Introduction

Imagine that you have to cross the busy Fifth Avenue in New York City. In order to safely cross the street you need to detect and anticipate the movement of other pedestrians in order to avoid walking into them; when and how much will they accelerate or decelerate, how will they react in response to external signs like street lights or obstacles and which way will they go in order to avoid walking into you? Crossing Fifth Avenue is clearly not as simple a task as it fi rst seems when we start thinking of the complex calculations that are necessary in order to navigate through a mass of independently acting subjects. Our brain handles tasks like this continuously without us giving it much thought, yet even the most advanced computers and robots are far from accomplishing anything like it.