Is the greenhouse effect like a greenhouse?

I am teaching a class at NYU this Spring that I am loving: It is called (with grandiosity) The Art and Science of Approximate Reasoning: Physics, Sustainable Energy, and the Future of Humanity. We are computing what we can about sustainability, from a physics perspective and with order-of-magnitude reasoning. The strangest thing about the class (for me) is that it is a writing class. So I am grading writing assignments, something I am certainly not qualified to do.

I have learned a huge amount in this class. Here's one tiny, tiny example:

How does a greenhouse work, and is it anything like how the greenhouse effect on Earth works? The answer is extremely cool. In both cases (greenhouse and greenhouse effect) the system permits visible radiation to enter, but deters infrared radiation from escaping. In this sense, the two effects are very similar.

However, there are critical differences: In the case of the greenhouse, much of the effect is that the glass walls cut off convective cooling to the upper atmosphere. That is, they trap the heat near the heated ground, not letting it mix with the rest of the atmosphere. This effect probably dominates over the infrared-reflectance effect of normal glasses and plastics. The fundamental point is that (to first order) the Sun heats the Earth, which then heats the atmosphere. If part of the Earth is put inside a glass box, that part gets to hold its heat without sharing it entirely with the surrounding atmosphere. And indeed, greenhouses control their temperatures through ventilation management (management of convection to the outside).

In the case of the greenhouse effect, increasing the CO2 (and water and methane and so on) in the atmosphere increases the opacity at infrared wavelengths, so the Earth can't cool radiatively as efficiently as when the atmosphere is more infrared-transparent. That certainly is an effect in greenhouses, but not the only effect, and probably not the dominant effect.

The Earth's atmosphere can only cool (to space) radiatively, so the greenhouse effect is purely an adjustment of that radiative cooling. The human-built greenhouse can cool radiatively or convectively, and it uses both of those mechanisms for heat management.

emission lines from stars

At the end of Mike Blanton's brown-bag talk at NYU yesterday, Matt Kleban asked: Why don't stars produce emission lines; why only absorption lines? Maryam Modjaz said "because they are hotter on the inside and cooler on the outside". That's true! But it is slightly non-trivial to see why the consequence is always absorption-lines only. And does it mean that if the stars were cold, condensed objects bathed in a hotter radiation field, they would produce emission lines? (I think the answer here might be "yes"; think of a gas cloud bombarded with ionizing radiation.) Also Kleban pointed out that actually the very outside of the Sun is in fact hotter than the surface, which is true, but it must be that this is just so optically thin it barely matters.

In some ways, the biggest paradox about stars is that they aren't all the same temperature: After all, the "surface temperature" of a star is the temperature around the place where the photosphere becomes optically thin; shouldn't this be around 10,000 K for all stars? After all, that's the temperature around which hydrogen atoms recombine (see, for example, the CMB). I don't know any simple answer to this paradoxical question; to my (outsider) perspective it seems like the answer is always all about detailed atomic physics.

what does a future doctor not need to know?

My big challenge in preparing my General Physics I syllabus is to figure out what to cut, when the majority of the students are pre-health. I cut thermodynamics, because we have learned that it is taught also in chemistry (and other places). I then wanted to add more material about fluids and elastic solids (pretty relevant to medicine, it seems), so what to cut? I ended up cutting most of rotation, spinning, and angular momentum. Why? To understand the body, you do need to know about torques (how does your arm work, static structures, and so on) but you don't really need to conserve angular momentum. Or do you? The centrifuge spins, but it doesn't have angular dynamics.

(I will be doing the centrifuge.)