Hi, rank newbie here, with my first post. This one is something I figure every first year student comes up with at some point, but I don't know enough keywords to Search for an answer. (I'm not a student except in the category "of life": this isn't assigned homework) I figger, using a bit of non-linear optics, the 2nd law of Thermodynamics can be circumvented. Obviously this isn't true, so let's take a looksee... --- Let's start with definitions, simplified for clarity. Temperature - overall power emission per areal unit. Black Body - absorbs all radiation, emits a predefined spectrum, related directly to power, with some non-sequitur to the experiment formula. Fluorescer - transforms a certain input range of frequencies into a certain output range. Basically, when photons are absorbed, a bit of internal heat is produced and lower frequency photons emitted. Bandpass mirror - passes only a certain frequency range, reflects the rest. ---- Next, the components of the device, also simplified, also for clarity... - A black-body ball, partially painted with a fluorescer (polkadots or stripes, your pick). - A bandpass mirror sphere, larger than the ball. Required is that the fluorescence output be the same frequency as the mirror's bandpass. ---- And conduct the experiment... Within an environment of a non-specified BB background radiation temperature... Place the ball within the sphere. That's it. ---- What happens... First let's backtrack a bit and compare the spectra of environment and ball before the experiment starts. Both integrate the same - the power outputs/temperatures are identical - but the spectra are different. While the environment spectrum is strictly BBR, the ball's spectrum should look like a spike was added to a lower-temperature BBR. (To bridge narrative somewhat seamlessly into the next bit, we can see that at the spike frequency the ball will be more energetic than the environment) The sphere only allow equilibration at its bandpass, which is at the spike. So, when the ball is placed within the sphere, there is more radiation exiting than entering, to start with. Eventually, power equilibrium (in vs out) at the bandpass/spike is reached, as the spike diminishes in size. (To clarify, the mechanism for the spike's decrease, but not cessation is the BB bit of the ball continuing to emit in the fluorescing absorption range (which reflects back onto the ball from the inside of the mirror).) ---- Result The ball gets colder. ---- So.... a) within the given framework of simplifed definitions and magically perfect components, will it work ? b) what property stops it from working in a real world ? with real, non-theoretical substances/components. Note that there's no "what will reduce the efficiency" question: the experiment was kept deliberately simplistic. In the grand scheme of things it shouldn't matter that bandpass mirrors have different properties at different angles, or that perfect black bodies don't exist. The only requirement is that the bandpass include some of the fluorescent emission and exclude some of the fluorescent absorption ranges.