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A better way to listen for messages from the stars
SETI is not going to be as simple as pointing a big radio dish at a star.
Posted July 27, 2012
I'm fascinated by an idea invented by Claudio Maccone, an Italian astronomer who works at the International Academy of Astronautics. In this powerpoint and in his book, he argues that SETI is not going to be as simple as pointing a big radio dish at a star.
We need, he says, a really, really big lens. And that lens can be our Sun itself.
The problem with searching with conventional radio antennas, Maccone says, is that we really could only pick up signals beamed directly at us with very powerful transmitters. Right now we’re trying to compensate for that by searching for signals that are so faint that we can only be sure they’re there by using statistical methods to distinguish them from noise. (It’s like making a camera supersensitive by holding the shutter open for a long time to let light accumulate.)
And the problem with sending messages with a big radio dish is that over the light-years, any signal we send is going to be so corrupted by noise that it will be unintelligible after just a few dozen light-years. That's why, in their "Cosmic Call" message, the astronomers Stephane Dutil and Yvan Dumas went to great lengths to create symbols that would resist noise degradation (see my blog entry about Cosmic Call here.) But even there, noise will overwhelm the signal a few tens of light-years out.
Maccone's come up with a doozy of a solution: use the Sun as a lens. He calls this idea FOCAL.
Think about how we do visual astronomy with telescopes. Our eyes can’t see Jupiter’s moons. So we use lenses that are much larger than our eyes. A lens gathers a large amount of light and focuses it into an image. It also works with radio waves. Light and radio are both forms of electromagnetic energy that are bent by the curvature of space.
It so happens that we have a really, really big lens at hand: the Sun. As Einstein figured out in 1915, the sun’s gravity will bend any electromagnetic wave that passes near its surface. The sun bends radio waves so that they meet at a focal point about 550 astronomical units away.
If we put a small antenna at that focal point, it will hear an enormously amplified signal from any radio source on the other side. Extremely faint signals will be heard loud and clear. (The illustration is on slide 4 of this powerpoint.)
Using the sun as a gravitational radio lens. (Maccone, slide 4)
It’s a really neat idea. If we wanted to examine a dozen target stars, or even a dozen whole galaxies, we could do it by sending a dozen small radio antennas to the focal point of the Sun’s gravitational lens on the opposite side. If we simply want to find out that there are other civilizations out there, we could aim them at entire galaxies and see if anything comes in.
How far is that focal point? An astronomical unit is the distance from the Earth to the Sun, which is 93 million miles. 550 AU is thus a little less than fifty billion miles away.
Well, how far is that? Voyager 1, which has just exited the solar system, is now about eleven billion miles from the sun. It’s taken it 35 years to travel that distance. However, Voyager 1’s trajectory wasn’t designed for speed. Maybe a nuclear-powered rocket or solar sail could reach that focal point fairly quickly, with some initial help from using Jupiter and the Sun itself as gravitational slingshots.
Fifty billion miles is a long way, but it's much closer than the nearest star, Alpha Centauri. Alpha Centuri is 25.8 trillion miles away, or 277,600 AU if you prefer. That's five hundred times further. We can’t achieve starflight anytime soon, but 550 AU is something we can probably do with technologies we have now.
"This lens can produce images which would take perhaps thousands of conventional telescopes to produce. It can produce images of the finest detail of distant stars and galaxies," Maccone writes in his powerpoint (slide 6.) "Every civilization will discover this eventually, and surely will make the exploitation of such a lens a very high priority enterprise."
Well, I hope so. It sounds like a terrific idea, and it doesn't sound like it would be all that hard to do. We already have a history of planning deep-space telescopes. The James Webb Space Telescope will be situated almost a million miles from earth. Maybe in a decade or two, we'll be able to think in even bigger terms.
(If you liked this blog entry, please check out my other ones on SETI:
