Most of us experience electromagnetic interference on a fairly regular basis. For example:
- If you put your cell phone down on your desk near the computer, I can hear loud static in your computer's speakers every time the phone and the tower handshake. In the same way, your car's tape player produces loud static whenever I make a call on your cell phone.
- When I dial a number on your home's wireless phone, I can hear the number being dialed through the baby monitor.
- It is not uncommon for a truck to go by and have its CB radio overwhelm the FM station I am listening to.
- Most of us have come across motors that cause radio or TV static.
An airplane contains a number of radios for a variety of tasks. There is a radio that the pilots use to talk to ground control and air traffic control (ATC). There is another radio that the plane uses to disclose its position to ATC computers. There are radar units used for guidance and weather detection, and so on. All of these radios are transmitting and receiving information at specific frequencies. If someone were to turn on a cell phone, the cell phone would transmit with a great deal of power (up to 3 watts). If it happens to create interference that overlaps with radio frequencies the plane is using, then messages between people or computers may be garbled. If one of the wires in the plane has damaged shielding, there is some possibility of the wire picking up the phone's signals just like your computer's speakers do. That could create faulty messages between pieces of equipment within the plane.
Many hospitals have installed wireless networks for equipment networking. For example, look at the picture of the heart monitor in How Emergency Rooms Work. The black antenna sticking out of the top of the monitor connects it back to the nursing station via a wireless network. If you use your cell phone and it creates interference, it can disrupt the transmissions between different pieces of equipment. That is true even if you simply have the cell phone turned on -- the cell phone and tower handshake with each other every couple of minutes, and your phone sends a burst of data during each handshake.
The prohibition on laptops and CD players during takeoff and landing is addressing the same issue, but the concerns here might fall into the category of "better safe than sorry." A poorly shielded laptop could transmit a fair amount of radio energy at its operating frequency, and this could, theoretically, create a problem.