Recently the Hawk-Eye technology has been used in tennis to judge close line calls. I had previously posted about Hawk-Eye, thinking at the time that it was the right way to go for tennis. In this post I will try and make a case otherwise :).
The problem with using Hawk-Eye in tennis lies in the fact that what it shows us is the 2d projection of the tennis ball. This is fine when the ball is traveling through the air. But when the ball hits the ground, it is not the 2d projection of the 3d ball which is the point of contact of the ball with the ground. The soft tennis ball gets partially squashed when it hits the ground, & it is the squashed portion of the ball which touches the ground. Thus the point (or region) of contact is actually a fraction of the 2d projection shown to us by Hawk-Eye. Moreover such a fraction would be hard to figure out using simple physics, since it's a chaotic system depending on a lot of factors like speed, spin, angle, surface, temperature etc.
Of course, one can argue that Hawk-Eye need not be perfect, but merely better than the human judges for it to be used. But testing such an assertion would be very tough, simply because there is no ground truth in this problem. Even if such a test comparing Hawk-Eye and human judges were to be performed, who decides whether Hawk-Eye or the human judge is "more right"? That would have to be a system better than both Hawk-Eye or humans. The existence of such a system would imply that we wouldn't have this argument in the first place. Ah paradoxes :).