Gabriel Popkin as an article at Inside Science about a study that looks at the possibility of intercepting communications between other alien civilizations. The idea is that communicating across interstellar distances is best done with lasers.
So far, the optical search for extraterrestrial intelligence has focused mainly on the hope of receiving—and recognizing—an intentional, laser-encoded message. Researchers use dedicated telescopes or mine astronomical data collected for other purposes, like the Sloan Digital Sky Survey, to search for light pulses that could not be produced by any known object like a star. So far, no one has reported a light pattern that suggests an extraterrestrial intelligence.
But rather than look for light beamed directly at us, astronomers could also try to intercept signals sent between two distant civilizations. If advanced beings have existed for millions of years, they may well have found each other and started talking. Eventually many light beams would penetrate the intergalactic darkness, creating a criss-crossing network of communication beacons. As our solar system revolves around the galactic center, could we meander into the path of one of these beams?
While an interesting idea, the study largely concluded that this is unlikely to happen.
Unsurprisingly, he found that the chance of intercepting another civilization’s messages increased as more civilizations joined the communications network. He also found that the interception probability increased dramatically as the angle through which the beams spread out increased.
But the probability of accidentally wandering through a beam remained small as long as the beams were narrow, or collimated, like a typical laser. The beams would have to spread out about 1,000 times more widely than a standard laser pointer—in other words, more like a flashlight beam—before we have a decent chance of intercepting them, Forgan says. Sending out such a wide beam would require far more energy than emitting a tightly collimated one.
This isn’t too surprising. Interstellar space is vast, and solar systems are (relatively) tiny. As the article discusses, the aliens would have to go expensively out of their way to increase the odds of being detected by a third party.
One thing I find interesting about these studies, is that they have an implicit assumption: that interstellar travel or exploration is impossible, even robotically. Because if it is possible and there are indeed hundreds of civilizations in the galaxy, then there would likely already be a communication relay in every solar system, and a vast interstellar communication web network. If so, then our system would almost certainly have multiple beams interacting with other nearby stars.
If the aliens don’t want to be detected, it is possible that they could keep their relay stations in the Oort cloud, the region of comets and other icy bodies extending for a couple of light years out from the sun. Of course, avoiding detection would have to be a major priority for them to keep their facilities so far away from the free solar energy of the sun. But they could have decided to do so as soon as they noticed a tool using species developing on the third planet. And if staying hidden is a priority, we’re unlikely to find them for a while.
But, is the assumption that interstellar travel is impossible a valid one? Or are SETI and other astronomers being overly pessimistic? Most people are aware of the speed of light limitation, that nothing can travel faster than light. But lamenting that issue is actually a bit of sour grapes, since we don’t even have the foreseeable technology to get to a significant percentage of the speed of light. Just getting to 10% of c (the speed of light) will require astounding amounts of energy.
But it’s hard to imagine that in the centuries and millennia ahead, that we won’t be able to cobble together some method of getting to at least 1% of c. There have been designs around for decades, such as the Orion Project, which would use nuclear explosions to propel a craft up to a high speed. And there are many more speculative designs out there that could conceivably improve on that.
The problem is that Orion would still take centuries to reach the nearest star. It’s easy enough for science fiction writers to wave their hands and imagine robotic probe machinery working that long, but engineering it is a different matter. Still, Voyager 1 is four decades out and is expected to work at least another decade, albeit in a very low power mode. Building a probe that could work for centuries would be difficult, but it remains an engineering challenge, not a fundamental limitation of physics.
And I think that’s why I remain optimistic that interstellar exploration will ultimately be possible, at least with robots. Because the issues to be overcome are engineering ones, not fundamental scientific ones. It’s hard to say whether those engineering challenges will be overcome in a century, a millennia, or farther out, but insisting that they never will seems unnecessarily pessimistic.
But as soon as we reach that conclusion, we’re back to wondering where everyone is (the Fermi Paradox). Either they (or more likely their robotic representatives) are already here in the solar system, and hiding, as described above or in some other way, or they’re simply not there. Or perhaps more accurately, the nearest neighboring civilization is millions of light years away in another galaxy, far enough away that they haven’t had time to reach us yet, if they ever will.