Accessory to War

by Neil DeGrasse Tyson

  But let’s say you want to move around at will, unobtrusively, rather than be locked in place like a window, yet also be functionally invisible. Nowadays you could coat yourself in a foam, fiber, or powder that reflects no light. An adversary would be prevented from illuminating you, although you would still block the scenery behind your position, and a clever hunter would be able to detect you by the exact absence of your human form rather than its presence. You could also cloak yourself in scales or mirrors that redirect the light that hits you, sending little or none of it back to the source—similar to the design principle of stealth aircraft, which redirect incident radar on their fuselage back out in many different directions. Another, quite recent option is a fabric made of tiny light-transmitting beads that can transpose an image from behind you to in front of you. To an observer, it’s as though you’re not there at all, the same effect as donning a Star Trek cloaking device. Other possibilities: If you’re an architect designing an offensively gargantuan skyscraper, you could cover it in LEDs that project the surrounding scenery minus its presence. If you’re a spy monitoring the doorway down the street, you might want to vanish into thin air, magician-style, via a clever sequence of lenses or mirrors placed between yourself and the doorway.16

  Achieving invisibility through temporary camouflage is an intuitive, imaginative tactic, with limited reliability. Achieving invisibility through stealth is a scientific tactic, grounded in an understanding of the physical laws of reflection and refraction as well as on centuries’ worth of discoveries about the many forms of light energy to which our senses have no access.

  By the end of the nineteenth century, we could no longer delude ourselves into believing that the universe communicates with us only through the narrow band of light available to the human retina. With the discovery of multiple bands of light, it became unthinkable to design a defense strategy solely around visible light or to explain the cosmos solely on the basis of observations done in visible light—like composing a symphony from only a single octave’s span of notes. A new term was needed—“astrophysics,” as distinct from “astronomy”—to clarify the difference between identifying the presence and position of celestial bodies and the more convoluted process of ascertaining their components, their mass, their paths, and their history. Light would become an encyclopedia. The effort to detect things too dim to register on the human eye would become the longest-running show in astrophysics.

  All this required new technology and new techniques. The astrophysicist sought detectors capable of capturing every wavelength; the warfighter sought offensive systems capable of exploiting those wavelengths and defensive systems capable of eluding them. The radio band felt like a good bet to both sides. For the military, pre-nukes, it became nearly indispensable; for space scientists, it offered new avenues to new information. Working together, they helped shape the course of World War II.

  Although the existence of radio waves was demonstrated as early as the mid-1880s, it took decades of competing theories from physicists and mathematicians, plus mounting experimental evidence, before scientists and engineers could work with them, control them, and exploit them. The first order of business was to understand their behavior: how some radio waves manage to travel intact around the curved surface of Earth and how the upper atmosphere—the ionosphere—affects their journey through space; the causes of radio noise, better known as static; the best shape and material for the antenna; whether the direction of transmission matters; whether the Sun and other celestial neighbors reflect or emit radio waves. And so forth.

  By 1919, the biggest transmission question had been answered: radio waves travel not because of being diffracted by Earth’s curved surface but because of being reflected by Earth’s ionosphere—a several-hundred-mile-thick series of layers in the upper atmosphere that seethe with charged particles (ions) produced by the Sun’s high-energy light knocking electrons off our atmosphere’s resident atoms and molecules. By 1937, the rest of the answers about transmission were mostly in place. Having approached the question differently, different researchers came up with different pieces of the total answer—and inadvertently advanced such varied endeavors as meteorology and mathematical theory. As one historian of science describes it, “They started from what they wanted to know, and found what they did not expect to learn.” While chasing a solution to a practical engineering problem, he writes, the US Navy ended up contributing to pure science.17

  Through the late 1930s, much theoretical and practical work focused on sending and receiving radio signals. Not until those twin problems were understood and mastered could the twin problems of detection and its avoidance even be addressed. But something else supremely important happened on the radio front in the 1930s—another practical project that resulted in another serendipitous contribution to science. In fact, it birthed a whole new branch of astrophysics.

  The object we know as a telephone started out as a device for relaying radio waves. Nowadays our mobile phones relay microwaves. Back in the medieval era of telephone communication, AT&T—the American Telephone and Telegraph Company—was a giant government-approved monopoly whose motto was “One System, One Policy, Universal Service.” AT&T’s first long-distance call within the United States took place in 1885 between New York and Philadelphia. Transatlantic calling service via two-way radio (also called radiotelephone) began in 1927, but the only place you could call that year was London. Transpacific calling began in 1934, to Tokyo. One big problem with long-distance service, aside from the price tag, was, as AT&T itself describes the situation, “Telephone service via available radio technology was far from ideal: it was subject to fading and interference, and had strictly limited capacity.”18 Another, double-edged problem was that few channels were available in the low-frequency, long-wavelength part of the radio spectrum, while the higher-frequency, short-wavelength part of it—the part that could carry much more information—was still unfamiliar territory, scientifically and technologically. Not until it was mastered could there be live FM stereo broadcasts from the Metropolitan Opera, which began in the 1970s.

  But let’s not get ahead of ourselves.

  In 1928 AT&T’s three-year-old R & D facility, Bell Telephone Laboratories, hired a young physicist named Karl Jansky to study Earth-based radio sources that might account for all the hissing and fading—the noise, the static—in terrestrial radio communications. After constructing a novel rotating antenna, tuned to capture a radio wavelength of 14.6 meters (frequency: 20.5 MHz), Jansky spent several years waiting for signals to drift by his receiver, studying the signal patterns, and scrupulously interpreting the results. In 1932 he published his preliminary findings.

  Jansky’s tone was modest and careful, his claims limited, his attention to fact honorable. In his 1932 paper, concerning “the direction of arrival and intensity of static on short waves,” he cites three identifiable types of static: one from local thunderstorms, one from distant thunderstorms, and an unidentifiable third, “a steady hiss type static of unknown origin” that seemed to be “associated with the sun.” In his 1933 paper, after a year of examining only that third type of static, Jansky stated that its origin lay far, far beyond the Sun. It must be somewhere “fixed in space,” he concluded, in a location either “very near the point where the line drawn from the sun through the center of the huge galaxy of stars and nebulae of which the sun is a member would strike the celestial sphere.”19 In short, approximately the heart of the Milky Way.20

  Every twenty-three hours and fifty-six minutes, Earth completes one rotation relative to the stars. Every twenty-three hours and fifty-six minutes, the center of the Milky Way returns to the same angle and same elevation on the sky when viewed from Earth. Every twenty-three hours and fifty-six minutes, Jansky’s fixed point in space hissed past his merry-go-round. Hence the inescapable conclusion that his fixed point in space was the center of the Milky Way. If the source had been our Sun, the interval between hisses would have been twenty-four hours, not four minute
s less.

  That was the birth of radio astronomy, though the end of Jansky’s career as a radio astronomer. Rather than agreeing to let him build the hundred-foot dish he proposed as a follow-up, Bell Labs—which now had answers to its practical questions and wasn’t about to start funding basic research—assigned Jansky to other tasks.

  Fortunately, a young radio engineer from Illinois, Grote Reber—who briefly fell victim to bad timing, having begun a job search just as the Great Depression was deepening—decided to forge ahead and build his own radio telescope in his own backyard. In 1938 Reber confirmed Jansky’s discovery, then spent the next five years making low-res maps of the radio sky all on his lonesome. Half a century later, Reber published a reader-friendly article titled “A Play Entitled the Beginning of Radio Astronomy,” in which (talk about timing!) he points out that Jansky

  was observing near the bottom of a low solar activity minimum. The ionospheric hole at 20.5 MHz was open from zenith to horizon day and night. A few years earlier or later, the observations would have been confused by ionospheric effects, particularly during the day. Jansky is an example of the right man at the right place doing the right thing at the right time.21

  Every band of light requires its own detection hardware. No single telescope can focus light of all bands. If you’re gathering X-rays, whose wavelengths are very short, your reflector will have to be supersmooth lest it distort the rays. But if you’re gathering radio waves, your reflector could be made of polished chicken wire that you’ve bent with your hands, because the irregularities in the wire would be smaller than the wavelength of the radio waves you’re trying to detect. The surface smoothness of the mirror simply needs to be commensurate with the scale of the wavelength you want to measure. And don’t forget about resolution: if you want a decent level of detail, your reflector’s diameter must be much wider than the wavelengths you want to detect.

  The detectors built by Jansky and Reber were the first effective radio telescopes—and the earliest invisible-light success stories. Glass mirrors were out of the question, because radio waves would pass right through them. The reflectors would have to be made of metal.

  Jansky’s hundred-foot-long contraption looked a little like the sprinkler system on a modern corporate farm. The antenna was a series of tall, rectangular metal frames, secured with wooden cross-supports and mounted on the front wheels and axles of junked Model-T Fords. Hooked up to a small motor, the whole thing rolled around a turntable, completing a full 360 every twenty minutes. Inside a nearby shed was a receiver equipped with an automatic temperature recorder that had been rejiggered to record the strength of the radio signals.22

  On the other hand, Reber’s telescope was a single nine-meter-wide dish, progenitor of generations of radio telescopes that rely on the dish—often parabolic, like a tilted half eggshell—to collect the incoming radio waves and then bounce them up to a receiver. In other words, the dish is an antenna that works like a mirror. What Reber achieved was detection; his apparatus wasn’t big enough to achieve good resolution. But in the early 1940s, merely detecting an invisible cosmic phenomenon was a huge step forward.

  Predictably, dish antennas soon got bigger and better. Mark I, the planet’s first really big radio telescope—a single, steerable, solid-steel dish 76 meters wide—saw first light in the summer of 1957 and is still on call at the Jodrell Bank Observatory in northwest England. More recent radio telescopes are not just big; they’re colossal. Built into a large natural sinkhole near the north-central coast of Puerto Rico is the 305-meter, non-steerable dish of the Arecibo Observatory. Completed in 1963, this spectacular construction—damaged but far from destroyed by Category 5 Hurricane Maria in September 201723—was under the supervision of the US Department of Defense until 1969.

  Initial funding for Arecibo traces to an anti-ballistic-missile program, Project Defender, supported by the Advanced Research Projects Agency. A precursor to the Strategic Defense Initiative, Project Defender addressed US worries that decoys would successfully prevent defensive action against intercontinental ballistic missiles. The Arecibo radio telescope held hope that the radar signature from an actual warhead passing through Earth’s ionosphere would differ enough from the radar signature of a decoy that the deadly missile could be identified and shot down. Oh, and the telescope could do astrophysics on the side.

  The shape of Arecibo’s curved dish is a segment of a true sphere rather than of a traditional paraboloid. Since the dish itself is stationary, an innovative movable detector—positioned high above the dish—serves to “point” the telescope toward different areas of the sky. The optics of a spherical surface uniquely permit this trick. In addition, Arecibo’s huge size assured it would detect extremely weak radio signals emitted by objects in deep space as well as by radio-busy layers of Earth’s own atmosphere, such as the ionosphere. Plus, the telescope not only detects radio signals; it can transmit them as well. These transmitted signals, beamed out to space in radar mode and then bounced back to Earth when they hit something reflective, can map the shapes and track the orbits of planets, asteroids, and comets.

  In 1974, the Arecibo telescope was the first to transmit, on purpose, a radio message to aliens—specifically, to a large and crowded cluster of stars in our Milky Way galaxy presumed to be orbited by planets that might host intelligent life. Another of the observatory’s many highlights was its role in the 1993 Nobel Prize in Physics, which went to Russell A. Hulse and Joseph H. Taylor Jr. for their 1974 discovery of a binary pulsar suitable for testing Einstein’s general theory of relativity.

  For almost fifty years, Arecibo held the title of world’s largest single-dish radio telescope. In 2016 that distinction passed to an even more spectacular construction: FAST, the Five-hundred-meter Aperture Spherical Telescope. Set into a huge limestone depression in a thinly populated, mountainous region of southwestern China, FAST’s dish is so large that, as the chief scientist at China’s National Astronomical Observatories put it, “if you fill it with wine, every one of the world’s seven billion people could get a share of about five bottles.” As with Arecibo, its shape is a section of a sphere, but that’s just an engineering detail. Because of its size, FAST can observe with much greater sensitivity than Arecibo can.24 At 500 meters in diameter, it enjoys nearly three times the collecting area of the 305-meter Arecibo telescope. Nothing in the world comes close. If something out there falls just below the detection limit of Arecibo, and FAST is pointed in that direction, it will easily extract the signal from the din of cosmic noise. So there’s a good chance that the first humans who will ever talk to aliens via radio waves will be Chinese astrophysicists. No nation, after all, has exclusive access to the universe.

  But when detail, more than dimness, is what skygazers seek, they turn instead to arrays of smaller dishes, spread across many kilometers of landscape. By pointing all the separate dishes at the same spot on the sky and cleverly combining their signals, these arrays—known as interferometers—achieve the equivalent resolution of one lone dish of unachievably wide diameter, equal to the extent of the array itself. “Supersize me” was the unwritten motto for radio interferometers long before the fast-food industry adopted the slogan, and they form a jumbo class unto themselves. Among their ranks, sprinkled around the world, are the Very Long Baseline Array (ten 25-meter dishes spanning five thousand miles from Hawaii to the Virgin Islands), the Giant Metrewave Radio Telescope (thirty lightweight mesh dishes, each 45 meters across, spanning sixteen miles of arid plains east of Mumbai, India), and the Atacama Large Millimeter/submillimeter Array (sixty-six dishes—some 12-meter, some 7-meter—clustered at an altitude of more than sixteen thousand feet in the driest region of the Chilean Andes).

  Not too far in the future, these enormous interferometers will be dwarfed by the Square Kilometre Array’s thousands of dishes augmented by battalions of fixed “aperture array” telescopes spread across wasteland—some looking, from a cloud’s-eye view, like gargantuan coins with sharply notched edg
es, others like miniature Eiffel Towers. Installed in a spiral configuration across southern Africa and western Australia, the SKA will have its headquarters at Jodrell Bank.

  Even with the finest detectors, there are limitations and irritations. While an extremely low frequency radio wave can be thousands of miles long, the largest individual radio telescope dishes are only several hundred meters across, and interferometer arrays cannot detect light whose wavelength is longer than the width of the array’s broadest dish. So, ultra low and extremely low frequency (ULF and ELF) radio waves pass across and through Earth undetected by the kinds of radio telescopes that astrophysicists know and love. Plus, various bands of detectable radio waves get degraded by terrestrial communication towers and other trappings of modern civilization. Then there’s the problem of turbulence in the ionosphere, whose various levels propagate radio transmissions as well as interfere with them and whose impact changes according to the time of day and the frequency of the wave.

  The ionosphere has figured prominently in the modern pursuits of both warmakers and space scientists. The Third Reich’s V-2 rockets—the world’s first ballistic missiles—had to pass through it unscathed before falling out of the sky onto their targets. Of equal military significance is the role of the ionosphere and its investigators in the history of radar, an acronym for radio detection and ranging.25

  Detection, of course, is simply about determining and/or confirming the presence of something. Ranging is about calculating its distance and direction. The idea is straightforward: transmit radio waves toward a distant object—an asteroid, the Moon, a bomber, a submarine—and see if any radio waves bounce back to you. If they do, the time delay as well as the intensity, frequency, and shape of the waves can tell you something about the object’s shape as well as how far away it is, in which direction it’s moving, and how fast. Nowadays, asteroids are the main cosmic targets for radar studies, allowing the interested party to map the size and shape of the rock and to establish precise orbital parameters lest we discover that one of them is headed toward Earth.