SETI: Search for Extra-Terrestrial Intelligence

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One of the most fundamental questions in science: Does life exist on other worlds?


The 'Wow!' Signal

What is the probability of extra-terrestrial life in the universe?

Where and How should we Look?

How would we communicate with them?

What would be the significance of discovering them?

The Wow! Signal

It was the night of August 15, 1977, when the historic 'wow!' signal gained notoriety. It's the most intriguing signal yet discovered by SETI. It was detected by the "Big Ear" Radio telescope in Ohio, while scanning our galaxy to seek out signals in the sky from extra-terrestrials. The computers picked up this signal coming from the direction of the constellation Sagittarius. And a series of characters were printed, '6EQUJ5', which indicated a rise and fall in the detected signal strength over a period of 72 seconds. It was a strong narrowband continuous wave signal. But was this from another civilisation? Could this have been an alien beacon, our first contact with extra-terrestrials? The Scientist, and then SETI volunteer, Jerry Ehman, was monitoring the readings that night. He circled the readings (after he regained his breath I suspect) and added a comment 'Wow!' beside them, out of sheer amazement. And henceforth, the 'Wow!' signal came into being.

According to some observers the signal had a potentially extra-terrestrial source, maybe a signal from an alien race! Although skeptics query the signal's origin and think it could be just random terrestrial interference or even a stray signal bouncing off an orbiting satellite or off some space debris.

The printout itself was a time-series representation of the signal amplitude. SETI used the numbers 1 to 9 followed by the letters A to Z to represent increasing signal strength levels. Each signal level was 1 sigma (standard deviation) above the baseline background noise level. So the signal strength reached a massive 30 (represented by the letter U) times the baseline sigma level at its peak.

The 'Wow' Signal

It was an unexplained signal then as it remains today. It had a 'label' of technology about it. But the signal was never detected again, despite extensive searches. For the next four weeks, Big Ear searched the same part of the sky meticulously, but nothing resembling or coming near to the mind-blowing wow! signal was detected. Over a hundred searches of the same region of the sky were subsequently carried out, using different radio telescopes, but to no avail.

How difficult is it to detect a signal? If extra-terrestrial signals were beamed in our direction, we would be unaware of the vast majority of these signals since we are not looking at all of the sky at all of the time. Also, when we are searching, we may not be scanning the sky at the right frequency. To be pointing a radio telescope at exactly the right location in the sky at the precise moment a signal is being transmitted and at the exact frequency of the transmission has enormous odds stacked against it. But at least with the 'wow!' signal we know the direction to point in. Or is it that we had only one chance to detect that signal when the Earth, and specifically the Big Ear radio telescope, was at a precise point in space at that precise time on August 15, 1977, and pointing in that precise direction - an occurrence probably never to be repeated again.

Another peculiar aspect to this signal was that the Big Ear radio telescope was set-up to carry out two scans of the same point in the sky, the second scan a few minutes behind the first scan. But the Wow! signal was not detected in the second scan. Could the signal be an intermittent signal so that it was not transmitting by the time the second scan took place? Or maybe the signal was pointing in a fixed direction, just as the Big Ear radio telescope itself was, and relying on the Earth's rotation to scan the sky. In that case, if the beacon was relying on its planet's rotation, then it would be pointing in a slightly different direction by the time the second Big Ear scan took place.

Or could 'scintillation-induced intermittency' be the reason behind the failure to detect the signal again (as suggested in studies by Cordes, Lazio & Sagan). Research on signal propagation through interstellar space has shown how signal amplification can be a result of a process called scintillation. First discovered by Astronomers while observing pulsars in the 1960, SETI Scientists have suggested that this may be a reason why the 'Wow!' signal was never detected again. In a similar process that causes stars to twinkle because of light refraction in the Earth's atmosphere, distant compact radio sources can fade in and out over periods of time due to the turbulent ionised interstellar medium. So could the 'wow!' signal have been amplified 30 times above the background noise levels by the ionised medium for just that brief period of time the first Big Ear scan occurred and have faded out by the time the second scan occurred a few minutes later, as well as the numerous scans of this region since?

Another puzzling aspect of the 'wow!' signal was its duration. As the radio telescope scanned the sky (or more accurately the Earth rotated), we would expect a Gaussian distribution of the output of any continuous detected signal. The signal would start weakly, then rise in intensity as the most sensitive part of the telescope pointed ever closer to the precise source of the signal, and then fall away in strength. And this should take 72 seconds for the Big Ear telescope. Why? Well this is the length of time Big Ear takes to scan any particular point in the sky. And this is precisely what happened. Would a reflected or stray terrestrial signal follow this pattern? It's certainly very unlikely if the source reflected from an orbiting satellite or space debris.

Also, the frequency of the signal was 1420MHz, which is regarded as in the best possible range for interstellar communications by intelligent extra-terrestrials.(See the cosmic waterhole).