A question society is increasingly going to have to face: As technological advances make intrusion and surveillance easier, where do we draw the line? More sensationally, do you want you and your family to be porn stars? A company called OSI Systems makes an X-ray body scanner, which it calls Rapiscan Secure 1000, for security at airports. The principle is simple. An airline passenger stands in front of a large box of a bit more than a man’s height. A narrow beam of X-rays at very low power sweeps across his body. A computer, using 3-D imaging, turns the data into a picture of the passenger, naked, on a monitor. And there is the rub. The objection many raise to Rapiscan is not that it doesn’t work, but that it works very well indeed. The pictures are of near-pornographic quality. Take a look. (archive.aclu.org/issues/privacy/Bodyscanner.html). You may not want your teenage daughter going through one of these searches. The pictures, from the Rapiscan site, make it clear that the system is effective. A concealed pistol stands out clearly, as do a wallet, snaps on hidden C4 explosive, and coins in pockets. So do rolls of fat, buttocks, and so on. It amounts to a black-and-white strip search. Can anything good be said about this machine? Yes, lots. And that is why it appeals to security people. It is quick. The company says a scan takes less than eight seconds. It is convenient for the passenger. Instead of embarrassing pat-downs, you just stand there for a few seconds. Importantly, it doesn’t detect only metal objects. Explosives are not metallic. Anyone familiar with weapons knows that sharp, strong knives of nonmetallic materials are available to anyone who really wants them. The scanner can detect ceramic firearms, drugs and other contraband. From a practical point of view, body scanners are a splendid idea. And they are safe. The X-rays for a scan, according to the company, amount to 20 minutes of normal background radiation. (A scan, says Rapiscan, averages less than three microrem of radiation.)The X-rays penetrate only about a 10th of an inch into the skin. The argument then is not whether it works or is safe — it does and it is — but whether we want to parade more-or-less naked in front of a stranger. The American Civil Liberties Union, which opposes body scans as routine parts of security screening, says, “Passengers expect privacy underneath their clothing and should not be required to display highly personal details of their bodies … .” If there is legal probable cause then scans should be permitted, says the ACLU. Rapiscan is aware of the issue of privacy, and routinely recommends that the person watching the monitor be in another room. That’s better than nothing. But will people buy it? Here we bump into the larger issue. As technology improves, so will the capacity to watch, to monitor, to record. Rapiscan, for example, aims at preventing terrorism and finding contraband. Both are legitimate concerns. For that matter, the government’s Total Information Awareness program may be intended to catch terrorists. But how watched do we want to be? If the security apparatus finds your unguarded e-mail about how you are cheating on your taxes, or the pirated Credence Clearwater Revival CD in your airport baggage — then what? Do they arrest you, or report you to the Internal Revenue Service? If they don’t, we have federal officials ignoring crimes. If they do, we cross the line between security checks and high-tech, pervasive, electronic policing involving cameras everywhere, automated reading of e-mail and X-ray searches. The machinery of security is not very different from the machinery of policing. The choice may be between minor crime and major regimentation.