*This article is part of a series on Complex Algebraic Geometry. See also:*

1. The Hardest Conjecture; 2. The Valley; 3. A Mathematical Daydream

Among the so-called *Millenium Prize Problems* – seven notoriously difficult mathematical problems, each open for decades, and each now carrying, courtesy of the Clay Mathematics Institute, a million-dollar prize – Kieth J. Devlin places the Hodge Conjecture last. “[A]n author should delay as long as possible introducing anything is likely to make his reader give up in despair,” Devlin writes in his book, The Millennium Problems: The Seven Greatest Unsolved Mathematical Puzzles of our Time. “There is no… path even to the problem’s front door.”

The Hodge Conjecture fascinated me even before I began studying mathematics. Its sheer inscrutability surely played a role. I sensed, behind the incomprehensible words and symbols of its Wikipedia page, a bafflingly deep, and coherent, world. I had to understand.

The problem’s formidability was only partly to blame. This world – which I envisioned – was not just expansive, but beautiful. I perceived, there, something like Dante’s “music of the heavenly spheres”.

My graduate study has taken me into a field of math relatively close to that which the Hodge Conjecture occupies. As I’ve explored the rich foothills of this towering mountain, its peak has become even more stunning and mysterious.

I will try to explain the conjecture to this blog’s lay readers.

# Smooth complex projective algebraic varieties

We will introduce this mathematical concept through a few successive steps.

### Smooth varieties

Let’s imagine a few things in our standard 3-dimensional space:

- A point, or two or three.
- A curve: start with a straight line, and then bend it or curve it.
- A surface: start with a flat plane, and then bend it in a smooth way, letting it wave and curve.

Actually, the 3-dimensional *ambient space* is not important here. Let’s take an *n*-dimensional ambient space instead (where *n* is, say, bigger than 3), and some number *k* smaller than *n*, and imagine:

- A smooth, curved “
*k*-dimensional object”: take a*k*-dimensional space, and then bend it.

Pattern? Each object’s *dimension* is indicated by the bullet point next to it.

It’s easy enough to see what it would mean to curve a point (nothing), a straight line, or a flat plane. Yet even three-dimensional space can become curved in mathematics – though we couldn’t see it – as can four-dimensional space (General Relativity posits that this is exactly what gravity consists in). This works for space of any dimension *k*.

I often hear protestations at this point: “When you start getting into higher dimensions, I get off.” I don’t know what these things look like any more than you do! I’m just willing to accept that they exist and can be studied mathematically. You can be too.

Thus, we have a:

Smooth, curved,

k-dimensional “object” inn-dimensional space.

We’ll call this a *smooth algebraic variety*. The story is not over yet.

### Complex *n*-space

Real numbers suck. The equation doesn’t even have any solutions! Among the many (apparently) pointless things you learned in high-school math, perhaps the deepest was that by passing to a new, larger number system – the complex numbers, in which has a square root, namely * *– we endow equations like the above with solutions. (What are they?)

We now work in an *n*-dimensional space in which each point is described by *n* *complex* coordinates, instead of the standard *n* real (Cartesian) coordinates. This changes the “material” of the space we work in. Just as the “missing solutions” to the equation (they’re and ) appear once we pass to the complex numbers, the “missing parts” of an algebraic variety of any dimension now arise with our passage to *n*-dimensional complex space.

Any complex number requires *two* (real) dimensions to describe (recall that each complex number lives in the complex *plane*, which consists of both a real and an imaginary axis). Precisely because a single complex number requires two dimensions to describe, the (real) dimension of our entire space will thus also double now, as will that of our points, curves, surfaces, and *k*-dimensional varieties. This makes everything harder to visualize. But it also makes them more complete.

### Projective space

“Yet there have been and still are geometricians and philosophers, and even some of the most distinguished,” argues Ivan, of Dostoevsky’s 1880 __The Brothers Karamazov__, “who… dare to dream that two parallel lines, which according to Euclid can never meet on earth, may meet somewhere in infinity. I have come to the conclusion that, since I can’t understand even that, I can’t expect to understand about God.”

*Projective space* is a non-Euclidean geometry in which, among other things, two parallel lines meet at infinity. Though its exact mathematical construction is challenging and perhaps uninstructive, the mathematical universe of projective space extends, and in some sense completes, our standard Euclidean space. The passage to projective space, indeed, offers another sense in which the naïve Euclidean picture represents only an artificial part of the whole.

This becomes evident, for one, in the characteristic eventual meeting of two parallel lines. In Euclidean space, two distinct lines *usually* intersect in a point, except in the special or “degenerate” case in which the lines become parallel and they don’t intersect at all. This pathology is actually an artificial artifact of the Euclidean setting. These lines, in some sense, actually* always* meet in exactly one point – provided, that is, that we work with the “whole” projective space. *Parallel* itself actually ceases to have meaning, here; whether two lines are in fact parallel depends only on the (arbitrary) way in which we ultimately truncate them using a restricted Euclidean lens.

Many situations, in fact, feature apparently disparate phenomena which unite upon passage to projective space. For example, a so-called *conic curve* in projective space reveals the disparate conic sections that we know only after it is artificially truncated using various Euclidean perspectives.

Complex projective space furnishes the most natural setting in which to study algebraic varieties.

### Closedness

It’s become part of folk-physics lore that if you head off into space in some fixed direction, you’ll eventually land back where you started. The universe, in other words, is *without boundary*, in the sense that you’ll never come up, as you go along, against some edge.

We want our algebraic varieties to be the same way. What does this look like?

- A point should… Any point is automatically closed.
- A curve shouldn’t just stop at a point; it should loop back upon itself.
- A surface should never cut off at an edge; it should wrap back upon itself, like (the exterior of) a blob in space.

…

- You get the idea.

Varities thus represent *k*-dimensional smooth closed objects living inside an *n*-dimensional complex projective space.

# Cohomology and Algebraic Cycles

Smooth, complex, projective algebraic varieties are elegant objects. The Hodge Conjecture asserts something deep about them.

### Cohomology and Hodge classes

*Cohomology* is a mathematical tool that extracts information about the shape of an algebraic variety and assembles it into a package, called a *cohomology group*, of entities called *cohomology classes*. Each cohomology group provides structural information about the variety.

*Each dimension *of any given variety, in fact, gets its own cohomology group, so that, for a fixed variety *X* of complex dimension *k* (and so real dimension 2*k*), we have cohomology groups which record the behavior of *X* in each of the separate dimensions *i = *0, 1, … , 2*k*.

W. V. D. Hodge, in the 30s, discovered – among the many fantastic phenomena which arise when we work with the complex numbers – a new one, now termed the *Hodge decomposition*, whereby each cohomology group of a complex projective variety decomposes into a combination of smaller groups which reflect, in even more detail, the structure of that algebraic variety. Each of the even-dimensional cohomology groups *i* = 0, 2, … , 2*k*, in particular, contains a singularly special sub-group whose constituent cohomology classes are known as *Hodge classes*.

### Algebraic cycles

Any algebraic variety admits, meanwhile, so-called *subvarieties*, which are exactly what they sound like: algebraic varieties of lower dimension embedded within the original variety. (Imagine, for example, a curve inscribed on/in a surface.)

There’s a natural process which associates, to any *i*-dimensional subvariety of a *k*-dimensional variety, a cohomology class of dimension 2*k* – 2*i*. This cohomology class, called an *algebraic cycle*, in some sense represents the contribution of that subvariety to the variety’s cohomology. More is true: This algebraic cycle is necessarily a Hodge class.

The Hodge conjecture is a question about the extent to which the cohomology of an algebraic variety is informed by its own subvarieties. Every subvariety determines a Hodge class. The conjecture asks the converse. Roughly:

Is every Hodge class the cohomology class of a subvariety?

The Hodge Conjecture, if true, would permit one to deduce deep structural information about an algebraic variety merely by understanding its cohomology groups, which are often easier to compute in practice.

# Beyond the Hodge Conjecture

The Hodge Conjecture (see [1, Conjecture 2.25] for a scholarly reference) is actually just the (admittedly redoubtable) gateway to an entire field of math, which could acceptably be termed *Hodge Theory*. This field is rich with marvelous results and outstanding conjectures. It’s also one of the hardest extant fields of math.

The standard Hodge Conjecture, for one, appears in a much deeper form in the mathematical titan Grothendieck’s legendary *Generalized Hodge Conjecture *(see [1, Conjecture 2.40]). The so-called *GHC* is so deep, in fact, that within it the original Hodge Conjecture, itself no easy assertion to grasp, becomes again buried in profound generality (this evokes also Grothendieck’s generalization of the Riemann-Roch theorem). This deeeper form is even more aesthetically fascinating.

Also worth mentioning is the GHC’s great cousin, the *Generalized Bloch Conjecture* (see [1, Conjecture 1.9]). The Generalized Bloch Conjecture’s assertion about smooth projective varieties is just as inspiring, if just as difficult to state, as is that of the Generalized Hodge Conjecture.

Beyond these two, one could point to a litany of fantastic papers by Claire Voisin, Hodge Theory’s most brilliant living servant [2] [3] [4]. Indeed, Hodge Theory has no greater force for progress, precision, and beauty than Claire Voisin. This informal article is dedicated to her.

References:

- Voisin: Chow Rings, Decomposition of the Diagonal, and the Topology of Families
- Voisin: The Generalized Hodge and Bloch Conjectures are Equivalent for General Complete Intersections
- Voisin: The Generalized Hodge and Bloch Conjectures are Equivalent for General Complete Intersections, II
- Voisin: Hodge Loci and Absolute Hodge Classes

I’ll use this post to offer a few broader comments about the reference to projective geometry in

The Brothers Karamazov. I’ve noticed a conflation — which perhaps originated in part with Dostoevsky and his contemporaries — between what I’ll roughly callcurvature, which measures a space’s geometry’s deviation from the Euclidean, anddimensionality, which measures how many mutually perpendicular lines any given point of a space admits. These notions are largely independent, though there are complications.Dostoevsky’s above quote continues: “I acknowledge humbly that I have no faculty for settling such questions, I have a Euclidean earthly mind, and how could I solve problems that are not of this world? And I advise you never to think about it either, my dear Alyosha, especially about God, whether He exists or not. All such questions are utterly inappropriate for a mind created with an idea of only three dimensions.” The apparent conflation persists in modern Dostoevsky scholarship, for instance in (even the title of) Liza Knapp’s otherwise highly compelling The Fourth Dimension of the Non-Euclidean Mind; Time in Brothers Karamazov or Why Ivan Karamazov’s Devil Does not Carry a Watch. Knapp writes: “Ivan was unable to accept the harmony of God’s universe because he was unable to understand the mystery of time – time being the ‘fourth dimension’ from which his three-dimensional, Euclidean mind barred him.”

In short, there is nothing non-Euclidean per se about higher-dimensional space, and neither need non-Euclidean geometries be higher-dimensional.

The appropriate modern framework within which to explore these ideas is perhaps Riemannian geometry, a field of math which studies Riemannian manifolds, curved and higher-dimensional generalizations of the standard three-dimensional Euclidean space we know. (I’ll speak about projective space later.) Einstein’s general relativity — in which gravity is nothing but the curvature of four-dimensional spacetime — is perhaps Riemannian geometry’s greatest modern vessel.

Curvature and dimensionality endure a double dissociation in Riemannian geometry, as they naturally should. We may have arbitrary-dimensional spaces which are perfectly flat (in which two parallel lines never meet). We can also have two-dimensional spaces which are curved (and in which they do).

The dissociation between these two notions is tempered and complicated by a few auxiliary facts. Curvature

doesinvolve extra dimensions, in the naive sense that a curved manifold ofkdimensions can only be placed into a flat space if that flat space has has more thankdimensions (this is easy to visualize whenk= 2). On the other hand, the higher dimension, sayn, of this “ambient” space has nothing to do with the “intrinsic” dimension of the manifold, and is less important mathematically. (A smooth curved surface in 3-dimensional space is 2-dimensional, not 3-dimensional.) Thus this higher ambient dimension is not as relevant mathematically.Projective space (one sort of space for which it could be said that two parallel lines meet, and perhaps that to which Dostoevsky referred) presents another sense in which these notions become complicated. I have avoided discussing projective space because it belongs not to the field of Riemannian geometry, but rather to that of algebraic geometry. In particular, projective space supports no notion of curvature; two parallel lines meet here instead because of the intrinsic construction of the space.

While projective spaces of any dimension — even 1, 2, and 3 — exist, projective spaces too require extra dimensions in a sense relevant more psychologically than mathematically. Indeed, projective space (typically) operates over the complex numbers, which require twice as many dimensions to depict as do the reals. Furthermore, the construction of projective space again requires an extra dimension, which too becomes irrelevant mathematically. In short: projective space is a strange world, and even if its mathematical dimension is small, its “human” dimension is always larger.

It remains to decide how best to interpret Ivan. The most mathematically consistent thing for him to say — let’s imagine Ivan as a modern mathematician — would perhaps have been that this world is a four-dimensional manifold (which we can’t visualize) with nonzero curvature (which is even harder to visualize), in which who’s to say what will come of any two parallel lines. Furthemore, time and space form indistinguishable continuum and gravity is the curvature of this continuum.

All of this goes to say, perhaps, that I am splitting hairs here — and that Dostoevsky (and his modern scholars) could have expressed their thoughts just as well by replacing references to the “Euclidean mind” and to “higher dimensions” by a generic appeal to the mathematical physics we simply cannot understand. I might as well, further still, perform this substitution myself and accept the point as given. This I do. In any case, it’s interesting to flesh out the matter mathematically.

“All such questions are utterly inappropriate for” my mind as well. And even if I came to understand them, there is always plenty more of the incomprehensible in math and physics to be had.

See the post above?