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Here's a thought experiment about the way that heat is transferred through radiation.

Humans can physically feel when a hot object radiates heat on them, such as a campfire or an infrared-based space heater. But can humans feel cold objects the same way?

Say that a scientist is working in a research station in the South Pole in winter, when the outside temperatures reach -80 celsius. (Feel free to also imagine it as 0.1K for the sake of the experiment.) Say that the research station is a big building with multiple floors and rooms within each floor. It has a strong environmental control system that keeps the air at a pleasnt 25 degrees all over the building. However, the outside walls of the building are still very cold, say -40 celsius. The internal walls are at room temperature.

Say that our scientist is walking from an internal room in the building to a room that has one or several external walls. Say that his body is surrounded by an equal 50-50 mix of external walls and internal walls. The air temperature is still 25 degrees. Will our scientist feel the coldness of the wall? Will the wall feel like it's radiating cold on the person?

Follow up question: Assuming the answers to the previous questions are yes. Say that we place a person in a closed room, where the walls on all sides, as well as the floor and ceiling are at a temperature of 0K. The air in the room is kept at 25 degrees celsius. The person is not touching the walls or the floor (say they have incredibly insulating shoes). Will that person freeze to death in minutes as they would in space?

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Yes they will feel the coldness, but it's the lack of heat radiation, not radiating coldness. Essentially, the scientist's body is radiating more heat than it is receiving, and it is this difference that they are feeling.

If the scientist doesn't know how heat and infrared radiation work, they may be hard pressed to tell that it's lack of heat radiation and not cold radiation just from what they can feel.

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  • $\begingroup$ But will this actually be felt? It seems the power of the IR emissions from body-temperatured object is so low that human sensors will be unable to notice the unbalance between body radiation and irradiation by the cold object. $\endgroup$
    – Ruslan
    Feb 26 at 5:51
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    $\begingroup$ @Ruslan Most definitely yes. On a clear, windless night it is easily felt and measured with an IR thermometer. The atmosphere radiates with effective temperatures of about -40 °C, even if the temperature of the air and the ground is about 0 °C. The top of your hand will feel much colder than the bottom which is irradiated by the ground, similar to (but weaker of course) standing close to a warm fireplace. $\endgroup$ Feb 26 at 11:32
  • $\begingroup$ @Martin'Kvík'Baláž What an IR thermometer measures is irrelevant when talking about sensitivity of human skin. Near the ground there's the air that can conduct the heat, especially from bottom to top due to the convection, so what you described doesn't really prove that at these temperatures the radiation is powerful enough to be sensed. $\endgroup$
    – Ruslan
    Feb 26 at 16:23
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While I have not done the numbers for your very particular follow up question (nevermind that it's impossible to reach 0K), I have every reason to believe the person will survive. Consider the night sky. It can be as cold as -40C. The difference in radiation between -40C and -273C is pretty minimal, so that's basically the same situation for a 37C human. We regularly survive with air temperatures far worse than 25C, so we have every reason to believe a human would survive in the situation you lay out.

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Regarding feeling the coldness: Sure, humans feel when their skin emits more IR radiation that is received. You can test this easily by opening a freezer and standing some ways away from it (avoiding the cold air itself).

For the follow up question, we can calculate it:

Assuming the person is naked, their outer surface is about 33°C = 306 K. If they are standing up, surface area is roughly 1.8 m². The total emitted black body radiation can be calculated as:

$$P_\text{net} = A \sigma \varepsilon \left( T^4 - T_0^4 \right)$$

Where $\sigma$ is the Stefan–Boltzmann constant and $\varepsilon$ is the emissivity, which is close to 1. With $T$ = 306K and $T_0$ = 0K, this gives 895 W. The human body internally produces about 200 W of heat in mild exercise or when shivering. This gives net transfer of 700 W.

The cooling effect of the emitted radiation is balanced by the heating effect of the air once skin temperature drops below 25°C. Based on a rough value of convection in otherwise still air of 20 W/m²K, the balance point is skin temperature of 12°C. This is too low for long term survival, so we can be sure that eventually the person will die. In the initial state the conduction to air would further cool the skin at 288 W, but skin temperature will quickly drop to near the air temperature.

Heat transfer by skin temperature graph

How fast will this happen? Based on thermal capacity of water and a body weight of 80 kg, with 700 W net heat emission, body temperature would drop by 8°C every hour. Hypothermia would begin in about 15 minutes and loss of consciousness in an hour. Human body's mechanisms to reduce blood flow to extremities will give a little more time by reducing internal heat conduction.

Compared to vacuum, cooling down would be slower because of less evaporation, but otherwise the situation is similar. Of course in vacuum you would suffocate long before freezing. Even a thin layer of clothing would greatly prolong the survival, and thick enough clothing would prevent freezing altogether.

Complicating factors:

  • Crouching down to a ball shape would reduce surface area by about half.
  • It is not realistic to keep air at 25°C close to the 0 K walls without significant air flow. Air flow would increase initial cooling, but bring in heat once skin temperature drops below 25°C.
  • Heavy exercise would temporarily increase body heat production and also air flow.
  • Emissivity of the walls matters too. If the walls were made of uncoated metal, they would reflect back some of the IR radiation.
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Will the wall feel like it's radiating cold on the person?

Nothing "radiates cold". Everything radiates some amount of infra-red radiation and hotter things radiate more IR.

If you are surrounded by a mixture of warm and cold walls with the ambient (air) temperature maintained at a constant value, then you might be able to notice the directional difference in the IR radiation reaching your exposed skin - although I think the difference in temperature between the warm and cold walls would have to be quite high for this to be noticeable. More likely you would notice directional convection currents in the surrounding air.

If you are surrounded by cold walls but the ambient temperature is kept constant then you will not freeze to death no matter how cold the walls are (as long as you are not in contact with the cold surfaces). Otherwise we would all freeze to death every night as soon as the sun sets.

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