After almost half a century of intergalactic eavesdropping, Search for Extra-Terrestrial Intelligence (SETI) has yet to report any signs of contact. Does this mean we're truly alone? Or is SETI aiming at a dead spot in the universe? SETI senior astronomer Seth Shostak says a 46-year long search is not the same as a thorough one and, no, he doesn't think we're alone.

"The number of star systems we’ve carefully examined is only about a thousand. That’s a trifling sample compared with the several hundred billion suns that stud the Milky Way, and of little statistical significance. It’s comparable to initiating a quest for Americans who play the oboe, but considering the search meaningful after interrogating only two people," is how he described it.

It's no surprise Shostak gets a lot of email from folks with their own ideas on why SETI has turned up empty handed after all these years. He took the top four reasons and added his opinion to each:

1. "You’re counting on the aliens using communication technology (radio, light) that’s oh-so-last century. They will be far beyond this."

In other words, SETI’s technical approach is wrong. Some have suggested looking for gamma rays, gravity waves, or taking advantage of "hyperdimensional physics." Shostak says gamma rays are wasteful since they require enormous amount of energy per bit. Gravity waves are difficult to produce produce ("You need to shake planets or something similar") and hard to detect. In addition gravity waves is not known to move faster than the speed of light.

As for "hyperdimensional physics" Shostak says that might work if they knew what it was. He is not discounting using methods based on undiscovered laws of the universe that will allow sending of bits from one place to another more cheaply than light and radio, or faster. But they're waiting for someone to discover these new laws first before they adjust their experiment accordingly.

2. "If hi-tech societies or thinking machines were out there, they’d have colonized the Galaxy by now. Clearly, we’re alone… lone… lone."

The Fermi Paradox assumes that if sophisticated societies are common, they should also be ubiquitous. But if you look out the window and don't see large animals with long, prehensile noses does that mean elephants don’t exist on this Earth? "To use the Fermi Paradox as a reason for the lack of a SETI signal is to make a very big extrapolation from a very local observation. Seems chancy to me," say Shostak.

3. "The aliens don’t want to communicate with us. Look at what we’re doing to the planet!"

Shostak says this is a self-centered view to think that what we do to our planet would matter to them.

4. "You SETI types are just looking in the wrong places. We know where the extraterrestrials are: on a planet in the Zeta Reticuli system."

According to Shostak he likes this explanation the best, even though it’s the worst. Zeta Reticuli is the star system that was the supposed hometown of aliens who reportedly abducted social worker Betty Hill and her husband in 1961. The system’s identification is based on a "star map" Betty drew after their release. But Shostak clarifies that, as a matter of fact SETI did look at both of Z. Reticuli’s stellar components during SETI's observing run in Australia ten years ago and "the aliens, for their part, remained coy."