Right now, sophisticated radio telescopes are listening to the cosmos, waiting for a transmission from an extraterrestrial civilization. The Allen Telescope Array operated by the SETI Institute is just one of many instruments that have been used in this search. Scientists and engineers are regularly developing new techniques to improve the performance of these instruments.

Millions of frequencies are monitored, but is this really the best way to discover the existence of extraterrestrial life? SETI researchers have traditionally looked for radio transmissions for some fairly sound reasons. Our own planet has used radio extensively for communications.

We also know that certain frequencies can travel across vast distances in space. That's how the science of radio astronomy was born. It seems fair to expect that extraterrestrials would also develop radio transmitters at some stage.

Radio searches will probably be the mainstay of searching for extraterrestrials for a long time. The technology is highly advanced. The theory behind this method is sound.

This shouldn't stop us from considering other ways of detecting extraterrestrials, and scientists are actively working to develop them. One alternative that's been growing for some time is optical SETI.

This involves the search for short, high-intensity laser pulses sent into space. Just as our own civilization has experienced the growth of optical technology and lasers for communications, it seems reasonable to assume that other planets could do the same. Lasers could also have certain advantages over radio transmitters, especially for communications specifically aimed at Earth.

Optical SETI is performed with optical telescopes fitted with special detectors. It's similar to the way conventional radio telescopes have been modified for SETI research with the use of additional instruments.

There could be more exotic ways of sending messages across the galaxy. Neutrinos are strange, almost massless subatomic particles that can travel through objects almost supernaturally. They are also difficult to detect. Neutrinos are produced naturally by stars and also by particle accelerators on Earth.

Right now, we don't have a full understanding of the properties of neutrinos, and it's difficult to work with them. Even detecting a handful of neutrinos requires a lot of complex gear. Nobody is using them for communications on Earth.

This doesn't mean that neutrino transmissions from an advanced extraterrestrial civilization aren't being broadcast by someone out there. So far, none of the experiments designed to detect neutrinos from space have spotted anything suspicious. SETI researchers themselves have not run any special experiments with neutrino detection, and this probably won't happen in the near future.

We could also consider gravity waves, distortions of space-time produced by large objects. We're struggling to detect gravity waves on Earth, even though relativity theory suggests that they exist. Generating gravity waves artificially would be extremely difficult.

Don't even ask for suggestions of how it could be done! It's a long shot, but using gravity waves for interstellar communications is still a possibility. Right now, it's on the far fringes of our search, and can't be attempted until the technology to detect gravity waves improves.

These techniques are varied, but all suffer from a common problem. All of these physical phenomena can be generated both naturally and artificially. How can we tell the difference? Astronomers have sometimes believed that they had intercepted artificial transmissions from space, only to later discover that they were seeing something natural.

The best example is the discovery of pulsars, which are naturally formed radio beacons. In some cases, the difference between a natural and an artificial transmission from extraterrestrials could be hard to distinguish.

The search for "exoplanets" orbiting other stars has opened other opportunities for discovering extraterrestrials with telescopes.

Scientists have already begun to search for evidence of large artefacts in space that could be constructed by extraterrestrial civilizations. These could include so-called "Dyson Spheres", enormous structures designed to absorb and convert the energy from a star into usable power.

These structures would affect the frequency of light emitted by stars that were surrounded by them. But are Dyson Spheres really practical? A civilization with the capability to build one could also have the technology to make them redundant.

Detecting a signal from extraterrestrials is difficult enough. But it's even more difficult to decode one. With no common language or protocols, it would not be clear if they were even deliberately trying to signal us.

Some scientists expect that any transmissions we receive could not be attempts to contact other civilizations, but just the spillage of regular transmissions from another world.

All sorts of theories on how we could develop a common code have been proposed but none can be tested without actual contact. Do we even know if they conceptualize the fundamentals of language or even mathematics as we do?

We don't know how extraterrestrials would try to communicate with us. We aren't even sure if they are trying. This can be frustrating for SETI practitioners, but it also points to the appeal of SETI. This is truly a bold venture into the unknown, and a source of inspiration to a world that sometimes thinks there is nowhere else to explore.

Dr Morris Jones is an Australian space analyst. He is a contributor to the new SETI book "Astrobiology, History and Society" from Springer. Read a free preview here