Alien Universe (30 page)

Where Are They?

Enrico Fermi was a brilliant Italian physicist who is known to the public as the man who led the team that first harnessed nuclear power under Stagg Field in Chicago on December 2, 1942. His impact in physics was actually much broader than that, and he has been honored (among many other tributes) by posthumously lending his name to the Fermi National Accelerator Laboratory, America’s preeminent laboratory for studying the basic building blocks of the universe. In addition to sheer brilliance, Fermi had a gift for trying to get at the bottom line, using simple estimators. We physicists call “a Fermi Problem” a question that is easy to ask, hard to know definitively, but able to be estimated by thinking it through. The most repeated example of a Fermi Problem is “How many piano tuners are there in Chicago?” By knowing the number of people in the city and then estimating how many households have a piano, how long a piano holds its tune, how long it takes to tune a piano, and the length of a work week, you can come up with a reasonable estimated answer. (Current estimate, about 125.)

Fermi lived in an elite academic world—an active mind surrounded by others of similar caliber. They would talk about all manner of things, looking at them from every angle, trying to get at the truth. From a casual lunchtime conversation, one of the most famous questions involving extraterrestrials was asked. The story goes something like this.

One summer day in 1950, Enrico Fermi was visiting the Los Alamos Laboratory, which had been the secret government facility at which much of the first nuclear weapons had been developed. He and three companions, one of whom was Edward Teller, were on their way to lunch. They were talking about a cartoon seen in the May 20 issue of
The New Yorker
, which explained a recent spate of thefts of trash cans in New York City as being perpetrated by Aliens taking them into their flying saucers. (The UFO mania of the late 1940s was still fresh in the public’s mind.) The conversation then meandered to Teller and Fermi bantering back and forth over the chances of mankind exceeding the speed of light in the next decade, with Teller suggesting a chance in a million and Fermi guessing 10%. During the stroll, the numbers changed as they intellectually fenced.

After sitting down to lunch, the conversation went in a different direction, with Fermi sitting there quietly. Fermi then suddenly burst out, saying “Where is everybody?” to general laughter, as they all instantly understood that he was talking about extraterrestrials.

The premise of Fermi’s paradox is the following. The Milky Way is about 13 billion years old and contains between 200 and 400 billion stars. Our own sun is only a little over 4 billion years old, suggesting that there have been stars around for a very long time. If Aliens are common in the galaxy, there has been plenty of time for them to have evolved—perhaps hundreds of millions of years or more before humanity—and have visited Earth. So where are they?

While Fermi’s outburst is the origin of the paradox, the question was revisited in 1975 by Michael Hart (leading some to call this the Fermi-Hart paradox). Hart published “An Explanation for the Absence of Extraterrestrial Life on Earth” in the
Quarterly Journal of the Royal Astronomical Society
. In this article, he explored some of the reasons why we hadn’t been contacted yet, from reasons of simple disinterest of the Aliens to either colonize the galaxy or to contact us to the idea that the Earth is being treated as a nature preserve. Perhaps some form of
Star Trek’s
Prime Directive applies, whereby civilizations are not contacted until they develop the capability for interstellar travel. As we recall from
chapters 3
and
4
, these kinds of explanations were offered in
The Day the Earth Stood Still
and, of course,
Star Trek
. What Hart was able to show was that technology wasn’t the problem. Taking some simple assumptions, Hart showed that a civilization that sent out two craft traveling at 10% of the speed of light to nearby stars and then spent a few hundred years developing infrastructure to build another pair of slow-moving starships could completely populate the Milky Way in just a couple of million years. Given the timescales involved, from the fact that there are stars that are billions of years older than the sun, it seems impossible that we should not have been visited before. If intelligent extraterrestrial life is even slightly common in the galaxy and only a few species have mankind’s curiosity and exploratory nature, it seems that we would know by now that we are not alone. Hart concluded that it was a distinct possibility that mankind might well be one of the earliest-developing intelligent species in the galaxy. In short,
The X-Files
tagline “We are not alone” could well be gravely incorrect.

Of course, the answer to the question is unknown and hence the reason why the term “paradox” is applied to it. Author Steven Webb explored the question in his delightful 2002 book
If the Universe Is Teeming with Aliens, Where Is Everybody? Fifty Solutions to Fermi’s Paradox and the Problem of Extraterrestrial Life
. Peter Ward and Donald Brownlee’s 2003 book
Rare Earth: Why Complex Life Is Uncommon in the Universe
is equally enjoyable, and this book takes the position that it is difficult for a planet to develop intelligent life. The
book describes the many ways in which planetary disaster can interrupt the development of sentient life on a planet.

No matter how carefully thought out, arguments of the sorts advanced in these books and others like them must defer to data. And to figure out what sorts of data are needed, it is helpful to have a guiding paradigm. The question of nearby extraterrestrial life has long been guided by a simple equation developed in 1961.

The Drake Equation

Frank Drake has been a leader in the field of searching for extraterrestrials for more than half a century. He started out as a radio astronomer using the National Radio Astronomy Observatory, located in Green Bank, West Virginia. While he did research into the physics of radio-emitting astronomical objects, he is most known for using his radio expertise to search for civilizations living on planets around nearby stars. The idea is quite simple. Humanity has been emitting radio or television broadcasts for about a hundred years. Since radio travels at the speed of light, this means that for a hundred light-years in all directions, a sufficiently advanced civilization might hear our broadcasts and know we’re there. Turning that logic around, we can instead listen to the cosmos using our own radio telescopes. If there are nearby civilizations with a comparable level of technology, it seems like we should be able to hear them. Since we don’t know if it will ever be possible to travel at speeds faster than light, perhaps the fastest way to communicate between stars will be radio communication. If there is a vast galactic civilization out there, perhaps we could intercept broadcasts from one star to another. Given that we have the right kind of equipment, we really should listen. Perhaps our first communication with an extraterrestrial civilization will be when we pick up a stray broadcast of an Alien version of
Gilligan’s Island
.

We will talk more about the history of searching for extraterrestrial life by way of radio in a short while, but the current subject is the Drake equation. In 1960, Drake had undertaken his first radio survey of nearby stars and had been asked by the National Academy of Sciences to convene a conference on the question of extraterrestrial life. This conference was held at Green Bank in 1961. Drake realized that he needed an agenda for the conference, so he put together his famed equation as a way to guide the discussion. He wrote down all the things you need to know in order to predict the number of extraterrestrial civilizations in the galaxy. As we will see, this equation doesn’t stem
from any particular theory of life formation but is essentially a classic Fermi problem. Because of the nature of the conference, it focused on the chances of receiving an extraterrestrial radio transmission.

The Drake equation is very simple

N
=
R
_*
×
f
_p
×
n
_e
×
f
_l
×
f
_i
×
f
_c
×
L

where:

N
= the number of civilizations in our galaxy from whom we might receive a radio broadcast

R
_*
= the average rate of star formation in the galaxy

f
_p
= the fraction of stars with planets

n
_e
= the average number of planets that can support life around those stars with planets

f
_l
= the fraction of those planets with habitable planets that develop life

f
_i
= the fraction of those planets with life that develop intelligent life

f
_c
= the fraction of stars with intelligent life that develop radio or other detectable technology, and

L
= the period of time during which the civilization will be detectable.

For a scientist, there are many potential criticisms one can level at the Drake equation. Obviously the relevant rate of star formation isn’t the rate we see now, but rather the rate several billion years ago. Further, the equation predicts the number of spontaneously arising civilizations. A weakness of this approach is that if some culture did develop interstellar travel, the equation quite ignores the creep of culture across the galaxy. For instance, even if the creation of intelligent life is exceedingly rare—so rare that only one culture developed our level of technology before we did—if they had developed the ability to travel interstellar distances and had humanity’s wanderlust, this could result in far more radio-emitting planets than the Drake equation suggests.

Still, the equation gives us a starting place and tells us the kinds of parameters that are important for researchers to consider as they try to figure out a reasonable guess for the number of technology-using extraterrestrial civilizations in the galaxy. It is perhaps obvious that most of these factors are not known at all, although we are not completely uninformed as to what constitutes a reasonable range of values.

We can take a look at the modern realistic guesses. To begin with, the rate of star formation (
R
_*
) is reasonably well determined from astronomical research. In addition, we are now able to estimate the fraction of stars around which planets form (
f
_p
). We will discuss these studies more a little later, but planet-hunting research leads us to believe that about half of stars develop some sort of planetary system. Our current technology preferentially finds systems in which a large planet orbits close to the star, and we find that something like 40% of the surveyed stars in our stellar neighborhood have this property. When we combine this observation with the fact that there are no doubt stars that have planets but without a huge planet like Jupiter, the actual fraction of planet-hosting stars is no doubt somewhat larger.

TABLE
7.1.
Values for the Drake equation showing different scenarios

Determining the number of habitable planets (or moons of gas giant planets) given that a planetary system has formed (
n
_e
) is much harder. Investigating this question is a hot research topic in the scientific community, with the launch in 2009 of the Kepler mission. By the time you read this book, what you learn here will definitely be out of date. You should keep your eye on this quickly evolving topic.

We don’t know the fraction of planets that are potentially habitable that actually host life (
f
_l
), but it would seem that this should be fairly high. The fact that life evolved on Earth so quickly after the planet cooled enough to allow liquid water suggests that life might develop easily. One can use statistical techniques to use the period of time it took life to develop on Earth to come
up with a lower limit on this fraction. Unless there are exceptional qualities of Earth that make it a nonrepresentative planet, it seems that the probability of an Earth-like planet developing life is higher than 20%.