596 PART 5^ |^ LIFE
Has Earth been visited by aliens? If you
conclude that there is likely to be life on other
worlds, then you might be tempted to use UFO
sightings as evidence to test your hypothesis.
Scientists don’t do this for two reasons.
First, the reputation of UFO sightings and
alien encounters does not inspire confi dence
that these data are reliable. Most people hear
of such events in grocery store tabloids, day-
time talk shows, or sensational “specials” on
viewer-hungry cable networks. You should take
note of the low reputation of the media that
report UFOs and space aliens. Most of these
reports, like the reports that Elvis is alive
and well, are simply made up for the sake of
sensation, and you cannot use them as reliable
evidence.
Second, the few UFO sightings that are
not made up do not survive careful examina-
tion. Most are mistakes and unintentional
misinterpretations, committed by honest
people, of natural events or human-made
objects. It is very important to realize that
experts have studied these incidents over
many decades and found none that are
convincing to the professional scientifi c com-
munity. In short, despite false claims to the
contrary on TV shows, there is no dependable
evidence that Earth has ever been visited by
aliens.
In a way, that’s too bad. A confi rmed visit
by intelligent creatures from beyond our solar
system would answer many questions. It would
be exciting, enlightening, and, like any real
adventure, a bit scary. Most scientists would
love to be part of such a discovery. But, scien-
tists must professionally pay attention to what
is supported by evidence rather than what
might be thrilling. There is not yet any direct
evidence of life on other worlds.Flying saucers from space are fun to think about,
but there is no evidence that they are real.26-2
UFOs and Space Aliens
recruited about 4 million owners of personal computers that
are connected to the Internet. You can download a screen saver
that searches data fi les from the Arecibo radio telescope for
signals whenever you are not using the computer. For informa-
tion, locate the seti@home project at http://setiathome.ssl.
berkeley.edu/.
Th e search continues, but radio astronomers struggle to hear
anything against the worsening babble of radio noise from human
civilization. Wider and wider sections of the electromagnetic
spectrum are being used for Earthly communication, and this,
combined with stray electromagnetic radiation from electronic
devices including everything from computers to refrigerators,
makes hearing faint radio signals diffi cult. It would be ironic if
humans fail to detect faint radio signals from another world
because our own world has become too noisy. One alternate
search strategy is to look for rapid fl ashes of laser light at optical
or near-infrared wavelengths. Such extraterrestrial signals, if they
exist, would have the advantage of being easily distinguished from
natural light sources but the disadvantage of being blocked by
interstellar dust. Ultimately, the chance of success for any of the
searches depends on the number of inhabited worlds in the
galaxy.
How Many Inhabited Worlds?
Given enough time, the searches will fi nd other worlds with civi-
lizations, assuming that there are at least a few out there. If intel-
ligence is common, scientists should fi nd signals relatively
soon—within the next few decades—but if intelligence is rare, it
may take much longer.
Simple arithmetic can give you an estimate of the number of
technological civilizations in the Milky Way Galaxy with which
you might communicate, Nc. Th e formula proposed for discus-
sions about Nc is named the Drake equation after the radio
astronomer Frank Drake, a pioneer in the search for extraterres-
trial intelligence. Th e version of the Drake equation presented
here is modifi ed slightly from its original form:
Nc = N*. fP. nHZ. fL. fI. fS
N* is the number of stars in our galaxy, and fP represents the frac-
tion of stars that have planets. If all single stars have planets, fP is
about 0.5. Th e factor nHZ is the average number of planets in
each planetary system suitably located in the habitable zone—
meaning, for the sake of the present discussion, the number of
planets per planetary system possessing substantial amounts of
liquid water. Th e conventional habitable zone in our system