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This is joked about all the time, but...

Can tin foil hats actually block anything? If they can, what frequencies? Is there any research into tin or aluminum foil and radio blocking or amplifying abilities when shaped into a hat?

If they really don't do anything, what would be better? Radio blocking in a hat design that is, not a Faraday cage suit (with matching tie).

Edit: people seem to be missing the tags, so I’ll clarify the questions – What frequencies (mainly on the radio range) can tin foil, when shaped into a hat, block or at least attenuate? If not, are there any frequencies that resonate/amplify within the hat? What would work better for signal blocking in this regard (to the head that is)?

Yes this is a funny subject but let's actually take a look at the physics. There are related questions to the hat, but I believe they fall outside the scope here. If someone wants to ask those on the exchange sites, go ahead.

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    $\begingroup$ Shouldn't this be redirected to Skepticism? $\endgroup$
    – Patrick Roberts
    Commented Sep 23, 2015 at 3:01
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    $\begingroup$ @PatrickRoberts I could've asked it over there, but I'm more intrested in the electromagnetic response than the hat's connection to conspiracy/crazy people in their attempt to block non-existant government mind reading rays. A more apt question for the latter might be how far away brain waves can be detected or something. $\endgroup$
    – Status
    Commented Sep 23, 2015 at 4:07
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    $\begingroup$ I'm already seeing this question generating some joke responses. Not that there's anything wrong with that, but remember that (1) answers should actually answer the question (and then you can supplement that with as many funny links as you want), and (2) comments are mostly meant for suggesting improvements to the post and requesting clarifications. Any idle discussion should go to Physics Chat. $\endgroup$
    – David Z
    Commented Sep 23, 2015 at 6:26
  • $\begingroup$ This seems to be a list-based question. It's also rather unclear what "better" (better at doing what? better in what way?) and "abilities" (psychic abilities? physical abilities?) mean here. $\endgroup$
    – Kyle Kanos
    Commented Sep 23, 2015 at 15:29

4 Answers 4

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A tin foil hat can block:

But, then again...

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    $\begingroup$ I would like to add x-rays. Depending on the thickness. And the energy. With 2mm of aluminium you should be able to block over 90% of X-rays below 30kV. Not sure, if 2 mm still can be considered foil. $\endgroup$
    – Dschoni
    Commented Sep 23, 2015 at 9:42
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    $\begingroup$ @Dschoni X-rays are electromagnetic waves, so it's already included. 2mm is not foil. $\endgroup$
    – Taemyr
    Commented Sep 23, 2015 at 11:04
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    $\begingroup$ 2mm would be "sheet metal", not foil. This lists 0.2mm but I also found references to 0.25mm for sale. $\endgroup$
    – JDługosz
    Commented Sep 23, 2015 at 16:46
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    $\begingroup$ This now has 3x the votes of my next highest rated answer, which was itself a bit vacuous. My careful, thoughtful answers tend to get low scores. Oh, well... $\endgroup$
    – Daniel Griscom
    Commented Sep 24, 2015 at 13:32
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    $\begingroup$ @DanielGriscom Hot network questions screw up voting. $\endgroup$
    – CodesInChaos
    Commented Sep 24, 2015 at 14:45
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Can tin foil hats actually block anything?

Anything? Sure. As already noted by Daniel Griscom's answer, tin foil can block several "things" including rain, alpha rays, and electromagnetic radiation with small enough wave length that the radiation cannot diffract around the edges of the hat.

If they can, what frequencies?

Since you mention frequency let's really focus on electromagnetic radiation. Foil, being a metal, is very good at reflecting electromagnetic radiation. You can see this quite easily by looking at a sheet of aluminum/tin foil: it is shiny and you may even see a reflection of yourself. Physically this happens because electrons can move easily within the foil. Electromagnetic radiation impinging on the foil causes these electrons to move, and that motion produces new electromagnetic radiation which generates new radiation which destructively interferes with the original incoming radiation. This leads to cancellation of the incoming wave so that it does not penetrate the foil, and creation of a new outgoing wave which is the reflection you see.

Now this isn't the whole story. Radiation with a wavelength at about the same size as the hat actually winds up resonating inside the hat. In an effect similar to a vibration of a violin/guitar string, radiation that fits nicely inside the metal cavity formed by the hat actually winds up creating electromagnetic fields which are bigger than the incoming field. You can get a rough idea of how this works on the Wikipedia page for resonance. It's the same effect as when you blow gently over a bottle and hear a loud hum sound.

Is there any research into tin or aluminum foil and radio blocking or amplifying abilities when shaped into a hat?

Yes, there is a sort of famous study from MIT where a student looked into this. They found the resonance effect mentioned above actually made the electromagnetic field strength inside that hats larger:

"For all helmets, we noticed a 30 db amplification at 2.6 Ghz and a 20 db amplification at 1.2 Ghz, regardless of the position of the antenna on the cranium. In addition, all helmets exhibited a marked 20 db attenuation at around 1.5 Ghz, with no significant attenuation beyond 10 db anywhere else."

Note that resonance is not the same thing as amplification. Amplification means an increase in power; you can only get that if you have a power source, as in a radio receiver which amplifies the signal coming in over the air waves. Resonance is an effect wherein (at least in this case) the incoming flow of energy winds up being sort of piled up into the hat so that the fields in the hat are larger than the fields coming in. It doesn't add new energy to the system, it just concentrates it in the hat.

If they really don't do anything, what would be better? Radio blocking in a hat design that is, not a Faraday cage suit (with matching tie).

Radio waves have pretty big wave length. For example, a 100 MHz wave has a length of 3 meters. If you want to block that out you need something like a Faraday cage with holes a substantial fraction smaller than that wave length.

P.S. The conclusions in that MIT study are hilarious and I recommend reading it.

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  • $\begingroup$ I have neighbours who lined their roofs with tinfoil for the same reason. Does it then resonate inside the bedrooms? I am thinking of rooms in the roof, with sloping ceilings. You mean they actually made the (whatever it was they were afraid of) worse? $\endgroup$
    – RedSonja
    Commented Sep 23, 2015 at 7:52
  • $\begingroup$ @RedSonja Yes, it's possible that putting foil on the roof has increased the electromagnetic field strength in some places. I'm basing that on general principle, not any specific data. $\endgroup$
    – DanielSank
    Commented Sep 23, 2015 at 8:54
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    $\begingroup$ Re lining roofs with tin? (Or aluminum?) foil: ask about their phone reception. Tin roofs are normally not "foil" but substantially thicker, so I'm not sure what you are referring to. I have aluminum paint on the inside of my roof. It's useful for its IR emission properties, not to block anything. $\endgroup$
    – JDługosz
    Commented Sep 23, 2015 at 16:56
  • $\begingroup$ Also, while increasing the frequency of em radiation makes it less susceptible to diffraction, the increased energy increases its ability to penetrate the foil. $\endgroup$
    – WhatRoughBeast
    Commented Sep 23, 2015 at 19:16
  • $\begingroup$ @WhatRoughBeast Doesn't that only really matter once you get up past x-rays? Certainly in the radio bands the penetration of fields within the metal is exponentially suppressed, so even if the skin depth gets a little bigger you still won't get through much. $\endgroup$
    – DanielSank
    Commented Sep 23, 2015 at 22:14
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I'd like to point out that blocking UV radiation is useful, as I for one have had sunburn on my head. Foil could be used as an improvised "beenie" hat if stuck out in the open, or as a black-out lining of a hat that is more decorative than opaque (e.g. knit fabrics).

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    $\begingroup$ Arguably, not just could tin foil be used, but might actually make a better hat to shield you from the sun, as it would tend to reflect heat away, whereas a traditional hat will tend to absorb heat from the sun. Since hats are often worn to shield you from the sun in hot weather, this property would be desirable. $\endgroup$
    – Hobadee
    Commented Feb 13 at 1:36
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Whereas a straight unfolded, uncrumbled piece of tin foil can block out some of the lower wave lengths permeating throughout the atmosphere, a folded or crumpled tin foil hat will not block out anything.

The reason is that tin (aluminium) foil is actually made up of two thin layers. There is a clear sheet in between the layers to act as a base in the manufacturing process. If folded or crumpled the aluminium (tin) foil breaks down and creates visible patches throughout the piece.

Try crumpling and then restoring a piece of foil and then hold it up to your face to see the breakdown spots emerge.

In this condition any wavelength would be able to penetrate the layers of foil making the idea of protection moot.

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  • $\begingroup$ Are you sure? I know the last rolling pass is done on two sheets (so each is half the thickness of the minimum that can be rolled) but they become two sheets. What kind of transparent base can withstand pizza-oven temperature, and would bind to the foil? $\endgroup$
    – JDługosz
    Commented Sep 23, 2015 at 16:36
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    $\begingroup$ I think you are confusing aluminum foil with this. The question refers to this. $\endgroup$
    – JDługosz
    Commented Sep 23, 2015 at 16:48
  • $\begingroup$ "If folded or crumpled the aluminium (tin) foil breaks down and creates visible patches throughout the piece" You should probably stick with theory based on the scope of the question, rather than practical. In that, let's say the foil (whatever type it is) is not crumpled, has no structural breakdown, (etc) :) Although it would be a valid point to note that different materials might make a difference. $\endgroup$
    – James
    Commented Sep 23, 2015 at 17:44
  • $\begingroup$ I think it's valid to bring up the construction methods as well. $\endgroup$
    – Nanban Jim
    Commented Sep 24, 2015 at 20:55
  • $\begingroup$ I demonstrate how tinfoil blocks radio and microwave frequencies on a regular basis using folded and crumpled examples. $\endgroup$
    – ProfRob
    Commented Feb 24 at 9:07

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