In Chapter 19 of There Are No Secrets author Wolfe Lowenthal comments that T'ai Chi "... is the subduing of the will to achieve understanding of softness, so that a slight, 75-year-old man, completely relaxed, can with a touch send a 250-lb. Judo champion flying." How in the world could such a thing happen, within the laws of physics?

An idea to pursue: model a person as a system, with sensors and actuators and time-delays. The sensors are the nerves and the brain; the actuators are the muscles; the time-delays are set by reflex and reaction lags. What is the simplest "interesting" such model? In "Artificial Wrestling: A Dynamical Formulation of Autonomous Agents Fighting in a Coupled Inverted Pendula Framework" Yoshida, Matsumoto, and Matsue propose what seems to be a far-too-complex system with multiple springs, controllers, actuators, sensors, and time delays.

Perhaps greater insight could come from something more primitive? Consider, for example, a single inverted solid-bar pendulum. A person is rather like a stick standing upright, kept from falling by small muscle movements that are controlled with a short time-delay based on inner-ear and other sensory inputs. If somebody could perturb that simple feedback-loop, maybe by applying a small force but on timescales shorter than the reaction time, could the system be driven into instability so the stick-person would fall down?

If so, what are the order-of-magnitude scales of the perturbing force and time, and how are they related? If, for instance, you react 50% faster than your opponent, do you only need 10% as much force to win a fight? What if you're ten times faster? What if you can only exert 1% of the other person's pressure? And what are the limitations of this approach? Surely a gnat can't derail a locomotive. But on the other hand, if all of the opposition's punches miss you, and you add an appropriate nudge a when a violent swing has just missed ... hmmmmm?!

_{(cf. The Complex Mathematics of Robot Wrestling" in MIT Technology Review June 2014) - ^z - 2014-07-23}