Lonely Planets

  Congressional critics easily found respectable scientists to quote, argu-

  ing that SETI was a waste of time.* The Hart objection contributed to

  Congress’s pulling the plug in 1981, and again in 1993.

  At a meeting entitled “Where Are They: A Symposium on the

  Implications of Our Failure to Observe Extraterrestrials,” held at the

  University of Maryland in November 1979, Hart forcefully reiterated

  his views. He did not argue that intelligent, technological species could

  not evolve elsewhere, only that the evidence suggests they have not yet

  done so.

  Stanford radio astronomer Ronald Bracewell amplified Hart’s point

  with an analogy from Earth history. He discussed the meaning of there

  being intelligent humans in both Africa and California. Bracewell

  pointed out that humans can and did walk from Africa to California,

  arriving perhaps around thirty thousand years ago.† This journey took

  much less time than it would for intelligent life to independently evolve

  in California. Comparing the timescales of evolution and interstellar

  migration, he concluded that intelligent life would be much more likely

  to travel between stars than to evolve separately on several of them

  within the same short time interval.

  Why, Bracewell asked, are there not multiple, independently evolved,

  intelligent, technological species on Earth? Not because life, left to its

  *Indeed, Senator Proxmire, in his attack on SETI, directly referenced a 1981 article in Physics Today by physicist Frank Tipler that repeated many of Hart’s arguments.

  †That is why there are reasonably intelligent creatures, adapted to the African savannas, basking in hot tubs all over California. Most of them, however, are descendants of later invaders who came by wagon, ship, and train and forced out the descendants of the original walkers.

  Fermi’s Paradox

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  own devices would not evolve technoids again. Rather, once humans

  appeared, they quickly spread around the Earth and occupied the intel-

  ligent, technological niche. By analogy, the galaxy has no species capa-

  ble of interstellar travel. Otherwise, they would have arrived here and

  changed a few things. Like Hart, Bracewell concluded that we are the

  first to reach this stage and it is our destiny to colonize the galaxy.

  These are well-argued points, but several alternatives are possible.

  For instance, perhaps there is only one intelligent race in the galaxy, but

  it is not ours. Maybe an advanced civilization long ago spread through-

  out the galaxy, but to them we are so clearly not intelligent, and inca-

  pable of meaningful conversation, that they don’t bother with us. To

  the truly intelligent species in the galaxy, we may not seem threatening

  or promising.

  I went to my first SETI conference, entitled “The Search for

  Extraterrestrial Life: Recent Developments,” at Boston University in

  June 1984. I had attended many forums on the same topic at science

  fiction conventions, but I had never been to a scientific conference

  devoted entirely to questions about intelligent aliens. For the keynote

  address, Philip Morrison gave a retrospective of the first twenty-five

  years of SETI. Participants devoted a lot of time and breath to the likely

  strategies and timescales of galactic colonization. The resurgent Fermi-

  Hart paradox led to almost as much discussion of interstellar travel as

  of radio communication.

  Frank Drake presented a new analysis of the Fermi Paradox, includ-

  ing a calculation of the energy required for interstellar travel. His con-

  clusion: intelligent beings do not colonize. He gave two reasons: (1)

  exponential growth is ultimately self-destructive, and (2) interstellar

  travel is just too expensive for any rational society to undertake. At the

  end of his talk, a smiling Frank Drake held up a T-shirt emblazoned

  with: “Absence of evidence is not evidence of absence.”

  Various opinions about the strategies and patterns of alien interstel-

  lar colonization were defended with analogies from human history:

  the rate of dispersal of human settlements from island to island by

  Polynesians in the South Pacific, and the mass emigration of the Irish to

  North America in the 1800s, were used as examples. These were com-

  bined with sophisticated mathematical models borrowed from ecology

  and physics. The spread of a civilization throughout the galaxy was

  modeled with the diffusion equation, which predicts, for example, how

  fast molecules of a gas will spread throughout a room, and has been

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  L o n e l y P l a n e t s

  used to simulate the spread of plant and animal species through a new

  habitat.

  To apply a diffusion model to the spread of species throughout the

  Milky Way, you have to make some assumptions about long-term rates of

  population increase, and the willingness of newly arrived colonists to start

  out on further colonizing expeditions. That’s where the examples from

  human history come in. But when it comes to trying to scientifically

  model the details of interstellar colonization, of course we’re reaching.

  When I was a kid, a friend and I had a snow-shoveling business. One

  winter I calculated that we could make $2,000 if it snowed x number of

  times and we had y customers and they each paid us z dollars per inch

  of snow. We’d be rolling in cash and could retire by the time we were

  twelve. At the end of that winter, I think we had shoveled in all of fifty

  bucks and a batch of stale homemade cookies from Mrs. Dolan across

  the street. Even when a problem is framed by a precise, quantitative

  formulation, it is easy to cook the numbers and reach any conclusion

  you want.

  However, even the slowest models of interstellar migration produce

  estimates of several hundred million years to populate a galaxy that is

  more than 10 billion years old. Thus the efforts of those trying to refute

  Hart actually end up supporting his central argument: if any interstellar

  colonization has occurred at all, even assuming a halting, aimless

  movement across space, it should still have had plenty of time to perco-

  late throughout the galaxy. It doesn’t matter if most societies do not

  colonize. If only one species, in the 10-billion-year history of the

  galaxy, had decided to start colonizing, then they could have long ago

  swept across the entire Milky Way.

  Despite Drake’s eloquent counterarguments, the SETI camp was

  forced into a defensive posture. Those who wish to argue against Fermi-

  Hart must argue that no interstellar colonization has ever occurred—a

  pretty extreme stance to take.

  There is no great logical response to Hart’s argument against the

  existence of ET civilizations, no killer retort showing that it must be

  wrong. The best answers are intuitive. It just seems wrong. It goes

  against the intuition shared by people over thousands of years, from

  ancient Greeks to modern geeks, that in such a vast universe we can’t

  possibly be alone. Either argument depends on untestable assumptions.

  At least the conclusions of the SETI camp can be tested by searching for

  Fermi’s Pa
radox

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  a message. Long shot or not, as those lottery billboards say, you can’t

  win if you don’t play.

  W H E R E T H E Y A R E

  Those who believe in the standard SETI model of many radio-

  communicating species but no interstellar colonizers have developed

  many arguments to explain the lack of an obvious extraterrestrial pres-

  ence on Earth. These excuses for absentee aliens can be grouped into

  two categories: physical and sociological.

  Physical explanations reason that aliens have never arrived on Earth

  because of some astronomical, biological, or engineering problem that

  makes interstellar travel impossible. The most obvious obstacles in this

  category are the immense distances to the stars and the great, perhaps

  prohibitive, time and energy it would take to travel between them.

  Science fiction writers and speculative scientists have invented

  numerous ways for future humans to circumvent this limit. Perhaps

  space travelers could be put in “suspended animation,” their metabo-

  lisms greatly slowed down until they reach their destination, when

  machines will awaken them (assuming they haven’t developed second

  thoughts about wanting human company, as HAL9000 did in 2001).

  Or frozen human zygotes could be brought on the journey, to be raised

  by “parenting machines” when a new, habitable world is only a few

  decades off.*

  The most plausible option may be to just let the journey take many gen-

  erations. A staple of SF, and also the subject of some “serious” scientific

  literature, is the “generation starship” or “world ship”: a large spacecraft

  with a breeding population of humans (and a balanced ecosystem of

  many other species), launched on a slow cruise toward a distant stellar

  port. The colonists who eventually arrive will be the remote descendants

  of those who set out. This is an old idea. Tsiolkovsky discussed it in The

  Future of Earth and Mankind, written in 1928. Robert Heinlein explored

  it in his novella Orphans of the Sky (1941).

  Even the notion that slow interstellar travel must take “several gener-

  ations” is too human-centered. An intelligent sequoia tree would have

  no problem with a thousand-year journey. We have no reason to believe

  *This was the mode of travel in Kurt Vonnegut Jr.’s “The Big Space Fuck” (1972).

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  that intelligent aliens, or even our descendants, will not have life

  expectancies of thousands of years. For such creatures a voyage of a

  few hundred years might not be such a big deal, as long as they have

  plenty of video games and munchies.*

  The distinction we draw between individual life span and multigener-

  ational societal life span may lose its meaning for a long-lived species

  (I’ll return to this thought in chapter 23, “The Immortals”). Is it so

  hard to imagine a sentient being that passes its memories and conscious

  identity on to its descendants, so that it does not really die? With our

  stories, books, photos, recordings, and computers we have already

  invented primitive forms of persistent memory. Maybe our future mem-

  ories will be passed on directly, in digital form. We may simply con-

  tinue life’s pattern of forming larger units of identity, attaining a more

  selfless attitude toward future generations, so that a successful arrival

  of the descendants is seen and felt as a completed journey by those who

  set out. Some techno prophets believe that immortality for human

  beings will become a reality in the next century or two. This will, no

  doubt, cause new problems, but it may solve the dilemma of the stars

  that we can see but do not have time to reach.

  Even if, for some unknown reason, living organisms cannot make the

  crossing, machines could do it. Soon, we’ll be capable of launching

  robot probes to the stars, and before long we surely will. What if it is

  machines that colonize the galaxy? Someone could build machines

  designed to mine the asteroids of distant stars and create more copies of

  themselves that in turn set off for still more star systems. Such devices

  are called Von Neumann machines, after the Hungarian mathematician

  John Von Neumann (1903–57), who worked out the theory of self-

  reproducing machines. Once launched, it would not be long (in galactic

  terms) before they reached every star system. We humans will soon be

  able to build Von Neumann machines, if we choose to. It is hard to

  imagine that a universe full of high-tech radio buffs will be empty of

  self-multiplying machine explorers. This presents a further dilemma for

  radio SETI advocates. If advanced aliens are out there in numbers large

  or small, might we be more likely to encounter their machine probes in

  our own solar system than their distant messages on the cosmic air-

  waves?

  *And an endless supply of hydroponic herb.

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  A L I E N S A I N ’ T M I S B E H A V I N ’

  If none of the physical explanations solve the Fermi Paradox, perhaps

  the answer lies in alien sociology.

  One popular sociological explanation is the “self-destruction hypoth-

  esis”: technological civilizations are inherently suicidal. They always

  blow themselves up, pollute themselves to death, or otherwise cash

  themselves in. If true, this solution contains bad news for those who

  look hopefully to ET civilizations for confirmation that long-term sur-

  vival with high technology is possible.

  The problem with the “self-destruction hypothesis” resolution of the

  Fermi Paradox is that it must apply to all species, everywhere in the

  galaxy. Suppose you estimate the chances of the human race surviving

  our technological adolescence at one in a hundred or even one in a

  thousand, and you imagine that this is typical of other technological

  races. If those one-in-a-hundred survivors go on to live for millions or

  billions of years, then—according to the Drake Equation—there should

  be a lot of them out there. It is possible to be pessimistic about the

  human prospect but remain sanguine about the likelihood of a galaxy

  full of the songs of the survivors.

  Another interesting sociological solution is the “contemplation

  hypothesis.” Maybe some combination of spiritual and technological

  progress removes the desire to physically explore and colonize the

  entire galaxy. After a certain point, advanced species may adopt a more

  passive, meditative attitude toward the universe. Such cosmic navel-

  gazers would be more interested in contemplation than colonization.

  Maybe the galaxy has not been fully colonized because all wise and

  ancient civilizations have realized that exponential growth is a dead

  end.* We can already see here on Earth how a philosophy of unrelent-

  ing growth will be self-limiting, and in the global environmental move-

  ment, we can even see the seeds of a new ethos developing. The choice

  is between limits we impose on ourselves and limits nature imposes on

  us. The same has to be true on an interstellar level.

  *Even if w
e expanded our domain at the speed of light—a pretty safe theoretical upper limit—and managed to colonize all available stars and planets within a sphere expanding at light speed, then, increasing our population at 2 percent per year, we will still run out of room and perish in our own wastes within the next millennium.

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  Of course, this contemplative philosophy could be taken too far. A

  policy of “extreme contemplation” is incompatible with long-term sur-

  vival. Creatures wishing to insure their survival for millions of years

  must be space-faring. At a minimum they must learn how to detect and

  deflect incoming asteroids and comets, lest they go the way of the

  dinosaurs. To survive unforeseen planetary disasters, they ought to dis-

  perse their population to more than one planet, or to space colonies.

  Finally, if they want to survive for billions of years, they cannot avoid

  interstellar colonization. They must move on before their sun dies. For

  a truly long-lived society, interstellar travel is not a luxury.

  It is quite reasonable to suppose that surviving civilizations will have

  developed habits of conservation and expansion that are thoughtful

  and not reckless. But, what is to prevent a “rogue civilization” from

  acting in a way that is at odds with the contemplative rationale? It

  doesn’t do much good to have a galactic Kyoto Protocol promoting

  responsible behavior if one powerful rogue planet decides it has no use

  for the treaty. This is the weakness of the contemplation hypothesis as a

  solution to the Fermi Paradox. A “universal sociological explanation”

  is required—one that applies to the behavior of every species that has

  ever come along and explored the galaxy. And how likely is that? I can

  imagine only one way that such a universal code of behavior can apply

  to all species in the galaxy—if it is codified in laws that are somehow

  being enforced.

  Is it possible that an advanced ETI knows about our existence and has

  decided that we should be left alone? We have seen what happens on

  Earth when a more technologically advanced, mobile society encounters

  an indigenous one. It often means the end of the locals. This leads us to

  the “zoo hypothesis”: aliens are out there, perhaps nearby, but they

  don’t want us to know about them, perhaps because of some alien pre-

  cept of noninterference.* Maybe the galaxy is actively regulated by a