Seth Shostak: Our final panelist is Jill Tarter, the director of SETI research at the SETI institute, and holder of the Bernard Oliver Chair for SETI.

Jill Tarter: My talk is about Technology Worlds. Maybe complex life somewhere else develops technology. I'd like to start with the unimaginable. Arthur C. Clarke, a fantastic science fiction writer and a very good scientist, is quoted as saying that "any sufficiently advanced technology is indistinguishable from magic."

So do we just have to give up about the unimaginable? I would say not. Don't try and predict the magic. Let's just do astronomy. Let's look at the universe every way we can, with every tool that we can use.

Astronomy has this wonderful history, starting 400 years ago with Galileo's first telescope. When we build a new instrument to look at a new cell in phase space, we look at the universe in a different way. We might find what the instrument was intended and built for, but often we find something totally unexpected. So I think that when it comes to the unimaginable, we just keep exploring our universe and see what it has to show us.

But if you want to stick to the imaginable, that's the world of science fiction. Again, the realm of Arthur Clarke, and let me start with Machine World.

The futurists on our planet talk about biological evolution and then bio-engineering, and the fact that our technology is evolving exponentially and our poor carbon-based brain power is not going to be able to keep up, and we are going to be soon turning ourselves into a machine through some amazing process. That's what the futurists call the singularity, and what the media call the Borg.

There's also Computer World. Suppose that information processing becomes the most important activity. You would deconstruct everything and then use nano-technology to reconstruct it into a series of layers of computational capability.

You would surround your star to capture all the energy, and the waste heat that comes from an inner layer would power the next layer out - creating matrioshka brains of information processing. Maybe that's the future.

What would be the observable consequences of such a system? First of all, you'd expect to find infrared excess heat around stars. We find lots of stars with such infrared excess. But this waste energy is actually coming from planetary systems, from debris disks and protoplanetary disks around stars. So far, we have no evidence for matrioshka brains.

The other thing is, if you're trying to be incredibly efficient in your computation, you've got to get rid of that waste heat. You might place your computer in the coldest places in the galaxy, and therefore we might find some strange emission coming from the edges of the galaxy or cold places within the galaxy.

Power production is another thing we do with our technology. There are three different types of Power Worlds we can conceive of. Type 1 manipulates the power that's incident on the planetary surface. Type 2 manipulates the power from its star. Type 3 has the ability to manipulate all the energy coming out of its galaxy.

We can expect Type 1 to have space colonies in orbit to harvest the insulation. We on Earth have been classified by Carl Sagan as a 0.7, meaning we're not advanced enough to even be a Type 1. Type 2, you don't want to just get the energy coming in the direction of the planet, you want to get all the star's energy, so that might involve a Dyson sphere. Type 3, well, the Seyfert galaxies, these marvelous explosive events, have been called the industrial accidents of the cosmos.

Perhaps a few Type 3 civilizations got it wrong. What would be the observational consequences of Power Worlds? Again, we'd expect to see infrared excesses around stars. We'd also expect to see coordinated explosions at large distances from one another.

If the power generation is more prosaic, and they are doing a lot of fission reactions to produce power, then they've got fissile waste. Maybe they dump the waste in their star, and then the star has enhanced emission lines of rare elements such as praseodymium or niobium.

If they are doing nuclear fusion, perhaps they've got orbiting fusion plants and a little bit of tritium leakage. Tritium has a radial line that's the analog of the hydrogen 21 centimeter line, and it only has a 12.5-year half life. So if you find that tritium line being emitted in space and it's not right next to a supernova, then maybe you found somebody's technology.

Traveling worlds. Maybe advanced technologies always get up and get out. The movie Contact told us about these wonderful transportation systems that were left by some builders early on. We often have science fiction stories about space ships.

There can be fast ships and slow ships, and even nanoships, small probes that might be inhabiting our solar system at places where the gravitational potential is at a minimum. It's a nice place to hang out for a hundred million years, waiting for the folks on the planet to wake up and smell the nanoprobes.

What would be the observational consequences of this kind of a world? In the movie version of Contact, there was 18 hours of recorded static in the traveler's video even though the trip seemed instantaneous to observers. In the book, the designers of the transportation system had left a message in the digits of pi.

Annihilation rockets might give us those unexplained phenomena known as gamma ray bursts. Slow ships need to fuel up every once in a while, so maybe they cut up an asteroid or two, and therefore if you find double asteroids, maybe the smaller one is actually a slow ship.

We've found double asteroids, but so far no one is suggesting that one of the rocks is a spacecraft. Lastly, the nanoprobes, if they're there in swarms, might be detectable with reflected sunlight or radar signals from Lagrange points.

The last place I'm going to talk about is Beacon Worlds. The kinds of signals transmitted might look almost like a pulsar, but a pulsar that changes its period periodically and regularly.

It might be a star that is blinking because there's some sort of artificial transit happening with a shape that can be distinguished from a planetary circular shape. These signals are almost natural, so a young technology exploring its universe might stumble on them but not recognize them as beacons.

There are the other kinds of signals, such as a single frequency on the radio dial. This is what SETI has been looking for since 1960. More recently, SETI has been looking for very bright optical laser pulses that last for a nanosecond. There might be some wide band optical communication systems that have been engineered, and we might just get in their beam.

You can think of building dedicated telescopes to find the obviously engineered signals. For the almost natural signals, we just need to do the astronomy.

We've discovered over 300 planets so far. However, that is a very small part of exploring our galaxy. All of those planets are within 300 light years of the sun.

SETI spent a decade looking for signals from a thousand stars out to 155 light years. We'll spend the next decade with the new Allen Telescope Array, purpose built for this job, to look at a million stars out to a thousand light years. But again, that is a disturbingly small part of the galaxy.

Still, I'm an optimist. I think if there are a modest number of worlds out there with life, and we're looking for them in the right way, then our tools are going to get good enough so that within a matter of decades we may be successful.