CHAPTER 26 | ASTROBIOLOGY: LIFE ON OTHER WORLDS 595
Although the 1974 Arecibo beacon was the only powerful
signal sent purposely from Earth to other star systems, Earth is
sending out many other signals more or less accidentally. Short-
wave radio signals, including TV and FM, have been leaking into
space for the last 60 years or so. Any civilization within 60 light-
years could already have detected Earth’s civilization. Th at works
both ways: Alien signals, whether intentional messages of friend-
ship or the blather of their equivalent to daytime TV, could be
arriving at Earth now. Groups of astronomers from several coun-
tries are pointing radio telescopes at the most likely stars and
listening for alien civilizations.
Which channels should astronomers monitor? Signals with
wavelengths longer than 100 cm would get lost in the back-
ground noise of our Milky Way Galaxy, while wavelengths
shorter than about 1 cm are mostly absorbed in Earth’s atmo-
sphere. Between those wavelengths is a radio window that is
open for communication. Even this restricted window contains
millions of possible radio-frequency bands and is too wide to
monitor easily, but astronomers may have thought of a way to
narrow the search. Within this broad radio window lie the 21-cm
spectral line of neutral hydrogen and the 18-cm line of OH
(■ Figure 26-11). Th e interval between those lines has low back-
ground interference and is named the water hole because H plus
OH yields water. Any civilizations sophisticated enough to do
radio astronomy would know of these lines and might appreciate
their signifi cance in the same way as do Earthlings.
A number of searches for extraterrestrial radio signals have
been made, and some are now under way. Th is fi eld of study is
known as SETI, Search for Extra-Terrestrial Intelligence, and
it has generated heated debate among astronomers, philosophers,
theologians, and politicians. You might imagine that the discov-
ery of real alien intelligence would cause a huge change in
humanity’s worldview, akin to Galileo’s discovery that the moons
of Jupiter do not go around the Earth. Congress funded a NASA
SETI search for a short time but ended support in the early
1990s. In fact, the annual cost of a major search is only about as
much as a single Air Force helicopter, but much of the reluctance
to fund searches stems from issues other than cost. Segments of
the population, including some members of Congress, consid-
ered the idea of extraterrestrial beings as so outlandish that con-
tinued public funding for the search became impossible.
In spite of the controversy, the search continues. Th e NASA
SETI project canceled by Congress was renamed Project Phoenix
and completed using private funds. Th e SETI Institute, founded
in 1984, managed Project Phoenix plus several other important
searches and is currently building a new radio telescope array in
northern California, in collaboration with the University of
California, Berkeley, and partly funded by Paul Allen of Microsoft
(■ Figure 26-12).
Th ere is even a way for you to help with searches. Th e
Berkeley SETI team (Note: they are separate from the SETI
Institute), with the support of the Planetary Society, hasstar, 4 light-years. Th e obvious way to overcome these huge dis-
tances is with tremendously fast spaceships, but even the closest
stars are many light-years away.
Nothing can exceed the speed of light, and accelerating a
spaceship close to the speed of light takes huge amounts of
energy. Even if you travel slower than light, your rocket would
still require massive amounts of fuel. If you wanted to pilot a
spaceship with a mass of 100 tons (about the size of a fancy
yacht) to the nearest star, and you traveled at half the speed of
light so as to arrive in eight years, the trip would require 400
times as much energy as the entire United States consumes in a
year. Don’t even think about how much fuel the starship
Enterprise needs.
Th ese limitations not only make it diffi cult for humans to
leave the solar system, but they would also make it diffi cult for
aliens to visit Earth. Reputable scientists have studied “unidenti-
fi ed fl ying objects” (UFOs) and have never found any evidence
that Earth is being visited or has ever been visited by aliens
(How Do We Know? 26-2). Humans are unlikely ever to
meet aliens face-to-face. However, communication by electro-
magnetic signals across interstellar distances takes relatively little
energy.
Radio Communication
Nature puts restrictions on travel through space, and it also
restricts the possibility of communicating with distant civiliza-
tions by radio. One restriction is based on simple physics: Radio
signals are electromagnetic waves and travel at the speed of light.
Due to the distances between the stars, the speed of radio waves
would severely limit humanity’s ability to carry on normal con-
versations with distant civilizations. Decades could elapse between
asking a question and getting an answer.
So, rather than try to begin a conversation, one group of
astronomers decided in 1974 to broadcast a message of greeting
toward the globular cluster M13, 26,000 light years away, using
the Arecibo radio telescope (see Figure 6-21b). When the signal
arrives 26,000 years in the future, alien astronomers may be able
to understand it because the message is anticoded, meaning that
it is intended to be decoded by beings about whom we know
nothing except that they build radio telescopes. Th e message is a
string of 1679 pulses and gaps. Pulses represent 1s, and gaps
represent 0s. Th e string can be arranged in two dimensions in
only two possible ways: as 23 rows of 73 or as 73 rows of 23. Th e
second arrangement forms a picture containing information
about life on Earth (■ Figure 26-10).
What are the chances that a signal like the Arecibo message
would be heard across interstellar distances? Surprisingly, a radio
dish the size of the Arecibo telescope, located anywhere in the
Milky Way Galaxy, could detect the output from “our” Arecibo.
Th e human race’s modest technical capabilities already can put us
into cosmic chat rooms.