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Comment by BugsJustFindMe | Hacker News Reader

Could someone here explain briefly (or point me somewhere) the basic process through which someone is able to look at the sky and reliably say "there is grease floating out there umpty billion miles away"? I understand things like spectral emissions showing particular atomic elements, but does that extend to complex molecules too?

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So, according to the abstract, this is looking at the absorbtion of a particular infrared band (3.4um). This band is presumably associated with the carbon-hydrogen bond, but I'm rusty on my IR spectroscopy so don't take my word on it. It's already known that there is attenuation of this band correlated with distance to a star, so they created molecules in the lab that mimic the absorbtion. This tells them how many molecules are needed to produce a particular amount of attenuation. From this, they can estimate the mass of a column between the Earth and a star, and extrapolate that to the Galaxy.

At the risk of sounding like a total idiot, could this explain some of the redshift seen at greater distances from earth. A.K.A explain away some of the expansion theory?

Very unlikely. They probably account for things like absorption when calculating redshift and the corresponding numbers that they're getting from their measurements. For instance, each star/galaxy has a spectral composition that can indicate what it's made of. This can be correlated with brightness for other stars and can be determined how bright it should be. Redshift would shift the entire spectral composition towards the red, while absorption would remove certain bands. There's probably many other ways they've verified redshift too that I'm not thinking of.

Science is miraculous. I couldn't even imagine being able to figure that shit out.

Almost everything is built on the work of those that came before.

The way projectfu describes it shows a clever but clear line of problem solving from 'we have x and we know y, how do we figure out z'.

Still miraculous, of course.

I think it's inspiring to see that it's not too different the fundamental problem solving process that many of us use in our daily lives.

Also don't forget that these scientists are deeply immersed in their fields. Every single one of them has been through a PhD program and is an expert in some domain, likely a narrow one. When you are that good at something, even your shower thoughts (and especially your shower thoughts) will be scientifically productive.

We joke at my company that our contractors must charge us for thinking about our problems in the shower, but I must admit that I sympathize and wouldn't blame them if they did.

Even in CS I'm always amazed that an "easy" algorithm problem we expect a new grad to solve was solved in the 70s by CS professors who published papers about the topic.

As a self taught developer with no formal education in CS, I'm amazed to know this. I'd love to read a paper outlining the solution of a rather simple algorithm (by today's standards). Sounds fascinating.

edmonds-karp, dijkstra search, kruskal mst, floyd-warshall, bellman-ford, tarjans least common ancestor, danzig's simplex method, etc. are all algos that were paper pubs throughout last century, covered in CLRS i.e. good undergrad algos class, and come up in interview style questions.

could this method detect really small concentrations? for example, in the 1 PPM range

In my understanding, the relevant quantity determining attenuation is the integrated amount of material in the column between the source and the observer. So, the small density is compensated by the large volume of column (distance to source).

This is tangentially related, but one of my favourite little spectral analysis factoids is that because of it Helium was discovered on the Sun before any samples of it were found on Earth. There's an explanation of the process in this link:

https://socratic.org/questions/helium-was-discovered-on-the-...

but does that extend to complex molecules too

Yes, there is a whole field of organic infrared spectroscopy. Before we had more powerful instruments like NMR, it was one of the key ways molecular structures were elucidated.

You can get a good sense as to what functional groups are on a molecule with IR alone.

I'd say that "grease" isn't an appropriate word. Grease is greasy. The substance is based on the close interaction of molecules. These molecules are to far away from each other for there to be anything we could point to as grease. It's like finding H2O and saying that there is water up their. It is really steam, very thin steam spread over billions of cubic kilometers. So this isn't really "grease".

"Grease" was chosen by the science writers, astronomers call this stuff PAHs just like a chemist would. It condenses into "dust grains", so no, it's not only a gas. These reason you can't see much in the plane of our galaxy is that these dust grains block visible light.

Grease is the word

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