CHAPTER 26 | ASTROBIOLOGY: LIFE ON OTHER WORLDS 599
What Are We? Matter and SpiritThere are over 4000 religions around the
world, and nearly all hold that humans have
a dual nature: We are physical objects made
of atoms, but we are also spiritual beings.
Science is unable to examine the spiritual
side of existence, but it can tell us about
our physical nature.
The matter you are made of appeared in
the big bang and was cooked into a wide
range of elements inside stars. Your atoms
may have been inside at least two or three
generations of stars. Eventually, your atoms
became part of a nebula that contracted to
form our sun and the planets of the solar
system
Your atoms have been part of Earth for
the last 4.6 billion years. They have been
recycled many times through dinosaurs,stromatolites, fi sh, bacteria, grass, birds,
worms, and other living things. You are
using your atoms now, but when you are
done with them, they will go back to Earth
and be used again and again.
When the sun swells into a red giant star
and dies in a few billion years, Earth’s
atmosphere and oceans will be driven away,
and at least the outer few kilometers of
Earth’s crust will be vaporized and blown
outward to become part of the nebula
around the white-dwarf remains of the sun.
Your atoms are destined to return to the
interstellar medium and will become part of
future generations of stars and planets.
The message of astronomy is that
humans are not just observers: We are
participants in the universe. Among all ofthe galaxies, stars, planets, planetesimals,
and bits of matter, humans are objects that
can think, and that means we can under-
stand what we are.
Is the human race the only thinking
species? If so, we bear the sole responsibil-
ity to understand and admire the universe.
The detection of signals from another
civilization would demonstrate that we are
not alone, and such communication would
end the self-centered isolation of humanity
and stimulate a reevaluation of the meaning
of human existence. We may never realize
our full potential as humans until we
communicate with non-human intelligent
life.SCIENTIFIC ARGUMENT
Why does the number of civilizations that could be detected
depend on how long civilizations survive at a technological
level?
This scientifi c argument depends on the timing of events. If you
turned a radio telescope to the sky and scanned millions of fre-
quency bands for many stars, you would be taking a snapshot of
the universe at a particular time. Broadcasts from other civiliza-
tions must be arriving at that same time if they are to be detected.
If most civilizations survive for a long time, there is a much greater
chance that you will detect one of them in your snapshot than ifcivilizations tend to disappear quickly due to, for example, nuclear
war or environmental collapse. If most civilizations last only a
short time, there may be none capable of transmitting during the
cosmically short interval when Earthlings are capable of building
radio telescopes to listen for them.
The speed at which astronomers can search for signals is limited
because computers must search many frequency intervals, but not
all frequencies inside Earth’s radio window are subject to intensive
search. Build a new argument to explain: Why is the water hole
an especially good frequency band in which to listen?CHAPTER 2CHAPTER 266 || ASTROBIOLOGY: LIFE ON OTHER WORLDSASTROBIOLOGY: LIFE ON OTHER WORLDS 599Summary
▶ (^) Consideration of the possibility that life exists elsewhere in the uni-
verse, and the study of life and its origin on Earth as it relates to life
on other worlds, is called astrobiology (p. 582).
▶ (^) Life can be defi ned as a process that extracts energy from the surround-
ings, maintains an organism, and modifi es the surroundings to promote
the organism’s survival.
▶ (^) Living things have a physical basis—the arrangement of matter and
energy that makes life possible. Life on Earth is based on carbon chem-
istry occurring in bags of water (cells).
▶ (^) Living things must have a controlling unit of information that can be
passed to successive generations.
▶ (^) Genetic information for life on Earth is stored in long carbon-chain
molecules such as DNA (deoxyribonucleic acid) (p. 584).
▶ (^) The DNA molecule stores information in the form of chemical bases
linked together like the rungs of a ladder. Copied by the RNA
(ribonucleic acid) (p. 585) molecule, the patterns of bases act as
recipes for connecting together amino acid (p. 584) subunits to
construct proteins (p. 584), including enzymes (p. 584) that are
respectively the main structural and control components of the life
process.