In the universe, the infrared window is particularly useful for probing dense clouds that contain stellar nurseries, within which infant stars are often enshrouded by leftover gas and dust. These clouds absorb most of the visible light from their embedded stars and re-radiate it in the infrared, rendering our visible-light window quite useless. This makes infrared especially useful for studying the plane of the Milky Way, because that’s where the obscuration of visible light from our galaxy’s stars is at its greatest. Back home, infrared satellite photographs of Earth’s surface reveal, among other things, the paths of warm oceanic waters, such as the North Atlantic Drift current, which swirls west of the British Isles and keeps them from becoming a major ski resort.
The visible part of the spectrum is what humans know best. The energy emitted by the Sun, whose surface temperature is about six thousand degrees above absolute zero, peaks in the visible part of the spectrum, as does the sensitivity of the human retina, which is why our sight is so useful in the daytime. Were it not for this match, we could rightly complain that some of our retinal sensitivity was being wasted.
We don’t normally think of visible light as penetrating, but light passes mostly unhindered through glass and air. Ultraviolet, however, is summarily absorbed by ordinary glass. So, if our eyes were sensitive only to ultraviolet, windows made of glass would not be much different from windows made of brick. Stars that are a mere four times hotter than the Sun are prodigious producers of ultraviolet light. Fortunately, such stars are also bright in the visible part of the spectrum, which means that their discovery has not depended on access to ultraviolet telescopes. Since our atmosphere’s ozone layer absorbs most of the ultraviolet and X-rays that impinge upon it, a detailed analysis of very hot stars can best be obtained from Earth orbit or beyond, which has become possible only since the 1960s.
As if to herald a new century of extended vision, the first Nobel Prize ever awarded in physics went to the German physicist Wilhelm Röntgen in 1901 for his discovery of X-rays. Cosmically, both X-rays and ultraviolet can indicate the presence of black holes—among the most exotic objects in the universe. Black holes are voracious maws that emit no light—their gravity is too strong for even light to escape—but their existence can be tracked by the energy emitted from heated, swirling gas nearby. Ultraviolet and X-rays are the predominant form of energy released by material just before it descends into the black hole.
It’s worth remembering that the act of discovery does not require that you understand, either in advance or after the fact, what you’ve discovered. That’s what happened with the cosmic microwave background. It also happened with gamma-ray bursts. Mysterious, seemingly random explosions of high-energy gamma rays scattered across the sky were first detected in the 1960s by satellites searching out radiation from clandestine Soviet nuclear-weapons tests. Only decades later did spaceborne telescopes, in concert with ground-based follow-up observations, show them to be the signature of distant stellar catastrophes.
Discovery through detection can cover a lot of territory, including subatomic particles. But one in particular virtually defies detection: the elusive neutrino. Whenever a neutron decays into an ordinary proton and an electron, a member of the neutrino clan springs into existence. Within the core of the Sun, for instance, two hundred trillion trillion trillion neutrinos are produced every second, and then pass directly out of the Sun as if it were not there at all. Neutrinos are extraordinarily difficult to capture because they have exceedingly minuscule mass and hardly ever interact with matter. Building an efficient, effective neutrino telescope thus remains an extraordinary challenge.
The detection of gravitational waves, another elusive window on the universe, would reveal catastrophic cosmic events. But as of this writing, these waves, predicted in Einstein’s 1916 theory of general relativity as “ripples” in space and time, have not yet been directly detected from any source. A good gravitational-wave telescope would be able to detect black holes orbiting one another, and distant galaxies merging. One can even imagine a time in the future when gravitational events in the universe—collisions, explosions, collapsed stars—are routinely observed. In principle, we might one day see beyond the opaque wall of cosmic microwave background radiation to the Big Bang itself. Like Magellan’s crew, who first circumnavigated Earth and saw the limits of the globe, we would then have reached and discovered the limits of the known universe.
Discovery and Society
As a surfboard rides a wave, the Industrial Revolution rode the eighteenth and nineteenth centuries on the crest of decade-by-decade advances in people’s understanding of energy as a physical concept and a transmutable entity. Engineering technology replaces muscle energy with machine energy. Steam engines convert heat into mechanical energy; dams convert the gravitational potential energy of water into electricity; dynamite converts chemical energy into explosive shock waves. In a remarkable parallel to the way these discoveries transformed earlier societies, the twentieth century saw information technology ride the crest of advances in electronics and miniaturization, birthing an era in which computer power replaced mind power. Exploration and discovery now occurred on wafers of silicon, with computers completing in minutes, and eventually in moments, what would once have required lifetimes spent in calculations. Even so, we may still be groping in the dark, because as our area of knowledge grows, so does the perimeter of our ignorance.
What is the cumulative influence of all this technology and cosmic discovery on society, aside from creating more effective instruments of destruction and further excuses to wage war? The nineteenth and early twentieth centuries saw the development of transportation that did not rely on energy from domestic animals—including the bicycle, the railroad, the automobile, and the airplane. The twentieth century also saw the introduction of liquid fuel rockets (thanks in part to Robert Goddard) and spaceships (thanks in part to Wernher von Braun). The discovery of improved means of transportation was especially crucial to geographically large but habitable nations such as the United States. So important is transportation to Americans that the disruption of traffic by any means, even if it occurs in another country, can make headlines. On August 7, 1945, for example, the day after America killed some seventy thousand Japanese in the city of Hiroshima, with tens of thousands more deaths following soon afterward, the front page of the New York Times announced, “FIRST ATOMIC BOMB DROPPED ON JAPAN.” A smaller headline, also on the front page, read, “TRAINS CANCELED IN STRICKEN AREA; Traffic Around Hiroshima Is Disrupted.” I don’t know for sure, but I would bet that day’s Japanese newspapers did not consider traffic jams to be a top news item.
Technological change affected not only destruction, of course, but also domesticity. With electricity available in every domicile, it became worthwhile to invent appliances and machines that would consume this new source of energy. Among anthropologists, one of the broad measures of the advancement of society is its per-capita consumption of energy. Old traditions die hard, though. Lightbulbs were a substitute for candles, but we still light candles at special dinners; we even buy electric chandeliers studded with lightbulbs in the shape of candle flames. And of course car engines are measured in “horse” power.
The dependence on electricity, especially among urban Americans, has reached irreversible levels. Consider New York City during the blackouts of November 1965, July 1977, and August 2003, when this decidedly twentieth-century luxury temporarily became unavailable. In 1965, many people thought the world was going to end, and in 1977 there was widespread looting. (Each blackout allegedly produced “blackout babies,” conceived in the absence of television and other technological distractions.) Apparently, our discoveries and inventions have gone from making life easier to becoming a requirement for survival.
Throughout history, discovery held risks and dangers for the discoverers themselves. Neither Magellan nor most of his crew remained alive to complete the round-the-world voyage in 1522. Most died of disease and starvation, and Magellan
himself was killed by indigenous Filipinos who were not impressed with his attempts to Christianize them. Modern-day risks can be no less devastating. At the end of the nineteenth century, investigating high-energy radiation, Wilhelm Röntgen explored the properties of X-rays and Marie Curie explored the properties of radium. Both died of cancer. The three crew members of Apollo 1 burned to death on the launchpad in 1967. The space shuttle Challenger exploded shortly after launch in 1986, while space shuttle Columbia broke up on reentry in 2003, in both cases killing all seven crew members.
Sometimes the risks extend far beyond the discoverers. In 1905 Albert Einstein introduced the equation E = mc 2, the unprecedented recipe that interchanged matter with energy and ultimately begat the atomic bomb. Coincidentally, just two years before the first appearance of Einstein’s famous equation, Orville Wright made the first successful flight in an airplane, the vehicle that would one day deliver the first atomic bombs in warfare. Shortly after the invention of the airplane, there appeared in one of the widely distributed magazines of the day a letter to the editor expressing concern over possible misuse of the new flying machine, noting that if an evil person took command of a plane, he might fly it over villages filled with innocent, defenseless people and toss canisters of nitroglycerin on them.
Wilbur and Orville Wright are, of course, no more to blame for the deaths resulting from military application of the airplane than Albert Einstein is to blame for deaths resulting from atomic bombs. For better or for worse, discoveries take their place in the public domain and are thus subject to patterns of human behavior that seem deeply embedded and quite ancient.
Discovery and the Human Ego
The history of human ideas about our place in the universe has been a long series of letdowns for everybody who likes to believe we’re special. Unfortunately, first impressions have consistently fooled us—the daily motions of the Sun, Moon, and stars all conspire to make it look as though we are the center of everything. But over the centuries we have learned this is not so. There is no center of Earth’s surface, so no culture can claim to be geometrically in the middle of things. Earth is not the center of the solar system; it is just one of multiple planets in orbit around the Sun, a revelation first proposed by Aristarchus in the third century B.C., argued by Nicolaus Copernicus in the sixteenth century, and consolidated by Galileo in the seventeenth. The Sun is about 25,000 light-years from the center of the Milky Way galaxy, and it revolves anonymously around the galactic center along with hundreds of billions of other stars. And the Milky Way is just one of a hundred billion galaxies in a universe that actually has no center at all. Finally, of course, owing to Charles Darwin’s Origin of Species and Descent of Man, it is no longer necessary to invoke a creative act of divinity to explain human origins.
Scientific discovery is rarely the consequence of an instantaneous act of brilliance, and the revelation that our galaxy is neither special nor unique was no exception. The turning point in human understanding of our place in the cosmos occurred not centuries ago but in the spring of 1920, during a now-famous debate on the extent of the known universe, held at a meeting of the National Academy of Sciences in Washington, DC, at which fundamental questions were addressed: Was the Milky Way galaxy—with all its stars, star clusters, gas clouds, and fuzzy spiral things—all there was to the universe? Or were those fuzzy spiral things galaxies unto themselves, just like the Milky Way, dotting the unimaginable vastness of space like “island universes”?
Scientific discovery, unlike political conflict or public policy, does not normally emerge from party-line politics, democratic vote, or public debate. In this case, however, two leading scientists of the day, each armed with some good data, some bad data, and some sharpened arguments, went head to head at the Smithsonian’s National Museum of Natural History. Harlow Shapley argued that the Milky Way constitutes the full extent of the universe, while Heber D. Curtis defended the opposing view.
Earlier in the century, both scientists had participated in a wave of discoveries derived primarily from classification schemes for cosmic objects and phenomena. With the help of a spectrograph (which breaks up starlight into its component colors the way raindrops break up sunlight into a rainbow), astrophysicists were able to classify objects not simply by their shape or outward appearance but by the detailed features revealed in their spectra. Even in the absence of full understanding of the cause or origin of a phenomenon, a well-designed classification scheme makes substantive deductions possible.
The nighttime sky displays a grab bag of objects whose classifications were not subject to much disagreement in 1920. Three kinds were especially relevant to the debate: the stars that are quite concentrated along the narrow band of light called the Milky Way, correctly interpreted by 1920 as the flattened plane of our own galaxy; the hundred or so titanic, roughly spherical globular star clusters that appear more frequently in just one direction of the sky; and third (or perhaps third and fourth), the inventory of fuzzy nebulae near the plane and spiral nebulae nowhere near the plane. Whatever else Shapley and Curtis intended to argue, they knew that those basic observed features of the sky could not be reasoned away. And although the data were scant, if Curtis could show that the spiral nebulae were distant island universes, then humanity would be handed the next chapter in its long series of ego-busting discoveries.
In a casual look at the night sky, stars appear uniformly spread in all directions along the Milky Way. But in fact, the Milky Way contains a mixture of stars and obscuring dust clouds that compromise lines of sight so that it becomes impossible to see the entire galaxy from within. In other words, you can’t identify where you are in the Milky Way because the Milky Way is in the way. Nothing unusual there: the moment you enter a dense forest, you have no idea where you are within it (unless you carved your initials into a tree during a previous visit). The full extent of the forest is impossible to determine because the trees are in the way.
Astronomers of the day were fairly clueless as to how far away things are, and Shapley’s estimates of distance tended to be quite generous, indeed excessive. Through various calculations and assumptions, he ended up with a galactic system more than 300,000 light-years in extent—by far the largest estimate ever made before (or since) for the size of the Milky Way. Curtis was unable to fault Shapley’s reasoning but remained skeptical nonetheless, calling the assumption “rather drastic.” Though based on the work of two leading theorists of the day, it was indeed rather drastic—and those theorists’ relevant ideas would soon be discredited, leaving Shapley with overestimates in stellar luminosities and, as a result, overestimates in the distances to his favorite objects, the globular clusters.
Curtis remained convinced that the Milky Way galaxy was much smaller than suggested by Shapley, proposing that in the absence of definitive evidence to the contrary, “the postulated diameter of 300,000 light-years must quite certainly be divided by five, and perhaps by ten.”
Who was right?
Along most paths from scientific ignorance to scientific discovery, the correct answer lies somewhere between the extreme estimates collected along the way. Such was the case here, too. Today, the generally accepted extent of the Milky Way galaxy is about 100,000 light-years—about three times Curtis’s 30,000 light-years, and one-third Shapley’s 300,000 light-years.
But that wasn’t the end of it. The two debaters had now to reconcile the extent of the Milky Way with the existence of high-velocity spiral nebulae, whose distances were even more highly uncertain, and which seemed to avoid the galactic plane altogether, earning the Milky Way the spooky alternative name “Zone of Avoidance.”
Shapley suggested that the spiral nebulae had somehow been created within the Milky Way and then forcibly ejected from their birthplace. Curtis was convinced that the spiral nebulae belonged to the same class of objects as the Milky Way itself, and proposed that a ring of “occulting matter” surrounded our galaxy—as is true of so many other spiral galaxies—and might be obliterating
distant spirals from view.
At that point, if I were the moderator, I might have ended the debate, declared Curtis the winner, and sent everybody home. But there was further evidence at hand: the “novae,” tremendously bright stars that occasionally, and very briefly, appear out of nowhere. Curtis contended that the novae formed a homogeneous class of objects that suggested “distances ranging from perhaps 500,000 light-years in the case of the Nebula in Andromeda, to 10,000,000 or more light-years for the more remote spirals.” Given those distances, those island universes would be “of the same order of size as our own galaxy.” Bravo.
Even though Shapley discounted the concept of the spiral nebulae as island universes, he no doubt wanted to appear open-minded. In his summary, which reads like a disclaimer, he entertained the possibility of other worlds:
But even if spirals fail as galactic systems, there may be elsewhere in space stellar systems equal to or greater than ours—as yet unrecognized and possibly quite beyond the power of existing optical devices and present measuring scales. The modern telescope, however, with such accessories as high-power spectroscopes and photographic intensifiers, is destined to extend the inquiries relative to the size of the universe much deeper into space.
How right he was. Meanwhile, Curtis openly conceded that Shapley might be on to something with his hypothesis concerning the ejection of spiral nebulae, and in the course of that concession, Curtis unwittingly managed to reveal that we live in an expanding universe: “The repulsion theory, it is true, is given some support by the fact that most of the spirals observed to date are receding from us.”
By 1925, a mere half decade later, Edwin Hubble had discovered that nearly all galaxies recede from the Milky Way at speeds in direct proportion to their distances. But it was self-evident that our galaxy, the Milky Way, was in the center of the expansion of the universe. Having been an attorney before becoming an astronomer, Hubble probably would have won any debate he might have had with other scientists, no matter what he argued, but he clearly could muster the evidence for an expanding universe with us at the center. In the context of Albert Einstein’s general theory of relativity, however, the appearance of being at the center was a natural consequence of a universe that expands in four dimensions, with time as number four. Given that description of the universe, every galaxy would observe all other galaxies to be receding, leading inescapably to the conclusion that we are not alone, and we are not special.
Space Chronicles: Facing the Ultimate Frontier Page 11