The Astronomy Book

NEW WINDOWS ON THE UNIVERSE 211

societies may already be trying
to make contact. The pair
suggested looking for signals
in the microwave spectrum,
identifying likely frequencies
and even potential places to start
the search for intelligent life.

A place to look
Cocconi and Morrison focused
on the “21-cm line,” a radiation
emission line (characteristic
wavelength) of a hydrogen atom.
In the high-frequency radio
(microwave) band, this 1420 MHz
radiation is emitted when protons
and electrons within a hydrogen
atom change their energy state.
Its discovery, in 1951, allowed the
distribution of hydrogen across the
galaxy to be mapped using radio
waves, which, unlike visible light,
are not blocked by dust clouds.
Since this line is universal,
Cocconi and Morrison argued
that it would be known to all
intelligent civilizations, and that any
search should begin by looking for
transmissions around this frequency
band. They predicted the most likely
form of transmission—a pulse-
width-modulated wave, like an FM
radio signal, on a loop like a Mayday
callout. The modulated wave would
carry a constant amplitude but

produce regular pulses of higher

frequency. A signal might cycle

over long periods, perhaps a

number of years.

Future searches

Cocconi and Morrison’s ideas

dominated the search for

extraterrestrial intelligence (SETI)

for decades. Acting on the article’s

recommendations, Frank Drake’s

pioneering 1960 experiment Project

Ozma, at the Green Bank observatory

in West Virginia, targeted the close

sunlike stars Tau Ceti and Epsilon

Eridani, scanning around the 21-cm

line. Sadly, the project failed to find

any convincing candidates. Today,

many question the wisdom of such

limited searches. Instead, SETI

researchers look for the chemical

or thermal signatures of advanced

civilizations, leakage from signals

not intended for us, and novel

methods of communication using

lasers or neutrinos. ■

CONSTANT AMPLITUDE

See also: Life on other planets 228–35 ■ Exoplanets 288–95

Giuseppe Cocconi and Philip Morrison

Giuseppe Cocconi was born

in Como, Italy, in 1914. After

World War II, he joined Cornell

University, New York. Working

with his wife, Vanna, Cocconi

demonstrated the galactic and

extragalactic origins of cosmic

rays. Later, he became director

of research at CERN (the

European Organization of

Nuclear Research) in Geneva.

Philip Morrison studied

at the University of California,

Berkeley, under Robert

Oppenheimer. During World War

II, he worked on the Manhattan

Project to build the first atomic

bomb, and famously shared a

car with the core of the Trinity

bomb as it was transported to

the test site. He later became

a vocal antinuclear campaigner.

He was a great popularizer of

science and narrated the 1977

documentary Powers of Ten.

Key work

1959 Searching for Interstellar

Communications

The Lovell (Mk 1) radio telescope

at Jodrell Bank, the third-largest radio

telescope in the world, was used

as part of the Project Phoenix SETI

program in the 1990s and 2000s.

Modulation is a method by which information can
be transmitted within a wave signal. The amplitude
is kept constant, while the frequency varies.

VARYING FREQUENCY