This is about a debate (argument??) I am having on another forum in which people are claiming heat can be transferred from greenhouse gases in a cooler atmosphere to a warmer surface (that allegedly warmed it). The so called back-radiation is presented as a positive feedback which warms the surface, that warmed the GHGs. That is not only a contradiction of the 2nd law it represents perpetual motion. Came across a thread on this forum about radiation and the 2nd law of thermodynamics. The link is here: https://www.physicsforums.com/threa...tion-and-second-law-of-thermodynamics.691278/ There was a response from Andrew Mason, I think it was, which went as follows: "An even better example would be a laser. The temperature of the laser device used to cut steel is much less than temperature of the steel that the laser strikes. But this does not violate the second law since the energy flow from the laser to the steel is not a transfer of thermal energy". That response is in agreement with my understanding of the general difference between infrared radiation and heat. IR transfers thermal energy from a warmer body to a cooler body but IR is NOT the heat itself. Heat is the kinetic (internal) energy of atoms in the respective bodies and does not flow through the air. IR is EM, and EM has distinctly different properties than heat. However, if you read certain textbooks on this subject they describe an infrared interchange between bodies of different temperatures and insist that the IR flow is heat. It can't be heat since IR, which is EM, has no property related to heat. IR is the transporting agent only and heat always remains local to the bodies, increasing or decreasing depending on the situation. Heat in atoms of solids like metals is largely related to valence shell electrons and resembles electric current more than it does IR. Heat is transferred in metals via electrons and in insulators via phonons. Either way, heat is an energy flow related to the vibrations of atoms and the waves they propagate. As such, it cannot travel through air or a vacuum as can EM. The 2nd law is clear as written by Clausius that heat can only be transferred from a warmer body to a cooler body without compensation. Put another way, by Clausius, heat cannot of itself transfer from a cooler body to a warmer body. Writers using pure radiative theory, a la Boltzman, are clearly ignoring the 2nd law. Some of them claim that the 2nd law is satisfied as long as a net energy (IR) flow between the bodies is positive. However, Clausius did not reference IR when he wrote the 2nd law, he referred only to heat transfer, Q, the temperature at which the transfer took place T, and work. He did reference internal energy and work but claimed they were not required to calculate external heat and work. In models they present of bodies exchanging heat, they do not state the temperature of the bodies or whether the bodies are independent heat sources. I have no idea what would happen if two stars (modelling blackbodies) with temperatures in the millions of degrees were close to each other but at temperatures of the Earth, where the surface allegedly warms greenhouse gases in the atmosphere, that is a far different story. Some people are getting entropy mixed up with the 2nd law and that convolutes the matter. However, Clausius made it clear when he coined the term entropy that it is a summation of infinitesimal quantities of heat dQ, at a temperature T, in a heat process. Whereas entropy has it's uses in chemical reactions it's not helpful when it comes to the 2nd law and radiative heat transfer, which is really about the direction of heat transfer. The 2nd law, no matter how it is stated, refers to heat, not EM.