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If you were invisible, would you also be cold? (Since light passes through you, so should thermal radiation.)

Additionally, I'd like to know if you were wearing invisible clothes, would they keep you warm? In my understanding, the heat radiation from the body would pass through the cloth.

Is it even necessary to be permeable for heat radiation in order to be invisible? Could there be a form of invisibility (hypothetically speaking, of course) that makes you permeable for light in the visible spectrum, but not for heat radiation? Can those two things be separated?

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    $\begingroup$ unrelated, but being invisible gives other problems as well. as you would be blind because light never touches your eyes. and lack of sunlight on your skin also is not healthy $\endgroup$ Dec 8 '21 at 7:31
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    $\begingroup$ Since you said that thermal radiation should pass through you, I'm assuming you're talking about invisibility in that spectrum as well. In that case, if your body is radiating heat, you're not invisible. $\endgroup$
    – Sandejo
    Dec 8 '21 at 23:27
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    $\begingroup$ If you were invisible in the infrared range then you wouldn't be emitting any IR radiation either, which might cause the opposite problem, namely overheating. (Though as most of the answers say, convection and conduction will also be in operation, presumably.) $\endgroup$
    – N. Virgo
    Dec 9 '21 at 6:25
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    $\begingroup$ "If you were invisible, would you also be alive?" $\endgroup$ Dec 9 '21 at 9:12
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    $\begingroup$ Air is invisible (right?) but sometimes hot. $\endgroup$
    – Pablo H
    Dec 9 '21 at 17:37
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Not necessarily. Glass is transparent and it can be heated with convection, conduction and some radiation. It will maintain any temperature around it.

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    $\begingroup$ If "invisibility" is just like glass, then the invisible man would still be detectable via IR (or other bands of EM radiation). $\endgroup$
    – Dave
    Dec 8 '21 at 16:29
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    $\begingroup$ @Dave I’m sure there are issues for what really qualifies for being invisible but even air which is very transparent can be heated or cooled. $\endgroup$ Dec 8 '21 at 16:32
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    $\begingroup$ @Dave : If the asymmetry of not-receiving IR radiation from the environment is relevant, then the invisible person is visible in the IR already. The question assumes this relevance. $\endgroup$ Dec 8 '21 at 21:25
  • $\begingroup$ Glass might be slightly colder than its immediate surroundings if everything around it is only maintaining its temperature due to incident visible light. This is why pavement can be hotter than the air temperature. The pavement is absorbing the incident light and heating it while the air above it allows the light to mostly pass through. A bright, sunny day would feel less warm to an invisible person than to a visible person. $\endgroup$ Dec 8 '21 at 21:47
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    $\begingroup$ Air. Don't forget air. $\endgroup$
    – JimmyJames
    Dec 9 '21 at 16:22
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Although the premise of this question is, well, questionable, I’ll answer because invisibility cloaks do exist. Of course, as of now, they work imperfectly, in impractical ways, and for only a narrow spectrum of light. And this last part is the key. Just because something bends light of one color does not mean it will bend light of another. When one colloquially discusses invisibility, visible light is implied; however, thermal radiation is primarily infrared. So if you’re standing at the center of an invisibility cloak, it will probably still radiate warmth.

And, of course, radiation is not the only way to transfer heat. There’s also conduction and convection, so let’s not forget those.

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    $\begingroup$ You have given me just the right keywords so I can keep looking! Sorry that my question seemed a little mal-worded, I'm not a physicist and I contemplated this matter for a piece of fiction writing. I will look into conduction and convection, and thanks for pointing out that readiation is primarily infrared. $\endgroup$
    – Alon
    Dec 7 '21 at 22:10
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    $\begingroup$ @Alon for more speculative discussion of science fiction, with an adjustable level of “real” physics, I recommend the Worldbuilding stack exchange. $\endgroup$
    – Gilbert
    Dec 8 '21 at 0:15
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    $\begingroup$ Thanks, I will have a look! $\endgroup$
    – Alon
    Dec 8 '21 at 0:42
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    $\begingroup$ "Of course, as of now, they work imperfectly" - I would argue that they work imperfectly because of fundamental physical laws. AFAIK there is not much development in the field in last, say, 10 years. $\endgroup$
    – dominecf
    Dec 9 '21 at 11:30
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    $\begingroup$ @dominecf Agreed. These cloaks will never work like they do in the movies. Although, generally their performance would improve with better fabrication capabilities leading to better control of the spatially dependent refractive index. $\endgroup$
    – Gilbert
    Dec 9 '21 at 12:13
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Two key points to remember:

  1. Radiation is not the only form of heat transfer. There's also conduction and convection.
  2. Being invisible doesn't only mean that you shouldn't absorb any light. It also means you shouldn't emit any.

From the perspective of heat transfer, making yourself invisible would be similar to wearing a skintight body suit made of reflective mylar (like emergency blankets).

You could definitely be kept warm by invisible clothes. Think about how the inside of a greenhouse stays warm.

The entire EM spectrum carries heat, but if by "heat radiation" you just mean infrared, then yes, you can absorb or reflect it while letting visible light pass.

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  • $\begingroup$ "Think about how the inside of a greenhouse stays warm." My very basic understanding of greenhouses is that they absorb more radiation than they let out. In other words, radiation gets "trapped" inside the greenhouse. If someone's magic invisibility made them completely transparent to radiation, then they wouldn't "trap" incoming radiation like a greenhouse. $\endgroup$
    – Stef
    Dec 9 '21 at 20:49
  • $\begingroup$ @Stef I think greenhouses would still work almost as well if they were completely transparent to all radiation. I think it's much more important that they limit conduction and convection. $\endgroup$ Dec 9 '21 at 22:39
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Thermal balance
Thermal balance of a human body has been discussed, e.g., in this question: Stefan-Boltzmann law applied to the human body. There are a few points to note (some already brought up in other answers):

  • Radiation is not the only mechanisms of heat loss (we need also to account for heat conduction and convection)
  • Clothing plays important role in mitigating heat losses (or preventing overheating).
  • The invisibility cloak is not necessarily equally transparent in both directions - i.e., it may contain the radiation emitted by the body, while not allowing the external radiation inside.
  • Invisibility may apply to different spectral ranges: optical invisibility (like in fairy tales), invisibility to radars (like B2 bombers, which are well visible to human eye), invisibility to infrared cameras, etc.

Thermal balance without external radiation
For the ambient temperatures below the edge of the thermal neutral zone (see the discussion in the thread linked above) the heat balance depends on absorption of the ultraviolet radiation. If the invisibility cloak does not allow penetration of this radiation, one would indeed feel uncomfortably cold. However, as mentioned above, invisibility mainly implies the lack of visibility in radio-infrared-visible part of the spectrum, i.e., any realistic invisibility cloak is likely to allow the ultraviolet radiation.

On the other hand, in conditions above the upper critical temperature of the thermal neutrality zone, one would rather be shielded from the external radiation, in order to avoid overheating, i.e., the discussion in the previous paragraph needs to be reversed mutatis mutandis.

Finally, these effects are minimized when one is shielded from the sunlight or during the night time.

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Update
I think that proper discussion of this question requires two key definitions:

  • what is to be invisible?
  • what is to feel cold?

Feeling cold
Feeling cold refers to the temperature balance, which is discussed at length in the post linked above and the links therein. It means balance between the heat obtained from the internal metabolism (digesting food) and radiation absorbed vs. the heat lost via radiation, convection and heat conduction. This is the theme developed in the maina nswer.

Invisibility
Invisible means not being seen by eyes or a detector. It is therefore not absolute - it depends on the range of the electromagnetic field detected and on the method of detection. Onecan distinguish at least three situations:

Infrared: Radiation emitted by the body
Infrared cameras typically detect teh radiation emitted by the body, which is hotter than the environment. Being invisible to infrared cameras means reducing the emission of the radiation this can be achieved by retaining heat inside, or reemitting it in different frequency range, or making sure that it is quickly dissipated (making the body blurred rather than invisible).

Optical: radiation scattered by the body
In conventional sense invisibility means not being seen by eyes or photo camera. In this case the light is the scattered one - e.g., the light of the Sun. The invisibility can then be achieved by making the object transparent, or mimicing the transparency via ingenious waveguiding or holographic systems. Again, we can talk about making the object blurred rather than truly invisible by fiddling with how the light is scattered.

Radio: active location
Another frequent use of invisibility is in the context of stealth airplanes. In this case the method of detecting is mainly the radar, which sends radio waves that are reflected by the airplane. Being invisible to the radar then means reducing the intensity of waves reflected in the direction of the radar or simply absorbing this radiation - in optical sense the object then appears black rather than invisible.

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Also, don't forget that not all heat comes to our bodies from outside. We convert a fair amount of sugar into heat.

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    $\begingroup$ This DOES answer the question, though perhaps it needs to be expanded somewhat. Unless you're sitting in the sun, or in front of a good file, radiation is only a minor source of heat gain or loss. $\endgroup$
    – jamesqf
    Dec 8 '21 at 18:17
  • $\begingroup$ a fair amount is putting it lightly, +1. Anybody got a percentage for that? Unless you live on the equator (even then) I'd presume it to be most. When I'm cold it's time to make some food and quit just drinking coffee all day. $\endgroup$
    – Mazura
    Dec 8 '21 at 20:55
  • $\begingroup$ Yep, the body is heated internally using the chemical energy in our food intake. Radiation actually has a negative contribution -- we emit more heat than we absorb. $\endgroup$
    – g.kertesz
    Dec 9 '21 at 10:34
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As most of us know from experience, radiation is not a major contributor to the body's heat gain or loss. Unless you're sitting in the sun, or in front of a good fire, radiative heat transfer is almost undetectable by the senses. Convection and conduction are the major methods of heat transfer.

You also need to remember that the body is a net heat producer. The metabolism of food produces heat. If we exercise, or shiver, we produce extra heat. We wear clothing, and/or perspire, to regulate body temperature. Clothing is almost entirely* concerned with regulating convection & conduction.

Perhaps a couple of real-world examples would help in understanding.

First, when I cross-country ski (an exercise that burns a lot of calories if done right) on a calm day, I can work up a sweat while wearing nothing but a light shirt and sweat pants. Stop for a while, and I'll need to haul out the down vest to keep warm. So radiation is not a factor here.

Second, when I go to the beach, I wear very little clothing, yet I don't become cold because my body heat is radiating away. If I go in the water, I may get cold because the water greatly increases convective heat loss.

So the bottom line is that if your invisible clothing was sufficient to keep you warm before it (and you) became invisible, it will still keep you warm when you are invisible.

*There's also perspiration,which I won't get into beyond saying that's why experienced people dress in layers when exercising outdoors in the winter.

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    $\begingroup$ Citation needed. Our experiences are often misleading; I believe radiation is responsible for about half our body's heat transfer. The reason that it's not very noticeable most of the time is because our environment tends to be a fairly uniform temperature and radiates back almost as much as we radiate away. It is however noticeable at night with clear skies - it can feel much colder than a cloudy day with the same air temps. $\endgroup$
    – Gene
    Dec 8 '21 at 20:47
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    $\begingroup$ The first paragraph is incorrect. The body emits about 1000:W of radiation and receives about 900 W. $\endgroup$
    – my2cts
    Dec 8 '21 at 22:48
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Thermoregulation

There are four avenues of heat loss: evaporation, convection, conduction, and radiation. If skin temperature is greater than that of the surrounding air temperature, the body can lose heat by convection and conduction. But, if air temperature of the surroundings is greater than that of the skin, the body gains heat by convection and conduction. In such conditions, the only means by which the body can rid itself of heat is by evaporation.

So, when the surrounding temperature is higher than the skin temperature, anything that prevents adequate evaporation will cause the internal body temperature to rise.[24] During intense physical activity (e.g. sports), evaporation becomes the main avenue of heat loss.[25] Humidity affects thermoregulation by limiting sweat evaporation and thus heat loss.[26]

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Warm dark matter is not what you may think, but rather a hypothetical component of dark matter, which may consist of, e.g., sterile neutrinos.

The point here is that temperature is essentially the average kinetic energy (at least for a gas), and it does not fundamentally matter whether the particles interacat with the electromagnetic field. Hence, a gas of neutrinos has a well-defined temperature, despite being invisible (to all wavelengths).

Note that while neutrinos and similar particles interact very little and so would probably not form molecules or complex structures, there not fundamental obstacle to imagine another sector of matter which has significant interactions in itself, but is decoupled from standard model matter.

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if you were wearing invisible clothes, would they keep you warm? In my understanding, the heat radiation from the body would pass through the cloth.

Clothes don't just block the radiated heat, they also stop conduction and convection with the ambient air. I haven't done the calculations but I suspect invisible clothes would only be marginally less warm.

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  • $\begingroup$ "I'm not nude in public, officer! I'm wearing invisible clothing!" $\endgroup$ Dec 9 '21 at 23:46

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