Continuation of this post.
Frank Tipler used the possibility of self-replicating spacecraft to argue against the existence extraterrestrial civilizations. If such civilizations existed, at least one of them should have already developed self-replicating probes. These probes would have overrun the galaxy -including our solar system- in a comparatively short time. We don't have evidence for these probes, and therefore it is likely that humans are indeed alone in the Universe.
Carl Sagan tried to answer his argument in his paper The Solipsist Approach to Extraterrestrial Intelligence. He argued that self-replicating probes (also called von Neumann Machines) would end up going rogue and posing a threat to the original builders. Advanced civilizations, being aware of the danger, would not build them in the first place:
I find Sagan's response unconvincing, for a number of reasons. Firstly, he assumes that all civilizations would care enough about their long term prospects to renounce the shorter term gains promised by a dangerous technology. This hasn't always been the case here on Earth (see the book Collapse by Jared Diamond.) And von Neumann machines only need to be invented once. What about a civilization that knows it is doomed due to some impending cosmic disaster, like a supernova? They wouldn't have a future that could be imperiled. What about two warring civilizations, like the ones depicted in Stanislaw Lem's novel Fiasco? They could resort to dangerous measures in their attempts to win the war.
Secondly, I doubt that "responsible" civilizations could defeat a rogue probe outbreak initiated by an irresponsible one. Von Neumann machines are likely to be though foes, because of their rapacious, unrestrained, endlessly proliferating nature. (If you want proof of this, try fighting the Slylandro probes in Star Control II.)
Lastly, maybe machines could be constructed that would never go rogue, even after 10^6 years. It's a matter of putting enough checks and controls in the design to ensure accurate replication. Of course, absolute safety is impossible, but perhaps the probability of a replication error could be reduced enough to make it unlikely to happen within the Universe's lifespan. Knowing that, civilizations would not be afraid to build them.
Frank Tipler used the possibility of self-replicating spacecraft to argue against the existence extraterrestrial civilizations. If such civilizations existed, at least one of them should have already developed self-replicating probes. These probes would have overrun the galaxy -including our solar system- in a comparatively short time. We don't have evidence for these probes, and therefore it is likely that humans are indeed alone in the Universe.
Carl Sagan tried to answer his argument in his paper The Solipsist Approach to Extraterrestrial Intelligence. He argued that self-replicating probes (also called von Neumann Machines) would end up going rogue and posing a threat to the original builders. Advanced civilizations, being aware of the danger, would not build them in the first place:
these implacable replicators will not stop until the entire Universe has been converted into ~10^47 von Neumann machines, which then presumably cannibalize each other. If anything like this were a real danger, an emerging interstellar civilization would be wise, as a matter of self-preservation, to take steps to prevent it. No civilization could be sure of the ultimate fidelity over > 10^6 years of the self-replication programs of the von Neumann machines, and whether they ultimately might pose as grave a threat to the planets of their builders as to any other world. [...] Thus, the prudent policy of any technical civilization must be, with very high reliability, to prevent the construction of interstellar von Neumann machines and to circumscribe severely their domestic use. If we accept Tipler's arguments, the entire Universe is endangered by such an invention; controlling and destroying interstellar von Neumann machines is then something to which every civilization -especially the most advanced- would be likely to devote some attention.
I find Sagan's response unconvincing, for a number of reasons. Firstly, he assumes that all civilizations would care enough about their long term prospects to renounce the shorter term gains promised by a dangerous technology. This hasn't always been the case here on Earth (see the book Collapse by Jared Diamond.) And von Neumann machines only need to be invented once. What about a civilization that knows it is doomed due to some impending cosmic disaster, like a supernova? They wouldn't have a future that could be imperiled. What about two warring civilizations, like the ones depicted in Stanislaw Lem's novel Fiasco? They could resort to dangerous measures in their attempts to win the war.
Secondly, I doubt that "responsible" civilizations could defeat a rogue probe outbreak initiated by an irresponsible one. Von Neumann machines are likely to be though foes, because of their rapacious, unrestrained, endlessly proliferating nature. (If you want proof of this, try fighting the Slylandro probes in Star Control II.)
Lastly, maybe machines could be constructed that would never go rogue, even after 10^6 years. It's a matter of putting enough checks and controls in the design to ensure accurate replication. Of course, absolute safety is impossible, but perhaps the probability of a replication error could be reduced enough to make it unlikely to happen within the Universe's lifespan. Knowing that, civilizations would not be afraid to build them.
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A rebuttal of Tipler's Argument, put forward by Carl Sagan and William Newman of Cornell University.1 They first pointed out that Tipler had underestimated the number of von Neumann probes there ought to be. With exponential growth, a single self-replicating probe could be expected to convert the entire mass of the Galaxy into copies of itself within 2 million years. Any species intelligent enough to build such a probe, Sagan and Newman argued, would also be intelligent enough to realize the danger of it and so would not embark upon the project in the first place. In the event of a von Neumann probe being released either accidentally or maliciously, it would be a prime duty of other, responsible civilizations, said Sagan and Newman, to stamp out the "infection" before it could spread.
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