von Neuman probes solve the travel time, cost, and distance objections for ETI even if light speed is the absolute limit.
For the opposite position (they shouldn't be here) to make sense, there has to be either no other civilizations, or our probe-sending habits are so rare that no other civilizations do it, or we are the first.
Any of those other stances seem vastly more improbable, as they assume that our sample-of-one society just happens to be ludicrously different than all others.
What if the time it takes to travel between stars is longer than most civilizations exist? What if we aren't that interesting to alien species or aren't worth the time and energy to discover? We've never sent a functioning probe to any other star and probably won't in my lifetime. And life may not be at the nearest star, it may be thousands of light years away. We may never have the resources available to do it.
The probe doesn't require its parent civilization to continue existing to keep working. It self-replicates. You don't have to "pick" where self-replicating probes go. Each can self-replicate, and you can get nigh-infinite probes for "free." A von Neuman probe finds its own resources.
We have only had complicated writing for a few thousand years. People have wondered if a technical civilization existed on earth a hundred million years ago would there be any thing around now as evidence. If it takes a million years of travel in the freezing, radioactive vacuum of space to reach another life form, you don't know that humans will be around long enough as a civilization to figure out how to actually build the thing that you are describing. I hope so, but I don't think its been proven to be possible yet.
But I do believe it is technically possible in principle to construct and build von Neuman probes. Once they work inside a solar system, then work on making them withstand, say, a few million years in orbit and see if they still work, once they survive a few million years "inactive" (or constantly repairing themselves or whatever it takes), send them off to the stars. But it would take a patience and scale of industry we humans literally can only vaguely dream of.
This is a fascinating talk that goes into the details of such an endeavor.
> ...scale of industry
They don't have to do that. The probe just has to make more "child probes" before it dies. You don't need a huge industry if they are self-replicating. You need only one working probe to start, the rest handles itself.
And it has to survive for millions of years until it reaches a new solar system without something to "feed" on. In many ways it seems easier to make a Neuman probe which can survive within a solar system than something which can travel to another.
Von Nuemann probes are possible in this physical reality for a relatively cheap cost and small time frame.
It makes sense to put probes that can watch for threats anywhere you think that threats can be. Species that do this have a greater chance of survival than species that don't.
We're predicting the behavior of civilizations a lot more advanced than us. We need a level of humility to realize that this is a lost cause, and a lot of the paradigms we take for granted will be shown to be false.
So while everything you said makes sense, the space of unknown unknowns is so large that we should moderate our confidence in the underlying assumptions downwards.
If humans had come first instead of dinosaurs and built von Neumann probes at that time, our probes could be saturating scores other galaxies--not stars, galaxies by now. That's with only a 250 million year differential in technology emergence. It isn't hard to imagine the emergence of technology varying by even more than that.
> Where do they get the resources to reproduce?
From the the star systems that they reach. Stars provide the energy and whatever detritus in the form of an asteroid belt that orbits the star will provide the material to build new probes.
We're already sending probes to Mars, so I imagine we would send VN probes if we had them, so it doesn't seem like much of a stretch.
Also there is signal degradation to consider. Depending on their broadcast strength, there is a limited range beyond which any signal they send will be indistinguishable from background noise. According to this quora post[0] that limit could be as little as four light years. It certainly would not work on any significantly pan-galactic scale. Lasers would fare little better, they would also diffuse and be absorbed by interstellar material.
Also, at that scale, whatever message gets sent may no longer be relevant, or factual, by the time it's received. A lot can happen in a million years. It's difficult for me to think of a message worth sending that also doesn't matter if it takes longer than the time it took humans to evolve to get from point to point.
[0]https://www.quora.com/How-far-do-radio-signals-travel-into-s...
My guess is that we would optimistically assume human survival and build a network capable of exploring the galaxy without us. Since lightspeed delays make real-time communication impossible, individual nodes would be programmed to replicate and explore, following a specific set of parameters. They would report their findings back to some central hub if they found anything interesting, which would probably be only rarely. No doubt for very surprising findings (eg advanced intelligent species) they’d have additional instructions to enter a holding pattern and observe until they received explicit commands. Which, as you point out, might never come.
I can’t speak to what aliens might do. humans would absolutely build this system the moment they had the technology to do it. The fact that we might not survive to use it would be acknowledged and then we’d do it anyway. If anything we’d see it as a fitting memorial to our existence.
That's the most bizarre statement I have seen in the context of VN probes.