Why is blackbody radiation continuous?

[TheCanadian]

Plasmas can emit radiation based on the acceleration of charged particles (which we generally consider as continuous), but for un-ionized matter compounds, transitions are quantized and photons have particular energies. At room temperature, collisional excitations are typically dominant. But if that is the case, what about blackbodies permits continuous spectra at all wavelengths? Why do certain materials (e.g. carbon nanotube structures) approximate well this behaviour despite being finite in size and available energy levels? Why do we not discuss/observe blackbody emissions as having discrete energies?

 

[DrClaude]

First, a blackbody is an idealisation. Real objects only approximate a blackbody spectrum; see for instance the spectrum of the sun: https://i.stack.imgur.com/tc0Mq.png

Second, only isolated systems can be seen has having well-defined energy levels. For a gas, you have to consider in particular Doppler broadening and collision broadening. In solids, the constituent atoms loose their individual character and energy levels become energy bands.

 

[Andy Resnick]

Plasmas can emit radiation based on the acceleration of charged particles (which we generally consider as continuous), but for un-ionized matter compounds, transitions are quantized and photons have particular energies. At room temperature, collisional excitations are typically dominant. But if that is the case, what about blackbodies permits continuous spectra at all wavelengths? Why do certain materials (e.g. carbon nanotube structures) approximate well this behaviour despite being finite in size and available energy levels? Why do we not discuss/observe blackbody emissions as having discrete energies?

I have a slightly different answer than Dr. Claude: When an electromagnetic field within a cavity is at thermal equilibrium (at some temperature T), the spectrum is named 'blackbody radiation'. The spectrum is continuous because in the thermodynamic limit, the cavity dimensions are much larger than wavelengths. Micro- and nano-cavities do not exhibit the usual blackbody spectrum:

https://www.osapublishing.org/abstract.cfm?uri=QELS-1995-QTuG19
https://www.ncbi.nlm.nih.gov/pubmed/17358533
https://www.osapublishing.org/abstract.cfm?uri=QELS-2003-QWA25