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rfmcdonaldKeith Wiley's paper on the Fermi paradox and, by extension, the development of extraterrestrial life to intelligence and technology and beyond, continues to fascinate me. It's all about his calculations regarding self-replicating probes, and the rapid speed at which--using even conservative estimates--our humble planetary system is populated by self-replicating probes which reached us.
Perhaps the number is absurd. As Wiley himself points out later in the paper, earlier researchers have convincingly argued that we haven't catalogued our solar system to nearly the resolution necessary to exclude an active extraterrestrial presence somewhere in the solar system. Robert A. Freitas Jr's 1983 paper "Extraterrestrial Intelligence in the Solar System: Resolving the Fermi Paradox" goes into great, if somewhat disturbing, detail.
Why indeed?
Even if self-replicating probes were actively replicating in our solar system right now, Freitas suggests, the likelihood that they would be detected from Earth may be relatively small: they need not be large, they need not remove large amounts of material, and the chances our astronomers might catch propulsion systems and the like in action may be small.
Freitas' argument may be challenged somewhat by the development of our astronomical observations since his writing. Just over a month ago, for instance, the argument was made that it's now possible to detect objects with the emission profiles of cities on Earth in the Kuiper belt of icy planetesimals beyond Neptune (and including Pluto).
This leads us to my [FORUM] questions of the weekend.
1. Let's say that tomorrow, you go to Google News and find that the site has aggregated a few tens of thousands of news items reporting on astronomers' detection of--let's be conservative--several hundred locations in the Kuiper belt with energy emissions strongly suggesting that technological artifacts are at work. How would you react?
2. Let's say that tomorrow, Google News reported that there were signs of defunct artifacts throughout the Kuiper belt. Would you feel better knowing they were quiet? Would you feel worse wondering why they were all quiet?
Discuss. It might be worth noting that whenever I've raised these questions in conversation, so far only one person other than myself has said that they'd be happy if the probes were up and running. Everyone else would prefer that the probes were dead, although--one told me--the situation of having the solar system being surrounded by alien probes whether dead or alive would be worrisome regardless.
To investigate how many SRPs have reached our solar system, we can use an equation which mirrors the method employed by the Drake equation (Brin, 1983;Walters et al., 1980), i.e., by combining fractional parameters and a starting value. Thus, on the assumption that each SRP mission targets one SRP per star, the number of SRPs that reach each star in the galaxy is Nr = Ns*fr*nr*Gr where Ns is the total number of stellar societies to ever arise (including colonies), fr is the fraction of societies that dispatch SRPs, nr is the number of missions they initiate, and Gr is the fraction of the galaxy that each mission reaches.
[. . .]
When we populate the equation with the lower and upper estimates, it yields Nr = 10(2)–10(11) SRPs in our solar system at this very moment. The absurdity of this result underlies the tremendous burden of the Fermi Paradox and demonstrates why many ETI-hopefuls have shied away from even the remotest
consideration of SRPs.
Perhaps the number is absurd. As Wiley himself points out later in the paper, earlier researchers have convincingly argued that we haven't catalogued our solar system to nearly the resolution necessary to exclude an active extraterrestrial presence somewhere in the solar system. Robert A. Freitas Jr's 1983 paper "Extraterrestrial Intelligence in the Solar System: Resolving the Fermi Paradox" goes into great, if somewhat disturbing, detail.
Detection of probes would be especially challenging, as these could in theory be located almost anywhere. A typical alien probe might be 1-10 metres in size - this is large enough to house a microwave antenna to report back to the senders, and to survive micrometeorite impacts for millions of years, but light enough to fly across the interstellar gulf without consuming unreasonable amounts of energy.
A spherical Solar System boundary enclosing the orbit of Pluto consists of 260,000 AU3 of mostly empty interplanetary space and 10(11) km2 of planetary and asteroidal surface area. To be able to say with any certainty that there is no alien presence in the Solar System, you have to have carefully combed most of this space for artifacts.
Currently the sky has been exhaustively surveyed to perhaps magnitude +14, the Palomar Schmidt Sky Survey extends to +21, and the best available magnitude limit for any telescope on Earth is about mv = +24. This means that at best, current surveys from Earth might have detected an unmoving, mirror-shiny, optimally-oriented 10-metre object orbiting 0.01, 0.25, and 1 AU from Earth, respectively. If the artifact is smaller, moving, black, or canted at a different angle then it will be even harder to see.
So we can only scan the nearest 4 AU(3) of space for probes, but we have at least 260,000 AU(3) to search. Even if the Palomar 200-inch telescope was employed exclusively to search for alien artifacts it could reach at most one-millionth of the necessary volume. Orbital space, in other words, is at least 99.999% unexplored for 1-10 metre objects.
A more realistic assessment suggests that the visual detection threshold for alien probes for present-day humanity includes only 10(-5)-10(-11) of the potential probe residence volume. This estimate assumes a random search pattern typical of past serendipitous observations which might have discovered alien artifacts if they were present. Radar and infrared measurements cannot substantially improve this current limit.
How about probes parked on planetary surfaces? Of the 0.1 trillion square kilometres of Solar System territory other than Earth, less than 50 million has been examined to 1-10 metre resolution. So 99.95% is still virgin territory as far as a search for extraterrestrial artifacts is concerned. If objects are buried somewhere or floating in a Jovian atmosphere, there is almost zero chance we could have found them up to now. Even huge 1-10 kilometre artificial alien habitats occupying the Asteroid Belt would appear visually indistinguishable from asteroids to terrestrial observers, and the Belt population itself is poorly catalogued. So it is exceedingly unlikely that we would have spotted an extraterrestrial artifact anywhere in the Solar System unless it was desperately trying to get our attention. And why should it bother to do that?
Why indeed?
Even if self-replicating probes were actively replicating in our solar system right now, Freitas suggests, the likelihood that they would be detected from Earth may be relatively small: they need not be large, they need not remove large amounts of material, and the chances our astronomers might catch propulsion systems and the like in action may be small.
Freitas' argument may be challenged somewhat by the development of our astronomical observations since his writing. Just over a month ago, for instance, the argument was made that it's now possible to detect objects with the emission profiles of cities on Earth in the Kuiper belt of icy planetesimals beyond Neptune (and including Pluto).
The first order of business is to show that searching for artificial lighting is possible within the Solar System, which Loeb and Turner approach by looking at objects in the Kuiper Belt. The technique is “…to measure the variation of the observed flux F as a function of its changing distance D along its orbit.” Working the math, they conclude that “…existing telescopes and surveys could detect the artificial light from a reasonably brightly illuminated region, roughly the size of a terrestrial city, located on a KBO.” Indeed, existing telescopes could pick out the artificially illuminated side of the Earth to a distance of roughly 1000 AU. If something equivalent to a major terrestrial city existed in the Kuiper Belt, we would be able to see its lights.
Objects of interest could be followed up with long exposures on 8 to 10 meter telescopes to examine their spectra for signs of artificial lighting, while radio observatories like the Low Frequency Array (LOFAR) or the Precision Array for Probing the Epoch of Reionization (PAPER) could be used to check for artificial radio signals from the same sources. Interestingly, the Large Synoptic Survey Telescope (LSST) survey will be obtaining much data on KBO brightnesses of the sort that could be plugged into Loeb and Turner’s methodology. Thus running a KBO survey as a tune-up of their methods would involve no additional observational resources.
This leads us to my [FORUM] questions of the weekend.
1. Let's say that tomorrow, you go to Google News and find that the site has aggregated a few tens of thousands of news items reporting on astronomers' detection of--let's be conservative--several hundred locations in the Kuiper belt with energy emissions strongly suggesting that technological artifacts are at work. How would you react?
2. Let's say that tomorrow, Google News reported that there were signs of defunct artifacts throughout the Kuiper belt. Would you feel better knowing they were quiet? Would you feel worse wondering why they were all quiet?
Discuss. It might be worth noting that whenever I've raised these questions in conversation, so far only one person other than myself has said that they'd be happy if the probes were up and running. Everyone else would prefer that the probes were dead, although--one told me--the situation of having the solar system being surrounded by alien probes whether dead or alive would be worrisome regardless.
