How/why is space between the sun and the earth so cold, when we can feel heat coming from the sun?
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I learned this from Robert Heinlein in “Have Spacesuit, Will Travel” in the 1980’s: Vacuum is NOT cold. Vacuum is NOT hot. Vacuum has no temperature.
If vacuum was cold then a thermos couldn’t keep soup hot.
Vacuum is simply the absence of molecules. Because there are no molecules, heat cannot travel by many of the means you are used to.
Example:
conduction, which is something touching something else to allow heat to transfer,
or
convection, which is really a special case of conduction because it’s just heat transfer to a fluid which then moves and carries molecules around so they can do more conduction.
The heat you almost always experience is actually one of these two types. Heat is really just vibration of molecules.
Radiant heat, that comes in the form of light, is just part of the electromagnetic spectrum (it’s light, whether you can see it or not) which interacts with matter and causes it to vibrate. That vibration is heat, and it is then transferred by the methods described above.
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