Chapter 19
The Quest for Extraterrestrial
19-1 Radio Searches and SETI
1. The search for extraterrestrial intelligence is based
on the detection of radio transmissions from other
life forms, because radio waves can travel through
gas and dust without significant degradation.
2. Signals are expected in certain frequency ranges
since otherwise natural sources of radio “noise”
would drown any extraterrestrial signals out.
3. The best radio frequency range is between 1000 and
10,000 MHz.
4. Organized searches for extraterrestrial signals
have been conducted since 1960 (Project Ozma,
Project Cyclops/Project Phoenix).
SETI@home started in 1999, and uses individual
computers around the world to check for
extraterrestrial signals amid the radio signals
received by the Arecibo Radio Telescope.
19-2 Communication with Extraterrestrial
1. The great distances between stars prohibit a
dialogue between intelligent beings in different parts
of the Galaxy.
2. Language problems might seem impossible to
overcome but we do have in common the physical
universe and its mathematical laws.
3. In 1974, a 10-minute radio message was sent to a
cluster of stars in the constellation Hercules, 26,000
light years away.
It contained some simple pictures and numbers that
describe humanity.
19-3 Letters to Extraterrestrials
The Pioneer Plaques
In the message, binary numbers,
drawings of a man, a woman,
the spacecraft, and our solar
system, and directions from our
solar system to pulsars, are
used to describe humanity and
our position in the Galaxy.
Courtesy of Pioneer Project, ARC, and NASA
1. A plaque on Pioneer 10 and
Pioneer 11 carries a message to
The Voyager Records
1. A more complete message is onboard Voyager 1
and Voyager 2. It includes 90 minutes of music
from around the world, greetings in 60 languages
and 118 pictures.
Courtesy of Pioneer Project, ARC, and NASA
Will the Message Be Found?
1. The chances that the messages will be found are
very slim.
The messages are really a symbol of hope,
preserving for ever “a murmur of an ancient
civilization that once flourished.”
19-4 The Origin of Life
1. According to the Haldane-Oparin hypothesis, all the
molecules necessary for the formation of life were present
soon after Earth’s formation.
2. The Miller-Urey experiment was the first test of the HaldaneOparin hypothesis.
Simulating the conditions of the early Earth, a mixture of water,
hydrogen, methane, and ammonia were put in a sealed container,
heated and exposed to electric sparks (simulated lighting
At the end of the experiment, molecules that form the basis for
proteins and molecules necessary for many life processes were
found in the mixture.
Fig. 19-4
3. Correcting for the gases in Earth’s early atmosphere (CO2,
water vapor and nitrogen), and using UV radiation, we get
similar results.
It seems that given the right chemicals and a source of
energy, we can produce the building blocks of life.
4. The Miller-Urey experiment did not produce life. We have
not been able to cross the gap between chemical and
biological evolution.
5. The molecular building blocks of life are also found in
interstellar dust and gas clouds, as well as in meteorites.
19-5 The Drake Equation
1. The Drake equation gives an estimate of the
number (N) of technologically advanced
civilizations in our Galaxy whose signal we might
be able to detect.
2. N = R*  fp  ne  f  fi  fc  L
None of the factors in the equation is well known.
Reasonable estimates result in N being in the
range from one to millions.
19-6 Where is Everybody?
1. If the formation of life is common in the Milky Way, there should
be a large number of civilizations like ours, so where are they?
2. Since we haven’t encountered any extraterrestrial civilization,
we must conclude either “they” choose not to reveal themselves
or “they” are very rare.
3. The most likely answer to the question “Where is everybody?”
is that the lifetime of a technological society is short.
4. Unless such a civilization survives 10,000 to 10 million years, it’s
unlikely such a civilization would exist at the same time as ours.

Slide 1