Beyond: Our Future in Space

by Chris Impey

  13

  Cosmic Companionship

  _______________________

  Number of Pen Pals

  We’ve seen how hard it is to leave the cradle of Earth, even for a short time. However, enough ingenuity and technology are being applied to the problem that it’s only a matter of time before we spread beyond our planet. This raises a series of questions. Are we the only species to travel in space? Are we the first? If we are not, how would we know? It will spur space exploration if we know we are not the first or the only species to spread our wings beyond the home planet.

  The question of cosmic companionship brings us back to the Drake equation and all its embedded uncertainty. Recall that the equation is a multiplicative set of factors incorporating astronomy, biology, and sociology and designed to give an estimate of the number of civilizations that are communicating or traveling through space at a given time.

  Exoplanet surveys suggest there are 10 billion Earth-like planets around Sun-like stars. That’s a vast number of “Petri dishes” in the Milky Way: locations with suitable physical conditions and the chemical ingredients for biology. Scientific arguments based on a sample of one are unreliable, but the fact that life formed on Earth as soon as suitable conditions arose is taken as evidence that habitability almost always means actually inhabited. A counterargument is that life only seems to have arisen once on this planet, but that argument is weak because other origination events may have been lost or concealed or outcompeted by the existing form of life. If life is found on Mars, either present or ancient, it will be good evidence that the fraction of habitable planets that host life is close to one. If we assume that for a moment, the Drake equation becomes N ~ fi x fc x L. Here, N is the number of civilizations in our galaxy that are currently able to communicate through space, fi is the fraction of planets with life that go on to develop intelligent life, fc is the fraction of those that can communicate through space, and L is the length of time that they endure or have such a capability.

  At this point, opinions diverge and uncertainty rules. Some biologists argue that fi is low because only a handful of the hundreds of millions of species on Earth developed intelligence. Others argue that biology has trended toward greater complexity over time, and there may be an evolutionary advantage to the development of brains. The fraction fc is even more controversial. There are intelligent species on Earth that can’t send signs of their existence into space—elephants, orcas, octopuses, and others. They could have hypothetical counterparts on other worlds. There are also many reasons why a technological civilization may choose not to travel in space, communicate, or somehow reveal its existence. We lose all traction on logic when we enter the realm of alien sociology.1

  The last term, L, is also imponderable. Anatomically modern humans originated 200,000 years ago, culture and language evolved 50,000 years ago, and the first civilizations date back to 10,000 years ago. We’ve had space travel and SETI for only about fifty years, which might suggest the number 50 as a lower bound on the lifetime factor, but in that technological surge we developed the ability to destroy civilization via nuclear weapons, so the lifetime may be short if technology renders a civilization unstable.2 Carl Sagan speculated that the last few factors were close to one, meaning approximately that N equals L (etched on Frank Drake’s California license plate), so civilization lifetime governs the number of potential pen pals. Sagan’s view of the Drake equation spurred his strong advocacy of environmental issues and his warnings about the dangers of a nuclear holocaust.

  The lifetime factor is a reminder that the universe contains real estate of time as well as space. The cosmos is 13.8 billion years old, and the Milky Way started forming soon after that. As generations of stars live and die, the galaxy increases its abundance of the heavy elements needed to make planets and biology. The galaxy disk formed nine billion years ago and Earth-like planets could have first formed then, giving them a 4.5-billion-year head start on the Earth. The Drake equation ignores the fact that civilizations could emerge more than once on a particular habitable planet. A civilization may be destroyed by disease or natural disaster or internal instability, but others might emerge over the eons. The Drake equation doesn’t distinguish between active communication and the passive creation of a detectable technological footprint.

  If biology is abundant but evolution rarely leads to intelligence and technology, or if technological civilizations are short-lived, then the number of pen pals in the galaxy is low. We might be truly alone. But if evolution almost inevitably leads to space travel and communication, or if technological civilizations are durable, then the Milky Way could be buzzing with activity.

  Finally, the Drake equation factors in only one galaxy. The Milky Way is not unique among the hundred billion galaxies seen within the limit of modern telescopes, so we extrapolate the local census of life into the vastness of the observable universe. Even if the number of intelligent civilizations in the galaxy is only a dozen, that still projects to a trillion throughout space and time. The total number of technological civilizations in the universe could be truly staggering.

  The Great Silence

  Speculation is fun, but science is all about data, and SETI researchers have been “listening” for artificial radio signals from nearby stars for more than half a century. What have they heard?

  Nothing. It’s referred to as “The Great Silence,” as if radio waves were audible or sound could travel through space. Radio astronomers have been listening for pulsed radio signals because radio waves are not produced by stars, they have low energy, and they travel easily across large distances in the galaxy. For all these reasons, it’s assumed they would be the tool of choice for a technological civilization trying to communicate. The targets are relatively close Sun-like stars that might have planets around them. Stars are the targets because their attendant planets wouldn’t be visible and they would be so close to the star that they couldn’t be distinguished on the sky. The data are analyzed by computers, and when the radio intensity is converted into sound, all they ever hear is static. White noise. Hiss.

  The epitome of SETI success was portrayed in the 1997 film Contact, based on Carl Sagan’s 1985 science fiction novel of the same name.3 The astronomer Ellie Arroway, played by Jodie Foster, is in the control room of the Very Large Array in Socorro, New Mexico, watching as the twenty-seven radio dishes tilt in the direction of the bright star Vega. She settles into her chair to listen as the radio signals are converted into sound in her headphones. Suddenly, pure static is interrupted by a thumping sound. Then a pause. Then two more thumps. The sounds come in clusters. Gradually it becomes obvious they’re prime numbers. The premise is clear. Stars don’t emit radio waves, so the radio signal must originate from technology on a nearby planet. Mathematics is assumed to be a universal language, and it’s implied that it would take intelligence for any species to calculate a prime-number sequence.

  Science fiction writer Ursula K. Le Guin imagined life on a planet around the star Tau Ceti (one of the two stars observed by Frank Drake in his Project Ozma in 1959).4 That civilization built its religion around mathematics, and its denizens “chanted the primes.” Arroway discovers that later signals include a huge amount of coded information, including the instructions for building a machine that can transport people through wormholes.

  Reality is more mundane. No convincing signal from ET has ever been detected.

  SETI began with the pioneers of radio. In 1899, Nikola Tesla observed repetitive signals in his coil transformer that he thought had originated from Mars. A few years later, Guglielmo Marconi also believed he had picked up messages from Mars. It’s likely they both were witnessing natural phenomena in the Earth’s atmosphere.5 Following Project Ozma, the Soviets did pioneering work and the largest experiment in the United States was a radio telescope the size of three football fields called “Big Ear” at Ohio State University. In 1977, a technician at Big Ear saw a booming signal on the printout and annotated it with an excla
mation. The “Wow!” signal never repeated and was never identified with a celestial source; scientists consider it a dead end. Radio SETI involves searching for narrow band signals, typically less than 100 Hertz wide. That’s because a signal confined to a narrow slice of the radio dial indicates a purpose-built transmitter—think of your car radio scanning to find a station. Natural sources of radio waves like pulsars and quasars spread their signal over a relatively broad frequency range.

  SETI has progressed in fits and starts. In 1959, Frank Drake scanned his single-channel receiver over a 400-kilohertz (kHz) band, a painfully slow way to search the spectrum. Paul Horowitz transformed the search in the 1980s. Horowitz was a wunderkind who became a ham radio operator when he was only eight. As a professor of electrical engineering and physics at Harvard, he wrote The Art of Electronics, considered the bible in its field. In 1981, he developed a 131,000-channel spectrum analyzer that could fit in a suitcase. By 1985, he increased that to 8.4 million channels, with funding assistance from film magnate Steven Spielberg. A decade later, a receiver equipped with custom digital signal processing boards was able to scan 250 million channels every eight seconds.

  Technological breakthroughs catalyzed SETI, but political headwinds slowed the progress. In 1978, SETI received one of the infamous “Golden Fleece” awards from Senator William Proxmire. He gave the award monthly to projects he thought were egregious wastes of public money, and he dumped particular scorn on the “search for little green men.” In 1981, he added a rider to the NASA budget that prevented the agency from doing SETI research. Carl Sagan persuaded Proxmire to relent, but in 1993 the program was killed again, this time by Nevada Senator Richard Bryan, who noted with satisfaction the end of a “great Martian chase” at the taxpayers’ expense.6 His action was hypocritical, since he later lobbied for government funding to upgrade a Nevada state highway that runs close to Area 51 (an iconic site for UFO conspiracy theorists) and name it the Extraterrestrial Highway. Since 1995, the program has continued with a mix of private and federal funding.

  There’s also been pushback against SETI within academia. Harvard biologist Ernst Mayr called SETI “hopeless” and “a waste of time,” and he criticized his colleague Paul Horowitz for drawing graduate students into such an endeavor. Sagan rebutted Mayr, but the search continues to elicit strong opinions.

  SETI uses both listening and signaling strategies. Inevitably, SETI is anthropocentric and its strategies are tightly coupled to our current capabilities. Its history mirrors the evolution of our technology. In 1820, German mathematician Karl Friedrich Gauss suggested cutting a right-angled triangle into the Siberian forest, creating a monument to the Pythagorean Theorem big enough to be seen from space. Twenty years later, astronomer Joseph von Littrow suggested digging trenches in geometric shapes in the Sahara Desert, to be filled with kerosene and set ablaze. Neither scheme was carried out. At the end of the nineteenth century, Jules Verne triggered a UFO scare in the United States; people read his fanciful fiction and reported seeing airships and dirigibles in the sky.7 By the middle of the twentieth century, the US Air Force had developed slender jets, so UFO sightings took the form of sleek metal cylinders and disks. Radio SETI dominated for decades until lasers became powerful enough that researchers realized they could be used for signaling—powerful lasers can outshine a star for very brief instants of time as seen from afar.

  Figure 53. The 305-meter radio dish at Arecibo Observatory in Puerto Rico represents the strengths and weaknesses of SETI. Our radio technology could detect transmitting Arecibo dishes far out into the galaxy, but this assumes alien civilizations using radio communication.

  We couldn’t have done SETI a hundred years ago, and we may use quite different strategies and technologies a hundred years from now (Figure 53). SETI will only succeed if technology isn’t a fleeting attribute of a civilization. As Philip Morrison pointed out in his seminal 1959 paper, “A detected signal tells us about their past and the possibility of our future.”

  Where Are They?

  It was 1950. Enrico Fermi was visiting the lab at Los Alamos, New Mexico, where the atomic bomb had been developed. He was having lunch with three colleagues and they were discussing two seemingly unrelated items in a magazine: a spate of UFO sightings in New York and the problem of trash-can lids disappearing from city streets. They all laughed as they imagined teenagers tossing the lids past windows of apartments, making the occupants think they’d seen a UFO.

  Then there was a brief pause, and Fermi said, “Where are they?”

  Fermi’s colleagues were used to his agile mind.8 He was called “The Pope” by other physicists—not because he was a Catholic but because they considered him infallible on the topic of physics. Fermi had given his name to a method of estimation that allowed scientists to get the rough answer to a problem even if they had little or no data to work with. In this instance, they realized he had rapidly combined a series of suppositions: the probability that life would arise on any Earth-like planet, the vast number of planets in the Milky Way, the amount of time available for the evolution of intelligence and technology, and space exploration as a likely endeavor of an advanced civilization. When he posed the question “Where are they?” he was saying we should be surprised that the galaxy isn’t littered with star voyagers. It’s called “the Fermi question,” and it’s as well-posed today as it was in 1950.

  Going a little further with this logic, we can argue that since humans got the ability to travel and communicate in space very recently, any civilization we encounter is likely to be more advanced than we are—unless we’re the first to reach this level of development.

  The Fermi question is provocative because it takes the failure of SETI and turns it into a poignant silence. Implicit in the question is the fact that all claims of UFOs as aliens visiting us have been without foundation. A vocal and persistent minority of the public claim sightings, encounters, or even abductions, but the scientific community believes these claims are baseless. I have spoken often enough in public to be accustomed to the UFO questions that tend to follow any astronomy talk. The most amusing (and exasperating to a scientist) involve diffuse government conspiracies and aliens kept “on ice” in Area 51. Anyone who reads the news should be skeptical that the US Government could keep such a huge discovery a secret. Carl Sagan put it best: Extraordinary claims require extraordinary evidence.

  The disconnect between the expectation that spacefaring aliens exist and the lack of evidence for them has been called a paradox. That’s a misuse of the word paradox, which is defined as any statement that contradicts itself. There are many ways spacefaring aliens can exist without our knowing about it.9 Absence of evidence is not evidence of absence.

  Let’s look at some plausible answers to Fermi—which are also ways to account for SETI’s Great Silence.

  The first and most basic explanation is that we’re alone. There are several variations of this explanation. In one, biology is a fluke and the vast numbers of potential sites for life are actually uninhabited. In another, evolution toward large brains and intelligence is highly contingent, so life almost always stays microbial. This contingency may also be astronomical. The Rare Earth hypothesis proposes that even though terrestrial planets are abundant, Earth-like planets with just the right conditions for complex life are rare. A stable long-term environment might depend on being in a part of the galaxy with the right amount of heavy elements and not too many encounters between stars, on being in a planetary system where the architecture protects against major impacts, on the planet being the right size for plate tectonics and being in a stable orbit with axial tilt stabilized by a large moon.10 In a third variation, the development of technology and space exploration is a very unlikely outcome as species evolve. Each of these options corresponds to setting one term in the Drake equation to a very low value. In all these possibilities, we’re the only intelligent, communicative civilization in the galaxy, so N = 1. There’s no one to talk to.

  The second p
ossibility is that we’re isolated. Perhaps technological civilizations do exist and some of them are plying the galaxy or sending and receiving electromagnetic signals. But if such civilizations are rare, we might be unaware of their existence. The Milky Way is 100,000 light years across, so if there are only ten civilizations actively exploring at any given time, the average distance between them is 10,000 light years. Texting with a 20,000-year pause between replies makes for a very stilted conversation. The information being received is old news and the sending civilization may not even exist by the time the message or probe is received. So space might be littered with the runes of dead civilizations. However, the assumption that they will use radio waves might be wrong (Figure 54). Isolation applies to time as well as space. As we’ve been learning, interstellar travel is expensive and difficult, so large-scale colonization may be beyond the capabilities of all but a few species around the galaxy.

  Figure 54. SETI uses the “water hole” between 1 and 10 GHz for listening and transmission because it’s a cosmically quiet part of the electromagnetic spectrum. Selection of this particular frequency range assumes that alien civilizations would use a similar logic.

  The third explanation is that the search isn’t good enough. Jill Tarter, the SETI pioneer who was a model for the Ellie Arroway character in Contact, has talked about the needle-in-a-haystack issue. The SETI haystack has nine dimensions: three of space and two each of polarization, intensity, modulation, frequency, and time. Tarter compared SETI to scooping a bucket of water from an ocean in the hope of catching a fish. That’s going to change as the Allen Telescope Array reaches its full potential. An irony of working in a field where detection capability improves exponentially is that each new search is better than the sum of all the searches that preceded it.11 The Allen Array will survey a million stars for artificial signals over a frequency range from 1 to 10 gigahertz.