On Intuition and Hessians

There's a bizarre thing that sometimes happens when I'm reading about physics or statistics or other technical subjects. Even if the concept I'm learning is completely fresh and foreign, I sometimes feel an uncanny sense for the ‘‘proper’’ way to approach a concept. But I'm often quite confused where my gut feeling comes from, especially if I've never encountered a topic before!

I attribute this feeling to the magic of intuition. As humans, we often feel a conviction that an idea is right or wrong even without a fully-fleshed-out explanation. Intuition lets us reason logically without any logical reasoning (!), and in my view, an ‘‘intuitive’’ understanding of technical knowledge can be more powerful than the details of particular logical steps.

In my opinion, if we want to apply our intuition to solving problems, we ought to be rather careful about how we build up a intuition so that our gut instincts align well with reality. We also ought to recognize which situations we can trust our intuition in, and which situations we can't.

These days, I find myself thinking carefully about where my intuition comes from. I feel that it's built up from (1) seeing lots of particular examples, (2) distilling the essence into simple language, and (3) reasoning by analogy for new examples. But if we don't want our intuition to run astray, we must refresh and recalibrate our intuition every once in a while by going over specific examples and hashing out the chain of logic. In my opinion, this recalibration process is a moral obligation for anyone who wishes to trust their intuition!

A few days ago, as I was reading some of Jim Sethna's work, I had yet another moment of ‘‘why the hell do I believe this idea even though I've never learned this topic before?’’ And instead of just letting it slide by, I decided to sit down and hash out exactly how my intuition arose. So here I am, fulfilling my moral obligation to calibrate my intuition.

In this post, I will discuss the curvature of multi-dimensional functions. For those of you familiar with the math, I will be talking about the eigenvectors and eigenvalues of (symmetric) quadratic forms. These sorts of concepts arise naturally in physics and in statistics when we want to talk about how high-dimensional functions behave near their minima.


High-dimensional functions

Many problems in science and engineering have a natural geometric interpretation in terms a multi-dimensional space. Today, I'll be considering two main examples:

Since we're so familiar with 3D space from our day-to-day lives, the geometric interpretation is a helpful tool for us to reason by analogy. For instance, when a bunch of particles are bouncing around inside a box, we know that the motion is described by a trajectory in configuration space. (Think about it – the configuration is changing continuously over time.) Or when we're trying to pick a good set of parameter values for a statistical model, it's akin to finding the best point in parameter space.

We can often define functions over these spaces. Such functions takes on different values at different points in the space. If we consider a ‘‘mattress’’ of balls and springs – a simple model of the nuclei in a metal, for instance – then each particular configuration of balls has an associated potential energy depending on how squished the springs are. We can thus think of the (potential) energy as a function of configuration space; it takes on a lower value at configurations where the springs are squeezed tightly and a higher value where the springs are loose. (Again, remember that points in configuration space have nothing to the real physical space we live in!). In the example of statistical models, we often think about a loss function the measures how well the model fits the data – sum-of-squared-errors loss, zero-one loss, whatever loss you like. A set of parameters that fits the data poorly results in a high loss; a good set of parameters will give a nice and low loss. In general, the loss function has different values at different places in parameter space.

These functions are rather complicated beasts to tackle, in part because high-dimensional spaces are immensely vast. The number of points in these spaces grows exponentially with the dimension! (think about the formula for length versus area versus volume…) So it gets rather unweildy to characterize the behavior of a function everywhere in a high-dimensional space. Thankfully, we typically (!) don't need to understand the behavior everywhere, because we only care about particularly nice regions of the space. In the example of many-body-physics, we expect that the trajectory will settle down and stay around a minimum of the potential energy surface, in much the same way that a bale of hay will roll to the bottom of a valley. (There are many caveats to this statement that I will ignore for simplicity.) In the example of statistical learning, we only care about the ‘‘best’’ choices of parameters that fit the data well, corresponding to regions of parameter space where the loss function is small. In both of these cases, all we care about is how a function behaves in a region of space around a minimum.

For the rest of this blog post, we will be examining how multi-dimensional functions behave in the neighborhood of a local minimum.

Mr. Taylor

We first review the mathematical language of functions of many variables. The takeaway of this section is that functions look like quadratic forms near their local minima.

One dimension

We begin with a function of a single variable f(x), and then we will generalize to higher dimensions.

Let x be a point on a 1D line, and f be a (smooth) function over the line. We want to study the behavior of f in the vicinity of a particular point x_0. In general, f might be a pretty complicated function, but if we zoom into the neighborhood of x_0, we have a better hope of understanding its local behavior.

Near this point, we can express x as x = x_0 + eta, where eta is a small displacement. Mr. Taylor tells us that the local behavior around x_0 is described by

 f(x) approx f(x_0) + f'(x_0) eta + frac 1 2 f''(x_0) eta^2 + O(eta^3)

Each successive term in the approximation gives you a better and better estimate of how f(x) behaves near x_0:

Note that if x_0 is a local minimum of f, then the first derivative f'(x_0) = 0 vanishes because the slope is flat. So the Taylor expansion no longer has a linear term; instead, the function looks like

 f(x) approx f(x_0) + frac 1 2 f''(x_0) eta^2 + ldots

This is the equation of a parabola. If f''(x_0) > 0, then the parabola points upwards and the function increases when you walk away from x_0 (to leading order); if f''(x_0) < 0 then it decreases as you walk away from x_0. The greater the magnitude of f''(x_0), the greater the ‘‘steepness’’ of the walls of the parabola.

We now understand the behavior of one-dimensional functions near their minima: they are parabolas. Now let us talk about higher dimensions.

Higher dimensions

In higher dimensions, the notion of a parabola generalizes to a quadratic form. As you may expect, a higher-dimensional function looks like a quadratic form in the vicinity of a local minimum. The reasoning is entirely analogous to the 1D case.

Points in a high-dimensional space can be labeled by vec{x} = (x_1, x_2, ldots, x_n). Our function f is now defined at every point in the space. Since high-dimensional space is so vast, it is fruitless to understand its behavior everywhere; we instead zoom in around a particular point vec{x_0} and study the behavior in the neighborhood of vec{x_0}.

Once again, the points near vec{x_0} can be described as vec{x} = vec{x_0} + vec{eta}, where vec{eta} is a small displacement. We want to write a Taylor expansion in vec{eta}, but this time around, it's not immediately obvious how to do so. So let us first understand how to take derivatives in higher dimensions.

The first derivative tells you how much f varies when you move by a little vec{eta}. Since there are many possible directions to move in higher dimensions, the amount that f changes by depends on which direction you choose to move in. If you move along the x_1-axis, then the function changes by frac {partial f} {partial x_1} eta_1; if you move along the x_2-axis, then the function changes by frac {partial f} {partial x_2} eta_2, and so on and so forth. If you move in an arbritrary direction, then you need to add up how much the function changes due to your movement along each particular axis. Thus the first-order approximation can be written as

 f(vec x) approx f(vec x_0) + sum_i frac {partial f} {partial x_i} bigg|_{vec{x_0}} eta_i + ldots,

where the sum over i is a sum over the different possible basis directions that you can move in the space. (In our physical 3D world, for instance, these three basis directions are up, right, and forward.) The displacement vec eta is due to small displacements eta_i in each of the x_i directions. The bar with the vec{x_0} means to take the value of the derivative at the point vec{x_0}.

Notice that the first-order term is generalization of a line to higher dimensions. In the case of one dimension, the linear correction was f(x) - f(x_0) approx f'(x_0) eta. In higher dimensions, since eta = (eta_1, eta_2, ldots, eta_n) can be a displacement in many possible directions, we have to add up the contribution from each of the directions of eta, so f(vec x) - f(vec x_0) approx sum_i partial_i f(vec x_0) eta_i.

As before, the first derivative vanishes when we're at a local minimum. In fact, the derivative must be zero along all possible directions in order for the function to have a minimum. (If there were a particular direction along which it wasn't zero, then the function would get smaller as you walked along that direction, so it couldn't be a minimum there.) So at a minimum, all the partial derivatives must be zero: frac {partial f}{partial x_1} = frac {partial f}{partial x_2} = ldots = frac {partial f}{partial x_n} = 0.

Since the linear term vanishes at a minimum, we must go to second order. Let us think about how the quadratic term looks like. The second derivative is the derivative of the first derivative. Since the first derivative has n different components (corresponding to each of the basis directions in the space), the second derivative must have n^2 different ‘‘components’’ because for each of the n components of the first derivative, you can take another derivative in any direction.

Thus the shape of the quadratic correction around the minimum of a multi-dimensional function is

 f(vec x) approx f(vec x_0) + frac 1 2 sum_i sum_j frac {partial^2 f} {partial x_i partial x_j} bigg|_{vec{x_0}} eta_i eta_j+ ldots,

This expression is analogous to

 f(x) approx f(x_0) + frac 1 2 f''(x_0) eta^2 + ldots

in the one-dimensional case.

Off the bat, I have to admit I can't come up with a compelling reason for why the expression takes this form. But it indeed turns out to be the case. In the next section I want to justify why frac 1 2 sum_{ij} Q_{ij} eta_i eta_j is such a natural object to consider.

What is a quadratic form?

Quadratic forms seem exotic at first, but once we see past the funny notation, we find that they are rather familiar. A few concrete examples of ellipses will help solidify our understanding. As we investigate ellipses more, we will discover if we pick our basis directions to line up with the axes of the ellipse, then their description becomes far simpler. This will be our clue into understanding the eigenvectors and eigenvalues of a quadratic form.

Let us work with a concrete 2D example to see how ellipses arise.

In two dimensions, the displacement vec eta can be written as vec eta = (x,y). A typical quadratic form looks like

 f(vec eta) = sum_{ij} Q_{ij} eta_i eta_j = Q_{xx} x^2 + (Q_{xy} + Q_{yx}) xy + Q_{yy} y^2.

For niceness let us rename some of the coefficents and write it as

 f(x,y) = a , x^2 + 2 b , xy + c , y^2.

Notice that the terms include all possible ways to multiply pairs of variables together. In this example, since we have x and y as our variables, the possible pairs are x^2, y^2, and the cross-term xy. If we added a z variable as well, then we would have to add a new z^2 term, as well as the cross-terms xz and yz. And so on and so forth. In general, a quadratic form is just a weighted sum of all the pairs of variables multiplied together.

Now let us turn to understanding the behavior of the function f(x,y) = a x^2 + 2 b xy + c y^2. There are a few ways to visualize a function of two variables. One way is to graph it as a surface over the x-y plane, where the height of the surface tells you the value of f(x,y). However, this surface lives in three dimensions, and it is very difficult to think and draw in three dimensions. So rather than trying to visualize this surface in its full three-dimensional glory, we typically slice it horizantally (like a CT scan) and draw the shape of the surface on the slice. The resulting ‘‘level sets’’ let you piece together a contour map of the surface.

The horizantal slices of the surface correspond to …

Thus we must answer the question: What are the shapes of the level sets of a quadratic form?

Diagonalizing the Hessian

Vibrations about equilibrium

Principal Component Analysis

Sloppy models

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