The term “line ball” is sometimes used to indicate an argument that could go either way. In tennis, though, any part of a ball that does hit any part of the line, is “in.” At the highest level of tennis, players flirt with the lines all the time, and there are many “line balls,” or, as they now seem to be called, close calls. The latter I discovered when watching broadcasts of the Australian Open matches. I also discovered, to my great surprise, that there were no longer any linesmen, or even any lineswomen. The Hawk-Eye system is automatically making the calls. And if anyone has questions about the call, there is an instant replay of the bounce of the ball, and super closeups of the impact mark of the ball on the court. Isn’t technology wonderful?

Looking for information about Hawk-Eye turns up these sources, among surprisingly few others:

and the wiki entry, Hawk-Eye.

The replay in “instant replay,” is not, strictly speaking, a “replay.”  It’s actually a “preplay.” There are 10 (in some reports 12) cameras in the Hawk-Eye system. None of them are anywhere near the lines. The cameras are arrayed around the court in high positions looking down on the play. They record at up to 340 frames per second, and feed that information to the associated computers. [What they are looking at is the ball strike and the subsequent trajectory of the ball. On that basis, the entire trajectory of the ball, including the bounce, is predicted. The ball could, in fact, be called “out” before it crosses the net.] As was pointed out in comments, the previous sentence is, as far as can be determined from inadequate documentation, incorrect. It is only the ball tracking on the approach to the bounce that is taken into account. [See comment by Sancho Panzer.]

That explains why Hawk-Eye never shows any actual footage of the ball in the vicinity of the line. It’s a simulation. This will be obvious to cricket lovers. Hawk-Eye for tennis is a development of the original product for cricket. Those balls do not go through the batman’s leg and on to, or past, the wicket.

For something upon which so much rests, there is a paucity of public information about the system. The software is proprietary, and not even the mathematical modelling has been revealed. Not many questions about calibration seem to have been answered, and things like the number of cameras differ in different reports. In an article in New Statesman from 2015, Harry Collins discussed the accuracy of the system, and the secrecy surrounding it.

So we telephoned the firm to talk about [accuracy] and we hit a wall. As sociologists of science we had spent decades chatting with scientists about this kind of thing but suddenly we were told this information was private and lawyers were on call.

Hawk-Eye had been creeping in through the outside courts for a few years, but in 2021 Covid-19 was the rationale for its full-scale introduction across all matches in both the Australian and US Opens. Think how many Covid deaths were prevented by getting rid of all those linesmen around the court.

Let’s suppose that there is a lot of money to be won or lost in betting on the results of major tennis matches. Now imagine that some disreputable but technically adept persons manage to gain access to the software of the system. These nefarious people introduce into the system a bias factor that can be invoked from the control system, or better yet, remotely. The bias is only applied to very close calls to reverse them, with the direction of the reversal depending, of course, on the player.

Who would notice? Players are being conditioned to mistrust their own perceptions and go with the Hawk-Eye call. For example, at the Aussie Open in 2021, Dominic Thiem said, “If the electronic call is out, the ball is out, so there’s no room for mistakes. I like it.” Spectators, live and remote, are shown the “definitive” animation almost immediately. A major match between the world’s top players will often come down to matters of millimetres, and those are the very players who are aiming close to the lines more often.

One of the videos referred to above mentions International Tennis Federation testing of the system in 2006. “Results showed the system to have a mean error of only 2.6mm when compared to a high-speed camera located on the playing surface.” So why not just use the high-speed camera? A competitor, Foxtenn, has done just that. Their high-speed cameras watch the lines, and their replays feature not just the simulation of point of impact, but the actual footage of the ball. But the imagery is coming from a computer, so anything is possible.

That’s the general problem. The world is presented to us as digital information mediated by computers, and all the virtue can be hacked out of the virtual. Once you accept that you must take on faith the video imagery that you see, just as you must the digital photo images, destination Dystopia is much closer.