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A tin-foil hat, also tinfoil hat, is a piece of headgear that some people wear because they believe it prevents mind control and/or mind reading, and find that it stops certain unpleasant experiences such as voices in their heads and apparent abduction by alien beings. While aluminium foil or tin-foil is 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 seems to be a key quality.
While there have been and still are many people who believe in the actual utility of such devices, the wearing of tin-foil hats has become a popular stereotype and term of derision, particularly in Internet culture. This draws on the stereotypical image of belief in mind control by ESP, microwave radiation or other technological means as part of the symptoms of paranoid schizophrenia. The delusion of "mind control rays" generally seems very real to those thus afflicted, and the making and wearing of improvised defences against the imagined rays is a commonly observed phenomenon in people afflicted by severe paranoid delusions.
There is a small amount of truth or reason to be found in the tin-foil hat story. 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 in 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 belong properly to the realm of pseudoscience.
Tin-foil hats in pop culture
The paranoid centaur Foaly, in Eoin Colfer's Artemis Fowl series of books, wears a tin-foil hat to protect from mind-readers. In an episode of The Simpsons, Bart (while paranoid under the influence of a drug to cure hyperactivity) wears a tin-foil hat.
In the film Lovesick, Dudley Moore plays a psychiatrist who gives a homeless patient some aluminum foil to "protect" the patient from the "mind control rays" his patient claims are bombarding him. In Total Recall, the hero (Douglas Quaid, played by Arnold Schwarzenegger) wraps a wet towel around his head to stop outgoing radiowaves from a transmitter inside his head. A recent appearance of tin-foil hats is the 2002 film Signs, where the children of the lead character wear tin-foil hats to avoid mind control.
On the popular news website slashdot, tin-foil hats are often referred to, especially when anything to do with RFID is announced. See: slashdot subculture.
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