von who?

Does Nuclear Pulse Propulsion—see previous post—solve the problem of travel between the stars? The Dyson paper proposes exploding H bombs behind a spacecraft at the rate of one every three seconds for ten days to achieve a speed of 10,000 km per second. At that speed, a ship would still take best part of 150 years to travel between the Sun and Proxima Centauri, the nearest star, a distance of four and a third lightyears. Even a one-way journey would require as much again propulsion to slow down at the other end, and the mass of this would have to be accelerated on the outward journey, nearly tripling the size of the craft and its cost. The legion questions regarding the practicalities of multi-generational crew survival in a very limited living area over such a long period are too obvious to need detailing.
 
It seems then that even nuclear pulse propulsion is impractical for the transport of biological species between the stars. But what about robots? The argument goes something like this: an advanced civilization would soon realize that interstellar travel is only possible for machines. Wishing to explore the galaxy, announce their presence, and proclaim their achievements, the civilization would conclude that the only way to do this would be to send out thousands of self-replicating ‘von Neumann’ machines on one-way journeys directed to different parts of the galaxy. 
 
A von Neumann machine can build an exact copy of itself; in this case, a robot spacecraft capable of travelling to the stars. It uses built-in instructions and the raw materials that it finds on the planets it where it lands. The complete machine includes all the robotic paraphernalia needed for finding, mining and refining the minerals and other materials required, as well as setting up factories to manufacture the parts and fuel needed for the spaceship and new robot crew. It must also be able to construct a spaceship capable of lifting off from the planet and travelling to the nearest stars. It would be programmed to continue manufacturing copies of itself, each one to be directed to a different nearby star. Given enough time, and if such an advanced civilization really did exist, the Galaxy should be full of von Neumann machines. 
 
Since Earth is not knee-deep in von Neumann machines, we can conclude that there are no advanced civilizations in the galaxy… Can’t we?