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Q & A: Where does blackbody radiation come from?

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Most recent answer: 01/08/2017
Q:
I have this explanation of the source of Electromagnetic radiation from a black body: "The radiation represents a conversion of a body's thermal energy into electromagnetic energy, and is therefore called thermal radiation. It is a spontaneous process of radiative distribution of entropy." But, that says nothing about what mechanism is ACTUALLY generating the electromagnetic energy. I'm wondering if it has to do with the movement of the [outer] electrons on the atoms that are being jostled around by the thermal kinetic energy. Maybe the atoms/molecules "ring" when they collide, and the sinusoidal movement of the electrons, caused by the ringing, is what generates the characteristic electromagnetic energy. Or, maybe the collisions (or repulsive forces due to close proximity of like [negative] charges) cause outer electrons to change quantum state, thus releasing a photon. Does science have anything to say about this?
- Steve (age 61)
Rockville, CA, USA
A:

First a slight correction. The radiation is not "a conversion of a body's thermal energy into electromagnetic energy". It is simply one of the many forms of thermal energy, and therefore must be present in thermal equilibrium. The reason is that entropy, the log of the number of possible quantum states, is maximal in equilibrium. Electromagnetic fields get more possible states when they get more energy, so they pick up thermal energy just like any other modes such as rattling of atoms. The phrase "radiative distribution of entropy" is also misleading, in that entropy is not a conserved quantity that is simply "distributed". Energy is conserved, and spontaneously distributes in a way that maximizes entropy.

Now to your main question. Say that the thermal radiation is allowed to escape to a cold outside. How does thermal energy get from other modes to make new thermal radiation? There's no single answer for all materials. In a plasma, for example, with a lot of charged particles moving more or less freely but bouncing off each other, a classical picture of radiation from accelerating charged particles is a pretty good way to think of it. There's no need for an individual event to give some sinusoidal ringing. Our choice to describe the broad-spectrum light in terms of sinusoidal components does not require that the sources be anything like sinusoidal. In other cases, it's more accurate to think of the actual quantum processes involved. In some cases, such as dilute gases atoms or molecules (neon, hydrogen,...) the quantum processes only allow rapid exchange of energy with electromagnetic fields in narrow frequency bands- the absorption/emission lines. Those do correspond to electrons falling from excited states to lower-energy states. In those cases you don't get the full blackbody spectrum, but just emissions in those narrow bands, like the beautiful colors from neon lights.

Mike W.


(published on 01/08/2017)

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