Tin foil hat

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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 home made headgear for protection has become a popular stereotype and byword for paranoia, persecutory delusions, and belief in conspiracy theories. This derision is popular in the mass media, used both to poke fun at genuine Mad hatters and undermine the credibility of skeptics who have offered explanations for contentious events such as assassinations of public officials, acts of terrorism, and political scandals. Tin foil hats have appeared in movies such as Signs and Futurama: Into the Wild Green Yonder.


The concept of a tin 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.[1]

Over time the term has been associated with paranoia and conspiracy theories.[2] They are worn by believers to prevent mind control by governments, spies, or paranormal beings, employing ESP or technologies such as microwave radiation.

Scientific basis[edit]

Electromagnetic radiation[edit]

The notion that a tin 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.[3] A well-constructed tin 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 tin 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 tin foil, as dictated by the skin depth, the distance the radiation can propagate in a particular non-ideal conductor. For half-millimetre-thick tin foil, radiation above about 20 kHz (i.e., including both AM and FM bands) would be partially blocked, although tin foil is not sold in this thickness, so numerous layers of tin foil would be required to achieve this effect.[4]

The effectiveness of the tin 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 a tin foil hat could attenuate incoming radiation depending on frequency. At WIFI frequencies - 2.4 GHz is attenuated by up to 90 dB; the effect was observed to be roughly independent of the relative placement of the wearer and radiation source.[5] At some microwave wavelengths, the skin depth is less than the thickness of even the thinnest foil.[6]

Tin foil hats are seen by some[who?] as a protective measure against the effects of electromagnetic radiation (EMR). Despite some allegations that EMR exposure has negative health consequences,[7] at this time no link has been established between the radio-frequency EMR tin foil hats are meant to protect against and subsequent ill health.[8]

Electromagnetic hearing[edit]

Humans are able to detect modulated radio-frequency electromagnetic signals in the microwave range, hearing them as sounds. The perceived source of induced sound is located inside of or directly behind the head of the recipient, regardless of the location of the transmitter. The effect is believed to be caused by interaction of the cochlea with microwaves.[9]

During the Cold War, electromagnetic hearing was clinically studied in the United States for applications including covert message transmission and use as a non-lethal weapon. As a declassified National Ground Intelligence Center document points out:

It may be useful to provide a disruptive condition to a person not aware of the technology. Not only might it be disruptive to the sense of hearing, it could be psychologically devastating if one suddenly heard "voices within one's head".[10]

In 1962, Allan H. Frey discovered that reception of the induced sound can be blocked by a patch of wire mesh (rather than foil) placed above the temporal lobe.[9]

See also[edit]


  1. ^ 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." 
  2. ^ "Hey Crazy--Get a New Hat". Bostonist. 15 November 2005. Retrieved 2007-04-05. 
  3. ^ Adey, W. R. (December, 1979). Neurophysiologic effects of Radiofrequency and Microwave Radiation, Bulletin of New York Academy of Medicine, vol. 55, no. 11,. pp. 1079–1093. 
  4. ^ Jackson, John David (1998). Classical Electrodynamics. Wiley Press. ISBN 0-471-30932-X. 
  5. ^ Rahimi, Ali; Ben Recht, Jason Taylor, Noah Vawter (17 February 2005). "On the Effectiveness of Aluminium Foil Helmets: An Empirical Study". Ali Rahimi. 
  6. ^ http://www.microwaves101.com/encyclopedia/skindepth.cfm
  7. ^ Lean, Geoffrey (2006-05-07). "Electronic smog - Environment - The Independent". London: Independent.co.uk. Retrieved 2009-06-09. 
  8. ^ "Safety and Health Topics: Radiofrequency and Microwave Radiation - Health Effects". Osha.gov. Retrieved 2009-06-09. [dead link]
  9. ^ a b 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. 
  10. ^ "Bioeffects of Selected Nonlethal Weapons". Nonlethal Technologies – Worldwide. National Ground Intelligence Center. 1998. 

External links[edit]