Simon Laub wrote:

IF WE are not alone in the Universe, why have we never picked up signals

from an extraterrestrial

civilisation? This long-standing puzzle, known as the Fermi paradox after

physicist Enrico Fermi,

who first posed the question, is still one of the strongest arguments

against the existence of

intelligent aliens.

Just in case someone else hasn't already made this point (which I doubt):

The entire SETI endeavor depends heavily on two assumptions:  One, that we can guess how the advanced alien civilizations communicate; and/or Two, that advanced intelligent civilizations want to communicate with more primitive civilizations like ours and so will choose a mode of communication that we already know about.

As for the first assumption:  We are only listening for electromagnetic signals, and more recently some types of laser signals.  But there is no reason to believe that intelligent civilizations thousands of years more advanced than we use those modes of communication anymore.  They may have long ago abandoned them in favor of something more advanced.  And even if they do use them, there's no reason to believe that they use them according to OUR guesses of frequency/wavelength (the hydrogen line and the famous "water hole").  That might be like people on Earth centuries ago guessing that advanced aliens might use really powerful smoke signals.

Already here on Earth, we are switching from broadcast TV to cable and direct broadcast satellite, and from broadcast radio to Internet radio.   So the "leakage" into space from broadcast EM may well decline sharply in centuries ahead.

As for the second assumption:  The whole point of SETI at all is that it assumes that advanced alien civilizations are going to make it as easy as possible for us to communicate with them--by making the mode of communication something that is well within our technological grasp. Trying to guess how they think is like trying to guess how God thinks. People in the 19th century wouldn't even have done a good job of imagining how we in the 21st century think.

If the SETI folks are right, then if and when an advanced intelligent civilization decides it wants to communicate with us, then it will be clever enough and inventive enough to send us a message that we will be unable to ignore.  Because they don't even know that we have a SETI program and so their message ought to be booming in on every TV and every radio receiver.