Furthermore, if a civilization on one of those planets hears us and chooses to respond, it would take as long for their signal to come back as it did for ours to go out.
That cuts the number of exoplanets near enough for us to expect a reply to a mere handful.
There are several hundred other near-enough stars that might have undetected planets, but that's still a tiny fraction of the galaxy, Shostak says.
"Most likely, the evidence that Homo sapiens have arrived on the scene has not reached anyone."
It's also possible that alien civilizations are merrily broadcasting their own TV series off into space.
But scanning the skies for such signals takes time, and time is a precious commodity at the large radio telescopes that are used for this task.
"The total number of stars we've been able to check out is 750 in a galaxy of a few hundred billion," Shostak said.
"Also, we've looked at only a small sample of [frequencies on] the radio dial. It's kind of like going to Africa and examining a spoonful of dirt and saying, Well, I guess there are no rhinos in Africa."
"They've All Gotten Cable"
Canadian science fiction writer Robert J. Sawyer isn't optimistic that we will ever make contact with intelligent aliens.
Sawyer is an award-winning author whose SETI-related novel, Rollback, is currently being serialized in Analog Science Fiction and Fact, the largest English-language science fiction magazine.
(This reporter is also a regular contributor to Analog.)
There is a tendency among hopefuls, Sawyer says, to presume that the universe is teeming with life.
After all, we exist, and there are a vast number of other stars that could support habitable worlds. Surely some of these planets have advanced civilizations.
"The reality is that we only know of one [civilization]," he said, referring to humans.
"To try to extrapolate any kind of trend from one data point would get you flunked out of any first-year statistics class. The fact we exist merely means we exist. That's all it means."
Sawyer thinks that the most likely reason we've not heard any response to our signals is that there's nobody out there to reply.
But that's not his only explanation.
"The second best answer is that they've all gotten cable," he said. In other words, aliens are using communications technologies that transmit in a manner we can't detect.
Another possibility is that the aliens have learned to keep a low profile. Sawyer compares it to keeping your email address secret to avoid being inundated with spam.
Maybe, he said, the aliens are thinking, "What, we don't have enough problems with our own crazies? Let's not invite the galaxy's crazies in [too]."
Stanley Schmidt, a former physics professor who edits Analog, relates the entire issue to a conundrum called the Fermi paradox, named for Nobel Prize-winning Italian physicist Enrico Fermi.
Back in the 1950s Fermi noted that there appear to be a lot of planets in the galaxy and that interstellar travel, while difficult, doesn't appear to be impossible.
So why aren't we hip deep in alien ambassadors?
"There have been lots of explanations," Schmidt said. One is that the aliens have a noninterference policylike the Prime Directive of Star Trek famethat requires them to leave us alone.
But Schmidt has his own suggestion, which he calls the Fermi plague.
"The more advanced technology is," he said, "the easier and more likely it is for one individual or a small group to do a lot of damage, up to and including wiping out a planet's population."
In that case, alien civilizations advanced enough to contact or even visit Earth have a higher risk of destroying themselves before noticing anyone else is out there.
"It's a disturbing hypothesis," Schmidt said.
Sawyer, the science fiction author, agrees, noting that most of the answers to Fermi's famous question"Where are they?"tend to be downers.
"If you want a more upbeat sort of thing, it may be that we are the first in our local neighborhood to emerge."
If so, "We have an obligation to survive, to get over our current difficulties and hang around for others when they get to the point we're at now," he said.
"The main message may be that civilizations are fragile things, and we should be doing everything possible to preserve and nurture our own."
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