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. (But the team who designed it pointed out that moderns digital storage devices like CDs and DVDs are already degrading, whereas the sturdy analog technology attached to Pioneer will endure for millenia.) 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. It is our "message in a bottle," tossed hopefully into the void of space.
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. 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 three decades has passed, and Pioneer 10 is now over 10 billion miles away from us, far 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, many people are aware that 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, these signals weaken so rapidly by the inverse square law that they fall below the radio "hiss" of the microwave background from the big bang before they even leave the Solar System. The spinning Earth does sends out radio waves that rise and fall several times per day due to the concentration of transmitters in the United States and Europe, butthe 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 every day. They have moved on from listening only to radio waves and have added 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 of success would be profound.