Tin foil hat

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Man wearing a tin foil hat

A tin foil hat is a hat made from one or more sheets of aluminium foil, or a piece of conventional headgear lined with foil, worn in the belief it shields the brain from threats such as electromagnetic fields, mind control, and mind reading. The notion of wearing homemade headgear for such protection has become a popular stereotype and byword for paranoia, persecutory delusions, and belief in pseudo science and conspiracy theories. Foil hats have appeared in the films Signs and Futurama: Into the Wild Green Yonder. Note that "tin foil" is a common misnomer for aluminium foil; packaging metal foil was formerly made out of tin before it was replaced with aluminium.[1]


The concept of a foil hat for protection against interference of the mind was mentioned in a science fiction short story by Julian Huxley, "The Tissue-Culture King", first published in 1927, in which the protagonist discovers that "caps of metal foil" can block the effects of telepathy.[2]

The use of foil headgear is often based on a belief that such hats prevent mind control by governments, spies, or paranormal beings that employ ESP or the microwave auditory effect. People in many countries who believe they are "Targeted individuals" (TIs), subject to government spying or harassment, have developed websites, conference calls, and support meetings to discuss their concerns; a favorite topic is headgear and shields made of foil or other metal.[3] Over time the term "tin foil hat" has become associated with paranoia and conspiracy theories.[4]

Scientific basis[edit]

Electromagnetic radiation[edit]

The notion that a metal foil hat can significantly reduce the intensity of incident radio frequency radiation on the wearer's brain has some scientific validity, as the effect of strong radio waves has been documented for quite some time.[5] A well-constructed aluminum foil enclosure would approximate a Faraday cage, reducing the amount of (typically harmless) radiofrequency electromagnetic radiation passing through to the interior of the structure. A common high school physics demonstration involves placing an AM radio on aluminum foil, 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 foil, as dictated by the skin depth, the distance the radiation can propagate in a particular non-ideal conductor. For half-millimetre-thick aluminum foil, radiation above about 20 kHz (i.e., including both AM and FM bands) would be partially blocked, although aluminum foil is not sold in this thickness, so numerous layers of foil would be required to achieve this effect.[6]

The effectiveness of the aluminum foil hat as electromagnetic shielding for stopping radio waves is greatly reduced by it not being 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. However the hat may be partially grounded by the conductive properties of the skin with which it contacts.

A study by graduate students at MIT determined that an aluminum foil hat could amplify or attenuate incoming radiation depending on frequency. At cellular phone frequencies – 2.6 GHz is amplified by 30 dB. At 1.6 GHz, a 30 dB amplification was observed. However, at 1.5 GHz, a 20 dB attenuation was observed, with no significant attenuation beyond 10 dB anywhere else.[7] At some microwave wavelengths, the skin depth is less than the thickness of even the thinnest foil.[8]

A belief also exists that aluminum foil is a protective measure against the effects of electromagnetic radiation (EMR). Despite some allegations that EMR exposure has negative health consequences,[9] no link has been established between the radio-frequency EMR that foil hats are meant to protect against, and subsequent ill health.

In 1962, Allan H. Frey discovered that reception of the induced sounds by radio-frequency electromagnetic signals heard as clicks and buzzes can be blocked by a patch of wire mesh (rather than foil) placed above the temporal lobe.[10]

See also[edit]


  1. ^ Berger, Kenneth R. (December 2002). "A Brief History of Packaging". University of Florida. Retrieved 29 June 2015. 
  2. ^ Huxley, Julian (1927). The Tissue-Culture King. Well, we had discovered that metal was relatively impervious to the telepathic effect, and had prepared for ourselves a sort of tin pulpit, behind which we could stand while conducting experiments. This, combined with caps of metal foil, enormously reduced the effects on ourselves. 
  3. ^ Weinberger, Sharon (January 14, 2007). "Mind Games". Washington Post. Retrieved 29 June 2015. 
  4. ^ "Hey Crazy--Get a New Hat". Bostonist. 15 November 2005. Retrieved 2007-04-05. 
  5. ^ Adey, W. R. (December 1979). "Neurophysiologic effects of Radiofrequency and Microwave Radiation". Bulletin of New York Academy of Medicine 55 (11): 1079–1093. 
  6. ^ Jackson, John David (1998). Classical Electrodynamics. Wiley Press. ISBN 0-471-30932-X. 
  7. ^ Rahimi, Ali; Ben Recht; Jason Taylor; Noah Vawter (17 February 2005). "On the Effectiveness of Aluminium Foil Helmets: An Empirical Study". Ali Rahimi. Archived from the original on 2010-07-08. 
  8. ^ Skin Depth – Microwave Encyclopedia – Microwaves101.com
  9. ^ Lean, Geoffrey (2006-05-07). "Electronic smog". The Independent (London). Retrieved 2009-06-09. 
  10. ^ Elder, Joe A.; Chou, C.K. (2003). "Auditory response to pulsed radiofrequency energy". Bioelectromagnetics (Wiley-Liss) 24 (S6): S162–73. doi:10.1002/bem.10163. ISSN 0197-8462. PMID 14628312.