An electromagnetic pulse (EMP) is a quick, powerful blast of electromagnetic energy that ranges across a significant portion of the electromagnetic spectrum. The most frequently cited source of an EMP is a nuclear weapon. Indeed, the easiest way to generate the energy for an EMP is through an abrupt chemical or nuclear explosion, and devices for creating EMPs in the absence of such an explosion are known to be relatively weak.
The general idea of an EMP is that it wreaks havoc on electronics, but leaves other physical structures mostly untouched. Real-life electromagnetic pulses released by high-altitude nuclear tests have fused power wires, triggered burglar alarms, and caused breakage in radios, TVs, and power lines as far as 1,500 kilometers (930 miles) away. The source of this disruption is a large-scale, intensely fluctuating magnetic field created when high-energy photons from an explosion knock electrons from their atomic orbits. This disruption becomes trapped within the earth’s magnetic field, leading to a coherent oscillating electric current.
In near-future science fiction, the EMP is seen as a potent superweapon that disables all sophisticated electronics in a national or even continental region. Because the EMP can extend all the way to the horizon with sufficient energy, a nuclear weapon detonated in near-earth orbit could in fact disrupt electronics in a huge region. In contrast, a nuclear weapon detonated close to the ground can do little damage with its EMP – for example, planes that drop atomic weapons continue to function even if they are within the line-of-sight of the nuclear explosion.
To release an EMP capable of doing damage to a wide area, the nuclear weapon that triggers it must be relatively large – in the range of 10 or more megatons, which is almost three orders of magnitude more powerful than the Hiroshima and Nagasaki nuclear bombs. For releasing a powerful EMP in a smaller area, the explosively pumped flux compression generator has been suggested, which uses high explosive to essentially achieve the same effect. The world’s militaries have tested prototypes of such a weapon, but it has not been widely deployed.
The EMP is presented in fiction or games as a weapon suitable for use against machines, cyborgs, or nanobots, but useless against biological creatures. In reality, machines can easily use processing and actuating elements based on mechanical and chemical principles rather than purely electrical ones, making them as immune as a human being to an electromagnetic pulse.