Space Chronicles: Facing the Ultimate Frontier (14 page)

The history of human discovery is a history of the boundless desire to extend the senses, and it is because of this desire that we have opened new windows to the universe. Beginning in the 1960s with the early Soviet and NASA missions to the Moon and the solar system’s planets, computer-controlled space probes—which we can rightly call robots—became (and still are) the standard tool for space exploration. Robots in space have several clear advantages over astronauts: they are cheaper to launch; they can be designed to perform experiments of very high precision without interference from a cumbersome pressure suit; and since they are not alive in any traditional sense of the word, they cannot be killed in a space accident. Nevertheless, until computers can simulate human curiosity and human sparks of insight, and until computers can synthesize information and recognize a serendipitous discovery when it stares them in the face, robots will remain tools designed to discover what we already expect to find. Unfortunately, profound insights into nature lurk behind questions we have yet to ask.

The most significant improvement of our feeble senses is the extension of our sight into the invisible bands of what is collectively known as the electromagnetic spectrum. In the late nineteenth century the German physicist Heinrich Hertz performed experiments that helped unify conceptually what had previously been considered unrelated forms of radiation. Radio waves, infrared, visible light, and ultraviolet were all revealed to be cousins in a family of light whose members simply differed in energy. The full spectrum, including all parts discovered after Hertz’s work, runs from the low-energy part, called radio waves, and extends, in order of increasing energy, to microwaves, infrared, visible (comprising the “rainbow seven”: red, orange, yellow, green, blue, indigo, and violet), ultraviolet, X-rays, and gamma rays.

Superman, with his X-ray vision, has few advantages over modern scientists. Yes, he is somewhat stronger than your average astrophysicist, but astrophysicists can now “see” into every major part of the electromagnetic spectrum. Lacking this extended vision, we would be not only blind but ignorant, because many astrophysical phenomena reveal themselves only in certain “windows” within the spectrum.

Let’s peek at a few discoveries made through each window to the universe, starting with radio waves, which require very different detectors from those found in the human retina.

In 1931 Karl Jansky, then employed by Bell Telephone Laboratories and armed with a radio antenna he himself built, became the first human to “see” radio signals emanating from somewhere other than Earth. He had, in fact, discovered the center of the Milky Way galaxy. Its radio signal was so intense that if the human eye were sensitive only to radio waves, then the galactic center would be one of the brightest sources in the sky.

With the help of some cleverly designed electronics, it’s possible to transmit specially encoded radio waves that can then be transformed into sound via an ingenious apparatus known as a radio. So, by virtue of extending our sense of sight, we have also, in effect, managed to extend our sense of hearing. Any source of radio waves—indeed, practically any source of energy at all—can be channeled so as to vibrate the cone of a speaker, a simple fact that is occasionally misunderstood by journalists. When radio emissions from Saturn were discovered, for instance, it was simple enough for astronomers to hook up a radio receiver equipped with a speaker; the signal was then converted to audible sound waves, whereupon more than one journalist reported that “sounds” were coming from Saturn, and that life on Saturn was trying to tell us something.

With much more sensitive and sophisticated radio detectors than were available to Karl Jansky, astrophysicists now explore not just the Milky Way but the entire universe. As a testament to the human bias toward seeing-is-believing, early detections of radio sources in the universe were often considered untrustworthy until they were confirmed by observations with a conventional telescope. Fortunately, most classes of radio-emitting objects also emit some level of visible light, so blind faith was not always required. Eventually radio telescopes produced a rich parade of discoveries, including quasars (loosely assembled acronym of “quasi-stellar radio source”), which are among the most distant and energetic objects in the known universe.

Gas-rich galaxies emit radio waves from their abundant hydrogen atoms (more than 90 percent of all atoms in the cosmos are hydrogen). Large arrays of electronically connected radio telescopes can generate very high resolution images of a galaxy’s gas content, revealing intricate features such as twists, blobs, holes, and filaments. In many ways, the task of mapping galaxies is no different from that facing fifteenth- and sixteenth-century cartographers, whose renditions of continents—distorted though they were—represented a noble human attempt to describe worlds beyond one’s physical reach.

Microwaves have shorter wavelengths and more energy than radio waves. If the human eye were sensitive to microwaves, you could see the radar emitted by the speed gun of a highway patrol officer hiding in the bushes, and microwave-emitting telephone relay towers would be ablaze with light. The inside of your microwave oven, however, would look no different than it does now, because the mesh embedded in the door reflects microwaves back into the cavity to prevent their escape. Your eyeballs’ vitreous humor is thus protected from getting cooked along with your food.

Microwave telescopes, which were not actively used to study the universe until the late 1960s, enable us to peer into cool, dense clouds of interstellar gas that ultimately collapse to form stars and planets. The heavy elements in these clouds readily assemble into complex molecules whose signature in the microwave part of the spectrum is unmistakable because of their match with identical molecules that exist on Earth. Some of those cosmic molecules, such as NH
₃
(ammonia) and H
₂
O (water), are household standbys. Others, such as deadly CO (carbon monoxide) and HCN (hydrogen cyanide), are to be avoided at all costs. Some remind us of hospitals—H
₂
CO (formaldehyde) and C
₂
H
₅
OH (ethyl alcohol)—and some don’t remind us of anything: N
₂
H+ (dinitrogen monohydride ion) and HC
₄
CN (cyanodiacetylene). More than 150 molecules have been detected, including glycine, an amino acid that is a building block for protein and thus for life as we know it. We are indeed made of stardust. Antoni van Leeuwenhoek would be proud.

Without a doubt, the most important single discovery in astrophysics was made with a microwave telescope: the heat left over from the origin of the universe. In 1964 this remnant heat was measured in a Nobel Prize–winning observation conducted at Bell Telephone Laboratories by the physicists Arno Penzias and Robert Wilson. The signal from this heat is an omnipresent, omnidirectional ocean of light—often called the cosmic microwave background—that today registers about 2.7 degrees on the “absolute” temperature scale and is dominated by microwaves (though it radiates at all wavelengths). This discovery was serendipity at its finest. Penzias and Wilson had humbly set out to find terrestrial sources of interference with microwave communications; what they found was compelling evidence for the Big Bang theory. It’s a little like fishing for a minnow and catching a blue whale.

Moving further along the electromagnetic spectrum, we get to infrared light. Invisible to humans, it is most familiar to fast-food fanatics, whose French fries are kept lukewarm under infrared lamps for hours before being purchased. Infrared lamps also emit visible light, but their active ingredient is an abundance of invisible infrared photons, which are readily absorbed by food. If the human retina were sensitive to infrared, then a midnight glance at an ordinary household scene, with all the lights turned off, would reveal all the objects that sustain a temperature in excess of room temperature: the metal that surrounds the pilot lights of a gas stove, the hot water pipes, the iron that somebody had forgotten to turn off after pressing crumpled shirt collars, and the exposed skin of any humans passing by. Clearly that picture is not more enlightening than what you would see with visible light, but it’s easy to imagine one or two creative uses of such amplified vision, such as examining your home in winter to spot heat leaks from the window panes or roof.

As a child, I was aware that, at night, infrared vision would reveal monsters hiding in the bedroom closet only if they were warm-blooded. But everybody knows that your average bedroom monster is reptilian and cold-blooded. Thus, infrared vision would completely miss a bedroom monster, because it would simply blend in with the walls and door.

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.