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I've been thinking about infrared radiation and noticing more and more how the human skin seems actually pretty sensitive to it.

You can easily feel a bonfire from several meters away, far away from where any convection would heat your skin.

When you open the hood of your car you can feel the heat from the engine even standing back a step or two (away from the updraft of hot air).

Now try this: hold the palms of your hands against eachother a couple of inches apart and keep them like that for a couple of seconds. Then slowly (to avoid wind cooling) lift the other palm so they no longer face eachother. Do you feel it? For me there's a noticeable difference in warmth.

Is that the skin detecting black body radiation from other skin? This could be easily blind-tested with a friend; you hold your palm out and look the other way, then see if you can correctly tell when your friend's palm is near you palm and when its not. Maybe the human skin is even able to detect black body radiation from another human standing behind her? Kind of like a sixth sense. Could explain the sensation of "i knew someone was there".

I've noticed also that when you stand close to a concrete wall that was heated by the sun, but the sun has just set, you can tell which direction the wall is just from the heat on your body.

Is this all placebo or does it actually work that way?

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    $\begingroup$ You left out a key word in your question "...another human standing even a few behind her?" A few what? Millimeters? Certainly. Centimeters? Sure. Meters? Probably not. $\endgroup$ – M. Enns Jul 21 '16 at 23:53
  • $\begingroup$ Sorry yeah i mistyped. Did you try the palm thing? This kind of blows my mind. I think it could be possible to detect a human body behind you at 1 meter. I should blind test this some day. What do you think? $\endgroup$ – Paul Jul 21 '16 at 23:57
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    $\begingroup$ Sure - try it out. $\endgroup$ – M. Enns Jul 21 '16 at 23:58
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    $\begingroup$ It depends strongly on ambient temperature. If it's reasonably cold then yes, of course you can. At 30°C? Probably not. $\endgroup$ – tfb Jul 21 '16 at 23:59
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    $\begingroup$ "Thermal" is synonymous with "blackbody" since anything other than blackbody won't have a unique temperature. In any event a person is close enough to a blackbody in the IR. $\endgroup$ – user10851 Jul 22 '16 at 3:49
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In principle, yes. In practice, it's going to depend on the sensitivity of the neurons involved.

The reason it could be felt is because what you feel is not temperature but heat flow (or changes in temperature). At least, that's one way to explain the classic experiment where a person lets one hand sit in hot water for a while, the other in cold, and then moves both hands to a lukewarm water bath. The hand that was in hot will feel cold, and vice versa. When two people stand close together it slows down the rate at which both are losing heat, so it can be felt, in principle. In practice, it's going to depend on all sorts of factors: how cold is the air in the middle, is the wind blowing, how close are they, etc?

Any experiment to measure this would need to be properly blinded with the person doing the sensing properly shielded from detecting the other person by other means (for example: hearing them, seeing their shadow, smelling them).

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The simple answer to your question is "yes". The heat receptors on the surface of your skin can feel the infrared radiation, and there are enough of them on your body to make it a pretty good directional heat detector.

An aside, but related: I used to wonder in high school why I could feel this one girl staring at me from clear across the cafeteria. It was creepy at the time, and it still is - how you can feel someone in a crowd staring at you. I finally wrote a blog entry to try to explain the phenomenon. It's way too long. I have been told that it's simultaneously boring and entertaining, and I'm sure there are holes in the science. But it may give OP a better answer to his question. Here's the link: "I Can Feel Her Staring at Me!"

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    $\begingroup$ Your "aside" is not related at all. $\endgroup$ – pela Sep 1 '16 at 20:05
  • $\begingroup$ Very interesting explanation about the staring girl! I have to say that debating how directional the skin's receptiveness to infrared radiation really is would be interesting. Woudln't there be some kind of pinhole or lens or something to achive such "directionality"? Because isn't the infrared picture that the skin "sees" very unfocused otherwise? It's hard to detect small details with unfocused vision. $\endgroup$ – Paul Sep 2 '16 at 14:09
  • $\begingroup$ I had to think about the directionality thing for a bit. It seems to me that the best directional detectors have the sensor array on a concave surface. The body, instead, has the sensor array (nerve endings) on a convex surface. In both detectors, the radiation detected by a sensor at an oblique angle to the source will be less than the radiation detected by a sensor pointed directly at the source. All of these sensory inputs get fed into the brain, which determines the direction of the source. I'm sure proximity also plays a part in the OP's example, but not in the staring scenario. $\endgroup$ – Ray Depew Sep 2 '16 at 20:59

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