A dozen years later, Joseph von Fraunhofer, a German physicist and top-notch glassmaker who had committed himself to producing the most distortion-free telescope lenses money could buy, made a major breakthrough in examining the spectrum of the Sun. He decided to place a prism in front of a lens and look at sunlight that had passed through both intermediaries. What he saw in 1814 were hundreds more dark spectral lines than Wollaston had seen in 1802. In experiments with different types of glass over the next couple of years, the lines always appeared in the same places on the spectrum. Today tens of thousands of these “Fraunhofer lines” are known to exist in the solar spectrum. They’re dark because the light that would otherwise show up at those specific wavelengths is being absorbed by the lower-temperature, outermost layers of the Sun. By contrast, certain bright lines that show up in the spectra of flames from laboratory experiments are the result of those specific wavelengths being emitted, rather than absorbed.
Not only did Fraunhofer assiduously map the solar spectrum; he also noticed that the position of two bright yellow lines in the spectrum of a sodium flame matched the position of two prominent dark lines in the solar spectrum. Moreover, he saw that the spectrum of the Sun matched the spectra of sunlight reflected from the planets but that the Sun and the other bright stars in the sky each had its own spectral signature. By some people’s standards, he made the first true spectroscope.102
Light was a topic of hot debate and cutting-edge research, and its fundamental nature remained elusive for much of the nineteenth century. Was it made of corpuscles, as Newton had argued, or of waves? Was it propagated through a ubiquitous, flexible, invisible medium? At what speed did it travel? Was it related to electricity? To magnetism? At mid-nineteenth century, spectroscopy didn’t yet exist as a specialty, but it soon would—thanks largely to the collaboration of two professors at the University of Heidelberg, the physicist Gustav Kirchhoff and the chemist Robert Bunsen (who, by the way, improved but did not invent the Bunsen burner). In the late 1850s they began to devote themselves to
a common work which doesn’t let us sleep. . . . [A] means has been found to determine the composition of the sun and fixed stars with the same accuracy as we determine sulfuric acid, chlorine, etc., with our chemical reagents. Substances on the earth can be determined by this method just as easily as on the sun.103
In 1859 Bunsen and Kirchhoff devised a way to superimpose the spectrum of a beam of light from a sodium vapor lamp on the spectrum of a beam of sunlight, thereby confirming Fraunhofer’s suspicion of a connection between two of his dark lines and the two bright-yellow sodium lines, forever linking the lab chemist’s table with the matter that occupies the farthest reaches of the cosmos. Over the next few years, by burning various substances on their Bunsen burner and passing the light through a spectroscope of their own design, they methodically mapped the patterns made by known elements, discovered several new ones, and enabled their students and other investigators to discover still more.
One person who probably rolled over in his fairly fresh grave when Bunsen and Kirchhoff began to publish their findings in 1860 was the French philosopher Auguste Comte, who in 1835, in the second volume of his six-volume Course on Positive Philosophy, boneheadedly declared the impossibility of gleaning any chemical information, or more than limited physical information, about the stars:
We understand the possibility of determining their shapes, their distances, their sizes and their movements; whereas we would never know how to study by any means their chemical composition, or their mineralogical structure, and, even more so, the nature of any organized beings that might live on their surface. . . . I persist in the opinion that every notion of the true mean temperatures of the stars will necessarily always be concealed from us.104
Had Comte been correct, astrophysics would not exist. But shortly after the publication of volume two of his magnum opus, spectroscopic revelations about Earth’s cosmic neighborhood began to multiply. Soon spectra would be not merely detected but also photographed, despite the challenge of grabbing enough photons of any given wavelength so that a line would actually register in the emulsion. Astrophotographs would capture previously unseen and unimagined attributes of distant celestial bodies. Thirty years before being discovered on Earth, helium would be discovered in the Sun’s spectrum and named for the Greek sun god, Helios. By 1887—four decades after two Bostonians daguerreotyped the star Vega in a hundred seconds—two French brothers, Paul-Pierre and Matthieu-Prosper Henry, took only twenty seconds to photograph a star ten thousand times dimmer.105
The Astrographic Congress of April 1887, convened by the French Academy of Sciences and attended by scientists from nineteen countries, marked the official marriage of photography and astronomy.106 During their eleven days in Paris, the delegates agreed to undertake a two-pronged international effort to use a standard instrument and standard methodology not only to map the sky photographically but also to precisely catalogue the two million brightest stars—a significant goal, given that the average unaided eye sees not much more than six thousand. The instrument of choice was one developed by the brothers Henry. The very next year, an American astronomer/physicist/aircraft pioneer, Samuel P. Langley, published a book titled The New Astronomy—although not everyone saluted the idea of newness. As one hidebound nineteenth-century astrophysicist wrote, “The new astronomy, unlike the old astronomy to which we are indebted for skill in the navigation of the seas, the calculation of the tides, and the daily regulation of time, can lay no claim to afford us material help in the routine of daily life.”107
The new astronomy needed a new journal and a new organization. In 1895 The Astrophysical Journal, an International Review of Spectroscopy and Astronomical Physics published its first issue. Four years later, the various subspecies of skywatchers came together to form the Astronomical and Astrophysical Society of America. Under truncated titles—The Astrophysical Journal and the American Astronomical Society—both the journal and the organization still thrive.
Today’s astrophysicists have at our disposal individual telescopes that collect seventy thousand times more light than Galileo’s first attempts at a spyglass, and spectrometers that can reveal hydrogen in a galaxy that dates back to the first billion years after the Big Bang. We’re also armed with an abundance of auxiliary tools and tactics: adaptive optics, digital detectors, supercomputers, devices for masking the overwhelming brilliance of a host star so that nearby planets can be detected, methods for separating signal from noise. But no matter the innovations, no matter the complexity of the technology, the twenty-first-century astrophysicist’s fundamental challenge remains the same as Galileo’s: to collect the maximum amount of light from extremely dim and distant objects, and then extract from that light as much information as possible. It’s how the contemporary astrophysicist—and the contemporary warfighter—wants to use the light that makes all the difference.
Astrophysicists deduce nearly everything we know of the contents and behavior of the universe from the analysis of light. Most of the cosmic objects and events we observe materialized long ago, and so their attenuated light arrives here on Earth after delays that stretch up to thirteen billion years. Since the observable universe now spans nearly 900,000 billion billion kilometers, and the actual universe is vastly larger than that, astrophysicists are proximity-challenged. Most of the objects of our affection lie forever out of reach and are, at best, barely visible from Earth. They don’t grow in a laboratory, they release stupendous energy, and they’re immune to manipulation. For the most part, they are accessible only by night. We can’t easily visit them in their natural habitat, and, beyond our solar system, it’s not yet possible to touch (or contaminate) them. Though smitten by the cosmos, we have no choice but to embrace it from multiple degrees of separation: when we want to know the motions of a star, we examine not the star itself, not an image of the star, not even the spectrum derived from the light recorded in an image of the star, but rather the shifts in the patterns
in the spectrum derived from the light recorded in an image of the star. A convoluted consummation.
So astrophysicists have learned to be lateral thinkers, to come up with indirect solutions. True, scientists in general are skillful problem solvers. Physicists can build a better vacuum chamber or a bigger particle accelerator. Chemists can purify their ingredients, change the temperature, try out a novel catalyst. Biologists can experiment on organisms born and bred in the lab. Physicians can question their patients. Animal behaviorists can spend hours watching clans of their favorite creatures. Geologists can scrutinize a hillside ravine or dig up sample rocks. But astrophysicists need to find another way, never forgetting that we’re the passive party in a singularly one-sided relationship.
Down here in our labs and offices, though, we become somewhat more aggressive, owing to our mutually advantageous alliance with the military. Many significant advances in our understanding of the cosmos are by-products of government investment in the apparatus of warfare, and many innovative instruments of destruction are by-products of advances in astrophysics.
As a group, astrophysicists don’t embrace a military approach to problem solving. Rarely do you find an astrophysicist thinking, I’ll do a or b so that it will someday help the military, or, I hope the military does x or y so that someday it will help me. The connection is more fundamental, more deeply embedded in the nature of the astrophysicist’s domain and the capabilities of the astrophysicist’s tools. Space—our turf—is the new high ground, the new command post, the new military force multiplier, the new locus of control, although in fact it’s not very new. Space has been politicized and militarized from the opening moments of the race to reach it.
The recurrent interconnections between sky work and war work have not gone unnoticed by either space scientists or space policy analysts. In his 1981 book Cosmic Discovery, Martin Harwit, director of the Smithsonian’s National Air and Space Museum from 1987 to 1995,108 profiles five turning points in the history of astronomy—the telescope, the birth of cosmic-ray astronomy, the birth of radio astronomy, the birth of X-ray astronomy, and finally the then-recent discovery of distant gamma-ray bursts. Only the account of radio astronomy’s early days includes no reference to military involvement. Harwit further points out that the discoveries of new phenomena often involved equipment originally designed for use by the military. British political scientist Michael J. Sheehan puts forth a related position in his 2007 book The International Politics of Space: “Space has always been militarised. Military considerations were at the heart of the original efforts to enter space and have remained so to the present day.”109
Much has been written about the making of the atomic bomb. The relationship between physics and war is clear: the ruler and the general want to threaten or obliterate targets; destruction requires energy; the physicist is the expert on matter, motion, and energy. It’s the physicist who invents the bomb. But to destroy a target, you have to locate it precisely, identify it accurately, and track it as it moves. That’s where astrophysics comes in. Neither protagonists nor accomplices, astrophysicists are accessories to war. We don’t design the bombs. We don’t make the bombs. We don’t calculate the damage a bomb will wreak. Instead, we calculate how stars in our galaxy self-destruct through thermonuclear explosions—calculations that may prove helpful to those who do design and make thermonuclear bombs.
Our utility is broad. We understand trajectories and orbits, and so we’re key to the launching of both spacecraft and space weapons. We’re skilled at the art and science of image analysis, especially at the limits of detection—a suite of techniques indispensable to the selection of targets and the interpretation of elusive evidence. We understand reflectivity and absorptivity, and so we’ve laid the groundwork for an entire industry of stealth matériel. We can distinguish an asteroid from a spy satellite by studying the differing wavelengths of light that they absorb and reflect. We know, by their light, which molecules inhabit which celestial bodies, and so we could spot an alien intrusion if one were suddenly to appear. We recognize the multi-spectral light signatures of naturally occurring collisions, explosions, impacts, magnetic storms, shock waves, and sonic booms, and we can differentiate them from dangers and catastrophes induced by a living agent.
But whether an astrophysicist’s work is done at the behest of the military or for the sake of science, the tools are the same. The techniques are the same.
After a swift yet peaceful journey of tens, hundreds, or thousands of light-years, the sharp pinpoint of light from a distant star reaches Earth’s lower atmosphere. A fraction of a second later, skygazers with telescopes see it as a fuzzy, jiggling blob, while naked-eye skygazers see it as a pleasantly twinkling, distant jewel. Back in 1704, Sir Isaac Newton was already worried that twinkling would hamper astronomers of the future:
If the Theory of making Telescopes could at length be fully brought into Practice, yet there would be certain Bounds beyond which Telescopes could not perform. For the Air through which we look upon the Stars, is in a perpetual Tremor; as may be seen by the . . . twinkling of the fix’d Stars.110
Newton went on to suggest that a mountaintop might be a good place to put a telescope, and he was right. But even given optimal placement, the atmosphere may not cooperate. Robert W. Duffner, an optics historian at the Air Force Research Laboratory in New Mexico, describes looking at stars through the atmosphere as akin to looking through the frosted glass of a shower stall: you see shapes but no detail.111
What happens when a star twinkles? The atmosphere is a tapestry of air patches with different temperatures and densities, and thus different optical properties. Each time a light wave crosses from one patch to another, it bends a little and slightly shifts direction. The scene resembles the fate of pond ripples moving across an untidy ridge of stones, which disturb the smooth shape of each wave crest before it reaches the shore. Under the influence of our undulating atmosphere, a star’s image will not only drift to and fro but will also change in brightness from one moment to the next. A time-lapse photograph will record a smeared circular blob; your eyes will record a twinkling star. In fully turbulent air, the patches are small and numerous, causing the star to twinkle ferociously.
What’s needed is a way to compensate for how the varying patches of atmosphere disrupt the starlight. This amounts to reconstructing our pristine pond ripple after it has passed over the rocks. To do this, you’ll have to record the light hundreds of times per second. Each time, you’ll need sufficient light to simultaneously track and correct for any ongoing atmospheric changes. To facilitate the corrections, you’ll need a luminous “guide star” for comparison, close enough to your target object to be influenced by the same patch of atmosphere at the same time. Alas, such stars are few and far between, unlikely to sit conveniently nearby on the sky. The solution? Create an artificial star. Send a powerful laser beam high above the stratosphere, where turbulence is minimal and there’s a continually replenished supply of sodium atoms left behind by vaporized meteors. Excite some of the sodium atoms so that they radiate back at you, and position this now-luminous spot exactly where it will serve you best.
Before the 1990s, anybody seeking high-resolution images of a star field or galaxy on a twinkle-ridden night had two obvious options. Plan A: Close the telescope dome and go to bed. Plan B: Raise several billion dollars, build a new telescope, launch it into orbit above the layers of atmospheric disturbance, and observe the universe from there. In 1990 Plan B gave us the Hubble Space Telescope, which offered a leap in resolution from the ground-based telescopes of its day as impressive as the leap from the unaided eye to Galileo’s first telescope.
But now there’s a less obvious remedy to the twinkling problem. Welcome to the field of adaptive optics. This innovation uses laser guide stars and deformable mirror surfaces to correct for the unwanted twinkling caused by Earth’s atmosphere. A matrix of push-pull pistons affixed to the back of a deformable telescope mirror continually adjusts the exact sh
ape in such a way as to correct for the transient atmospheric turbulence, canceling out the patch-to-patch, moment-to-moment atmospheric variations. All adaptive optics systems also include a second, non-segmented mirror to monitor and correct the wandering of the image due to larger-scale motions in our atmosphere. Rounding out the system components, adaptive optics uses beam splitters, interferometers, monitoring cameras, and of course specialized software. The whole contraption is costly and complex. It’s also strikingly effective, enabling the sharpness of ground-based images to rival that of images taken from space.112
Did civilian astrophysicists make adaptive optics a reality? No. But that was not for want of trying. From the 1950s onward, astrophysicists developed concepts and potential solutions. But while they were still focusing on possibilities, the US Department of Defense was secretly achieving results—through classified research funded and conducted from the late 1960s through the late 1980s by organizations such as the Defense Advanced Research Projects Agency, the Air Force Research Laboratory and Phillips Laboratory at Kirtland Air Force Base in New Mexico, the Air Force Maui Optical Site, Itek Optical Systems near Hanscom Air Force Base in Massachusetts, the Air Force’s Rome Air Development Center in New York, MIT’s Lincoln Laboratory, the Visibility Laboratory at Scripps Institution of Oceanography, and the Strategic Defense Initiative. Additional expertise came from the top-secret national-security science advisory group called the Jasons. Formed in 1960 and comprising MacArthur geniuses, Nobel laureates, and prominent academic physicists, the Jasons provide the military with bleeding-edge ideas for how to wage war, end war, and prevent war. From their earliest years of summertime meetings, there have always been a few Jasons whose specialty is the cosmos.113
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