Hacker News Reader

Comment by gns24 | Hacker News Reader

This makes no sense to me. The article states that there is one frequency of radiation that will pass through the atmosphere, so the device radiates at that wavelength. But why would it make any difference? The atmosphere might reflect most radiation back to earth, but not back to the point it came from. So it shouldn't matter what frequency the device is radiating - its radiation will be reflected elsewhere, and radiation from elsewhere will be reflected to it.

View on HN

IANAPhysicist, but AFAIU, radiators work both ways. That is, they're also absorbers. So on the frequencies that don't pass through the atmosphere, "your heat" may end up elsewhere on the planet, but you'll also catch radiation emitted everywhere else. You'll eventually reach equilibrium with the average amount of heat that is "available" on this frequency. On the other hand, if your radiator is tuned to the frequency that passes through the atmosphere, you emit your heat, but don't absorb heat in return. The equilibrium point you'll be trying to reach is that of deep space.

You basically design a material that is "black" (good absorber-emitter, conversely: bad reflector) in the infrared window[0] and "white" (good reflector/bad absorber-emitter) in the ranges where the bulk of the incoming radiation is.

That way you maximize the amount of energy it can shed and minimize the amount it absorbs. You're not cheating thermodynamics, you're just doing better than for example titanium dioxide paint or metal foil normally do because those also reflect as much sunlight as possible but at the cost of of not shedding heat through IR radiation.

[0] https://en.wikipedia.org/wiki/Infrared_window

The absorption and emission spectra of a material are equal, mostly. There are some exceptions and small errors, but for the most part a material that emits a specific set of wavelengths of light will absorb practically the same wavelengths.

The converse could be said that if you use the right material, it will absorb significantly less radiation from the atmosphere while emitting a similar amount.

However there are some bands with very high absorption and if you avoid emitting in that band you could possibly experience locally lower air temperatures or lower levels of a particular band. (I am only guessing that this would be a significant amount)

I don't understand your point maybe. The atmosphere can't reflect the radiation back -- it is unable to absorb energy in those ranges of wavelengths. The radiation passes through the atmosphere in to space unhindered.

> But why would it make any difference?

I think because at that frequency range the apparent temperature of the sky is low.

I’ve always wondered that too.

Reply on news.ycombinator.com