Beyond: Our Future in Space (32 page)

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 exclamation. 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.

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.