In the light of recent wild speculations, here is my take on the “Fermi Paradox”, adapted from my book Alone (aka The Reason Why). In a nutshell, the Fermi Paradox (which is not really a paradox but a puzzle) asks why, if technological alien civilizations exist, they have not visited us. And this is the reason why it is a puzzle:
Hungarian-born John von Neumann, who among many other things worked on the Manhattan Project, which developed the first nuclear bomb, played a key role in developing the puzzle in this form,.
Von Neumann’s interest in the idea of a computer that could solve any problem led him to think about the nature of intelligence and of life. Intelligent beings are in a sense universal Turing machines, since they can solve many different kinds of problems, but they have the additional ability to reproduce. Was it possible in principle for there to exist self-reproducing Turing machines in a non-biological sense? Von Neumann proved that it is indeed possible. The process involves just a few simple steps. First, the computer program stored in the memory banks of the machine instructs the machine to make a copy of the program and store it in some sort of memory bank (nowadays, we could imagine this to be an external hard drive). Then, the program instructs the machine to make a copy of itself, with a blank memory. Finally it tells the machine to move the copy of the program from the storage device into the new machine. Von Neumann showed, as long ago as 1948, that living cells must follow exactly the same steps when they reproduce, and we now understand this in terms of nucleic acids as the ‘program’ and proteins as the ‘machinery’ of the cell. First, the DNA is copied. Then, as the cell divides in two the copy of the DNA is moved into the new cell.
A self-reproducing non-biological automaton is now often referred to as a ‘von Neumann machine.’ Von Neumann, though, did not live to see these ideas developed. He died of cancer in 1957 at the age of 53. It was Ronald Bracewell who suggested that such probes could be used to explore the Milky Way, and the American Frank Tipler who presented the full force of the argument which shows how quickly such von Neumann machines could visit every interesting planet in the Galaxy.
The key point is that a technological civilization only has to build one or two probes in order to colonize (by proxy) the entire Milky Way. Such a probe would be programmed to use the raw materials that it found among the asteroids and other cosmic rubble in a planetary system, plus the energy of the parent star, to build copies of itself, and send those copies off to explore other planetary systems. One or two probes sent from Earth to the asteroid belt between Mars and Jupiter could mine the raw materials there to make a fleet of identical probes which could set off to explore nearby stars, maintaining contact with home by radio. Each time one of them arrived in a new planetary system, as well as reporting its findings it would set about building copies of itself and repeating the process. Making the very modest assumption that probes could travel at one fortieth of the speed of light and that they were programmed to seek out stars with planets, it would take less than 10 million years (in a Galaxy 10 billion years old) from the construction of the first probe to visit every interesting planet in the Galaxy. And all it costs is the construction of one initial probe (or at most a few copies, as backups).
Even with our present technology, we could get a decent-sized probe to the nearest star. It would involve sending the probe on a close flyby of Jupiter, using the gravity of Jupiter like a slingshot to speed it up and send it diving past the Sun, where the Sun’s gravity would give it a further boost, sending the probe out of the Solar System at a speed of about 0.02 per cent of the speed of light. When the probe arrived at its target star, it could use the gravity of the star and its planets to slow itself down. It would take several thousand years for the probe to reach its target, but if civilization still existed on its home planet and anyone was interested, it could be programmed by radio to construct not just a replica (or replicas) of itself but an improved, faster version (assuming technology back home had advanced in the millennia since the probe was launched). It might take thousands of years for any interesting news to come back from the first probe, or probes; but as they reproduced and spread faster and faster through the Galaxy news about different planetary systems would come flooding in several times each year.
This is so nearly within our present technological ability that it is quite clear that within a couple of decades at most (unless civilization collapses) we will be able to start this process. And by ‘we’, I don’t necessarily mean the full might of government-sponsored agencies such as NASA. Individuals such as Paul Allen already pay for radio telescopes to search for ET; the Paul Allens of the next generation may well be able to pay to explore (or at least start the exploration of) every planet in the Galaxy. Such an individual might well be hoping to get news from the nearest planetary systems within their own lifetime, rather than caring much about what happens millions of years from now. But since it literally costs no more to explore the entire Galaxy than to explore the nearest planetary system, who could resist going the whole hog?
That is why the possibility of constructing von Neumann probes is such a powerful argument that we are alone in the Milky Way. All the arguments that ‘they’ are out there but are, for whatever reason, avoiding contact with us require that every technological civilization is working together to keep their presence secret. But it isn’t just the case that you only need one civilization to break ranks and send a few probes out into the Galaxy. All it needs is for one individual being to send one probe and every interesting planet will be visited in a few million years.