Read more: The face of first contact: What aliens look like, What Voyager's golden record tells ET about Earth

Editorial: Hello ET, we come in peace

THE cosmos is quiet. Eerily quiet. After decades of straining our radio ears for a whisper of civilisations beyond Earth, we have heard nothing. No reassuring message of universal peace. No helpful recipe for building faster-than-light spacecraft or for averting global catastrophes. Not even a stray interstellar advertisement.

Perhaps there's nobody out there after all. Or perhaps it's just early days in the search for extraterrestrial intelligence (SETI), and we're listening to the wrong star systems or at the wrong wavelengths.

There is another possibility, says Douglas Vakoch, head of the Interstellar Message Composition programme at the SETI Institute in Mountain View, California, which ponders the question of how we should communicate with aliens. "Maybe everyone's listening but no one is transmitting. Maybe it takes an audacious young civilisation like ours to do that."

So should we start sending messages into the void? And if so, how can we make ourselves understood to beings we know nothing about?

One astronomer has already stepped up to the challenge. Alexander Zaitsev at the Russian Academy of Science's Institute of Radio Engineering and Electronics in Moscow has sent four interstellar messages since 1999, each beamed to no more than a handful of nearby sun-like stars. Zaitsev's efforts are pretty small scale, however, and so far his group is more or less alone except for some low-power operations offering to send your message to the stars for a fee.

Read more: Earth calling: A short history of radio messages to ET

That could be about to change. Perhaps bored with spending so long hearing nothing, the wider SETI community is starting to consider a more active approach. They will get together to discuss whether to go for it at a meeting in April in League City, Texas. Vakoch, who will chair these sessions, is all in favour. "I have long held the position that after broad-based international consultation, we should be doing active SETI," he says.

It's an approach that worries ex-astronomer and science fiction author David Brin, who was a member of the International Academy of Astronautics SETI panel until 2006. He resigned when the committee backtracked on the wording of a protocol that called for discussion before deliberately broadcasting into space. "I dislike seeing my children's destiny being gambled with by a couple of dozen arrogant people who cling to one image of the alien," says Brin. Since then three other members have quit for similar reasons. Vakoch has some sympathy with Brin's point of view. "These issues are much too important and too complex to be resolved after only a few days of discussion."

If the enthusiasts for active SETI get their way and there is a real effort to send a message, the next question is: what should we say?

Some early attempts to communicate with aliens - including the plaques attached to NASA's Pioneer 10 and 11 spacecraft, and the phonograph records on the Voyager probes - were really only symbolic efforts. More likely to be received one day is the powerful radio broadcast devised by SETI pioneer Frank Drake and sent from the Arecibo telescope in Puerto Rico in 1974. We have a long wait for contact if we rely on this, though: the message won't reach the distant star cluster it was aimed at, M13, for 25,000 years. It was also a very brief message, containing only 210 bytes of information.

"These are greeting cards," says Seth Shostak, who is a senior astronomer at the SETI Institute. "It is nice to get a greeting card, but hard to decipher if it's in a language you don't understand, because the amount of material is so limited."

Interstellar geek speak

Although the Arecibo message is cunningly constructed, it is difficult even for a human to understand. The signal contains a series of 1679 bits, a number chosen as it is equal to the product of two prime numbers - 23 and 73. The hope is this will prompt an acute alien recipient to arrange the 1s and 0s into a 23-by-73 rectangle. Doing so reveals a rather complicated picture, which is supposed to give some basic information about our chemical and biological make-up, our civilisation and the solar system (see illustration).

To me it looks a bit like a small person with a big head, four eyes and eight bushy eyebrows. So the only bit I've got right is the "person", and there I had a big advantage. "It is much worse if you don't have lot of context, if you don't even know about Homo sapiens," says Shostak.

So where might we find some common ground with ET? Perhaps in mathematics. This idea goes back about three centuries, when it was suggested that we could hail beings on the moon by cutting a diagram of Pythagoras's theorem into the forests of Siberia.

In 1960, Dutch mathematician Hans Freudenthal proposed an interspecies language called Lincos that starts with simple mathematical statements - a series of beeps to represent 1 + 1 = 2 for example. It then uses these to define logical relationships, eventually building up to more complex concepts of time and space.

More recently in the 1990s, mathematician Carl DeVito and linguist Robert Oehrle, both then at the University of Arizona in Tucson, made an attempt to speed up this process by assuming that respondents have some knowledge of physical concepts. In DeVito's scheme, once some mathematical and logical symbols have been defined, we would launch into a description of the periodic table, and then discuss energy in terms of specific heats of the elements and so on. They argue that any civilisation capable of receiving our radio signals must have some knowledge of physics to build radio receivers.

While interstellar geek-speak might be fine for sending details of your solar system and the latest technology, it's not so easy to move on to more abstract ideas, such as human nature and culture. How would you convey a simple sentiment like "we come in peace"?

How do you convey a simple sentiment like 'we come in peace' to an alien species?

"It seems that sooner or later one must fall back on pictures," says DeVito. Images might be the only way to convey to an alien what we're talking about, and to show them what our world is like. They wouldn't need to have vision like ours, merely some way to understand spatial patterns in at least two dimensions. Then they could attach symbols to meanings, like a child with a picture book.

And we can do better than "cows go moo". Paolo Musso, a philosopher at the Pontifical Urbaniana University in Rome, Italy, has outlined one way to combine mathematics and images to tell a moral tale using the process of analogy. First, arithmetic is used to establish signals that mean right and wrong, defined by sums that are right and wrong. These can then be applied to cartoons showing what we consider to be good and bad actions.

Many of these approaches start with counting, but what if the slimy ones don't have our concept of arithmetic? "There is no guarantee that if ETs have mathematics it will be commensurable with ours," says Vakoch. He and others have instead suggested a more creative channel of communication: sending examples of our culture. It is possible, Vakoch says, that aliens will develop a taste for Toulouse-Lautrec paintings or Meade Lux Lewis boogie-woogie numbers, and then we will have done something worthwhile. In 2008 NASA transmitted a song by The Beatles at the star Polaris, while artist Joe Davis has aimed other sounds and messages at two nearby stars.

Zaitsev's longer messages - Cosmic Call 1 and 2, and Teen Age Message - combine several of these ideas. Each message consists of a beam of radio waves sent from the Evpatoria radio telescope in southern Ukraine, which encoded each bit of information by shifting the transmission frequency slightly up or down. A bit can represent a black or white square, and a number of them can be built up pixel by pixel into a series of pictures like the original Arecibo message. Some of them form a rather sketchy "bilingual image glossary" - a dozen hand drawings meant to show concepts including people, family, nature and games, with English and Russian words attached. Zaitsev has also included an excerpt of theremin music to soothe, or perhaps irritate, alien listeners.

Give them porn

But the main content was a series of 127-by-127 pixel images forming the"interstellar Rosetta Stone", developed by astronomers Yvan Dutil and Stephane Dumas from Defence Research and Development Canada in Valcartier, Quebec. Like Freudenthal's and DeVito's approaches, the images start with arithmetic and build from there. They also include graphs to illustrate physics, as well as sketches of the solar system, Earth's topography and the man and woman from the Pioneer plaque.

As well as these complex signals, Zaitsev has transmitted a more lightweight message. In "A message from Earth", sent in 2008, contributions from users of social networking site Bebo were directed at a single planetary system around the star Gliese 581. If anyone there is listening, they can expect to receive it in 2029 - followed by 26,000 unsolicited text messages collected byCosmos magazine and transmitted last year. At 50 to 60 kilobytes, there is more information in the Cosmic Calls than in the Arecibo message, giving alien cryptologists and linguists something to work with. To give them a helping hand, a lot of what has been sent is redundant.

"Redundancy really helps," says Shostak, as it allows a recipient to make a guess about the meaning and then check it, like in a crossword. He suspects that all the polite efforts to be understood might be unnecessary. "A lot of people wonder what we should send. Music, mathematics or pictures? My first thought is it probably doesn't matter," he says.

Instead, Shostak suggests that we just gabble. "My conclusion is that you would just send them the Google servers. That's an enormous amount of information, much of it redundant and pictographic. Much of it is pornographic too, but I expect they could handle that." (Although it raises questions like, can Earth handle a trillion orders for Viagra?)

According to Shostak's calculations, sending so much stuff becomes practical if we assume that ET is very good at listening and has the technology to pick up a faint, rapidly changing signal against the background noise of the galaxy. If they have truly vast radio dishes, then we can send them encyclopedias and even libraries in a sensible amount of time.

Shostak also points out that an interstellar message is quite likely to be a one-way deal: it may not even reach its destination until long after our civilisation has ended. So as this is probably the only chance of telling an alien world about humanity, we might as well say as much as we can.

Of course, it would be much better if we could actually chat, as this would allow us to gradually teach each other our languages and histories. But interstellar distances make that nigh on impossible unless we can somehow send a representative.

Sociologist William Bainbridge of George Mason University in Fairfax, Virginia, is trying to develop avatars, human personalities encoded in software. If he succeeds, we could send one into deep space, perhaps programmed into an interstellar probe. Or if we can find a way to tell receivers how our software works, we could even send avatars to them via a radio beam.

Whatever we decide, the next problem is where to aim our message. Broadcasting loudly to the whole sky would use ludicrous amounts of power, far beyond our capabilities today. Instead, active SETI would target promising star systems. A sun-like star is a start; planets are better. Space telescopes such as Kepler should be able to detect planets that are Earth-size, and future telescopes should eventually be able to pick out those with liquid water on the surface and oxygen in their atmospheres.

This still might not be a well-targeted address list, however, because civilisations may be very thinly spread. "Even if you found a list of 10,000 Earth-like worlds, that might not be good enough," says Shostak. "We've had biology for 4 billion years, and radio telescopes for 40 years; that's 1 in 100 million." If the technological window for aliens is as short as ours then we might have to transmit to 100 million Earths before anyone hears us.

For practical reasons, Shostak thinks we should wait and listen. "The bottom line is that for the foreseeable future, the only decent targets are people that have contacted you. Let them do the heavy lifting."

In the meantime, planning our own message could help to focus the minds of SETI experts on what kind of communication we should be looking out for. If Earth's efforts are anything to go by, we can expect a basic maths lesson and some pictures of naked aliens.

Read more: Earth calling: A short history of radio messages to ET

Read more: The face of first contact: What aliens look like

Editorial: Hello ET, we come in peace

Stephen Battersby is a writer based in London