Chris Anderson is an entrepreneur; former editor-in-chief of Wired; co-founder and

GRADIENT DESCENT Chris Anderson Chris Anderson is an entrepreneur; former editor-in-chief of Wired; co-founder and CEO of 3DR; and author of The Long Tail, Free, and Makers. Life The mosquito first detects my scent from thirty feet away. It triggers its pursuit function, which consists of the simplest possible rules. First, move in a random direction. If the scent increases, continue moving in that direction. If the scent decreases, move in the opposite direction. If the scent is lost, move sideways until a scent is picked up again. Repeat until contact with the target is achieved. The plume of my scent is densest next to me and disperses as it spreads, an invisible fog of particles exuded from my skin that moves like smoke with the wind. The closer to my skin, the higher the particle density; the farther away, the lower. This decrease 1s called a gradient, which describes any gradual transition from one level to another one—as opposed to a “step function,” which describes a discrete change. Once the mosquito follows this gradient to its source using its simple algorithm, it lands on my skin, which it senses with the heat detectors in its feet, which are attuned to another gradient—temperature. It then pushes its needle-shaped proboscis through the surface, where a third set of sensors in the tip detect yet another gradient, that of blood density. This flexible needle wriggles around under my skin until the scent of blood steers it to a capillary, which it punctures. Then my blood begins to flow into the mosquito. Mission accomplished. Ouch. What seems like the powerful radar of insects in the dark, with blood-seeking intelligence inexplicable for such tiny brains, is actually just a sensitive nose with almost no intelligence at all. Mosquitoes are closer to plants that follow the sun than to guided missiles. Yet by applying this simple “follow your nose” rule quite literally, they can travel through a house to find you, slip through cracks in a screen door, even zero in on the tiny strip of skin you left exposed between hat and shirt collar. It’s just a random walk, combined with flexible wings and legs that let the insect bounce off obstacles, and an instinct to descend a chemical gradient. But “gradient descent” is much more than bug navigation. Look around you and you'll find it everywhere, from the most basic physical rules of the universe to the most advanced artificial intelligence. The Universe We live in a world of countless gradients, from light and heat to gravity and chemical trails (chemtrails!). Water flows along a gravity gradient downhill, and your body lives on chemical solutions flowing across cell membranes from high concentration to low. Every action in the universe 1s driven by some gradient drive, from the movement of the planets around gravity gradients to the joining of atoms along electric-charge gradients to form molecules. Our own urges, such as hunger and sleepiness, are driven by electro-chemical gradients in our bodies. And our brain’s functions, the electrical signals moving along ion channels in the synapses between our neurons, are simply atoms and electrons flowing “downhill” along yet more electrical and chemical gradients. 104 HOUSE_OVERSIGHT_016907