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
UFO Investigations Manual, told the UK's Daily Mail newspaper 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 New Scientist 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?").
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.
No comments:
Post a Comment