Blackbodies are physical structures that absorb all electromagnetic radiation and emit what scientists call blackbody radiation. They are ideal emitters, so at a given temperature, they emit as much or more energy in all frequencies than other bodies of the same temperature. The energy they emit is diffused isotropically, so it's the same in every direction. Every other physical body's ability to emit radiation is measured in relation to a blackbody. That measurement is called emissivity, and blackbodies have an emissivity coefficient of 1.
Blackbodies don't actually exist in nature (though there are objects that come close enough as to be called blackbodies by scientists), but the physical description is a hole in a box.
Picture of blackbody representation from Wikipedia website on blackbodies
Light can go in at any wavelength, but because the walls are opaque to radiation and the hole is small, the light most likely won't get out. Any radiation the box emits will be a function of temperature and will radiate from all sides of the box.
Given this description of a blackbody, can the average human be called one? Of course not (technically)! We're visible, for one, which means we don't absorb all kinds of electromagnetic radiation. We aren't spherical, so any energy we do emit isn't emitted isotropically. Still, astronomers like to assume that a lot of things are blackbodies. Is the emissivity coefficient of the human body close enough to 1 to be considered a blackbody?
The interesting question is whether or not our chemical composition would lead one to believe that humans are blackbodies. All solid substances are considered grey bodies, meaning their emissivity coefficients are between 0 and 1. I'm curious to see if an object's emissivity coefficient can be found by adding up all of the emissivities of its components (taking into account percent composition).
The average human body is mostly oxygen, carbon, hydrogen, and nitrogen (in order of decreasing presence).
From Wikipedia's page on the composition of the human body
Some of those elements, because they're not metals, don't actually have emissivity coefficients. Thinking I had reached a dead end, I almost gave up. But then I realized that the internal composition doesn't matter anywhere near as much as the external composition. In other words, I figured that I should actually be looking at the chemical composition of human skin.
According to a study done in 1927 on the composition of human skin (which, despite its age, was the most straightforward data I could find), the average 30-year-old human's skin is composed of water, calcium, magnesium, each making up about 1% of the skin.
A quick google search told me that the emissivity coefficient of water is .95 (from infrared-thermography.com). Given this information and the (rough) chemical composition of human skin, it's safe to say that the emissivity coefficient of human skin is very close to 1.
Therefore, the human body is, in fact, a very close approximation of a blackbody.
Well done Moiya, you raise an interesting question here! Now you've made me curious: what is our peak emission wavelength? How much energy per second do emit? (And, as a result, how many slices of chocolate cake can I eat that will be compensated by just blackbody emission...)
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