a new baby. And she is beautiful! Her parents, Vincent Borrelli, SaĂŻd Jabrane, Francis Lazarus, Boris Thibert and Damien Rohmer, who formed the HĂ©vĂ©a project, have named her *the first C1 isometric embedding of the flat square torus*. Sexy, right? Have a look at the first video ever taken of her:

OK, her name is a bit long so Iâll just call her *the HĂ©vĂ©a Torus*.

She is beautiful indeedâŠ But what is she?

Amazingly, she was first imagined a century and a half ago, by her ancestors Carl Friedrich Gauss and Bernhard Riemann, in 1854. But it took a century for her great grandfather John Nash to actually prove that she could one day be conceived, although he didnât specify how. Another 20 years later, her grandfather BenoĂźt Mandelbrot laid the foundations of a new kind of geometry which hinted at an actual possible conception. But, weirdly enough, it took the advent of computers to finally procreate her, after 5 long years of top-level mathematics research! And yet, it all started with a very simple problemâŠ

## Bending the Flat Square Torus

These days, Iâm spending way too much time playing the game on the right called Netwalk where a network needs to be built. The game is played within a square. One tricky aspect is that whenever you get out of the square on the right, you reappear on the left. Like in PACMAN, or in these awesome games.

What do these games have to do with the video above?

Amazingly, the network universe and *the HĂ©vĂ©a Torus* are geometrically identical!

Youâre kidding, right? I mean, on the right we have a square, and the HĂ©vĂ©a Torus isâŠ well, a weird twisted shape!

Youâre right. From our perspective, these are very different. But, from the perspective of some being stuck within these 2D worlds, there is absolutely no difference! The Netwalk world and *the HĂ©vĂ©a Torus* are intrinsically mathematically (nearly) identical.

OKâŠ I admit, thereâs a slight difference in terms of accelerations in these geometries. Weâll get to thatâŠ

HummâŠ I have the biggest troubles imagining myself living in the HĂ©vĂ©a TorusâŠ

Youâre not the only one! It took two of the greatest giants of mathematics to figure out what it meant to live within a torus. The first one is Carl Friedrich Gauss, also known as the Prince of mathematics, who famously proved the *Theorema Egregium*, which you can learn more about by reading Scottâs article on non-Euclidean geometry. But, more importantly, it was Bernhard Riemann, Gaussâ disciple, who unlocked the wider geometry of so-called manifolds by providing a powerful new picture of what geometry could be.

What is this Riemannâs picture of geometry youâre talking about?

In short, a Riemannian manifold is a space, such that each local neighborhood of a point of that space looks flat. The torus of the video is an example of a 2-dimension manifold, also known as surface. Around each point, if you zoom sufficiently, then your surface will look like a 2-dimensional sheet of paper. And, on this sheet of paper, lengths and angles are the same as on actual sheets of paper!

More precisely, a scalar product must be defined on each local neighborhood. In the middle of the image above, we have a 2D section of a 6D Calabi-Yau manifold, which is widely studied by string theorists. Find out more about Riemannian geometry with my article on the spacetime of general relativity.

Now, involving lengths and angles makes things complicatedâŠ so letâs start with a simpler kind of manifold: The topological torus.

The topological torus?

Topology is the art of forgetting the geometrical concepts of angles and lengths. This means that topology allows stretching sheets of paper without effectively *changing* it.

Can you give an example?

Sure! Topologically, it is straightforward to transform the Netwalk square into a torus. We merely have to glue together opposite sides. This is whatâs done below:

Iâve tried to make a torus with the Netwalk square but I miserably failed after hours of ridiculous attemptsâŠ Sorry for that!

This procedure of gluing is also described by this beautiful video by Geometric Animations:

Find out more about these gluing operations with my article on PoincarĂ© conjecture. You can also learn more with my article on topology.

## Nashâs Isometric Embedding

The trouble is that, as you can feel it while watching this video, it seems impossible to glue opposite sides without stretching the square we started with.

Is that much harder?

It is! This no-stretching requirement corresponds to an **isometry**. To get a sense of the constraint that isometry represents, check this awesome video by Colm Kelleher on TedEd, which explains how isometry can prevent the tip of your pizza from falling down:

As the video explains it, itâs impossible to bend a sheet of paper into a sphere, or into a potato chip. This is because of the **curvatures** of these shapes. And the classical smooth donut-like torus is highly curved too, so itâs impossible to bend the flat Netwalk world into that classical torus. An isometric bending of the Netwalk world requires to transform it into an non-curved torus!

HummmâŠ Iâm now beginning to think itâs impossible to bend a square into a torus!

Youâre not the only one! 60 years ago, most mathematicians even thought it was impossible! But no one could prove it. In fact, when deeply annoyed by his young, brilliant and pretentious colleague, MIT mathematician Ambrose Warren rashly retorted to him: â*If youâre so good, why donât you solve the [100-year-old isometric] embedding problem for manifolds*â, which the bending of the Netwalk world with no stretching is an example of. Warren was hoping it would thereby keep his colleague busy for the rest of his life. But shortly later, that colleague was claiming he had solved it!

Mathematically, an embedding is an injective map of a manifold into some larger space, typically Rn, where n is at strictly greater than the dimension of the manifold. This map needs to be an immersion, which means that its derivatives (which are linear applications) must always be injective.

Who was that colleague?

He was the founder of game theory, future winner of a Nobel prize in economics and future hero of the movie *A Beautiful Mind*âŠ John Forbes Nash.

And he immediately solved a 100-year-old problem?

Not reallyâŠ As I said, the young Nash was a pretentious douchebag. After all, he had just completed a revolutionary 28-page PhD thesis, and he was visibly smarter than most of his colleagues. So, he figured out that only a prestigious problem was worthy of his time. One that would mark History. So, to test whether the isometric embedding problem was of that kind, Nash looked at the reactions of his colleagues as he told them he had cracked it. And surely enough, it seemed that solving the isometric embedding would mark History. So, Nash decided to give it a try.

Did Nash actually solve it?

Amazingly, he did! Nash might have been showy, heâs also definitely a world-class mathematician.

So whatâs the answer? Can a square be bent into a torus?

The answer is yes. It can. Sort ofâŠ Iâve explained it all in this Science4All video:

I uploaded the above video the day before Nash sadly passed away in a car accident. Given how little praises I gave in the video above, I made another tribute video to honour this beautiful mind.

So, as it turned out, Nash proved that we could, provided the bent torus had an infinite number of points on which acceleration could not be defined!

What does that mean?

Hereâs an illustration of what that means. Imagine a skater rolling down a slope:

In the first case, there is a huge discontinuity in the slope at the green point. In some sense, the slope cannot be defined at that point. In the third case, the slope is so smooth that the skater doesnât feel anything special at this point. Finally, and most interestingly, in the second case, at the green point, the skater will suddenly feel a force acting on him which was not here before. Nash proved that this second case must happen at an infinite amount of times on the bent torus.

Mathematically, this second case corresponds to the green point having no second derivative. You can learn about the basics of derivatives in my article on differential calculus.

Nashâs claim means that Newtonâs laws of mechanics would have no sense in the bent torus on an infinite amount of points, kind of like general relativity no longer makes sense at singularities better known as black holes!

Waw! That sounds sick! And hard to imagine!

I know! In fact, it was so sick that Nash couldnât provide an example of a bent square torus! His proof was not constructive. He merely showed that there must be a way to bend a square into a torus, but didnât say howâŠ

## Mandelbrotâs Fractals

Weirdly enough, itâs much easier to imagine a bent torus for which acceleration is undefinable everywhere.

Really? That sounds so weirdâŠ

To understand what that could mean, letâs look at some other sick geometrical objects: Non-differentiable continuous curves.

Do you mean curves which are continuous everywhere but differentiable nowhere? Is that even possible?

First examples of such curves were given in the 19th century. They were called monsters. I guess that, Historically, one of the first examples of a monster was that of the function â2âncos(4nx). But the more popular example nowadays is the Koch snowflake, as beautifully explained in the awesome show *Fractals â The Hidden Dimension* by NOVA. Or, even better, the curve formed by the British coastline:

Oh! So this is what fractals are: Non-differentiable continuous curves!

Exactly! And it took another genius to unveil the magic of fractal geometry. This genius is **BenoĂźt Mandelbrot**.

Learn more with Thomasâ article on fractals.

Curiously though, Mandelbrotâs ideas werenât popular at first. His first papers on this new kind of geometry were so unusual that the mathematical community rejected them. Referees claimed that the papers, despite displaying pretty images, did not contain actual mathematics. Because the community was not ready for his new ideas, Mandelbrot decided to write a book on his own. This best-seller was so amazing that it quickly became iconic, not just for young mathematicians, but in popular culture as well!

Now, the thing with fractals is that itâs very easy to make a shape of any length. For instance, you can draw a âcurveâ that looks exactly like a circle, but actually has a length of 4. This is what Vihart did in this awesome video:

So, following Mandelbrotâs (and Vihartâs) ideas, itâs not too hard to imagine a torus whose lengths perfectly match that of the Netwalk world. In other words, through a fractal process, itâs (at least theoretically) possible to describe a simple procedure to fold the square of the Netwalk world into an origami torus-like â*zigfinihedron*â which glues opposite sides.

So why did it take 5 years to actually construct it?

Look carefullyâŠ *The HĂ©vĂ©a Torus* is not a *zigfinihedron*! What the HĂ©vĂ©a team was searching for was a smoother bending of the torus, which, although had no second derivatives, still had continuous tangent planes at all points! And the trouble of fractals is that they are precisely rough objects with no such continuous tangent planesâŠ

So why did you bring that up?

Somehow we need a trade off, between the smoothness of classical surfaces and the roughness of fractals. This trade off is what the HĂ©vĂ©a team called **smooth fractals**.

## The HĂ©vĂ©a Torus

The idea of smooth fractals isnât far from what was hinted at by John Nash. His embedding theorem stated that if a manifold could be topologically embedded in a higher dimension space, as we have done it for the Netwalk world, then this embedding could be smoothly corrugated like Vihartâs *zigfinigons* to end up isometric, while having tangent planes at all points. Whatâs more, Nash proved that the amplitudes of the corrugations could be as small as we wanted.

Technically, Nash still required a C1 continuity, which means that tangent spaces must vary continuously. Moreover, the initial embedding needs to be a shrinking of the manifold. Furthermore, Nash only proved this result, provided that the dimension of the manifold was 2 smaller than the dimension of the space it was embedded in (and it would thus not work for our torus). The refinement to all embeddings is due to Nicolaas Kuiper, and is known as the Nash-Kuiper theorem. Finally, Nash also proved a Ck isometric embedding theorem, which says that we can still embed a manifold even with greater continuities, provided the higher space we embed our manifold in is of much greater dimension.

So, following Nash and Mandelbrotâs ideas, the HĂ©vĂ©a team has added several layers of corrugation to a classical torus embedding. This is whatâs illustrated below, where the 3 first layers have been added sequentially:

All the images of this section are taken from the Hevea project webpage.

Letâs have a closer look!

Just like Vihartâs drawings, *the HĂ©vĂ©a Torus* is theoretically obtained by adding an infinite amount of such corrugations. But, computationally, the HĂ©vĂ©a team merely added 5 layers, as any greater layer would in fact be imperceptible given the resolution of the image.

But, is the HĂ©vĂ©a Torus really a Netwalk world with no stretching?

Thatâs hard to verifyâŠ but yes. Below is an image which displays the correspondence between lines in the Netwalk world and lines in *the HĂ©vĂ©a Torus*:

Amazingly, the black and green loops actually have the same lengths in both figures. Granted, itâs a bit less obvious on *the HĂ©vĂ©a Torus*, but thatâs because the black loop has a more fractal-like structure, which makes it longer than it seems to be!

WaitâŠ Is the HĂ©vĂ©a Torus really smooth?

Yes! The key to obtain the smoothness of our corrugated torus is to have the amplitudes of successive corrugations decreasing faster than their âwavelengthsâ. By opposition, Vihartâs drawings had rather one-to-one ratios. For instance, at 3:50, she divided amplitudes of corrugations of her curves by 2 while she made twice many of them. Instead, by carefully adjusting the faster decrease of amplitudes of successive corrugations, the HĂ©vĂ©a team managed to guarantee the C1 continuity of *the HĂ©vĂ©a Torus*!

Iâm not sure this convinces meâŠ

One way to see that is to take the shape of the black loop of the figure above (which corresponds to a meridian), in the 3D space *the HĂ©vĂ©a Torus* is embedded in. Letâs compare that to a Koch snowflake.

Importantly, as opposed to the non-differentiable Koch snowflake, the meridian of *the HĂ©vĂ©a Torus* is still smooth enough to have (continuous) tangents at all points.

## Letâs Conclude

Instead of recapitulating, Iâll let the great James Grime sum up all weâve discussed here:

Now, the result of *the HĂ©vĂ©a project* is a spectacular achievement of over 160 years of mathematical pondering. Obviously, this article only mentions the few mathematicians who made the most stunning breakthroughs in this investigation. But, unfortunately, it also dramatically fails to account for the hundreds of other mathematicians who have shaped and reshaped our intuitions of curves and surfaces, in a deeper but less obvious fashion. For me, this amazing silent build up is the source of the magic of mathematics. Think about it. Thanks to many unsung mathematicians, we now have in front of our eyes an object that the great Gauss, Riemann, Nash and Mandelbrot couldnât even imagine, even though they had been searching for it all along. I hope you feel privileged and dismayed by the following precious images of *the HĂ©vĂ©a Torus*âŠ

I should say that even though combining Nash and Mandelbrotâs ideas sound reasonably doable, it is actually a huge and difficult endeavor. Once again, the HĂ©vĂ©a team had to spend 5 years to come up with *the HĂ©vĂ©a Torus*, after several unsuccessful attempts. It was a tedious and tricky work, which involved very modern mathematics, including and especially MikhaĂŻl Gromovâs homotopy principle. This gives me one last occasion to stress how big an achievement the conception of *the HĂ©vĂ©a Torus* is. Congrats to the HĂ©vĂ©a team!

Let me leave you with my favorite image of *the HĂ©vĂ©a Torus*, taken from its interior, with light coming from the opposite sideâŠ Astonishing!

One last thoughtâŠ Can you believe that cutting through *the HĂ©vĂ©a Torus* twice yields a square?

Regards