The
The search for extraterrestrial intelligence (SETI) is based
entirely on speculation. The only way to move beyond mere speculation is to
obtain some real data, to conduct an experiment, to gather supporting evidence. We have already been listening for messages
and we have even sent our own on space probes and with radio waves. So what is our strategy? What is the best message to send? What type
of communication should we use? There are millions of possible targets in the
galaxy, where should we beam our signal?
It is important to be aware that, unlike other scientific
endeavors, we cannot adequately justify a search on strictly rational grounds.
The case for SETI cannot be made in the same way as the case for building a
particle accelerator or for sequencing a gene.
To confront this reality, SETI optimists, such as Carl Sagan, have
argued that the results of a search for extraterrestrial intelligence will be
significant, no matter what the result is. However, a failure to detect
intelligent life will not prove that we are alone. The truth is that SETI is a scientific
gamble. The odds are stacked against us, but the stakes are incredibly high.
Our role as sentient beings in this expansive universe will be profoundly
affected by whether or not we are alone. And unless we conduct an experiment,
we will never know.
In 1972, the Pioneer 10 spacecraft was launched toward
Jupiter, and it has since become the first human artifact to leave the solar
system. With it we attached our first message, a plaque carrying a greeting to
any civilization that might find it. The plaque shows a naked man and woman
next to a silhouette of the Pioneer spacecraft. The top of the drawing shows
the spin transition of the hydrogen atom, and the bottom shows the trajectory
of the spacecraft within the solar system. The radial pattern represents the
position of the solar system within the Milky Way, by triangulation among a set
of 14 pulsars. At the time it was launched, the plaque created quite a stir.
Many complained about the government-sanctioned nudity, and feminists objected
to the fact that the man's hand was raised in greeting, but not the woman's.
Also, some people found the pulsar map to be obscure, wondering how aliens
would decipher it if they could not. The plaque illustrates our first
difficulties in trying to encapsulate the essence of human beings in a short,
concise message.
Five years later, in 1977, the Voyager spacecraft was slated
for launch. A team headed by Carl Sagan
and Frank Drake designed a new message to be sent into space with it. This time
they included music in addition to images. As Lewis Thomas has said, "I
would vote for Bach, all of Bach, streamed out into space, over and over again.
We would be bragging, of course, but... we can tell the harder truths
later." Instead of a plaque, this time a gold-anodized record was attached
to the spacecraft. Both images and
sounds were encoded onto the record and instructions were etched on the cover.
Now, just a few decades later and well before reaching another planetary
system, the record technology used to encode the message is obsolete on Earth.
In an attempt to represent the diversity of humans and the natural environment,
a wide variety of images were included on the record. Unfortunately, perhaps in response to the
complaints of the naked pair on Pioneer’s plaque, censorship prevailed and NASA
vetoed one picture involving nudity. The sound selections included many natural
sounds such as those made by whales, rain, footsteps, and a kiss. They also included music such as Bach, jazz,
and rock and roll, but also much non-Western music. Spoken greetings in 55 different languages
are followed by a message from the United Nations Secretary General. Despite
the efforts, it is difficult not to see the record as a message to us, rather
than to an alien civilization.
Since the primary purpose of the spacecraft was exploration
and not communication with an extraterrestrial civilization, it’s important to
contemplate the time and effort put into creating the messages. The messages are very much like bottles
tossed into the ocean. No one knows how long they will be lost at sea before
being picked up by an unsuspecting seafarer.
Almost two decades has passed, and Pioneer 10 is now about 3.5 billion
miles away from us, beyond the orbit of Pluto. Even traveling at about 6 miles
per second, it will take 100,000 years to reach the nearest star to the Sun.
How can we build an interstellar probe that travels at more
than the current limit of 10 miles per second, which is only 1/20,000 of the
speed of light? Unfortunately, the laws of physics work against us. The energy
required to accelerate a small payload of 100 kg to one-tenth the speed of
light exceeds one year's output from all the power plants on the Earth! To
reach 99% of the speed of light using the most efficient energy source
imaginable, the annihilation of matter and antimatter, requires a spacecraft
with 40,000 times as much mass in fuel as in payload. The energy requirements
for transmitting electromagnetic radiation are far less restrictive. The
kinetic energy of a radio wave photon is 1012 times less than the kinetic energy of an
electron traveling at 99% of the speed of light.
Rather than create the technology to launch something that
can accelerate to the speed of light, why not use something that naturally
achieves that velocity? Electromagnetic
waves (or photons) are the preferred carriers of information for just this reason,
and they are easy to transmit, modulate, and receive. The only drawback is that
photons range in frequency and wavelength over a factor of 1020 from radio waves
to gamma rays. How do we choose a single optimum frequency for communication
from such a large range? Luckily, nature has provided us some guidance. First,
radio waves contain the least amount of energy per photon, and so are the most
efficient to produce. Secondly, photons with optical frequencies or higher
suffer absorption and scattering by gas and dust in the interstellar medium.
The best penetration through these barriers is achieved by radio waves.
Finally, when we measure the spectrum of cosmic radio "noise", it
shows that the quietest region on the dial is the zone around 1000 MHz (a
thousand million Hertz, or 109 Hz). At lower radio frequencies, radiation from
high-energy electrons in the Milky Way contaminates the signal. At higher radio
frequencies, there is a rising noise source due to the cosmic background
radiation. The quiet zone — in other words, the frequency range where naturally
occurring cosmic noise is low — also contains the frequency of the spin
transition of cold hydrogen, the most abundant element in the universe.
Even if we accept the arguments for radio communication, we
still face a challenge. Due to the nature
of the beast, we have a classic "needle in a haystack" problem. There
is a range of thousands of MHz in the zone in which cosmic noise is low. We do
not know in advance what the bandwidth or range of frequencies of a signal
should be. To send a large amount of information, the bandwidth should be
large. A single TV channel has a range of 6 MHz, and an FM radio station
transmits over a range of 200 kHz (200,000 Hertz). On the other hand, to transmit information as
efficiently as possible, a narrow bandwidth should be used. It doesn’t matter
how narrow the signal is originally, radio waves will scatter in interstellar
space that smears the signal to a width of about 0.1 Hz. Smearing is analogous
to the blurring of an optical image as it passes through the Earth's
atmosphere. Therefore, when searching for a message from space, we must examine
thousands or perhaps millions of targets, each of which must be searched over
billions of separate frequency channels!
And when composing a message, we must decide what is
the most efficient channels and targets to use.
Communicating via radio waves presents a unique set of
challenges when compared to similar messages in pictorial form on plaques. How
do we convey a picture over radio waves?
How will the radio waves be interpreted?
We haven’t spent much time in designing a message. In fact, for fifty years we have been
inadvertently leaking radio, radar, and television signals into space, creating
a bubble of radio energy expanding outward from the Earth at the speed of
light. Dilute signals of I
Love Lucy have crossed the paths of approximately 1000 stars, and The Brady Bunch has
reached several hundred. Only a few dozen star systems have been treated to
episodes of Seinfeld.
The joke goes that aliens are not visiting us because they have received our
radio and television broadcasting and have so far seen no signs of intelligent
life. In fact, the spinning Earth sends out radio waves that rise and fall
several times per day due to the concentration of transmitters in the United
States and Europe. Realistically, the content of our weak transmissions could
not be deciphered due to the overwhelming radio din from other cosmic sources.
Moreover, the two largest sources of our radio leakage have diminished.
Powerful early warning radar has being dismantled due to the end of the Cold
War, and TV transmissions are moving towards fiber and cable.
What is the best way to communicate? Scientists at the SETI Institute contemplate
this very question everyday. They have
moved on from listening primarily to radio waves and have moved on to create a
new program called Optical SETI, which will look for single, short-lived bursts
of optical light from our neighbors. The
search is vast, the stakes are high, but the results will be profound.
Recommendations:
THE ORIGIN OF LIFE ON EARTH
EVOLUTION AND INTELLIGENCE
HISTORY OF SPACE EXPLORATION
THERMAL RADIATION
EARLY COSMOLOGIES
WAYS OF REPRESENTING DATA
THEORIES
UNIVERSAL EXPANSION