I'm not saying ghosts exist but, in the spirit of the season, let's assume for a moment they do. Certainly many people believe that and some of them have come up with devices to detect them. So how are those supposed to work?
Technological methods for detecting ghosts rest on the assumption that these spectral interlopers are not purely spiritual or psychic phenomena, made of something that modern physics has yet to get even the slightest whiff of. If that were the case, you couldn't photograph a ghost, or make it twiddle a needle on a meter. For these things to happen - and plenty of people claim that they do - the ghost must have some physical effects.
For instance, ghosts are reported to make a room grow cold, so the well-equipped ghost hunter carries a thermometer to detect this. One theory here is that ghosts need energy to manifest themselves which they draw from the heat in the surrounding air. (The theory seems to violate the laws of physics, by the way, but seeing as it's a ghost, I'll let it off with a warning.)
Most ghost-detecting gadgets rely on electromagnetism. That's not an unreasonable approach. We associate electricity and magnetism with technology but really electromagnetism is everywhere, responsible for most physical phenomena we experience. Light, ultraviolet, infrared, microwaves, radio waves, X-rays and gamma rays are all electromagnetic waves, and it is electromagnetism that binds atoms and molecules together and drives their movements. Electromagnetism is of supreme importance to the material world and, if ghosts exist, they too are probably electromagnetic entities.
It's not surprising, then, that many ghosthunters wield meters to measure the EMF, or electromagnetic field. Elsewhere these devices are used for a variety of things like locating live wires behind walls.
Electromagnetic ghosts could in principle affect cameras, too, since light is just one kind of electromagnetic wave. Cameras are typically sensitive to more than just visible light and some are designed this way, like thermal cameras which capture invisible infrared waves. Cameras are a must for any serious ghosthunter to provide a record of not just what they have seen, but also what they have not.
Audio recorders are common tools, too, to capture electronic voice phenomena, or EVPs, which are the aural equivalent of a ghostly picture. You don't hear an EVP live - they don't exist as sound waves. But when recordings of what should be silence are turned up very loud, some claim to hear ethereal voices behind the hiss of static. The ghost must directly manipulate the signals in the recorder, a rather impressive feat of pure electromagnetism.
That's the theory, anyway. But the problem with measuring physical effects is that they can also have ordinary physical causes. Draughts cause cold; ordinary objects and people cause EMF disturbances; optical tricks and specks of dust cause mysterious images in cameras; and random noise causes the human brain, with its penchant for finding patterns in chaos, to perceive voices. These gadgets certainly reveal something to ghosthunters - but whether it is a ghost or the hunters' own preconceptions is quite another matter.
- © Fairfax NZ News