Fleets of robotic space probes from alien civilisations could already have arrived in our solar system, a mathematical study has found.
The study quotes scientists who warn that we may not be able to detect them with our technology - the probes may also be so hi-tech they can “conceal” themselves.
Earth’s own Voyager probe has only just reached the edge or our solar system - but older alien civilisations could have launched probes long ago, using knowledge far beyond ours, "slingshotting" round stars using their gravity, according to a paper in the International Journal of Astrobiology.
Duncan Forgan and Arwen Nicholson of the University of Edinburgh analysed how a “fleet” of probes could travel through space - and says that probes which “self-replicate”, building new versions of themselves from gas and dust in space, would have had time to arrive here already.
But the fact we have not seen any may mean our galaxy is a lonely place, Dr Forgan warns, "The fact we haven't seen probes of this type makes it difficult to believe that probe building civilisations have existed in the Milky Way in the last few million years"
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The probes would only need to travel around 10% of the speed of light to explore our whole galaxy within 10 million years.
“Interstellar probes can carry out slingshot manoeuvres around the stars they visit, gaining a boost in velocity by extracting energy from the star's motion around the Galactic Centre,” the researchers say.
Our own Voyager probes used a similar “slingshot” technique using large planets in the solar system - but using the gravity of a star would offer a far bigger boost.
The discovery - using computing power that would not be available to earlier generations of scientists, ignites once again the most famous question in the search for extraterrestrial life - the "Fermi Paradox".
In 1952, the physicist Enrico Fermi posed the question ‘Where is everybody?’ The new paper shows that it is technically possible, at least, that alien probes could be here - once again begging the question of why they have not communicated with us.
“We can conclude that that a fleet of self-replicating probes can indeed explore the Galaxy in a sufficiently short time.. orders of magnitude less than the age of the Earth,” the researchers say.
The probes would have to be programmed so as not to arrive in large numbers - “it could be interpreted as hostile, causing panic,” say the researchers.
Other scientists have warned that they may already be here.
“Extraterrestrial artifacts may exist in the solar system without our knowledge simply because we have not yet searched sufficiently,” wrote Jacob Haqq-Misra of the Rock Ethics Institute in a 2011 paper. “Searches to date of the solar system are sufficiently incomplete that we cannot rule out the possibility that nonterrestrial artifacts are present and may even be observing us.”
Forgan quotes NASA space expert Robert Freitas, who suggested in 1983 that any probes in our solar system may have been designed not to be seen, using technologies far beyond those available to us.
Forgan says, “The probe camouflages itself so as to set up a threshold test of the technology or intelligence of the recipient species, where the test must be met before the spec ies is allowed to communicate with the device.”
“Evidence in the form of ‘spent’ or destroyed probes is less likely, as any civilisation attempting interstellar exploration are presumably skilled engineers, and would send probes with the ability to self-repair due to the large travel distances and times required for such a task, giving the probes a very long life-span.”
Dr. Anders Sandberg, of the Future of Humanity Institute at Oxford University, suggests a more depressing answer to the question of why we have never seen evidence of extraterrestrial life, “Beyond a certain technological level, civilizations can spread not just across their own galaxy but across enormous intergalactic distances. There are millions or billions of galaxies from which a civilization could have reached us, if it were established early,” Sandberg says.
The answer to why no one has contacted us yet may be simple - intelligent civilisations tend to wipe themselves out.
“If advanced societies wipe themselves out, or decide to not go exploring, they need to converge to this outcome with extremely high probability, since it only takes one that escapes this fate to fill the universe,” says Sandberg.