[ExI] Von Neumann Probes

[Ben Zaiboc]

John K Clark wrote:

> You don't need interstellar travel to make a Dyson sphere/swarm, and something like that should be very noticeable, but we have noticed nothing. And any technological civilization worth its salt should be able to get a Von Neumann Probe moving at 1% the speed of light because its mass would be very small, and so it could get from one side of the galaxy to the other in just 10 million years, a blink of the eye cosmically speaking. But just how much would a Von Neumann Probe weigh? 


"Very noticeable"? I'm not sure that is true.
How near would a Dyson sphere or swarm need to be before we could stand a chance of detecting it?
It's a tricky question, because it depends on the conditions (density of stars, interstellar dust, etc.). I think the idea is that we would only be able to detect the waste heat from such a thing, correct? So we'd be looking for an anomalous infrared signature from the location of a single star, which of course is invisible to us. I don't see how this would be 'very noticeable'. Wouldn't this signature be lost amongst the infrared radiation from billions of other stars? We have detected what we think are anomalous signatures from the Tabby group of stars which are, if I recall, about 1k light-years from us. Somewhere between 1.5k and 0.5k ly? The galaxy is something like 100k ly across, and we are near the outer edge. So if we guess that we could detect a Dyson swarm within say 2k ly, but no farther, that would mean that, um, I don't know how to do the maths, but I can draw a circle 100mm across to represent the galaxy seen from the top, on a scale of 1k ly per mm, then a dot about 26mm from the centre, to represent roughly where we are, and a circle around that, 2mm in diameter, to represent the area we can reasonably expect to be able to detect a Dyson swarm from its heat signature. That's probably less than 1% of the galaxy. No, that's definitely much less than 1% (0.025%?). I'd conclude that noticing nothing under these circumstances means nothing. Well, it means there are no Dyson swarms in our local tiny corner of the galaxy.

Am I missing something that would mean we could reliably detect any Dyson swarms in the other 99.975(ish)% of the galaxy? Or indeed any respectable percentage of the galaxy?

Even if we expand the hypothetical detection range by x100, we could still only be able to reliably look at less than maybe 3% of the galaxy.

To make things even more difficult, there's also the matter of directional radiation of waste heat. I think someone here said at some time that it would make more sense to radiate waste heat out of the plane of the ecliptic than in it. We probably wouldn't be able to detect that (would we? Could back-scatter from dust and gas reveal it?).

So overall, I'm not feeling optimistic about the chances of detecting even nearby Dyson swarms. Maybe under-construction ones (maybe that's what the Tabby stars are), but not completed ones.


The other thing I'm trying to understand is how a bacterium-sized probe travelling at 1%c would last more than a few decades in interstellar space. Just one single collision with a grain of dust would destroy it. I'd think, for this reason, that von Neumann probes would need to be at least millimetre or perhaps centimetre scale objects, capable of maintaining an active power source to be able to self-repair. Maybe a few hundred sent as a group might work.

They would have to be capable of some data-processing and navigation, so they could tell when to, and be able to, decelerate into orbit around a suitable star, and then scan for, and dock with, suitable raw material and start converting it into more probes.

Presumably there would be protocols for deciding when to stop reproducing and start making other things, like data-centres. That should be simple, something like bacterial quorum-sensing algorithms would be fine (I'm also wondering how many stellar systems have the right materials, in the right proportions, not locked up in large gravity wells, to enable Avogadro's No. of probes to be constructed?).

Then what? Maybe my imagination is limited here, but I'd assume that building a physically modest structure capable of receiving and transmitting signals on a reliable long-range wavelength would be the goal, so that the intelligent entities that designed and spread this whole system could come and visit.

If our own solar system had one, two, or even a few dozen of these data centres already, how would we know?

Ok, I can see one answer. They wouldn't be just building modest data centres, they would be building more Dyson swarms, using the available matter in each system. I'd expect some kind of intelligence to be involved in making a decision first, though, unless we assume they'd simply not care about wiping out any existing ecosystems. Assuming any intelligent life wouldn't want to wantonly destroy information in this way, perhaps we can assume they'd avoid doing that to systems with existing life (so we'd be back to undetectable data centres in those systems).

So where's Fermi's Paradox now? There doesn't seem to be one (or at least it needs to be re-stated to: "Where is everybody (in the tiny fraction of one percent of our one galaxy in which we could possibly detect another civilisation that overlaps ours in time)?").

Or, unlikely as it seems, we might really simply be the first.

Will we ever know? Maybe not. Probably best to just assume that we are.

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Ben