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Sun, 27 Apr 2008 13:10:15 -0400
http://www.technologyreview.com/Infotech/20569/
Technology Review - Published by MIT
May/June 2008
Where Are They?
Why I hope the search for extraterrestrial life finds nothing.
By Nick Bostrom
People got very excited in 2004 when NASA's rover Opportunity discovered ev=
idence that Mars had once been wet. Where there is water, there may be life=
. After more than 40 years of human exploration, culminating in the ongoing=
Mars Exploration Rover mission, scientists are planning still more mission=
s to study the planet. The =ADPhoenix, an interagency scientific probe led =
by the Lunar and Planetary Laboratory at the University of Arizona, is sche=
duled to land in late May on Mars's frigid northern arctic, where it will s=
earch for soils and ice that might be suitable for microbial life (see "Mis=
sion to Mars," November/December 2007). The next decade might see a Mars Sa=
mple Return mission, which would use robotic systems to collect samples of =
Martian rocks, soils, and atmosphere and return them to Earth. We could the=
n analyze the samples to see if they contain any traces of life, whether ex=
tinct or still active.
Such a discovery would be of tremendous scientific significance. What could=
be more fascinating than discovering life that had evolved entirely indepe=
ndently of life here on Earth? Many people would also find it heartening to=
learn that we are not entirely alone in this vast, cold cosmos.
But I hope that our Mars probes discover nothing. It would be good news if =
we find Mars to be sterile. Dead rocks and lifeless sands would lift my spi=
rit.
Conversely, if we discovered traces of some simple, extinct life-form--some=
bacteria, some algae--it would be bad news. If we found fossils of somethi=
ng more advanced, perhaps something that looked like the remnants of a tril=
obite or even the skeleton of a small mammal, it would be very bad news. Th=
e more complex the life-form we found, the more depressing the news would b=
e. I would find it interesting, certainly--but a bad omen for the future of=
the human race.
How do I arrive at this conclusion? I begin by reflecting on a well-known f=
act. UFO spotters, Ra=EBlian cultists, and self-=ADcertified alien abductee=
s notwithstanding, humans have, to date, seen no sign of any extraterrestri=
al civilization. We have not received any visitors from space, nor have our=
radio telescopes detected any signals transmitted by any extraterrestrial =
civilization. The Search for Extra-Terrestrial Intelligence (SETI) has been=
going for nearly half a century, employing increasingly powerful telescope=
s and data-=ADmining techniques; so far, it has consistently corroborated t=
he null hypothesis. As best we have been able to determine, the night sky i=
s empty and silent. The question "Where are they?" is thus at least as pert=
inent today as it was when the physicist Enrico Fermi first posed it during=
a lunch discussion with some of his colleagues at the Los Alamos National =
Laboratory back in 1950.
Here is another fact: the observable universe contains on the order of 100 =
billion galaxies, and there are on the order of 100 billion stars in our ga=
laxy alone. In the last couple of decades, we have learned that many of the=
se stars have planets circling them; several hundred such "exoplanets" have=
been discovered to date. Most of these are gigantic, since it is very diff=
icult to detect smaller exoplanets using current methods. (In most cases, t=
he planets cannot be directly observed. Their existence is inferred from th=
eir gravitational influence on their parent suns, which wobble slightly whe=
n pulled toward large orbiting planets, or from slight fluctuations in lumi=
nosity when the planets partially eclipse their suns.) We have every reason=
to believe that the observable universe contains vast numbers of solar sys=
tems, including many with planets that are Earth-like, at least in the sens=
e of having masses and temperatures similar to those of our own orb. We als=
o know that many of these solar systems are older than ours.
From these two facts it follows that the evolutionary path to life-forms ca=
pable of space colonization leads through a "Great Filter," which can be th=
ought of as a probability barrier. (I borrow this term from Robin Hanson, a=
n economist at George Mason University.) The filter consists of one or more=
evolutionary transitions or steps that must be traversed at great odds in =
order for an Earth-like planet to produce a civilization capable of explori=
ng distant solar systems. You start with billions and billions of potential=
germination points for life, and you end up with a sum total of zero extra=
terrestrial civilizations that we can observe. The Great Filter must theref=
ore be sufficiently powerful--which is to say, passing the critical points =
must be sufficiently improbable--that even with many billions of rolls of t=
he dice, one ends up with nothing: no aliens, no spacecr
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