The Newtonian physics (or not) of Angry Birds Space
Ca-Caw! Experts weigh in on the basic principles featured in the birdiverse
The Angry Birds have arrived … IN SPACE, still aimed toward exacting revenge on a cadre of bright green pigs who’ve waddled off with the birds’ eggs.
(If that sentence seems like a preposterous assembly of words, an explanation: That is the premise of Angry Birds, a game for smartphones and tablets first released in 2009 that quickly became popular worldwide. The most recent spinoff, Angry Birds Space, was, ahem, launched with an announcement from the International Space Station in March, and within three days more than 10 million people had downloaded the game.)
The new game is set in microgravity, and the astro-birds and space-pigs are negotiating a Birdiverse filled with cratered spheres, sinister-looking gears, and objects vaguely resembling icy moons. Instead of tracing parabolas en route to pig fortresses, the curmudgeonly birds — including a few newbies — trace arcing paths around these planet-like bodies.
Acted upon by circular gravitational forces, properly aimed birds can sometimes loop around several spheres in a dizzying figure-eight pattern before laying siege to encamped pigs, which are sometimes sitting upside-down on the bottom of a planet.
The extra dimensions make this version a bit trickier than the original, but Angry Birds Space helps by projecting a flung bird’s path — at least, for a short distance. And, the game’s designers have inscribed a circle around each object with a gravitational field. Once the bird is within the circle, its path changes, and the bird appears to be tossed around by both gravity and some kind of atmospheric drag.
“It’s a very intuitive, dynamical system they’ve set up,” says Jeffrey Parker, a mission designer and navigator at the Jet Propulsion Laboratory in Pasadena, Calif., who is currently working on the GRAIL mission. He likes the game because it combines addictive fun with what he does for work. “At NASA, we spend a lot of time thinking about just where to pull back that slingshot, which precise angle to use, and what speed to give a spacecraft.”
Though the game is charming — and quite fun — Parker and some other folks who know a thing or two about physics suggest that its details belie a rather non-Newtonian universe. Then again, snorting pigs and a bird that sounds like a Wookie — the big green one — wouldn’t make a sound in space. Neither would exploding TNT. “There’s also no such thing as an exploding bird,” Parker notes.
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But some of the more realistic characteristics of the game don’t quite obey Newton’s laws. For starters, the gravitational fields appear to have a uniform magnitude instead of weakening with distance from an object’s center (called the inverse square rule). That seems to be a casualty of making the game fun. “Otherwise, gravity would be too weak at the edges of the field,” says Erin Catto, the game physics programmer who created Box2d, the physics engine that runs behind the curtain at Angry Birds.
Box2d simulates the interactions of rigid bodies — birds, pigs, crates, balloons — with defined velocities, masses, and densities. Every fraction of a second, the program simulates a world of these bodies, calculating how all the forces in play (things like friction, collisions and gravity) interact to influence the bodies’ motion.
Though the original Angry Birds was set with a constant gravitational force acting over a flat surface, Catto says that simulating isolated, circular fields using Box2d would be easy. “They’re adjusting that force probably 30 times a second,” he says. When a bird approaches or circles an object, “they determine that force based on proximity to these planets.”
Only within a planet’s sphere of influence is a bird subject to the field’s strength. Before that, the bird travels in a straight line, transitioning to a curved orbit inside the gravity sphere. “There’s a dramatic change between going along in free space, and suddenly, bam – you’re sucked in,” Parker describes.
This isn’t quite right either. “There’s no special distance at which gravity starts working,” says Nick Moeckel, a theorist at the United Kingdom’s University of Cambridge, who recently modeled the paths of planets orbiting binary stars. “If birds were launched at pigs in real life, their trajectories would be affected by the planets no matter where they are.”
And unless there’s some kind of drag, a body in orbit will trace the same curved path, over and over, Moeckel says. In Angry Birds Space, the feathery orbit decays and sends the bird spiraling down to eventual impact, suggesting that the bodies do contain some kind of atmosphere. Further evidence: the pigs aren’t enclosed in bubbles on these worlds. But they are when floating in space. (And when the bubbles pop, the pigs turn blue, freeze, and fracture into icy pig-bits. “I thought that was a cute touch,” Parker says. “Space is really cold.”)
Atmospheres affect orbital trajectories, as was clearly demonstrated by Russia’s Phobos-Grunt spacecraft in January.
In the game, the birds follow elliptical paths around their planets, even though the paths are variable. That’s what should happen, if you’re captured by a planet with an atmosphere. “They got the ellipse right, and the fact that you move faster when you’re closer to the planet or asteroid,” Parker says. “That’s good.”
But, Newton said that objects in motion tend to remain in motion, which is one big difference between Angry Birds and the real world. Birds launched into a space-vacuum ought to continue winging along the same path until they encounter an object. But they don’t. “When anything in the game moves through free space, or around a body, there appears to be drag,” Parker says. If a bird is moving at a slow speed, it’ll come to a stop, just millimeters from where you wanted it to go.
“In real life, that never happens,” Parker says. “It’s like you’re shooting through not free space, but something that’s not as heavy as water, some kind of a low-density fluid that just slows everything down.”
Similarly, Newton also described how every object with gravity will exert a force on every other object. Normally, one wouldn’t expect to see the tug of a bird’s gravity on a planet — the bird is just too small — but a cluster of planets, which appear in some of the game’s levels, will all be tugging on one another. In reality, the planets might even be orbiting one another — which would make the game super-hard — but in some cases, says Parker, this principle, if incorporated correctly, would lead to a big crunch.
But that might be OK. “It’s good that they don’t actually pull on one another in Angry Birds,” Parker says, of the clustered planets. “Otherwise, you’d start the level and they’d all collapse and crash into each other, and you’ve have no more level, and that would be no fun.”