Spectroscopy Is the Rosetta Stone to the Universe

In last night's Cosmos, Neil deGrasse Tyson explained how the spectroscope was first invented. We'll fill you in on how far it's come since then. Stretchy time is involved.

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Apr 7 2014, 6:40pm
Image: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)

Every week, Becky Ferreira, your hostess with the cosmostest, hones in on the most important science and history topics the hit show Cosmos glosses over. Previously: Astronomer Caroline Herschel Has Been Snubbed for a Century.

Neil deGrasse Tyson introduced last night's Cosmos episode, “Hiding in the Light,” by explaining that “the age and size of the cosmos are written in light.” Where DNA is the language of life and mathematics is the backbone of nature, light is the universe's trusty Pony Express. It sprints across the cosmos at the fastest speed possible, carrying multitudes of information wherever it goes. Studying it is like deciphering a “secret code,” as Tyson put it—a code that it has taken millennia to crack. Indeed, we're still finding new answers in light's ethereal signatures.

“Hiding in the Light” profiled many brilliant opticians, including the pacifist philosopher Mozi and the great Arab polymath Alhazen—a perfect representative of the Islamic Golden Age. This period of intense intellectual activity is glossed over all too often in the history of science, despite its enormous contributions to our modern world (read Jim Al-Khalili's The House of Wisdom for more).

But the episode mainly focused its aperture on the rags-to-riches tale of Joseph von Fraunhofer. Though William Hyde Wollaston was the first scientist to note the dark absorption lines on the sun's optical spectrum, Fraunhofer was the guy who saw the big picture, and he is undeniably the father of spectroscopy.

Fraunhofer shows off his spectroscope in this photogravure from a painting by Richard Wimmer.

The vertical shadows on stellar spectrums catalogued by Fraunhofer now bear his name, and they are nothing less than an astronomical Rosetta Stone—a stellar alphabet that spells out the contents of celestial objects. Whether you are observing absorption lines, which tell us what substances are absorbed by interfering gases, or emission lines, which tell us what substances are emitted by an object, spectroscopy ingeniously isolates the molecular composition of distant objects.

Not surprisingly, this technique has applications far beyond getting the skinny on star stuff, and that's what I want to focus on here. Spectroscopy may have its roots in astronomy, but the field has diversified into an expansive juggernaut with fingers in practically every scientific pie. Broadly speaking, it is simply the study of radiated energy on matter, and boy, have scientists ever have found exotic ways to riff on that basic premise.

Take force spectroscopy, which studies the behavior of single molecules, or even atoms, when stretched by “optical tweezers” forged by high-energy lasers. It's almost like scientists put these tiny objects on torture devices in order to get them to squeal about their inner workings. And squeal they have, the poor things. For one thing, force spectroscopy has been a boon to genetic research, because it can finely measure the elasticity of DNA and RNA.

Then there's differential acoustic resonance spectroscopy (DARS), which throws a little love out to the sonic spectrum. DARS has made it possible to to measure the velocity and wave flow of sound through materials with impressive precision. The trick is to measure sound frequency and wave flow in a vacant, fluid-filled column, then dunk in the object you want to study. Comparing the measurements of the empty column against the occupied column gives you the object's acoustic properties, almost like a spectroscopic version of Archimedes' Eureka-inducing experiment in water displacement.

But by far the weirdest spectral outlet I've ever come across is Time-Stretch spectroscopy, or “dispersive Fourier transform” as it is more formally known. I can't really explain this one well, because my brain short-circuits whenever I try to understand it. I don't have the neural processing power to grasp how Time-Stretch converts pulses of light into temporal waveforms, but that's what it does. This allows for real-time analysis of the optical spectrum, providing valuable insight into chaotic spectral noise. 

Fraunhofer lines of stars, nebulae, and the sun. Image: Dodd, Mead and Company

Spectroscopy gets weirder and weirder the more it integrates cutting-edge tech into its arsenal. I'm sure there are far more peculiar applications than the ones I mentioned here, and I hope some of them will be brought up in the comments. 

But as Tyson emphasized in last night's episode, we are only the latest generation to be baffled by light's elusive radiance. We'll need to continue finding ways to dig deeper into its eccentricities, and spectroscopy is the torch that will illuminate the way. Even so, we have a long way to go before we're out of the dark.