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Natural Science Forum / Physics / Particle Physics / April 2005Do photons have temperature?
They have mass they have energy, momentum... but do they have classical thermodynamic properties?(For example, temperature, entropy, enthalpy,etc) If true, doesn't that mean a electromagnetic field has a temperature?? I don't remember hearing about that anywhere before... Yes they have. All you assume is fully right > They have mass they have energy, momentum... but do they have classical > thermodynamic properties?(For example, temperature, entropy, > enthalpy,etc) > If true, doesn't that mean a electromagnetic field has a temperature?? > I don't remember hearing about that anywhere before... Are there any equations for it or something... it's funny that we don't often here about the temperature of photons or magnetic fields... tianshuo@gmail.com wrote in news:1112357938.160556.254970 @g14g2000cwa.googlegroups.com: > Are there any equations for it or something... > it's funny that we don't often here about the temperature of photons or > magnetic fields... heat radiation from a black body can be calculated: http://www.egglescliffe.org.uk/physics/astronomy/blackbody/bbody.html You can take the wavelength at the 'peak' of radiation for a particular temperature of a black body.  Signaturebz please pardon my infinite ignorance, the set-of-things-I-do-not-know is an infinite set. bz+sp@ch100-5.chem.lsu.edu remove ch100-5 to avoid spam trap > Are there any equations for it or something... E.g. Planck's radiation law for black bodies. That gives the spectral energy density of a photon gas at a fixed temperature. [snip] Bye, Bjoern > They have mass they have energy, momentum... but do they have classical > thermodynamic properties?(For example, temperature, entropy, > enthalpy,etc) > If true, doesn't that mean a electromagnetic field has a temperature?? > I don't remember hearing about that anywhere before... It is true, but recall that thermodynamic properties can only be assigned for colelctions of objects. It does not make sense to speak of the temperature of a single photon (although one can be assigned based on energy). Blackbody radiation is what you get for an electromagnetic field at a certain temprature, or equivalently, in equilibrium with a thermal reserviour at temperature T. SignatureAndrew Resnick, Ph.D. Department of Physiology and Biophysics Case Western Reserve University >They have mass they have energy, momentum... but do they have classical >thermodynamic properties?(For example, temperature, entropy, >enthalpy,etc) >If true, doesn't that mean a electromagnetic field has a temperature?? >I don't remember hearing about that anywhere before... Temperature is a thermodynamic concept, a property averaged over many particles. A photon gas has a temperature. Probably the most famous one is the cosmic background radiation. The temperature is the average photon energy, and you can use Boltzmann's constant to convert from energy to degree kelvin, if you like. Signature"The average person, during a single day, deposits in his or her underwear an amount of fecal bacteria equal to the weight of a quarter of a peanut." -- Dr. Robert Buckman, Human Wildlife, p119. I'm not quite sure how a photon gas behaves. (Yes, I am talking about a collection of photons, not just one) By common sense, I think that photons in a a laser should have a higher temperature--but is that true?? How about temperature of fields (not temperatures in fields)? I'm not too sure if you can say there are so. Since fields are usually behavior from wavelike interactions. Can I just calculate a temperature of a field from the temperature of a photon and the Maxwell equations? > I'm not quite sure how a photon gas behaves. (Yes, I am talking about a > collection of photons, not just one) That is described in detail in most textbooks discussing statistical mechanics. > By common sense, I think that photons in a a laser should have a higher > temperature--but is that true?? You can only talk about the temperature of a photon gas when that gas is in thermal equilibrium. The photons in a laser aren't. > How about temperature of fields (not temperatures in fields)? See a book on Quantum Field Theory at finite temperature. ;-) > I'm not too sure if you can say there are so. Since fields are usually > behavior from wavelike interactions. And what is this supposed to mean??? > Can I just calculate a temperature > of a field from the temperature of a photon and the Maxwell equations? As already several people explained, a single photon has no temperature. Just as a single atom or molecule has no temperature. A quote from the post you replied to here, but which you apparently did not read or not understand: "Temperature is a thermodynamic concept, a property averaged over many particles." Bye, Bjoern > They have mass they have energy, momentum... but do they have classical > thermodynamic properties?(For example, temperature, entropy, > enthalpy,etc) > If true, doesn't that mean a electromagnetic field has a temperature?? > I don't remember hearing about that anywhere before... Temperature is a macroscopic concept derived from what humans feel from the skin. Photon is a substance that causes the temperature of an object to rise. In the world of photons, I don't think there is a concept of tempertature. Does the heat have temperature? Now you realize it sounds stupid. > > They have mass they have energy, momentum... but do they have classical > > thermodynamic properties?(For example, temperature, entropy, [quoted text clipped - 4 lines] > Temperature is a macroscopic concept derived from what humans feel from the > skin. It is macroscopic, but it has nothing to do with what you feel on your skin. What you feel is heat flow, not temperature. > Photon is a substance that causes the temperature of an object to rise. Well, it can cause this, but that isn't the definition of photon. > In the world of photons, I don't think there is a concept of tempertature. Sure there is. A photon gas can be treated as a thermal system with a temperature. > Does the heat have temperature? That wasn't the question. > Now you realize it sounds stupid. It wasn't a stupid question at all. > They have mass they have energy, momentum... but do they have classical > thermodynamic properties?(For example, temperature, entropy, > enthalpy,etc) > If true, doesn't that mean a electromagnetic field has a temperature?? > I don't remember hearing about that anywhere before...\ Did you ever wonder "wtf is a photon, anyway?" You see how subtle shades of a concept can change the entire meaning of a statement? :) Light is just a bad habit; photons are the hallucinations that follow. Richard Perry |
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