A tin-foil hat, also tinfoil hat, is a general term for a piece of headgear made from one or more sheets of tin foil, aluminium foil, or other similar material. Some people wear the hats in the belief that they act to shield the brain from such influences as electromagnetic fields, or against mind control and/or mind reading. Such hats are very uncommon in mainstream society, as the injuries they might guard against are highly speculative, and their effectiveness in preventing such harm would be dubious even if the danger were plausible. Instead, the concept has become a popular stereotype and term of derision; in Internet culture, the phrase (sometimes as the abbreviation "TFH") serves as a byword for paranoia.
The delusion of "mind control rays" or other invasive mental activity may seem very real to those afflicted with severe paranoid delusions, and such persons have been known to make and wear improvised defences against the imagined invasion. A placebo effect may even convince the sufferer that the device actually works. While aluminium foil and tin-foil are traditional, less fragile materials such as 3M Velostat (a kind of metallised plastic) and metal window-screen mesh are now more commonly used. Electrical conductivity is seen as a key quality.
There is a small amount of truth or reason to be found in the rationale for a tin-foil hat. A well constructed tin-foil enclosure would approximate a Faraday cage, reducing the amount of (notionally harmless) radiofrequency electromagnetic radiation inside. A common high school physics demonstration involves placing an AM radio on tinfoil, and then covering the radio with a metal bucket. This leads to a noticeable reduction in signal strength. The efficiency of such an enclosure in blocking such radiation depends on the thickness of the tin-foil, as dictated by the skin depth, the distance the radiation can propagate in a particular non-ideal conductor. For half-millimeter-thick tin-foil, radiation above about 20 kHz (i.e., including both AM and FM bands) would be partially blocked.
The effectiveness of the tin-foil hat as an electromagnetic shield for stopping radio waves is greatly reduced by the fact that it is not a complete enclosure. Placing an AM radio under a metal bucket without a conductive layer underneath demonstrates the relative ineffectiveness of such a setup. Indeed, because the effect of an ungrounded Faraday cage is to partially reflect the incident radiation, a radio wave that is incident on the inner surface of the hat (i.e., coming from underneath the hat-wearer) would be reflected and partially 'focused' towards the user's brain. While tin-foil hats may have originated in some understanding of the Faraday cage effect, the use of such a hat to attenuate radio waves belongs properly to the realm of pseudoscience.
A (somewhat tongue-in-cheek) study by graduate students at MIT determined that a tin-foil hat could either amplify or attenuate incoming radiation depending on frequency;http://people.csail.mit.edu/rahimi/helmet/ the effect was observed to be roughly independent of the relative placement of the wearer and radiation source. Note that GHz wavelengths are well below the putative skin depth of even the thinnest foil.
Tin foil hats are seen by some as a protective measure against the effects of EMR, or electro magnetic radiation. At this time, no link has been verifiably proven between EMR exposure and subsequent ill health, however EMR exposure has many alleged effects.http://news.independent.co.uk/environment/article362557.ece
Paranoia | Hats | Mind control | Pseudoscience | Safety clothing
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