Some astronomers are not looking for stars, planets or galaxies. They want to know if there is other intelligent life in the universe. This field of study is known as SETI (Search for Extraterrestrial Intelligence).
Scientists think one way we might find life beyond Earth is with radio waves. We have been sending out radio and television waves from Earth for more than a century. Anybody in the Universe with a sensitive enough receiver could listen in on what we have been sending to each other. Even though radio waves travel at the speed of light, it could take millions of years for these messages to be received.
The Universe itself makes a lot of electronic ‘noise.’ That is what radio astronomers listen to. SETI works to filter out artificially-produced radio waves from the noise of the Universe. All human-made radio waves are narrow-band. This means they are transmitted on a narrow part of the electromagnetic spectrum. It is like when you want to tune in to a radio station. You turn the dial and hear static until the signal comes through clearly. That’s what SETI researchers are doing. But they don’t know what the signal they are listening for will sound like. They predict that any signal would be repeated, so other stations could tune in to the signal. This would show that a signal wasn’t a random noise. A repeating signal could have been created by an intelligent life form. Without a repeated signal, people will not be convinced that extraterrestrial intelligence has contacted us.
It’s hard to know what a message from an extraterrestrial intelligence would be like. Did you know that we humans have tried to write one ourselves? In November 1974, a message was sent from the Arecibo radio telescope in Puerto Rico. The message was aimed at a group of stars called M13 that are about 21 000 light-years away. The message was made up of 1 679 binary numbers. It was arranged in a 23-column by 73-row run-length coding image. The message contained information about us humans. The picture showed the numbers 1-10, DNA, a human, the solar system, and the radio telescope that sent the message. If you look at the message, you can see how hard it might be to decipher alien radio signals. Even if the Arecibo Message is heard and a reply is sent, we won’t get it for at least 42 000 years! If we ever do find somebody else in the Universe, it’s going to be hard to have a conversation.
The Universe is an awfully big place. So is the electromagnetic spectrum. There are billions of radio frequencies. However, there is one part of the electromagnetic spectrum, in the 1-10 gigahertz (GHz) range, where there is very little natural background noise. SETI researchers think any extraterrestrials would also know this and send signals in that range. Almost all SETI research looks at those wavelengths. There are still billions of wavelengths in that range, though. Searching for a needle in a haystack is easy compared to SETI!
There are only a few radio telescopes in the world. This means that it is hard to get time on them. SETI searches often ‘piggyback’ on other radio astronomy. For example, they may search through data gathered by other researchers. SETI researchers do get some observing time on large radio telescopes like the giant 305 m dish in Arecibo.
Before radio telescopes had been invented, some scientists wondered whether radio receivers might pick up transmissions from space. In 1899, Nikola Tesla heard some strange signals on his Tesla Coil receiver. He thought those signals came from the planet Mars. Guglielmo Marconi and Lord Kelvin also thought radio signals could be used to communicate with beings on Mars.
In August 1924, Mars passed closer to Earth than at any time in more than a century. Astronomers on Earth wanted to use this chance to listen for radio messages from beings on Mars. Astronomer David Peck Todd convinced the United States government to declare a “National Radio Silence Day.” This happened over 36 hours from August 21-23, 1924. For five minutes every hour, high-powered radio transmitters were supposed to shut down. This would make it easier to pick up any signals sent by Martians. Cryptologists waited to decipher any messages that might come in. Sadly, no signals from Mars were heard then. Today there are so many radio and television signals in the air that filtering out human-made ‘noise’ is a big part of SETI searches. The quiet airwaves those astronomers had in 1924 is something that modern SETI researchers can only dream of!
Modern SETI began in 1960. In that year, astronomer Frank Drake conducted Project Ozma, which was named after the ruler of Oz. Drake pointed a 26 m radio telescope at two nearby stars for about four months. They listened to the 1.4 GHz frequency for signals. No signals were heard, but Drake’s work inspired many SETI research projects over the past 60 years.
You can even help out with SETI in your own home! One of the biggest challenges for SETI researchers is analyzing all the data gathered. In 1999, the University of California at Berkeley launched the SETI@Home program. This lets the public help with SETI. You can download a program that lets your computer analyze data with its unused power. The results are then sent back to SETI researchers.