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Comment by brundolf | Hacker News Reader

It blows my mind a little bit that spectrosocopy is a thing that just happens to be afforded us by physics, and how much less we'd know about the universe if it didn't exist. "Oh, every element has a totally unique signature and all you need to do is look at it to tell what it is, no matter how many miles or light-years away." It's the kind of thing where if it only existed in a sci-fi novel, it would be considered a far-too-convenient plot point.

https://en.wikipedia.org/wiki/Emission_spectrum

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It's actually not all that surprising if you approach it from a different angle.

Consider that light is nothing more than electromagnetic radiation; it is generated or absorbed only by changes in electromagnetic potential.

Now also consider that electron orbitals in molecules are discrete and quantized: you can be in this orbital (with such-and-such potential energy) or in that orbital (with a slightly different potential energy) but you cannot exist between those two orbitals.

Therefore, light can only be absorbed or emitted by a molecule if it can precisely force an electron into a different orbital, which in classical terms means that it only works if light has a particular frequency.

As you might imagine, sticking protons (which generate electron fields) at different positions within a molecule, or equivalently adding protons to an atomic nucleus, will change the permissible orbitals very slightly. Consequently, the light that an atom or molecule can emit or absorb will change as well.

How are they able to see the signature water gives off through all the noise that other types of molecules must be sending back? Isn't there even more noise to deal with when doing this over even bigger distances?

To answer your second question first:

Stuff being in the way will of course diminish the signal (by direct absorption or by scattering it). But space is pretty empty. The regular air on Earth contains ~10^19 particles per cm³, and it can take several km of air to start seriously interfering with light. A dense portion of space contains something like 10^4 particles per cm³, and a less dense region maybe 10^-4 particles per cm³. So, once you leave the atmosphere, the noise that's destroying the signal is simply far, far, far weaker than the signal.

Now for your first question. As I mentioned in my first comment, the spectral lines arise from different orbitals. What this means is that there isn't a single line associated with a molecule, but actually several lines. So if you think you see a line from one molecule, you should be able to find the other associated lines, and the more of the lines you can find, the stronger the evidence.

As a couple of people have noted, this isn't a perfect process, so you can get into arguments if your identification is actually correct or not.

Yes, we are really good at denoising. There can sometimes be a couple of possible different "solutions" to the spectroscopy curve, ie. different combination of elements that would lead to the same curve.

It is an extension of our eyesight. We see objects as being a specific color because they selectively absorb or reflect particular wavelengths of light. Our tri-color vision system can seem some large band gaps and our brain calls that a color. Spectroscopy just sees smaller gaps, smaller absorption lines.

But all kinds of complicated material-properties can affect the color our eyes see (to my understanding). That alone wouldn't be useful in this way. It's the fact that these signatures exist at an elemental level, and are consistent across all contexts, and can be teased apart at a cosmic scale that makes the technique so powerful.

Spectroscopy isn't simple. There are huge grey areas. Very well-defined signatures often overlap into a blurry mess of lines. Even within a single element, say hydrogen, there can be different signatures for different ionization states. Rather than clean detections there is often much opinion and debate, for years, before spectroscopy evidence is agreed as a legitimate detection of a substance.

What blows my mind is that travelers from across the galaxy can enter eyes here at Earth's surface to be seen as stars in the night sky.

What blows my mind is how little spectroscopy fingerprint data is available in the public domain!

To wit, considering how ubiquitous camera sensors are, you'd think that we'd have the ability to do spectrosocopic analysis on our smart phones with a simple/cheap adapter by now.

Well you're constantly getting free signals. They just happen to be signals with only three very wide band filters. But you can still identify some materials with them. Like cat fur. :)

Hmmm,

I am fairly certain that this does exist, I remember almost purchasing a spectroscopy adapter for my iPhone way back around 2014. I didn't buy it and I guess it wasn't too popular.

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