Wednesday, September 14, 2011

Masers in Space, or Cool Stuff You Didn't Know Existed

I think the title of this post pretty well sums up what it's going to be about.

What is a maser?

I am going to go ahead and assume that you've all at least heard of lasers and probably experienced them in a pointing at things from a distance context. Did you know Einstein developed the theoretical underpinnings of lasers in 1917? He sure did a lot of fundamental physics for someone who didn't like the idea of quantum uncertainty.

Although it's now written all in lower-case, LASER was originally an acronym: Light Amplification by Stimulated Emission of Radiation. The acronym is actually a fairly good summary of how lasers work; they emit light (a form of radiation), which has been amplified thanks to stimulation. Yeah, OK, rephrasing it like that doesn't actually help. On the other hand, going into the quantum mechanics of it won't either. Briefly, a laser works thusly:
  1. You need an optical cavity, which is just a fancy way of saying a container (usually a metal tube) with mirrors at either end and filled with the right kind of material (which varies depending on the colour/wavelength you want to get out).
  2. You put some photons (particles of light) into it (or some other form of energy, like electrical, which will ultimately lead to photons) and they bounce back and forth thanks to the mirrors. The input light should be of a similar wavelength to what you want to emit.
  3. While the photons are bouncing around, the material inside your cavity — called the "gain medium" — absorbs some of them and then re-emits them at a very specific frequency/wavelength. The reason for the specific part is because quantum physics dictates that the intervals between different energy states (in the gain medium, in this case) have very specific and discrete values. The properties of your laser will dictate exactly which energy transition (and hence which
  4. So you end up with a whole lot of photons of the same frequency* bouncing around inside your cavity. At some point, you have more photons being emitted than are being absorbed and you can reap the rewards of your lasering. 
  5. When you let these photons out of the cavity, you are releasing a whole lot of light which is exactly the same wavelength/frequency/colour and, thanks to the properties of the cavity, which is perfectly in synch (you can think of it as focussed, which is also true but actually a different property). That's why lasers are singularly coloured and why more energetic lasers can burn.
That's nice, you may be thinking, but what in space does this have to do with masers? Well, dear readers, it is no coincidence that "maser" sounds very much like "laser". It's a bastardisation of the acronym, standing instead for Microwave Amplification by Stimulated Emission of Radiation. The principle is the same but with microwave radiation instead of optical** (and because of the longer wavelengths involved, the industrial construction is a bit different). Interestingly enough, the first laser built was actually a maser. (See the history section on the laser wiki article I also linked to above.)

* If you're wondering why I keep randomly switching between frequency and wavelength, it's because they're interchangeable via a simple formula and we scientists tend to just use whichever we feel like in the context. The formula, if you're wondering is speed of light = frequency x wavelength. And the speed of light is constant (more or less).

** All forms of electromagnetic radiation are in principle the same, just longer or shorter wavelengths. From the shortest — gamma rays, X-rays, UV light — to optical light that we can see with our eyes, to the longer wavelengths — infrared, microwave, radio waves — it all works the same way just with different amounts of energy involved.  

Um, so where's the space part?

Glad you asked! This is where it gets really cool. (Arguably lasers are already cool, but bear with me.)

 In space, anywhere where there are large collections of molecules — for example in molecular clouds (d'uh), around dying stars, and some planetary atmospheres — the following sequence of events isn't uncommon:
  1. Some external source excites a molecule. The source is probably starlight, but there are a few other things that could also do it. Beside the point at the moment. The term excitation refers to the molecule absorbing a single particle of light (photon) and thus increasing its internal energy level.
  2. At some point, the molecule will spontaneously de-excite, emitting a photon of exactly the energy difference between the excited and less excited energy states. (This could be the energy of the original photon it absorbed, but in the case of masers often the exciting photon jumps the molecule up several energy levels and it drops down them one by one.)
  3. The emitted photon goes on to be absorbed by another molecule of the same type (where there's one there's almost always going to be a bunch more) or it can stimulate another molecule to de-excite by exactly the same amount. (It's a bit of quantum magic, but it does happen.) And now you have two photons of the same energy.
  4. Loop steps 2–3.
  5. A whole lot of light of exactly the same frequency will escape the cloud (and some of it will shine at Earth, so that we can see it).
Sound familiar? Yep. There are natural masers in space.

If you are curious, some of the sorts of molecules which often exhibit masing* are water, carbon monoxide, methanol, hydroxide, molecular hydrogen, formaldehyde, ammonia, and a bunch more. We can easily tell the difference between these molecules because the exact energies of the light they emit is specific to the molecule. It's even possible to tell the difference between different isotopes of oxygen, nitrogen, carbon, etc. And yes, that means all those molecules are floating around in space or surrounding dying stars with other stuff ejected during their death throes.

* Science likes making up words.

One more thing...

Image credit: NASA/ESA
A cool link: Giant reservoir of water found surrounding a quasar (discovered thanks to water masers, although it doesn't explicitly say so in that article). Above is the image to go with it, because NASA/ESA are good at finding artists to draw cool things in space.

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