Impossible Motion in Video Games

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Photo by Marco Nürnberger.

Modern video games will often go to great lengths to make the physics of their environments as realistic as possible; for example, first-person shooters use sophisticated engines just to animate enemy deaths.  There were no such expectations in early arcade games.  Certainly, a fully realistic physical model would have been beyond the capabilities of the primitive processors used in early arcade cabinets, but it's likely that much of the "bad" physics in these games was actually by design.

One of the most common cheats in video game physics is what I’ll call the “perfect” controller. For example, the Galaga ship shown below can instantly change from moving one direction to another, without changing speed, just by changing the direction that you’re pushing the joystick.

Galaga ship(1981) demonstrating typical video game physics.

To rapidly change direction like this requires a large acceleration, and according to Newton’s second law, a large acceleration imparts a large force. It's the same force you feel when you make a sudden stop in your car, but for a rapidly moving spacecraft, it would be magnified many times over. This is one reason why fighter pilots and astronauts need to undergo special training and stay in excellent physical condition. The acceleration experienced by an occupant of the ship above would be enormous, far greater than any experienced by a real pilot or astronaut. In fact, a sudden stop or change in direction is effectively equivalent to running into a wall, and we all know what happens when a fast-moving vehicle does that.

A clip from Rally-X (1980), where the car makes sudden changes in direction that in the real world, would feel like running into a wall.

However, not all of the early arcade games used such simplistic models for the motion of game objects. Asteroids, shown below, featured a ship that took a full 2 seconds to reach its top speed. In video game time, this is a virtual eternity and in fact, one of the primary challenges of Asteroids is learning to navigate the asteroid field without the ability to stop quickly.

Asteroids (1979) demonstrating motion with finite acceleration.
But is it realistic? The ship in Asteroids can traverse a distance about 20 times its length in a second, so if we suppose that it’s meant to simulate a spaceship the size of the space shuttle (37 m), then its top speed is about 740 meters per second. As I mentioned earlier, the ship takes about two seconds to reach this speed, corresponding to an acceleration of about 370 meters per second squared, or about 37 times the acceleration due to of gravity (g’s). This is much more than a human being could tolerate. Fighter pilots and race car drivers usually don’t exceed accelerations of 5 g’s and the maximum that a human being can handle is usually quoted as being about 10 g’s.

So even Asteroids, with its two-second time to top speed, is an unrealistic simulation of real-world physics. Other early games that feature acceleration, like Defender and Joust, were even more generous, allowing the player to reach top speed in less than a second. But before you criticize game designers for not being realistic, look at what happens when we bring Asteroids down to a realistic speed:

Maybe realism isn't all that it's cracked up to be.