Oh, hardly any of it. Most of SETI runs "piggyback" on some radio receivers, just picks up signals from wherever they happen to point. And the sky is vast in the radio spectrum, and our telescopes have to point at some particular narrow spot.
The Allen Telescope Array is a dedicated array of radio telescopes involved in SETI.
Good list of some of the searches underway and planned in radio spectrum here:
Search the Skies - How SETI Works
There's optical SETI as well,
Basically our only chance of spotting ETIs through SETI is if they are far in advanced of us technologically (that is likely of course) - and that they also use their advanced technology to build an extremely powerful radio telescope.
If they no longer use radio, some other form of signalling maybe somethign we haven't discovered yet - or if they use efficient tight beam communication, we wouldn't spot them.
If they use efficient compressed data streams also, then we wouldn't notice them easily - wouldn't be clear at first that it is data - because the best compression of data is indistinguishable from noise.
An easy to spot pulse type signal is an inefficient way of communicating data over long distances.
So in SETI they are searching for beacons that have been designed by ETIs specifically to alert other ETIs to their existence or to share information with them, or the likes of an automated radio transmitter transmitting an archive of their civilization for posterity etc.
Very unlikely to be able to "eavesdrop" on communications directly between ETIs, either civilizations communicating directly to other stars they know about that are inhabited - or communicating with spaceships. Only if we were on "line of sight" with this communication.
But - it is probably unlikely that the galaxy is already colonized by ETIs, or if it is, they surely are not interested in Earth or Earth like planets or sunlike solar systems for their own use - because if they were, coincidence is far too great that they could have arisen at exactly the same moment in geological time as us, to within a thousand or even a million years in billions of years - they would be here by now, and would already be using Earth and chances are that we would not have evolved.
There is also the interesting search for Dyson Spheres. Hoping to find ETI megastructures - which would be more obvious perhaps as anomalous heat signatures, even with an efficient technology unless deliberately designed to be hidden from view, then you'd expect to see the waste heat from their settlements if large scale.
Hunting for Alien Megastructures
Also - searching from the far side of the Moon, radio quiet no interference from Earth would help a lot.
But we are a long way away from the capability of detecting an Earth like civilization at same stage as us unless very close by. So the only hope is if for some reason ETIs are easier to detect than us. Either through megastructures, or because they deliberately send out signals that can be detected by early stage civilizations like ourselves - or because they are already close by (as in the film Contact) - but that last case a bit surprising as if so why aren't they here already?
I think another possibility is that they are explorers, not colonizers. If so then if they also have long lived civilizations and individually long lives, the nearest ETI may be dozens of light years away, or tens of thousands of light years away - and we may be due a visit some time in the next few thousand years. Or next few million years, or whatever.
I think everyone agrees it is a bit of a longshot. But because of the potential "pay off" if we detect them, many feel it is well worth making the attempt even so. Like a one in a thousand, or million chance of a huge "jackpot".
There might be some ETI out there sending a powerful, focused, radio or an optical beacon that regularly intercepts the Earth that we can hope to spot. And there just possibly might be ETI megastructures out there (if they go in for them) that we could spot also.
As for ETIs close enough to detect by more ordinary methods - not deliberately signalling us and trying to make contact - I think myself a bit unlikely just because of the argument, that if they are so close, why aren't they already here?
But with a small chink of hope there, what if they are explorers - and very numerous explorers, but not colonizers, like millions of them that are continually exploring the galaxy but never settle anywhere, except small temporary outposts, perhaps as a deliberate policy because of the potential unpredictable and maybe disastrous effect of colonization on the galaxy? Then some of them might be close enough to contact by more ordinary methods.
The Allen Telescope Array is a dedicated array of radio telescopes involved in SETI.
Good list of some of the searches underway and planned in radio spectrum here:
Search the Skies - How SETI Works
There's optical SETI as well,
Basically our only chance of spotting ETIs through SETI is if they are far in advanced of us technologically (that is likely of course) - and that they also use their advanced technology to build an extremely powerful radio telescope.
If they no longer use radio, some other form of signalling maybe somethign we haven't discovered yet - or if they use efficient tight beam communication, we wouldn't spot them.
If they use efficient compressed data streams also, then we wouldn't notice them easily - wouldn't be clear at first that it is data - because the best compression of data is indistinguishable from noise.
An easy to spot pulse type signal is an inefficient way of communicating data over long distances.
So in SETI they are searching for beacons that have been designed by ETIs specifically to alert other ETIs to their existence or to share information with them, or the likes of an automated radio transmitter transmitting an archive of their civilization for posterity etc.
Very unlikely to be able to "eavesdrop" on communications directly between ETIs, either civilizations communicating directly to other stars they know about that are inhabited - or communicating with spaceships. Only if we were on "line of sight" with this communication.
But - it is probably unlikely that the galaxy is already colonized by ETIs, or if it is, they surely are not interested in Earth or Earth like planets or sunlike solar systems for their own use - because if they were, coincidence is far too great that they could have arisen at exactly the same moment in geological time as us, to within a thousand or even a million years in billions of years - they would be here by now, and would already be using Earth and chances are that we would not have evolved.
There is also the interesting search for Dyson Spheres. Hoping to find ETI megastructures - which would be more obvious perhaps as anomalous heat signatures, even with an efficient technology unless deliberately designed to be hidden from view, then you'd expect to see the waste heat from their settlements if large scale.
Hunting for Alien Megastructures
Also - searching from the far side of the Moon, radio quiet no interference from Earth would help a lot.
But we are a long way away from the capability of detecting an Earth like civilization at same stage as us unless very close by. So the only hope is if for some reason ETIs are easier to detect than us. Either through megastructures, or because they deliberately send out signals that can be detected by early stage civilizations like ourselves - or because they are already close by (as in the film Contact) - but that last case a bit surprising as if so why aren't they here already?
I think another possibility is that they are explorers, not colonizers. If so then if they also have long lived civilizations and individually long lives, the nearest ETI may be dozens of light years away, or tens of thousands of light years away - and we may be due a visit some time in the next few thousand years. Or next few million years, or whatever.
I think everyone agrees it is a bit of a longshot. But because of the potential "pay off" if we detect them, many feel it is well worth making the attempt even so. Like a one in a thousand, or million chance of a huge "jackpot".
There might be some ETI out there sending a powerful, focused, radio or an optical beacon that regularly intercepts the Earth that we can hope to spot. And there just possibly might be ETI megastructures out there (if they go in for them) that we could spot also.
As for ETIs close enough to detect by more ordinary methods - not deliberately signalling us and trying to make contact - I think myself a bit unlikely just because of the argument, that if they are so close, why aren't they already here?
But with a small chink of hope there, what if they are explorers - and very numerous explorers, but not colonizers, like millions of them that are continually exploring the galaxy but never settle anywhere, except small temporary outposts, perhaps as a deliberate policy because of the potential unpredictable and maybe disastrous effect of colonization on the galaxy? Then some of them might be close enough to contact by more ordinary methods.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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