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MIT student blogger Sam M. '07

You don’t know what a robot is? by Sam M. '07

I dig Roger Ebert. Roger Ebert digs MIT. Thus, I dig MIT.

Roger Ebert is my favorite film critic; I think he’s got an honest face, he truly cares about movies, and I usually agree with his reviews, except that he vastly underrates both Galaxy Quest (one of the best movies ever! Alan Rickman!) and Army of Darkness (Bruce Campbell always deserves better than two stars).

Now, Roger Ebert didn’t like “War of the Worlds“, and not just because of the whole Tom Cruise/Oprah thing. Rather, he was just profoundly bothered by the alien invaders’ use of hulking, unstable tripods to invade Earth. Ebert concludes, at the end of his two-star review:

All of this is just a way of leading up to the gut reaction I had all through the film: I do not like the tripods. I do not like the way they look, the way they are employed, the way they attack, the way they are vulnerable or the reasons they are here. A planet that harbors intelligent and subtle ideas for science fiction movies is invaded in this film by an ungainly Erector set.

So intrigued was Ebert by the concept of a three-legged robot, he returned to the subject in his biweekly “Movie Answer Man” column–and guess who he asked for scientific advice.

Seeking an expert opinion, I contacted Jessica Banks, a Ph.D. candidate at the MIT Computer Science and Artificial Intelligence Lab, whose thesis involves a robot with one point of contact. She consulted her colleague Dan Paluska, a Ph.D candidate at the MIT Media Lab, an expert on robot-legged locomotion, who was featured on the cover of Wired magazine.

I guess that advising a film critic on the scientific plausibility of a Steven Spielberg movie isn’t quite as prestigious as creating a new form of matter or bioengineering human tissues, but it’s always great to see those three letters popping up in unexpected places.

Banks and Paluska’s full response, as printed in Ebert’s online column, appears below.

They began by pointing out, “Your comment, ‘If evolution has taught us anything, it is that the limbs of living things, from men to dinosaurs to spiders to centipedes, tend to come in numbers divisible by four’ is wrong and misleading. Numbers of limbs are divisible by two, due to the principle of bilateral symmetry to which nature adheres.”

I meant of course to write “two” instead of “four” but was attacked by a brain cloud. My online review has been corrected. Banks and Paluska continue with a fascinating discussion of the functions of three legs among both living and mechanical creatures, which I am printing in full on Here are some bullet points:

–“A three-legged chair or table is very stable when it is still. However, the answer isn’t so easy when one considers three-legged locomotion. Things have a right and a left, a front and a back. This has to do with the fact that animals tend to travel in a certain direction, facing forward when doing so. Having an even number of legs allows animals to be balanced as they travel forward.”

–“There is a rhythm to walking and running that may be difficult to achieve with a three-legged machine. A kangaroo is the closest thing to a three-legged animal because it uses its tail. However, its tail is not the same as its legs and the tail does not touch the ground when the kangaroo is hopping.”

–“The argument that nature didn’t ‘come up’ with such a creature doesn’t hold much water. Nature didn’t come up with the wheel for locomotion, either. We could, for instance, imagine a three-legged creature that stood still and upright for the vast majority of its life. However, it would be hard to imagine such a robot being efficient at locomoting over any significant distances.”

–“The height of the tripods and the fact that they are top-heavy makes it plausible that one would fall if one of its legs was damaged, especially if the alien was in motion at the time of injury. This doesn’t really say that much, though; considering the fact that if you were to kick one of my legs while I was running or even give it a forceful unexpected blow when I was just loitering about, I would most likely fall to earth as well.”

–“So who knows if it is practical or not for a robot to walk on three legs? Ultimately, it would all depend on the system as a whole (speed, passive stability, simplicity, energy consumption, navigability, human-exterminating-ability, etc.), the available technologies (sensors, computation, actuators, etc. ), the environment in which the robot was supposed to perform, and, well, who was funding it.”

2 responses to “You don’t know what a robot is?”

  1. Spencer says:

    I was not too keen on that movie either.

  2. Mitra says:

    Wow, a comment from Spencer! I’m green with 5-hour-UROPing jealousy.