It is possible to conclude, after a little reading up on the matter, that Wilhelm Conrad Röntgen discovered X-rays because he was color-blind. Right there in his two-room laboratory, cramped with apparatus, a big clock ticking on the wall, inside the physical sciences building at the University of Würzburg, November 8, 1895, just before suppertime. The usual suspects: a pear-shaped glass container called a Crookes tube (covered with black cardboard and sucked fairly clean of its air, a process that took a whole day); cathode rays (a piece of technology now used mostly to make TV sets, but in those days a new and experimental form of electricity); and a screen painted with something called barium platinocyanide (a form of the poison). In his cathode ray experiments, Röntgen had decided to juice up the intensity of the rays, all the better to see them. What he didn’t yet realize was that the juiced-up tube now emitted an electromagnetic jet so fast and with such shallow wavelengths that it could pass through just about anything and, in passing through some things more easily than others, cast shadows. That day, a jet of energy, never before observed, struck the barium platinocyanide and burned the color green.

A month later, in early December, the tannenbaum trimmed in an upstairs parlor, Röntgen summoned his wife down into the lab. He had told no one of the rays that he dubbed X. Röntgen threw the switch and trained the machine on his wife’s hand for a full fifteen minutes. He developed the plate. On it was burned the image of finger bones, skinless knuckles and the nearly opaque shadow of Bertha’s ring. The first and most important application, medical in nature, of Röntgen’s ray was thus suggested.

Two days after Röntgen went public, Thomas Edison’s vast stable of scientists and experimenters were set to the task of finding a way to leverage a marketable device out of X-ray radiation. Edison eventually built several fluoroscopes, which were something like X-ray goggles. You looked through the fluoroscope at your hand, say, and you saw the bones, alive and moving. He took his device on the road. At the National Electrical Exhibition in New York, a trade show, thousands of people mobbed his portable fluoroscope. Clarence Dally, perhaps the inventor’s closest staffer, spent hours and hours manning the exhibit and working next to the X-ray generator back at the lab. Within a short period of time, burns began appearing on his skin. His fingernails stopped growing, his hair slowly fell out and finally, in 1904, he died from exposure to Röntgen’s rays. Edison ceased any and all X-ray research. (On a lighter note, in 1896, having read that many X-ray researchers had been losing their hair, one exceptionally deductive entrepreneur, a Frenchman named Gaudoin, had the bright idea to direct the new radiation at women seeking the removal of unwanted facial hair. Gaudoin achieved a considerable customer base for his lab-salon. Successfully beamed, his clients paid, left and waited. They returned for a second treatment. Their beards remained. M. Gaudoin did not. He refunded some fees, then escaped with the rest.)

Three companies now manufacture X-ray machines for use in US airports—Rapiscan, Smiths Heimann and L-3 Communications Holdings. The machines to inspect carry-on bags cost around fifty thousand dollars apiece. Inside each is a cathode tube, not so different from Röntgen’s pear-shaped Crookes, which blows electrons at near light speed toward a target. Complicated things happen, the most important of which is that X-rays effervesce and pass through Louis Vuitton, Coach, Samsonite, Tumi and other garment bags stitched with initials. The rays strike an X-ray detector, generally made of tungsten, which sends them over to a filter, which then sieves out the weakest of the rays, like geeks picked last in gym-class basketball. The strongest rays move on to another detector. Shadows of varying darkness are cast. Intel Pentium processors and a software program called TIP (Threat Image Protection) do the rest of the work. The results appear on a computer monitor.

Organic materials glow red and orange. Inorganics glow blue and green. Because metal is dense, a bowie knife or a Smith & Wesson will show up in clear, visible outline. Between February and December 2002, checkpoint personnel in the US confiscated, astonishingly, 37,504 box cutters from passengers undaunted about packing a tool that has now become a totem of aviation terror. There are, however, more dangerous objects to be on the lookout for. Stuff that shows up organic might, for instance, be a plastic explosive (or it might be soap.) The checkpoint X-ray is just a decision-making tool. The real responsibility lies with the person gazing at the computer screen.

The first X-ray machines designed to look inside airline baggage were installed in the 1970s, in the United States, after a rash of hijackings of US aircraft by Cubans loyal to Castro. Initially, airport X-rays were used only to detect the tools of the pirate—the gun and the knife. Not bombs. Then, in December 1988, during Pan Am Flight 103, a timer on a plastic explosive, hidden in a Toshiba radio in a suitcase with no accompanying passenger, reaches zero. Some of the 259 people on board are incinerated immediately. Others survive the explosion only to plummet, buckled into their seats, twenty-five thousand feet into the roofs and sods of the Scottish town of Lockerbie. In response to the disaster the airports of Europe, and especially of the United Kingdom, go on an antiterrorism crusade, installing huge machines—CAT scan–like in capability—that use X-rays and powerful computers to screen all checked baggage for explosives, in addition to other measures like putting armed policemen at every gate. Britain thus becomes the world exemplar for combining passenger ease with tight security. The US waits until 1996, with the passing of the Federal Aviation Reauthorization Act, to even begin a plan to do the same. The funding necessary to install all the checked-baggage machines is provided in a desultory fashion. On September 12, 2001, all this changes.

The US federal government now employs the forty-five thousand people who operate the X-ray machines that scan carry-on bags. Following the fury of the post-9/11 investigations, they no longer earn $4.50 an hour, but $11.00. Those beleaguered private security firms who once handled all luggage screening and checkpoint security have been essentially banished from the business. Operators (who must now pass muster) report to an arm of the Department of Homeland Security called the Transportation Security Administration. The TSA claims that 100 percent of all checked baggage on US airlines is now screened—compared with less than 5 percent pre-9/11—but not all of it by machines. Bomb-sniffing dogs and human beings rifling through underwear are still our last line of defense in a few airports.