I have always assumed an interstellar Von Neuman probe would be a fairly large autonomous factory starship containing maintenance bots, smaller robotic in system spacecraft that can do detailed geological survey of systems mapping promising asteroids and moons, deploy mining equipment and bring mined raw materials back to factory for refining and fabrication into spare parts, computer chips, new factory modules, engines, solar panels, more in system transport craft. It may take multiple decades or century for it to have built up enough infrastructure for commanding AI to contemplate starting fabrication of new Von Neuman probe.Formless wrote: ↑2020-10-22 07:40pm The problem with Von Neuman probes is that the theory is based on overly simplistic assumptions that do not work in the real world. Namely the theory does not account for the distribution and accessibility of heavy elements needed for high technology, let alone self-replicating technology. We now know that there are many planets with masses in between that of Earth and Neptune, and those planets probably serve to trap much of the heavy elements of the universe underneath a much stronger gravitational field than that of Earth's. The self replicating probe idea requires the probe to constantly seek out sources of fuel and building materials, and find solutions to liberating those resources in order to make more probes. Which means we can compare it quite handily to living organisms. And you know what takes up the vast majority of a living organism's time? Eating! The probe and its offspring would have to contend with not only the vastness of space and surviving constant bombardment by cosmic radiation that could cause serious damage to its electronic brain-analogue (which in turn can be compared to damage to the DNA of a cell for self replication purposes), but it has no ability to identify the low mass objects like asteroids and dwarf planets that it needs to feed on from interstellar distances because of the same physical limitations our own telescopes have to put up with. So the idea of a single self-replicating organism analogue in space is actually pretty ludicrous. It needs an entire infrastructure of specialized machines like telescopes, computer brains, and probably specialized engines that resemble a partial Dyson swarm in order to actually propel itself from system to system in order to actually work.
A typical sci fi nanobot swarm that converts any material into more of itself is highly unrealistic.
Maybe experiments with bending of spacetime or wormholes or gravity or fabricating Kugelblitz black holes. That kind of stuff that we are fairly sure would require energy far above to what is available to us or even full Type I civilization.Formless wrote: ↑2020-10-22 07:40pm As far as I'm concerned generalizations like this are so vague they are insufficient to answer the point. If we can't imagine something that actually requires the energy levels of a Type II or Type III civilization, the hypothesis is flawed on a conceptual level. We cannot assess the probability of finding an alien experiment unknowable to our own science.
For stationary or semi stationary applications it is perfectly fine to not to try to squeeze out every last percentage point of efficiency. Look at aircraft and cargo ships, engineers go to great lengths to design lightest aircraft reasonably possible while ships are just made of steel because it is cheap. Who cares if it burns a bit more cheap diesel fuel because it is not built from lightest carbon composites available. In a space setting equivalent to an ocean cargo ship may be a space habitat. While it may have fairly powerful engines it is not supposed to need a lot of delta v so mass and energy efficiency may be somewhere after durability, longevity, ease of maintenance and cost.
Wouldn't it be far more than few hundred gram scale payload if you had exawatt scale lasers driving the sail IIRC there are real life proposals to send tiny gram scale light sail spacecraft out of our solar system using only GW class lasers. As far as visibility goes I think that would apply to any interstellar engine unless it is very close. A rocket based starship we would see across interstellar distance only if it's exhaust would be pointed directly at us.Formless wrote: ↑2020-10-22 07:40pm Ah, but lightsail starships of the type you are talking about are actually puny. We're talking about something that might very well be the size of a cell phone strapped to a sail the diameter of the moon (and thus far more massive than the payload even if we assume its ridiculously thin). There is no question why we aren't seeing these kinds of ships. Unless the ship is aimed directly at us so that we can see spillage from the laser driving it, its basically invisible.