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It really is surprising that life could be so rare. But then you realize that the chance of us detecting life from a distant star comes in only very fine bands: in other words our post-SETI civilization needs to be within the light-distance of the star at or after they first produce radio communications but before they start using something far more advanced for communication that we don't yet know how to detect. Plus they need to not destroy themselves. And we need to actually look right at their signal, and notice it!

Basically what this means is: SETI is looking for stars that have developed intelligent life and radio roughly exactly as many years ago as they are light-years away from us. This minimizes the ease of finding significantly.

We could assume civilizations always use advanced forms of radio after they develop it but then why haven't we found life? We assume there must be a limit to the usefulness of radio in that sense. After all, we're moving to fiber etc.

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


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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