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In 1950, Nobel prizewinning physicist Enrico Fermi posed his famous paradox: if extraterrestrial intelligence exists, why haven’t we found it?

Why indeed? It is not as if we haven’t been trying. The search for extraterrestrial intelligence (SETI) has been going on for over half a century. It has mostly drawn a blank. But once in a while there is a flurry of excitement. Here are some of the highlights.

Read more: “Is the answer to life, the universe and everything 37?”

First contact

On 8 April 1960, Cornell University astronomer Frank Drake pointed a 26-metre radio telescope at two nearby stars. The telescope – based at the US National Radio Astronomy Observatory (NRAO) in West Virginia – was tuned to a frequency of 1420 megahertz, the wavelength of radiation naturally emitted by hydrogen in space. Thus began Project Ozma, the first experiment explicitly designed to look for aliens.

Drake was hoping to detect radio waves sent by an extraterrestrial civilisation. He chose the emission frequency of hydrogen because it is the most abundant element in the universe, and hence an obvious signal for any intelligent civilisation trying to get itself noticed by another.

Although the stars – Tau Ceti and Epsilon Eridani – were considered promising candidates, being nearby and sun-like, Project Ozma detected nothing in over 150 hours of observation.

In 1972, astronomers at NRAO had a second go, this time using a bigger telescope that collected as much data in a minute as the older one could in 19 years. They sporadically monitored more than 650 stars for four years, again searching for the hydrogen signal – and again finding nothing. But the Ozma projects established SETI as a credible discipline and set the scene for many more attempts.

The Wow! signal

One of the projects inspired by Ozma was the “Big Ear” programme at Ohio State University, which ran from 1973 to 1995. On 15 August 1977, its 79-metre dish picked up a powerful burst of radio waves from the general direction of Sagittarius.

The burst lasted 72 seconds and was very close to the emission frequency of hydrogen – considered a likely candidate for alien messages. When astronomer Jerry Ehman saw the signal recorded on a computer printout, he circled it in red pen and scrawled “Wow!” on the sheet of paper.

The set-up of the telescope made it hard to work out exactly where the burst came from, but the general patch of sky was identified.

The “Wow!” signal remains the most promising putative alien signal ever detected by SETI. But despite extensive searches of the same patch of sky it has never been seen since.

Radio ga-ga

In 2007, astronomers at West Virginia University discovered a previously unknown celestial phenomenon: a super-intense, very brief burst of radio waves apparently originating outside our galaxy.

The Fast Radio Burst lasted for just 15 milliseconds but released more energy than the sun emits in about a month. Calculations suggested that it came from an object no more than 1500 kilometres across.

At the time there was no obvious explanation for the FRB. Astronomers speculated that it came from a single cataclysmic event, such as the final collapse of a dying black hole or the merger of two neutron stars.

A handful of other FRBs have since been detected but there is still no agreed explanation.

Inevitably, the gap has been filled by speculation that FRBs are messages from aliens. Earlier this year, Nigel Watson, author of the

, that FRBs could be evidence of a “vast alien communication network”. Or, he said, it could just be an as-yet-unknown astronomical phenomenon.

Messages closer to home

In the absence of a smoking gun from the sky, some alien hunters have looked for signs on our doorstep. For a while, crop circles – strange geometric patterns that began to appear in arable fields in southern England in the 1970s – were claimed by many people to be messages from ET. They are now known to be the work of artists and pranksters.

Around a decade ago a slightly more serious idea began to circulate: perhaps there are alien messages in our DNA. As Paul Davies, author of The Eerie Silence: Renewing our search for alien intelligence, wrote in 91av in 2004: “Might ET have inserted a message into the genomes of terrestrial organisms, perhaps by delivering carefully crafted viruses in tiny space probes to infect host cells with message-laden DNA?”

A decade on, we have no evidence whatsoever that ET did this. In the past couple of years the idea has been revived in a slightly different form: a pair of Kazakh researchers have proposed that the genetic code would be a better place to plant a signal, and even claim to have found what they call “the Wow! signal of the genetic code” (see “Is the answer to life, the universe and everything 37?“).

Comin’ atcha

Our failure to detect alien messages has not deterred us from sending some of our own. As soon as humans learned to communicate using radio waves we began unwittingly broadcasting to the stars: the earliest radio shows are now about 100 light years away and counting.

The first deliberate attempt to contact ET was in 1972, with the launch of NASA’s Pioneer 10 space probe. This carried a gold-plated aluminium plaque bearing pictures of a male and a female nude and graphical information about the origin of the craft, in the (extremely unlikely) event of it being intercepted by aliens. Pioneer 10 is currently on the fringes of the solar system on course for a star 68 light years away. Pioneer 11, launched in 1973, also carries a plaque.

A more detailed message was loaded onto the Voyager 1 and 2 probes, launched in 1977. Both carry a gold-plated copper disc encoding over 150 images and sounds from Earth, greetings in 55 languages, and brief excerpts of music. (In 2010, Voyager 2 started sending some unusual signals back to Earth, prompting speculation that it had been hijacked by aliens. It turned out to be a computer glitch.)

The chances of any of these physical messages ever being picked up is remote in the extreme, but they are not our only attempts at contact. At the dedication of Puerto Rico’s Arecibo radio telescope in November 1974, a digital message was beamed towards the star cluster Messier 13 (M13) 25,000 light years away. The message – encoded in microwaves – included the numbers 1 to 10, various graphical representations of DNA and its constituent atoms, and drawings of a human figure, the planets of our solar system and a radio telescope. The message is currently just over 40 light years from Earth. It has yet to receive a reply.

Since then, numerous other messages have been broadcast to the stars, including a giant Doritos advert beamed from the Arctic archipelago of Svalbard to a solar system 42 light years away. Such messages are controversial, however: some scientists believe we should keep our heads down so as not to alert malicious aliens to our feeble presence.