The forbidden light

In the early nineteenth century, two scientists -- Joseph von Fraunhofer and Charles Wheatstone -- independently observed something strange; if you heated up samples of various elements, they emitted a light spectrum that contained strong peaks at certain frequencies, showing up as bright lines instead of a continuous rainbow of colors.

It quickly became obvious that this property could be used to identify the presence of different elements in mixed samples.  In fact, helium was discovered when French astronomer Georges Rayet found emission lines in the solar spectrum that didn't correspond to any other known element, making it the only element in the periodic table first detected somewhere other than on Earth.  (The name helium comes from the Greek Ἥλιος, meaning the Sun.)

Figuring out why this phenomenon occurred, though, took almost a hundred years.  The explanation, due in large part to the work of Danish physicist Niels Bohr, has to do with the fact that the electron shells in atoms are quantized -- there are only certain allowed energy levels, so an atom has to absorb a particular frequency of light in order for one of its electrons to jump to the next level (or, conversely, to drop to a lower level, the atom has to emit a photon of a particular frequency).  This simultaneously explained the specificity of emission spectra and the odd phenomenon of absorption spectra, where broad-spectrum light passing through transparent substances shows dark lines where certain frequencies are absorbed, effectively subtracting them from the beam.

So each element has its own distinctive "fingerprint" of spectral lines, which is how researchers here on Earth can determine the chemical composition of distant stars, and even the constituents of the atmospheres of exoplanets.

The emission spectrum of iron [Image is in the Public Domain]

However -- as usual -- even this rather complex model has some unexpected twists.

Very rarely, the electrons in atoms will undergo forbidden transitions, resulting in light being emitted that should not be possible from the element in question.  (A simple analogy is if you were climbing a staircase, and somehow were able to go up by one-and-three-quarters steps.)  These transitions are highly unstable (just as your attempted ascent would be), and the electron almost instantaneously collapses back into one of the allowed energy states, but when it does so the atom emits a frequency of light you wouldn't expect.  So these aren't so much forbidden as they are extremely improbable; in ordinary situations, their contribution to the light spectrum is vanishingly small.

But in very high energy conditions, where the electrons are bouncing all over the place millions of times per second, you begin to see a significant contribution from forbidden transitions.

The reason this comes up is because of a study of a Seyfert galaxy named MCG 01-24-014.  Seyfert galaxies, named after American astronomer Carl Keenan Seyfert who studied them extensively, look superficially like ordinary spiral galaxies, but have an active galactic nucleus.  This latter name is a massive understatement, mostly because astronomers shy away from calling something "Holy Shit This Thing Is Super Bright, No Really You Have No Idea How Bright It Is."  The center bit of a Seyfert galaxy has a luminosity equal to the luminosity of all the stars of the Milky Way put together, and is thought to be the result of large quantities of material falling rapidly into a supermassive black hole.  Most of the light emitted is outside of the visible spectrum -- thus their ordinary appearance through a telescope -- but when viewed in other frequency ranges, it becomes obvious how weird they are.  

The Circinus Galaxy, one of the best-studied Seyfert galaxies [Image is in the Public Domain courtesy of NASA/JPL]

And MCG 01-24-014 is really peculiar -- emitting far more light from forbidden transitions than even an average Seyfert galaxy would.  So whatever is powering its galactic core is running full-throttle.

The forbidden light of Seyfert galaxies provides us with yet another example of "you think you understand, then nature throws you a curve ball."

Sometimes you hear the criticism levied at scientists that all the technical details somehow take away from the wonder of simply looking up and delighting at the beauty of the night sky.  I can't speak for anyone else, but for me, the exact opposite is true.  I can still go outside on a clear winter's night and look up at my favorite naked-eye astronomical object -- the Pleiades -- and fully appreciate how lovely it is, but my enjoyment is increased further by knowing that it's a cluster of recently-formed hot blue supergiant stars inside the wispy strands of a reflection nebula.  

Understanding and appreciation shouldn't be inversely proportional.  The more I know, the more I wonder at the beauty, complexity, and strangeness of this universe in which we live.  The only frustrating part about it all is the limitation of my mind in comprehending it all.

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Looking for a biosignature

From Gordon's Obsession #1 (paleontology) yesterday, today we're moving on to Gordon's Obsession #2: outer space and extraterrestrial life.

No, unfortunately, I'm not about to announce we've located the Andorian home world.  Maybe next time.

We're returning to this favorite topic of mine because of a paper this week in Astrophysical Journal Letters by a team led by Lisa Kaltenegger, of the Carl Sagan Institute of Cornell University, right in my part of the world in upstate New York.  Kaltenegger et al. describe models they've developed to refine how we look for Earth-like exoplanets -- by trying to figure out what our own world would have looked like from the depths of space during its entire geological history.

In "High-Resolution Transmission Spectra of Earth Through Geological Time," Kaltenegger's team recognizes the phenomenon we've discussed here before -- that seeing farther out into space means seeing further back into time.  An intelligent, technological alien species as little as 150 light years away wouldn't have any way of knowing that the Earth hosted a complex civilization with its own sophisticated scientific and technological capabilities, because they'd be seeing us as we were 150 years ago -- before the invention of long-distance radio-wave communication.  To them, the Earth would be a small rocky planet that was entirely silent, and apparently, devoid of life.

So are we making the same mistake with the exoplanets we're seeing?  And is there a way to get beyond that, and find "biosignatures" -- detectable traces of life on a far-distant world?

The key, says Kaltenegger, is in the world's atmosphere.  As the light from its host star passes through the thin envelope of gases surrounding the planet, the light is altered; each kind of gas has a specific set of frequencies it can absorb, and those are selectively removed from the stellar light, creating a dark-line or absorption spectrum.  This gives a fingerprint of what gases are there -- and, potentially, tells us what's going on down on the planet's surface, including whether or not there's anything alive.

The data they're using comes primarily from two sources -- the orbiting James Webb Space Telescope, and the Extremely Large Telescope out in the Atacama Desert of Chile.

I don't know about you, but the name of the latter always makes me laugh.  I'm picturing the scientists coming up with a name for the observatory after it was complete:

Scientist #1:  So, what are we gonna name our telescope?
Scientist #2:  How about naming it after Edwin Hubble?
Scientist #1:  No, that one's already taken.
Scientist #2:  Well, what's this thing's most outstanding feature?
Scientist #1:  It's extremely large.
*pause*
Scientist #1 and #2, together:  Heyyyyyy......!

But I digress.

Kaltenegger's team is looking for the presence of highly-reactive gases -- oxygen being the most obvious example -- that wouldn't be in an atmosphere unless something was continually pumping it out.  While there could be a non-biological way to inject large quantities of oxygen into an atmosphere, the better likelihood is some analogue to photosynthesis.

In other words, life.

The nice thing about this approach is that the presence of oxygen would have been detectable here on Earth over a billion years ago -- thus, potentially detectable by technological aliens from up to a billion light years away.  That's quite a window.  "Even though extrapolations from our findings suggest that one out of five stars hosts a planet which could be like Earth, it would be extremely surprising if all of them were at our Earth’s evolutionary stage," Kaltenegger said.  "So taking Earth’s history into account to me is critical to characterize other Earth-like planets."

What the team did is predict what the absorption spectra of the Sun's light would look like after passing through the Earth's atmosphere during the various periods of our prehistory -- the anoxic period (prior to the evolution of photosynthesis), the time during which aerobic life was present but uncommon, the transition to the land & evolution of plants, and so on, up through the Industrial Revolution, when (as James Burke put it in After the Warming) "instead of the atmosphere doing things to us, we started doing things to it."

The technique is not without its difficulties, however, most notably that the absorption spectrum of one of the biologically-produced reactive gases they studied -- methane -- is awfully close to that of water.  So teasing apart what's the signature of a ubiquitous compound, and what's the actual fingerprint of life, may not be simple.

What's certain is that we've only scratched the surface of what's out there.  At present there are a few more than 4,000 exoplanets identified, a lot of which are gas-rich Jovian planets that are likely not to have a solid surface.  (The reason for this is that the two main techniques for locating exoplanets, stellar occlusion and detection of a "wobble" in the star's position, work much better if the planet in question is large, biasing us against detecting small rocky worlds like our own.)  But if Kaltenegger is right that twenty percent of stars have Earth-sized planets, that's a lot of potential homes for alien life.

I don't know about you, but to me, that's tremendously exciting.  Even if we can't detect Vulcans and Klingons and Andorians yet, we might just be able to see if there's life at all out there.

And I'd be satisfied with that.  Just knowing we're not all alone in the cosmos would be reassuring, even if we don't know what that alien life is like, or whether they might be looking back at us through their own telescopes.

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This week's Skeptophilia book recommendation is an important read for any of you who, like me, (1) like running, cycling, and weight lifting, and (2) have had repeated injuries.

Christie Aschwanden's new book Good to Go: What the Athlete in All of Us Can Learn from the Strange Science of Recovery goes through all the recommendations -- good and bad, sensible and bizarre -- that world-class athletes have made to help us less-elite types recover from the injuries we incur.  As you might expect, some of them work, and some of them are worse than useless -- and Aschwanden will help you to sort the wheat from the chaff.

The fun part of this is that Aschwanden not only looked at the serious scientific research, she tried some of these "cures" on herself.  You'll find out the results, described in detail brought to life by her lucid writing, and maybe it'll help you find some good ways of handling your own aches and pains -- and avoid the ones that are worthless.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]