130 PH V SICS OF THE IMPOSSIBLE
More specifically, Drake's equation calculates the number of civilizations in the galaxy by multiplying several numbers together, including
• the rate at which stars are born in the galaxy,
• the fraction of these stars that have planets,
• the number of planets for each star that have the conditions for life,
• the fraction of planets that actually develop life,
• the fraction that develop intelligent life,
• the fraction that are willing and able to communicate, and
• the expected lifetime of a civilization.
By taking reasonable estimates and by multiplying these successive probabilities, one realizes that there could be between 100 and
10,000 planets in the Milky Way galaxy alone that are able to harbor
intelligent life. If these intelligent life-forms are uniformly scattered
across the Milky Way galaxy, then we should expect to find such a
planet just a few hundred light-years from Earth. In 1974 Carl Sagan
estimated that there might be up to a million such civilizations within
our Milky Way galaxy alone.
This theorizing, in turn, has provided added justification for those
looking to find evidence for extraterrestrial civilizations. Given the favorable estimate of planets capable of harboring intelligent life-forms,
scientists have begun seriously to look for the radio signals such planets may have emitted, much like the TV and radio signals that our own
planet has been emitting for the past fifty years.
LISTENING TO ET
The Search for Extraterrestrial Intelligence (SETI) project dates back
to an influential paper written in 1959 by physicists Giuseppe Cocconi
and Philip Morrison, who suggested that listening to microwave radiation of a frequency between 1 and 10 gigahertz would be the most
suitable way to eavesdrop on extraterrestrial communications. (Below
1 gigahertz, signals would be washed out by radiation emitted by fast-
More specifically, Drake's equation calculates the number of civilizations in the galaxy by multiplying several numbers together, including
• the rate at which stars are born in the galaxy,
• the fraction of these stars that have planets,
• the number of planets for each star that have the conditions for life,
• the fraction of planets that actually develop life,
• the fraction that develop intelligent life,
• the fraction that are willing and able to communicate, and
• the expected lifetime of a civilization.
By taking reasonable estimates and by multiplying these successive probabilities, one realizes that there could be between 100 and
10,000 planets in the Milky Way galaxy alone that are able to harbor
intelligent life. If these intelligent life-forms are uniformly scattered
across the Milky Way galaxy, then we should expect to find such a
planet just a few hundred light-years from Earth. In 1974 Carl Sagan
estimated that there might be up to a million such civilizations within
our Milky Way galaxy alone.
This theorizing, in turn, has provided added justification for those
looking to find evidence for extraterrestrial civilizations. Given the favorable estimate of planets capable of harboring intelligent life-forms,
scientists have begun seriously to look for the radio signals such planets may have emitted, much like the TV and radio signals that our own
planet has been emitting for the past fifty years.
LISTENING TO ET
The Search for Extraterrestrial Intelligence (SETI) project dates back
to an influential paper written in 1959 by physicists Giuseppe Cocconi
and Philip Morrison, who suggested that listening to microwave radiation of a frequency between 1 and 10 gigahertz would be the most
suitable way to eavesdrop on extraterrestrial communications. (Below
1 gigahertz, signals would be washed out by radiation emitted by fast-