(FORTUNE Magazine) – Ever wonder what exactly would happen if, settled in your airplane seat during a long flight, you fired up your cell phone? Sure, the flight attendant told you not to use it while aboard the aircraft, and you remember hearing something about it being against government regulations. But is it actually, you know, life-threateningly dangerous?

This is a good question. "There's a good probability that absolutely nothing would happen and that no one in the airplane would know a thing about it," says John Sheehan, formerly of RTCA, a nonprofit think tank that focuses exclusively on aviation electronics issues and frequently advises the FAA. "There is, however, a remote possibility that it could affect some system in the airplane."

If Sheehan sounds as if he's on the fence when it comes to saying whether or not it's dangerous to use your cell phone while airborne, that's because he is. The truth is that neither he nor anyone else knows for sure just how dangerous it is--or if it's dangerous at all. Oddly enough, it appears that no one has ever actually scientifically tested a cell phone on an airplane. The study that comes closest was conducted by RTCA. Submitted to the FAA last summer, the study was based on a three-year project in which scientists tested the relationship between a plane's avionics system and various portable electronic devices (PEDs) such as CD players, Game Boys, radios, and so forth--but not cell phones or any other two-way "intentional emitter." "We wanted to," says Sheehan, who ran the project, "but we ran out of time and money." The study recommended that the FAA ban the use of cell phones and other PEDs during "critical phases of flight" such as takeoffs and landings; the FAA rejected this advice and lets each airline create its own policy.

Practically speaking, the FAA's decision doesn't matter, since a 1992 FCC rule specifically makes using a cell phone while airborne illegal anyway, according to Steve Markendorff, deputy chief of the FCC's commercial wireless division. The reason, however, has nothing to do with airplane passenger safety and everything to do with the technology of the cellular system. When you make a cell call, an electronic message is sent from your phone and picked up by the nearest cell site (i.e., those really big antennae that have become an integral part of our American landscape). The message is then transmitted through the system until it reaches the person you called. When the caller is on the ground, buildings, mountains, and other objects block communication with more than one site at a time, and everything works fine. "When you're in the air you can communicate with hundreds of cell sites at once and overload the system," explains Art Prest, vice president of science and technology for the Cellular Telecommunications Industry Association. Thus the real reason you can't make cell calls on an airplane: It isn't good for the system.

To be fair, determining whether a cell phone--or any other PED--is safe to use on planes really is difficult. It's all very complicated, and no one actually knows for sure. According to Robert Lilley, director of the Avionics Engineering Center at Ohio University, the issue of cell phone/plane safety is determined by the electromagnetic relationships among three variables: the plane, the cell phone, and the sky. The problem is that the electromagnetic qualities of each of the variables are either unpredictable or changeable. No two planes are exactly alike--the slightest difference in a plane's configuration can result in different electromagnetic characteristics. The same goes for cell phones: Each unit can be different. And the sky, the most variable variable of all, is a veritable quilt of electromagnetism. Put all this in an equation, and you have an electromagnetically unique jet, filled with passengers yapping on electromagnetically unique cell phones, hurtling through a sky in which electromagnetic conditions are changing constantly. It's almost impossible to quantify the risk of airborne cell phones.

So is it or is it not dangerous to use a cell phone in flight? "Look," says Dr. Lilley of Ohio University, "in the case of cell phones on airplanes, there are two good reasons not to do it. One, we know the cellular ground system can't handle it very well. Two, the airplane may not handle it very well. To me that's two strikes against doing it, and that's as far as I want to go." Us too. --Erin Davies