Where is ET? After nearly 50 years of searching for intelligent signals from space, astronomers have yet to receive a single verifiable signal of intelligent origin.

We live in the Milky Way galaxy, which contains roughly one thousand billion stars. There are about 100 billion galaxies in the known universe. Astronomers now know that planets around stars other than our sun are common. Some astrophysicists project that the universe could contain as many as 100 quintillion planets (that is a one followed by twenty zeros), although those that might contain the conditions necessary to harbor life are rare. So far, in the 21st century we have confirmed the existence of 4,000 or so in our immediate galactic neighborhood.

Why the sky is so silent is one of the big cosmological questions in light of these numbers, first proposed by Nobel physicist Enrico Fermi, now called “The Fermi Paradox” or “The Great Silence.”

There are 4,000 or so radio telescopes in operation around the world, although not all are engaged full time in SETI (search for extraterrestrial intelligence) research. Much of SETI research “rents” time on existing radio telescopes, as done in the movie “Contact.” These huge telescopes scan radio frequencies from 1 to 10 gigahertz (GHz), which includes the so-called ‘water hole’ in the range of 1,420 to 1,720 MHz (megahertz).

The water hole is so named because hydrogen atoms and hydroxyl molecules can combine to form a molecule of water, the most essential molecule for life as we understand it.

The problem with interpreting the radio waves that we do receive is that natural cosmic sources broadcast over a wide frequency range. Because signals produced by communication technology are in a much narrower bandwidth than these cosmic signals, the signals must be processed to extract information from them.

SETI programs break the radio spectrum into millions of very narrow bandwidth channels and use Fourier transforms to eke out specific narrow frequency peaks.

Even with all of the technology devoted to SETI, the great silence remains. Many astronomers have speculated on the reasons for it.

Alien technology is advanced enough to know not to influence our primitive society, or not to get involved with intelligent life forms. It is the Prime Directive of the Star Trek world. Maybe Earth is just an uninteresting backwater.

It may be that ET is ignoring us, as if we are in a cosmic zoo or wildlife sanctuary where they can observe us without interfering. Maybe we are just as an ant colony in the Amazon!

They could have developed technology that we do not have yet or would not recognize such as modulated gamma radiation or neutrino beams.

Our own galaxy and the universe are old enough for other civilizations to have come and gone, or they have destroyed themselves before they had time to develop a level of technology that would allow interstellar communications.

They may have realized, as physicist and cosmologist Steven Hawking warned, that broadcasting their location is not an intelligent thing to do. After all, in our own human history, things have not gone well when a primitive society met more technologically advanced invaders. Our own Hawaiian Islands are an excellent example of that.

Or maybe space is just too spacious. We have scanned only a small portion of the sky. If Earth’s ocean represents the vastness of space, then we have sampled merely one cup.