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Over the past 50 years, several SETI projects have scoured the cosmos but have yet to turn up anything conclusive. What do you make of this cosmic radio-silence?

A lot of people do think of and refer to this as a silence, but I certainly don't. Because of the limitations in equipment and money, we've carefully explored very little of the sky. Yes, the first SETI experiment was done more than a half century ago, but you probably have to look at a few million star systems at very high sensitivity before you score a success. We haven't carefully examined anywhere near that numbera few thousand at most.

The thing to keep in mind is that we're still in the very early days when it comes to the search for extraterrestrial intelligence. Saying there's a silence is a bit like if Columbus, looking to discover a new continent, only sailed 10 miles off the coast of Spain before turning back to say, "Nothing out there! I guess that whole exploration gig isn't going to work out."

Still, you've said before that we'll likely find some evidence of extraterrestrial intelligence within the next two decades. What makes you so confident?

If you're trying to determine when we're going to succeed with SETI, that really depends on only two questions. First, how many societies are out there broadcasting signals strong enough for you to find? And second, how quickly are you examining these star systems to find them?

My guess that we'll succeed in the next two decades is based on the fact that with improvements in digital electronics and computerswhich are getting better and cheaper, following Moore's lawwe will be continually sifting through the sky faster. And you can extrapolate how fast we'll be able to search, assuming we have the money, in the next decade or two.

As for the number of civilizations out therehow many needles are we looking for in this haystacknow that we don't know. But there are certainly thoughtful guesses, including those made by people like Frank Drake and Carl Sagan. And if you just look at the range of guesses, then this is where I've come to the conclusion that, if this experiment is well founded, if the ideas of SETI are legitimate in the sense that they make sense and this is realistic, then it's going to succeed rather quickly if it's going to succeed at all.

What exactly are we looking for?

A lot of people assume we're looking for complex patterns in radio signals, like the value of pi, a list of prime numbers, or the Fibonacci [sequence]. But what we're really looking for is just any radio signal that some sort of transmitter could have madeone that's restricted to a very narrow band of frequencies. This is because the cosmos makes a lot of radio waves and other noise, but those are all over the dial, which is a sign of bad engineering, because you're wasting a lot of energy. We're looking for something that is, so to speak, carefully manicured.

Now there are also other SETI experiments looking for other types of signals beyond those made of radio waves, particularly very short flashes of visible light. That makes sense because light and radio both cost more or less the same amount of energy per bit of information when you're sending a signal.

Admittedly, these are not the only way to do SETI. You could also just build a number of very large telescopes to look for signs of astroengineering. That would be expensive and very difficult, but perfectly legitimate. I also get emails all the time from people saying that we should be looking for gravity waves, or using neutrino detectors, and lots of other ideas. But it seems to mebecause of the cost and the fact that we know it workslooking for radio signals and other electromagnetic communication is the way to go.

Last week NASA practically doubled the number of known exoplanets. Will this affect where we search through the cosmos?

Oh yes, and I think that the biggest change is this. In the past, people spent a lot of time putting together lists of star systems they thought were the best candidates to search for signals. And those were systems that had stars like the sun, and stars that weren't too young. But now we're learning that you can pretty much forget those lists, because essentially all stars have planets. And in terms of which ones have planets that are hospitable for life, we still don't know exactly, but it's beginning to look like a fairly high percentage. The preliminary numbers are somewhere between 10 and 30 percent.

It takes time for light to travel, so looking out into space is also looking back in time. Won't any intelligence or civilization we discover be ancient or even long gone?

Well, it kind of doesn't matter. I studied Latin in high school, and I was reading stuff from Cicero. And that signal took a few thousand years to get to me. But I was still interested in what he had to say. Also, keep in mind that if there are a lot of intelligent societies out there right now, tens or hundreds of thousands, then the nearest ones could be just a couple hundred light-years away. Of course, even that distance means you're not going to get into conversation.

What if we do find a signal? What happens?

A lot of people in this country assume that it'd be kept quiet, because "the public couldn't handle it," or something like that. And we know that's wrong, because the media has been all over every false alarm we've ever had.

What would we learn? Well, in the beginning, maybe not much. If you're getting information from a society that's more advanced than oursand they might be much more advanced than oursit strikes me as uncertain that we might ever really understand anything. Actually comprehending the message could be hard, and we might never figure it out.

But forget the message. Just knowing that there's somebody else out therethat what's happened on this planet has also happened in many other placesthat might change our lives in a very subtle way, but it's interesting to know and worth looking for.

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