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Historically, SETI – the search for extraterrestrial intelligence – involves scanning the sky for radio signals that another civilization is deliberately sending. The simplest would be a constant blast in all directions, but in a narrow range of frequencies, similar to early radio broadcasts – like a constant hum that would tell a listener it is artificial. From light years away, we would not be able to get any other information – all we would be able to tell from Earth is that a signal was there and where it was coming from, not what it says.
But David Messerschmitt at the University of California, Berkeley points out that such a continuous signal would take a tremendous amount of energy. Assuming aliens have utility bills, he says, they would use a different strategy.
They would also probably want to say something more than "we are here" by adding information to the signal. All attempts to send messages to ET from Earth have contained information, sometimes a lot of it. One of the most famous, the Arecibo message sent in 1974, which encoded details about humans, DNA, the solar system and more.
"All our discussions about transmitting ourselves include information, and how to encode it such that ET can understand our message and what to include in the message," Messerschmitt says.
To do this most efficiently, instead of a constant, narrow-band signal, Messerschmitt argues that ET would beep out short bursts in a wider range of frequencies – a broadband signal. This would take less energy to transmit, and could encode information.
Current SETI searches are not designed to pick up information in that kind of signal, notes Seth Shostak, director of the Center for SETI Research at the SETI Institute in California.
"The problem is that… encoding a message; means that any signal would vary quickly," Shostak says. "To see such variations – to get the information in the signal – requires having enough sensitivity to see changes in a 10th or 100th of a millionth of a second. That requires antennas with collecting areas maybe 10,000 times larger than necessary to detect a steady signal." No such antennas currently exist that would pick up the variations more than a few light years away.
But Messerschmitt thinks there's a workaround. Existing software, such as that used for the SETI@Home project, which processes millions of signals using idle home computers, could be adapted to extract information from a signal. SETI@Home looks at many channels at once seeking narrowband signals, but it could be programmed to look for broadband ones instead.
He doesn't think this means the current approach to SETI should be halted, but rather expanded to also look for this alternative form of signal.
"I would not advocate putting all our eggs in one basket," says Messerschmitt. "We really don't know what ET is up to."
Journal reference: Acta Astronautica, DOI: 10.1016/j.actaastro.2014.11.007