How Sensitive Must SETI Arrays Be to Detect Extraterrestrial Signals?

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In summary, the possibility of a signal from advanced ET's being picked up by SETI arrays is low due to the sensitivity of the receivers and the power needed to transmit a signal. It is possible to transmit a powerful signal if it is aimed specifically at the SETI arrays, but this is unlikely to happen.
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Canada Bob
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TL;DR Summary
SETI
Fermi asks "why don't we hear from them" could that be because they don't "shout" loud enough or maybe our ears are sensitive enough to pick up a signal emitted by advanced ET's within just our own backyard.

Is it down to the sensitivity of the SETI arrays, or the strength of a signal from within our own galaxy to be picked up by the SETI or any other array on Earth ?

How "loud" would a signal have to be to travel from the further reaches of our own galaxy to be picked up by SETI, how much power would that transmission need ? is it possible to transmit such a powerful signal ?

Yea, I know, it's 6 of one and half a dozen of the other, hopefully there's an overlap as to how strong a signal could be and how sensitive a receiver could be ?

Bob.
 
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Canada Bob said:
Summary:: SETI

How "loud" would a signal have to be to travel from the further reaches of our own galaxy to be picked up by SETI, how much power would that transmission need ? is it possible to transmit such a powerful signal ?
That's not answerable without knowing how directional and focused the signal is. Think of an old fashioned lighthouse with only a flame as a light source. Ships 20 or more miles out to sea can see the flash of light.

But even with a highly focused beam, I think the only chance is from nearby stars.
 
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IIRC, this is why there's now efforts to search for laser-borne signals.

One 'gotcha' on collimation etc is to use system star as Einsteinian lens.
IIRC, our 'focus' at ~550 AU, ~3 light-days, would give 'a few metres' resolution imaging of stars and planets out to 50-some light-years, 'kilometres' to thrice that.

Snag is each target system would probably need its own observatory and out-riders.

Conversely, the same 'lensing' could be used to practicably transmit towards those systems without requiring 'absurd' beam energies...

Down-side is improbability of intercepting such unless aimed our way...

Could be said that getting out to 'focus' represents a major 'filter'...
 
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According to Seth Shostak, SETI searches aren't really looking for radio: they are looking for radar, presumably as part of a meteor early warning system. There is incentive to make the beam as powerful as you can and as directional as you can ( @anorlunda's point).
 

FAQ: How Sensitive Must SETI Arrays Be to Detect Extraterrestrial Signals?

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