The conspicuous absence of extraterrestrial intelligence (ETI) known as the Great Silence (or the Fermi paradox) raises the question: where are they? A civilization capable of interstellar travel could colonize the galaxy in 1 to 100 million years. The galactic thin disk is ~10 billion years old, so enough time has elapsed for an advanced civilization to have colonized the Milky Way several times over by now. The fact that we have not been visited leads some people to conclude that intelligent life is unique to Earth.
The rarity of intelligence or life in the galaxy is one explanation for the silence in the skies, but many other resolutions to the Great Silence have been suggested. ETI may exist but have no desire to communicate, or perhaps no one else is around beacause intelligent civilizations inevitably destroy themselves. Then again, maybe they're stealthily watching and waiting for us to cross a technological threshold before making contact!
Interstellar Probes
Humans will likely develop the technological capability to launch interstellar probes within ~100 years. Exploration by remote probes provides an in situ follow-up to missions that aim to detect extrasolar planets (such as Kepler) and characterize their atmospheres as possibly indicative of life (such as the Terrestrial Planet Finder). Interstellar travel to these newly discovered worlds might be a distant possibility, but in the near-term we are apt to explore our region of the galaxy with remote unmanned probes.
Probes can also serve as a backup device to archive human cultural achievements. A catastrophic event, such as a pandemic or large asteroid impact, could conceivably lead to irreversible information loss: the burning of the library of Alexandria is one such example. But unlike Alexandria, our modern digital storage is easily duplicated and transported. Probes may be useful as a means of protecting our accumulated knowledge from catastrophic risk on Earth.
Most SETI (Search for Extraterrestrial Intelligence) efforts aim to detect intentional ETI broadcasts at radio or optical wavelengths. However, broadcasting our own presence to potential ETI listeners will require unprecedented intergenerational cooperation and foresight, so exploration of the galaxy via interstellar probes may be well underway before we even begin our own active SETI broadcasting program.
Probes in the Solar System
Technological ETI have not yet colonized the galaxy, but enough time has elapsed for their probes to have reached the Solar System. We can only speculate on the motives of technological ETI, but exploration by small probes is within the realm of possibility for future humans. If ETI choose to remotely explore the galaxy, then their use of interstellar probes is at least a plausibility.
Defunct Probes: Relics of past exploration, a defunct probe no longer communicates with its home world. Defunct probes may originate from an extinct civilization that collapsed long ago or from an extant civilization that has lost the ability or interest to maintain the probe. Our own Pioneer and Voyager spacecraft will soon be defunct drifters in interstellar space.
Autonomous Probes: An active exploratory probe will periodically communicate with its home world to transmit information and receive new instructions. Autonomous probes are operational and must originate from an extant civilization.
Intelligent Probes: Self-replicating probes (also known as von Neumann machines) are a candidate for rapid exploration of the galaxy because they can use raw materials (such as asteroids) to construct additional exploratory probes. Intelligent probes may have the ability to observe, learn, adapt, hide, and even converse.
Search Strategies
Long-lived interstellar probes would have a limiting size of ~1 to 10 meters, which makes discovering an ETI probe in the Solar System analogous to finding a needle in 2000 tons of hay. Even so, probe activity and communication may be detectable, especially in known stable orbits.
Lagrange Points: Low-maintenance orbital positions are favorable for long-lived probes, especially the five Lagrange points between any two massive bodies. Searches of the stable Earth-Moon L4/L5 libration points have found no artifacts to date and comprise most of the Solar System SETI effort thus far. ETI probes could conceivably reside at any stable Lagrange point, though, and the outer Solar System (such as Jupiter-Sun L4/L5) might be a cleaner orbit and a better vantage point.
Asteroid Belt: Small probes have a plethora of hiding places in the asteroid belt, while intelligent probes also find the raw materials necessary for self-replication. Searches for infrared excesses in the asteroid belt may reveal the activity of ETI probes. Additionally, intermittent communication between a probe and its home world might be detected as anomalous microwave phenomenon using radio telescope arrays.
For Solar System SETI, piggybacking with other observing programs is an ideal strategy to complement directed searches of known stable orbits. Scanning the Solar System for probes is a colossal task laden with null results, but the discovery of a single ETI artifact would resolve the Great Silence once and for all. Maybe they're already here, and we just haven't looked.
Comments
It's worth reiterating that the Fermi paradox rests upon an extremely arrogant presumption -- that technologically advanced civilizations would necessarily seek interstellar travel. As if interstellar travel would be the greatest achievement of mankind (obviously) and only ignorant barbarians would fail to obsess over it to the point that they would overcome the massive physical barriers to interstellar flight.
My feeling is that if a technologically advanced society developed a sustainable economy that allowed each member to live a fulfilling life, then why would they bother with the risks and deprivation of interstellar travel? Any species with a sustainable global economy would probably have quenched any expansionist impulse that they may have had in their less sophisticated stages of development.
Regardless of that point, I still think that this is an interesting article. The detection of ET would be so profound that it is definitely worth keeping our eyes open for them.
My feeling is that if a technologically advanced society developed a sustainable economy that allowed each member to live a fulfilling life, then why would they bother with the risks and deprivation of interstellar travel? Any species with a sustainable global economy would probably have quenched any expansionist impulse that they may have had in their less sophisticated stages of development.
Regardless of that point, I still think that this is an interesting article. The detection of ET would be so profound that it is definitely worth keeping our eyes open for them.
Anonymous (not verified) | 04/09/10 | 12:39 PM
The galactic thin disk is ~10 billion years old, so enough time has
elapsed for an advanced civilization to have colonized the Milky Way
several times over by now.
I'm sorry, but what is the basis for this statement? The only civilization that we definitively know of hasn't been around for nearly that long, so why should there be an assumption of 10 billion years for colonization?
As Marshall pointed out, the presumption that natural selection leads to intelligence, which in turn leads to a cultural intelligence capable of space travel and colonization is hardly a foregone conclusion and is extremely unlikely even when viewed against the background of human evolution. How many hundreds of thousands of years did humans exist before they got to a point of where they had a culture that supported mass developments, and then developed a methodology of science, and then finally discovered a means of long-range communication?
Intelligence is not a biological destination and yet this is precisely what is presumed when discussing ETI. There is nothing now or ever which has indicated that intelligence (such as humans) is anything except a short-term anomaly. While we may not like to consider that possibility, it nevertheless represents a high probability given what we know about all biological systems.
Gerhard Adam | 03/13/10 | 15:54 PM
Jake,
A good piece, as usual.
As a minor quibble, the Lagrange points are not a particularly good place to hide a small probe for a long time. As you say, Earth-Moon L4 & L5 are clean of non-anthropogenic probes down to ~1 m size. The survey bounds on the Jupiter Trojans are in the ~1 km range, so a probe could hide out there. However, unless that probe is actively adjusting its orbit, radiation pressure would push it off station. This merely means that we should keep our eyes open everywhere else as well.
A good piece, as usual.
As a minor quibble, the Lagrange points are not a particularly good place to hide a small probe for a long time. As you say, Earth-Moon L4 & L5 are clean of non-anthropogenic probes down to ~1 m size. The survey bounds on the Jupiter Trojans are in the ~1 km range, so a probe could hide out there. However, unless that probe is actively adjusting its orbit, radiation pressure would push it off station. This merely means that we should keep our eyes open everywhere else as well.
Michael Busch (not verified) | 04/01/10 | 15:53 PM
Why send a probe to a neighboring star? By the time it's half-way there it will be overtaken by something better that our descendents have sent. So why bother?
In politics, belief that our descendents will solve our current problems is the axiom of choice.
Patrick Lockerby | 04/10/10 | 12:43 PM
First contact with an intelligent alien species, would invariably result in the destruction of one or the other. There is no plausible scenario that doesn't involve a tremendous risk for little or no benefit.
Gerhard Adam | 04/10/10 | 13:07 PM
This conversation has already been broached at this article for the Institute for Ethics and Emerging Technologies where I raised the similar concerns. Those with the idea that there must be intelligent life in the universe and thus they must have developed to the point to even have deployed probes, ignore the very same constraints that they themselves put on those who believe that intelligent life has already been here. Setting aside the fact that there is no scientific evidence for extraterrestrial origins for UFOs, the level of skepticism present here could be matched or exceeded on any arbitrary world you want to suggest. In our own history there have been skeptical elements that have worked against technological advancements. What if they had succeeded? Skepticism does not have a scientific basis. It can be prompted by a religious bias as well, and often has been.
There so many things working against the development of even the level of civilization that we have now, let alone anything more. It all comes down to mindset - why try to travel in space? For that matter, why try to fly? We take it for granted now that flight is possible, but one forgets how many attempts were made before there was success and how many people thought it would never work. Add to that the amazing short sightedness in realizing its potential, once it had been accomplished. What if, in the wake of the many fatal failed attempts at flight, there was a law past - for the public safety, to make attempts at manned flight illegal, just so no one else need lose their lives? That didn't happen here, but if the way people think now, was that way then, we still would be stuck on the ground. What if that scenario has happened elsewhere? No manned flight, probably no attempts at rockets either. Add skepticism to mix (for any random number of reasons) and you have a planet in lock down.
So we are confronted with the facts that religion, philosophy as well as a science based skepticism, could all equally as well prevent civilizations from not only becoming spacefarers but even having any interest in life in outer space. It has almost happened here. So just as the odds are in favor of life being in the universe, the odds are equally against the development of advanced technology by any arbitrary intelligent life. If they are like the most advanced countries here, the chances of their having a spacefaring civilization are remote, after all there is only one country from this planet that has even been on the moon or sent probes to the outer regions of our solar system. Why hasn't anyone else done this? The answers could be easily be applied to intelligent life elsewhere.
So expectations of any advanced civilizations, based strictly on numbers of galaxies and the numbers of worlds in them, are Pollyanna-esque at best, and the result of a complete cluelessness, of the results of various forms of skepticism, at worse. I'm not saying that we shouldn't search, just pointing out the irony of the premise in the first place.