The discussion centers on the characterization of temperature in non-equilibrium thermodynamics, particularly regarding bodies with arbitrary emission spectra. It establishes that temperature is well-defined only in thermal equilibrium, as per Planck's law, and can be locally defined in non-equilibrium systems under certain conditions. The conversation highlights that while blackbody radiation can be assigned a temperature, other forms of radiation, such as laser light, cannot. The concept of temperature is further clarified through the relationship between electromagnetic energy and kinetic energy of particles, emphasizing that temperature can exist even before massive particles are formed, as illustrated by theories like baryogenesis.
PREREQUISITESPhysicists, thermodynamic researchers, and students studying non-equilibrium thermodynamics or the fundamental concepts of temperature in various physical systems.
How about a monochromatic laser? In that case, we don't talk about temperature, we talk about frequency and energy.pervect said:
I don't see how we can ever relate temperature to the radiation itself until it interacts with matter... or am I missing something obvious here?anorlunda said:
Yes I am aware of blackbody radiation. The radiation has a certain power output and energy density, but temperature is a measure of average particle kinetic energy in some phase of matter. Then again, I suppose you could measure the average energy of the photons in blackbody radiation and call it a temperature... the fact that it is blackbody radiation, thus traveling in all directions equally and at the same energy density, allows this to be done, similar to how a given quantity of matter must be in thermal equilibrium to have a meaningful temperature measured across its entirety? I've always thought of temperature as a property of matter caused by radiation in some way, but I can see how temperature can be related here. Is my understanding above correct?anorlunda said:
What I am saying is that it is either the emitter or the receiver that has a temperature, in the sense that the average kinetic energy of these particles (with mass) can be measured, compared to the radiation itself, in which only its average electromagnetic energy can be measured. My impression from what you are saying (and what I've googled) is that blackbody radiation (i.e. the radiation itself, not the emitter which obviously has a temperature) can have its own temperature, despite there being no way to measure the average kinetic energy of the photons.anorlunda said:
That's right. For example particle genesis at the time of the big bang. According to that theory, before there were any massive particles of any kind, there was temperature.Comeback City said:
pervect said:
This makes more sense now... I had always read (not quite understood though, haha) about there being temperature scales immediately following the big bang, but I never made the connection that massive particles hadn't theoretically formed yet. Thank you for the clarificationanorlunda said:
