How Our Brains Play Angry Birds With The World

New research suggests we may use an "intuitive physics engine"—similar to those found in video games—to make rapid everyday inferences.

Every day our brains make lightning quick surveys of the physical world whether we realize it or not. We know how dishes would fall off the tray if the waitress doesn't carry it carefully. We know it's time for a child to come down from the tree if we see a branch bend a certain way. We know where to release the Angry Bird to beat the level we've been playing since the subway door closed. Most of the time, at least, we know what's coming.

Cognitive scientists have proposed any number of theories for how people perform these advanced evaluations so successfully, regardless of their grades in high school physics. Given the widespread affection for physics-related games, people seem to have some innate appreciation for the physical complexity of these concepts and moments. But the precise mental process we use remains a bit of a mystery.

A trio of MIT scientists has suggested a new theory: people possess an "intuitive physics engine" capable of running rich physical simulations on everything around us. These simulations are similar to the ones computers run during video games with two exceptions. They take into account uncertainty and they trade precision for speed; in technical terms, they're approximate and probabilistic.

"What happened in the past, what will happen—the way you form those judgments is based on these simulations supported by an intuitive physics engine," Peter Battaglia, lead author of a paper on the idea, recently published in Proceedings of the National Academy of Sciences, tells Co.Design. "It's a very powerful way of making these predictions."

The intuitive physics engine theory says that our brains develop three-dimensional geometric models of real-world scenes we wish to evaluate. These models consider static variables like the shape and spatial arrangement of objects in the scene, dynamic ones like motion and friction, and external factors like gravity. Using approximate rules of physics, our intuitive engines can simulate how these scenes will look a moment later, and suggest a probable outcome—as if our brains were playing Angry Birds with the world.

Battaglia and colleagues tested the theory in a series of experiments designed to gauge how we understand everyday scenes. One test was called "Will It Fall?" As the name suggests, study participants looked at about 60 different arrangements of building blocks and determined whether or not each would fall.

Image: Courtesy of Peter Battaglia

Other experiments tweaked the scene a bit (adding blocks with varying weights) and asked different questions (such as which direction the blocks will fall). The most complicated scene showed test participants a table with red and yellow blocks on top and asked them to imagine someone bumping into the table hard enough to knock them off. They had to decide whether red or yellow blocks were more likely to fall.

Image: Courtesy of Peter Battaglia

Across five of these psychophysical tests, people responded in a way that matched up well with the predictions made by an intuitive physics engine and much less well with other potential cognitive models. These rejected alternatives included a "ground truth" model, which runs a pure and precise physics simulation, as opposed to an approximate and probabilistic one. These ground truth models don't account for uncertainties like a gust of wind, or a minor misalignment.

Take responses to one "Will It Fall?" test that showed a stack of precariously balanced blocks (below). In fact, the blocks are arranged in a way that would not fall over, which a perfect ground truth physics model would recognize. But test participants believed the stack would topple over because their intuitive physics engine considered—and evidently accepted—the possibility of a slight but critical imbalance.

Image: Courtesy of Peter Battaglia

"It would take so many coincidences for the thing to stand up that basically you can say, 'I don't believe those things happened. I think this should fall over,'" Battaglia says.

The intuitive physics engine will have to pass subsequent experimental tests to hold up, but the recent research is a promising start. If it does stand as a solid explanation of physical intuition, Battaglia hopes educators will revise their teaching methods in response to the theory, perhaps by incorporating games into their curriculum. "It'd be great if you could play a video game and learn physics at the same time," he says.

Welcoming Angry Birds in the classroom—there's something our intuitive engines didn't see coming.

[Image: Flickr user Tomaž Štolfa]