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Invisible Light

And therefore as a stranger give it welcome.

There are more things in heaven and earth, Horatio,

Than are dreamt of in your philosophy


Before 1800 the word “light,” apart from its use as a verb and an adjective, referred just to visible light. But early that year the English astronomer William Herschel observed some warming that could only have been caused by a form of light invisible to the human eye. Already an accomplished observer, Herschel had discovered the planet Uranus in 1781 and was now exploring the relation between sunlight, color, and heat. He began by placing a prism in the path of a sunbeam. Nothing new there. Sir Isaac Newton had done that back in the 1600s, leading him to name the familiar seven colors of the visible spectrum: red, orange, yellow, green, blue, indigo, and violet. (Yes, the colors do indeed spell Roy G. Biv.) But Herschel was inquisitive enough to wonder what the temperature of each color might be. So he placed thermometers in various regions of the rainbow and showed, as he suspected, that different colors registered different temperatures.†

Well-conducted experiments require a “control”—a measurement where you expect no effect at all, and which serves as a kind of idiot-check on what you are measuring. For example, if you wonder what effect beer has on a tulip plant, then also nurture a second tulip plant, identical to the first, but give it water instead. If both plants die—if you killed them both—then you can’t blame the alcohol. That’s the value of a control sample. Herschel knew this, and laid a thermometer outside of the spectrum, adjacent to the red, expecting to read no more than room temperature throughout the experiment. But that’s not what happened. The temperature of his control thermometer rose even higher than in the red.

Herschel wrote:

[I] conclude, that the full red falls still short of the maximum of heat; which perhaps lies even a little beyond visible refraction. In this case, radiant heat will at least partly, if not chiefly, consist, if I may be permitted the expression, of invisible light; that is to say, of rays coming from the sun, that have such a momentum as to be unfit for vision.††

Holy s#%t!

Herschel inadvertently discovered “infra” red light, a brand-new part of the spectrum found just “below” red, reported in the first of his four papers on the subject.

Herschel’s revelation was the astronomical equivalent of Antonie van Leeuwenhoek’s discovery of “many very little living animalcules, very prettily a-moving”††† in the smallest drop of lake water. Leeuwenhoek discovered single-celled organisms—a biological universe. Herschel discovered a new band of light. Both hiding in plain sight.

Other investigators immediately took up where Herschel left off. In 1801 the German physicist and pharmacist Johann Wilhelm Ritter found yet another band of invisible light. But instead of a thermometer, Ritter placed a little pile of light-sensitive silver chloride in each visible color as well as in the dark area next to the violet end of the spectrum. Sure enough, the pile in the unlit patch darkened more than the pile in the violet patch. What’s beyond violet? “Ultra” violet, better known today as UV.

Filling out the entire electromagnetic spectrum, in order of low-energy and low-frequency to high-energy and high-frequency, we have: radio waves, microwaves, infrared, ROYGBIV, ultraviolet, X-rays, and gamma rays. Modern civilization has deftly exploited each of these bands for countless household and industrial applications, making them familiar to us all.

After the discovery of UV and IR, sky-watching didn’t change overnight. The first telescope designed to detect invisible parts of the electromagnetic spectrum wouldn’t be built for 130 years. That’s well after radio waves, X-rays, and gamma rays had been discovered, and well after the German physicist Heinrich Hertz had shown that the only real difference among the various kinds of light is the frequency of the waves in each band. In fact, credit Hertz for recognizing that there is such a thing as an electromagnetic spectrum. In his honor, the unit of frequency—in waves per second—for anything that vibrates, including sound, has duly been named the hertz.

Mysteriously, astrophysicists were a bit slow to make the connection between the newfound invisible bands of light and the idea of building a telescope that might see those bands from cosmic sources. Delays in detector technology surely mattered here. But hubris must take some of the blame: how could the universe possibly send us light that our marvelous eyes cannot see? For more than three centuries—from Galileo’s day until Edwin Hubble’s—building a telescope meant only one thing: making an instrument to catch visible light, enhancing our biologically endowed vision.

A telescope is merely a tool to augment our meager senses, enabling us to get better acquainted with faraway places. The bigger the telescope, the dimmer the objects it brings into view; the more perfectly shaped its mirrors, the sharper the image it makes; the more sensitive its detectors, the more efficient its observations. But in all cases, every bit of information a telescope delivers to the astrophysicist comes to Earth on a beam of light.

Celestial happenings, however, don’t limit themselves to what’s convenient for the human retina. Instead, they typically emit varying amounts of light simultaneously in multiple bands. So without telescopes and their detectors tuned across the entire spectrum, astrophysicists would remain blissfully ignorant of some mind-blowing stuff in the universe.

Take an exploding star—a supernova. It’s a cosmically common and seriously high-energy event that generates prodigious quantities of X-rays. Sometimes, bursts of gamma rays and flashes of ultraviolet accompany the explosions, and there’s never a shortage of visible light. Long after the explosive gases cool, the shock waves dissipate, and the visible light fades, the supernova “remnant” keeps on shining in the infrared, while pulsing in radio waves. That’s where pulsars come from, the most reliable timekeepers in the universe.

Most stellar explosions take place in distant galaxies, but if a star were to blow up within the Milky Way, its death throes would be bright enough for everyone to see, even without a telescope. But nobody on Earth saw the invisible X-rays or gamma rays from the last two supernova spectaculars hosted by our galaxy—one in 1572 and another in 1604—yet their wondrous visible light was widely reported.

The range of wavelengths (or frequencies) that comprise each band of light strongly influences the design of the hardware used to detect it. That’s why no single combination of telescope and detector can simultaneously see every feature of such explosions. But the way around that problem is simple: gather all observations of your object, perhaps obtained by colleagues, in multiple bands of light. Then assign visible colors to invisible bands of interest, creating one meta, multi-band image. That’s precisely what Geordi from the television series Star Trek: The Next Generation sees. With that power of vision, you miss nothing.

Only after you identify the band of your astrophysical affections can you begin to think about the size of your mirror, the materials you’ll need to make it, the shape and surface it must have, and the kind of detector you’ll need. X-ray wavelengths, for example, are extremely short. So if you’re accumulating them, your mirror had better be super-smooth, lest imperfections in the surface distort them. But if you’re gathering long radio waves, your mirror could be made of chicken wire that you’ve bent with your hands, because the irregularities in the wire would be much smaller than the wavelengths you’re after. Of course, you also want plenty of detail—high resolution—so your mirror should be as big as you can afford to make it. In the end, your telescope must be much, much wider than the wavelength of light you aim to detect. And nowhere is this need more evident than in the construction of a radio telescope.

Radio telescopes, the earliest non-visible-light telescopes ever built, are an amazing subspecies of observatory. The American engineer Karl G. Jansky built the first successful one between 1929 and 1930. It looked a bit like the moving sprinkler system on a farmerless farm. Made from a series of tall, rectangular metal frames secured with wooden cross-supports and flooring, it turned in place like a merry-go-round on wheels built with spare parts from a Model T Ford. Jansky had tuned the hundred-foot-long contraption to a wavelength of about fifteen meters, corresponding to a frequency of 20.5 megahertz.†††† Jansky’s agenda, on behalf of his employer, Bell Telephone Laboratories, was to study any hisses from Earth-based radio sources that might contaminate terrestrial radio communications. This greatly resembles the task that Bell Labs gave Penzias and Wilson, thirty-five years later, to find microwave noise in their receiver, as we saw in chapter 3, which led to the discovery of the cosmic microwave background.

By spending a couple of years painstakingly tracking and timing the static hiss that registered on his jury-rigged antenna, Jansky had discovered that radio waves emanate not just from local thunderstorms and other known terrestrial sources, but also from the center of the Milky Way galaxy. That region of the sky swung by the telescope’s field of view every twenty-three hours and fifty-six minutes: exactly the period of Earth’s rotation in space and thus exactly the time needed to return the galactic center to the same angle and elevation on the sky. Karl Jansky published his results under the title “Electrical Disturbances Apparently of Extraterrestrial Origin.”†††††

With that observation, radio astronomy was born—but minus Jansky himself. Bell Labs retasked him, preventing him from pursuing the fruits of his own seminal discovery. A few years later, though, a self-starting American named Grote Reber, from Wheaton, Illinois, built a thirty-foot-wide, metal-dish radio telescope in his own backyard. In 1938, under nobody’s employ, Reber confirmed Jansky’s discovery, and spent the next five years making low-resolution maps of the radio sky.

Reber’s telescope, though without precedent, was small and crude by today’s standards. Modern radio telescopes are quite another matter. Unbound by backyards, they’re sometimes downright humongous. MK 1, which began its working life in 1957, is the planet’s first genuinely gigantic radio telescope—a single, steerable, 250-foot-wide, solid-steel dish at the Jodrell Bank Observatory near Manchester, England. A couple of months after MK 1 opened for business, the Soviet Union launched Sputnik 1, and Jodrell Bank’s dish suddenly became just the thing to track the little orbiting hunk of hardware—making it the forerunner of today’s Deep Space Network for tracking planetary space probes.

The world’s largest radio telescope, completed in 2016, is called the Five-hundred-meter Aperture Spherical radio Telescope, or “FAST” for short. It was built by China in their Guizhou Province, and is larger in area than thirty football fields. If aliens ever give us a call, the Chinese will be the first to know.

Another variety of radio telescope is the interferometer, comprising arrays of identical dish antennas, spread across swaths of countryside and electronically linked to work in concert. The result is a single, coherent, super-high-resolution image of radio-emitting cosmic objects. Although “supersize me” was the unwritten motto for telescopes long before the fast food industry coined the slogan, radio interferometers form a jumbo class unto themselves. One of them, a very large array of radio dishes near Socorro, New Mexico, is officially called the Very Large Array, with twenty-seven eighty-two-foot dishes positioned on tracks crossing twenty-two miles of desert plains. This observatory is so cosmogenic, it has appeared as a backdrop in the films 2010: The Year We Make Contact (1984), Contact (1997), and Transformers (2007). There’s also the Very Long Baseline Array, with ten eighty-two-foot dishes spanning 5,000 miles from Hawaii to the Virgin Islands, enabling the highest resolution of any radio telescope in the world.

In the microwave band, relatively new to interferometers, we’ve got the sixty-six antennas of ALMA, the Atacama Large Millimeter Array, in the remote Andes Mountains of northern Chile. Tuned for wavelengths that range from fractions of a millimeter to several centimeters, ALMA gives astrophysicists high-resolution access to categories of cosmic action unseen in other bands, such as the structure of collapsing gas clouds as they become nurseries from which stars are born. ALMA’s location is, by intention, the most arid landscape on Earth—three miles above sea level and well above the wettest clouds. Water may be fine for microwave cooking but it’s bad for astrophysicists, because the water vapor in Earth’s atmosphere chews up pristine microwave signals from across the galaxy and beyond. These two phenomena are, of course, related: water is the most common ingredient in food, and microwave ovens primarily heat water. Taken together, you get the best indication that water absorbs microwave frequencies. So if you want clean observations of cosmic objects, you must minimize the amount of water vapor between your telescope and the universe, just as ALMA has done.

At the ultrashort-wavelength end of the electromagnetic spectrum you find the high-frequency, high-energy gamma rays, with wavelengths measured in picometers.†††††† Discovered in 1900, they were not detected from space until a new kind of telescope was placed aboard NASA’s Explorer XI satellite in 1961.

Anybody who watches too many sci-fi movies knows that gamma rays are bad for you. You might turn green and muscular, or spiderwebs might squirt from your wrists. But they’re also hard to trap. They pass right through ordinary lenses and mirrors. How, then, to observe them? The guts of Explorer XI’s telescope held a device called a scintillator, which responds to incoming gamma rays by pumping out electrically charged particles. If you measure energies of the particles, you can tell what kind of high-energy light created them.

Two years later the Soviet Union, the United Kingdom, and the United States signed the Limited Test Ban Treaty, which prohibited nuclear testing underwater, in the atmosphere, and in space—where nuclear fallout could spread and contaminate places outside your own country’s perimeter. But this was the Cold War, a time when nobody believed anybody about anything. Invoking the military edict “trust but verify,” the U.S. deployed a new series of satellites, the Velas, to scan for gamma ray bursts that would result from Soviet nuclear tests. The satellites indeed found bursts of gamma rays, almost daily. But Russia wasn’t to blame. These came from deep space—and were later shown to be the calling card of intermittent, distant, titanic stellar explosions across the universe, signaling the birth of gamma ray astrophysics, a new branch of study in my field.

In 1994, NASA’s Compton Gamma Ray Observatory detected something as unexpected as the Velas’ discoveries: frequent flashes of gamma rays right near Earth’s surface. They were sensibly dubbed “terrestrial gamma-ray flashes.” Nuclear holocaust? No, as is evident from the fact that you’re reading this sentence. Not all bursts of gamma rays are equally lethal, nor are they all of cosmic origin. In this case, at least fifty bursts of these flashes emanate daily near the tops of thunderclouds, a split second before ordinary lightning bolts strike. Their origin remains a bit of a mystery, but the best explanation holds that in the electrical storm, free electrons accelerate to near the speed of light and then slam into the nuclei of atmospheric atoms, generating gamma rays.

Today, telescopes operate in every invisible part of the spectrum, some from the ground but most from space, where a telescope’s view is unimpeded by Earth’s absorptive atmosphere. We can now observe phenomena ranging from low-frequency radio waves a dozen meters long, crest to crest, to high-frequency gamma rays no longer than a quadrillionth of a meter. That rich palette of light supplies no end of astrophysical discoveries: Curious how much gas lurks among the stars in galaxies? Radio telescopes do that best. There is no knowledge of the cosmic background, and no real understanding of the big bang, without microwave telescopes. Want to peek at stellar nurseries deep inside galactic gas clouds? Pay attention to what infrared telescopes do. How about emissions from the vicinity of ordinary black holes and supermassive black holes in the center of a galaxy? Ultraviolet and X-ray telescopes do that best. Want to watch the high-energy explosion of a giant star, whose mass is as great as forty suns? Catch the drama via gamma ray telescopes.

We’ve come a long way since Herschel’s experiments with rays that were “unfit for vision,” empowering us to explore the universe for what it is, rather than for what it seems to be. Herschel would be proud. We achieved true cosmic vision only after seeing the unseeable: a dazzlingly rich collection of objects and phenomena across space and across time that we may now dream of in our philosophy.

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