Did these guys just repeal the second law of thermodynamics and solve the global warming problem to boot? [ Science vol. 355 pp. 1023 – 1024, 1062 -1066 ’17 ] Heady stuff. But they can put a sheet of metamaterial over water during the day in Arizona and cool it by 8 degrees Centigrade in two hours!
How did they do it? Time for a little atmospheric physics. There is nothing in the Earth’s atmosphere which absorbs light of wavelength between 8 and 13 microns (this is called the atmospheric window). So anything radiating energy in this range sends it out into space. This is called radiative cooling. It doesn’t work during the day because most materials absorb sunlight in the visible and near infrared range (.7 -2.5 microns) heating them up. Solar power density overwhelms the room temperature radiation spectrum shorter than 4 microns. So for daytime cooling you need a material reflecting all the light shorter than 4 microns, while being fully emissive for longer wavelengths.
This work describes a metamaterial– https://en.wikipedia.org/wiki/Metamaterial — in which small (average diameter 4 microns) spheres ofSiO2 (glass) are randomly dispersed in a polymer matrix transparent to visible and infrared light. The matrix is 50 microns thick. The whole shebang is backed by a very thin (.2 micron) silver mirror. So light easily passes through the film and is then bounced back by the mirror without being absorbed.
Chemists have already studied the Carnot cycle, which gives the maximum efficiency of a heat engine. This is always proportional to the temperature difference between phases of the cycle. That’s why the biggest thing about a nuclear power plant is the cooling tower (and almost as important). Well few things are colder than the cosmic microwave background (2.7 degrees Centigrade above absolute zero).
So while the entropy of the universe increases as the heat goes somewhere, locally it looks like the second law of thermodynamics is being violated. No work is done (as far as i can tell) yet the objects spontaneously cool.
Perhaps the physics mavens out there can help. I seem to remember Feynman and Wheeler once saying something to the effect that radiation is impossible without something around to absorb it. If I haven’t totally garbled the physics, it almost sounds like emitter and absorber are entangled.
Anyway beaming heat out into space through the atmospheric window sounds like a good way to combat global warming.
No wonder DARPA supported this research.