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Also, this alien civilization needs to be broadcasting a signal we have a likely chance of detecting, something we're apparently not doing:

http://www.seti.org/faq

Are we also sending any signals?

We conduct a passive experiment, designed only to look for signals, not to send them. However, humankind has been unintentionally transmitting signals into space – primarily high-frequency radio, television, and radar – for more than fifty years. Our earliest TV broadcasts have reached about one thousand nearby stars, although any alien viewers would have to build a very large antenna to detect them.

SETI researchers have not been very interested in broadcasting because of the long time one has to wait for a reply. If the nearest civilization is 100 light-years away, we would have to sit around for 200 years for a reply to a deliberate broadcast.

...

If an extraterrestrial civilization has a SETI project similar to Project Phoenix, could they hear Earth?

In general, no. Most earthly transmitters are too weak to be detectable by Phoenix-type equipment at the distance of even the nearest star. The exceptions are some high-powered radars and the Arecibo broadcast of 1974 (which lasted for only three minutes). To detect "leakage" radiation similar to our own will require instruments that are many times more sensitive than what we now have.

I never thought of the SETI guys as being particularly crackpot, but these answers make little sense to me. They're admitting that even if there's another roughly equivalent SETI project on an Earth-like planet 25 ly away, neither of us will hear each other because we're too impatient (!!) to continuously send a transmission and wait for a response?

I know very little about the propagation of microwaves, but my guess is that SETI doesn't want to admit that real interstellar communication would require vastly more signal power or vastly better detection capabilities than our civilization is capable of mustering.




An assumption of SETI is that we're listening for slightly older civilizations which would use much more energy (many orders of magnitude more). And it's a quite reasonable assumption.

Responding would be possible, if we knew the direction - a focused beam in direction of a specific star can be a billion times stronger than our "leakage", but we can't transmit to all stars, or 1% of all stars, or 0.01% of all stars - there are just too many of them.

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