The unreasonable effectiveness of deep learning2021/11/20
The search for building and understanding intelligent machines has been under way for thousands of years and appear fundamental to the human condition . Deep learning models have recently been heavily dominating this search for artificial intelligence (AI), as demonstrated by the go-playing Alpha Go , the narrating GPT-3 , and many more. This is an odd development for many since computational graphs are old news, and because no ground-breaking heureka moment has guided the development. Rather, deep learning is guided by a primitive brute-force optimization mechanism.
Why is it that deep learning and deep neural networks have become so popular despite their simplicity? If there is no theoretical foundation to build on, why are they so unreasonably efficient?
In this essay, I will argue the answer is: part coincidence, part skill. Motivating the skillfulness, I will first define deep learning (DL) and attempt to explain the phenomenon from a first-principle's approach. In this light, I will describe the "coincidental" part by revisiting sets of problems DL is particularly good at, so we can finally arrive at an answer to our question: why are deep networks so useful?
Even though many DL proponents promote its similarities to the human brain, the building block of deep networks have little to do with nervous systems. Deep networks can be roughly defined as large computational graphs with weighted nonlinearities.
In fact, eerily similar approaches was attempted throughout 1960-1990, with little success. What makes deep networks interesting, though, is that they can approximate any function to any desired degree of approximation . This striking fact was discovered in 1986 and tells us that deep networks can solve any problem, if they are big enough. Granted, that is a big if, and this has largely been driving the explosive growth of the graphics processing units (GPUs) market: the bigger network, the better approximations.
It still took several decades before the idea of a universal function approximator could be used to solve problems with any form of practical relevance. Theoretically, this was already solved in 1986, where 7 showed that large nets could be "trained" by searching for optimal configurations in a sea of possibilities. However, it was not until the early 2000's that sufficiently powerful hardware came along to build and search through massive deep networks now up to hundreds of trillions of parameters .
Deep Learning first principles
It may come as a surprise how this type of searching yields meaningful networks. Fortunately, physicists and mathematicians helped resolve parts of the conundrum with two ancient ideas: symmetry  and equivariance .
It turns out that symmetry is a useful property during learning. If you are to, say, identify a cat in an image, it should not matter how big the cat is. Or where in the picture the cat is. In other words, we want to find a way to learn a way to look at cats that are symmetric under scale (size) or translation (location in picture). This symmetry is normally called invariance, because the idea that a cat is in the picture does not vary if the cat is very large and dead-center, or really small in the lower left corner. We identify cats independently from those distortions, .
We can write this down more precisely by looking at some images that we are looking to classify into a binary decision using a network , we can therefore say that the guess without a distortion , should be the same with the distortion:
We can take this one step further by generalizing the invariance, so that the networks can operate on distorted images, while retaining the distortion . Imagine for a while, that we are no longer identifying cats, but colouring them, as shown in Figure 1. In that case, invariance will not do because we need to include the distortion in the output. Relating this to the example of the cat, we cannot always think that the cat is in the center of the image and colour its outline there. Rather, we need to correctly scale and translate the outline in the output image, that is, we need to preserve the structure of the distortion. When we want to mask our cat (as in Figure 1), our structure preserving network should be written as and fulfill1:
Our network is now a structure preserving map, also known as equivariant maps. They are fundamental to, for instance, topology and graph theory.
Suitable problems for Deep Learning
It has been shown that deep networks are particularly good at preserving symmetry and structures [10; 9]. This kind of preservation allows for the detection of symmetries and structures, but also the mapping of symmetries and structures between domains such as image, 3d-space, language, audio and many others .
This is obviously relevant for classifying and masking cats, but what about everything else "AI"? What about other types of intelligence like bodily intelligences, causal reasoning (what is the source of the structure), abstract thinking, and so on? Is that all a matter of preserving mapping? Maybe. We do not know . Please find out.
Why did Deep Learning become so popular?
Returning to the idea of a universal function approximator, these networks are precisely exercising the approximation of domain mappings. They find appropriate responses such as "CAT", a colouring mask, or even a synthesized voice command. Pause here for a second and imagine how many problems can be solved this way. Is language a mapping of concepts to voice or text ? Is programming a mapping of concepts to code ?
By now, the reader is hopefully convinced of the usefulness of the approach. And plenty of evidence exists in applications for speech synthesis, self-driving cars, facial recognition, object detection, fake blob opera, etc. This is the coincidence: pioneers of the abovementioned principles could not possibly have known the extend to which their findings would generalize.
Figure 2 plots the relative popularity of two search terms: "artificial intelligence" and "deep learning" . It appears the general public lost interest in "classical" AI in the beginning of the 00's only to slowly move towards deep learning-driven AI in the mid 10's. Interestingly enough, this coincides with the first papers on invariances and structure preservations in 2014/15.
It would be presumptuous to exclude other factors, and the advent of commercialized hardware that unlocks the training of large deep nets, is certainly one such influence. But the fundamental properties we have discussed here, are not only interesting because they explain what is feasible, they also hint at what is not. Perhaps that is why deep learning is losing popularity towards the end of 2020?
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