Determined: A Science of Life without Free Will

by Robert M. Sapolsky

Here’s a more complex example: An ant forages for food, checking eight different places. Little ant legs get tired, and ideally the ant visits each site only once, and in the shortest possible path of the 5,040 possible ones (i.e., seven factorial). This is a version of the famed “traveling salesman problem,” which has kept mathematicians busy for centuries, fruitlessly searching for a general solution. One strategy for solving the problem is with brute force—examine every possible route, compare them all, and pick the best one. This takes a ton of work and computational power—by the time you’re up to ten places to visit, there are more than 360,000 possible ways to do it, more than 80 billion with fifteen places to visit. Impossible. But take the roughly ten thousand ants in a typical colony, set them loose on the eight-feeding-site version, and they’ll come up with something close to the optimal solution out of the 5,040 possibilities in a fraction of the time it would take you to brute-force it, with no ant knowing anything more than the path that it took plus two rules (which we’ll get to). This works so well that computer scientists can solve problems like this with “virtual ants,” making use of what is now known as swarm intelligence.[*],[7]