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Comment by jancsika | Hacker News Reader

> He contends that there likely isn't a sentient civilization within about a billion light years of us because the signature of Dyson spheres would be unmistabkable and unmissable.

If a civilization has the intelligence and foresight to build a Dyson sphere, how much more intelligence and foresight does it need to hide one?

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See [1] and [2] where the idea of hiding a Dyson sphere is covered in depth. It essentially means breaking the laws of thermodynamics, specifically entropy. Hit and object with light from a star and it heats up. It radiates away that heat in a predictable way.

[1] https://www.youtube.com/watch?v=QfuK8la0y6s

[2] https://www.youtube.com/watch?v=HlmKejRSVd8

> It radiates away that heat in a predictable way.

Unless that heat energy itself is also caught and used.

That is exactly what is meant by "It essentially means breaking the laws of thermodynamics".

Waste heat must be radiated away, though thermodynamics would be quite content if that radiation is anisotropic.

For example, consider a perfectly insulating black box with small hole. Observers on the opposite side from the hole will not observe any radiation from the interior of the box.

I don't think there's any physical limit on collimating the outgoing radiation to hide from everyone outside a narrow beam. It's "just an engineering problem".

Actually, I believe the law of Etendue[1] covers that. Basically, any focusing of the area of light must be balanced by a widening of the angle of that light.

1: https://en.wikipedia.org/wiki/Etendue

In terms of hiding an emitter, the relevant quantity is the beam solid angle in the far field. This is wavelength dependent which implies temperature dependence if we're talking thermal radiation.

The lower the temperature, the longer the wavelength and the wider the beam (given fixed emitter geometry). So there's a tradeoff between hiding your Dyson sphere vs extracting the most energy from the source.

Have there been any studies about whether Dyson swarms harvesting under 0.1% of the star's output are detectable, especially if the designers want to minimize the chance of detection?

I suspect that civilisations that make it to the Dyson swarm stage are on average a little biased towards environmentalism in order to survive past their terrestrial industrial stage.

Doesn't make sense to put a giant target on yourself until you can discount the dark forest hypothesis either, especially not when you've just beaten incredible odds to become a near-immortal truly space-faring civilisation.

Once it is established there is a safe way to roam around and do construction logistics the dark forest hypothesis doesn't make much sense. A real advanced civilisation would have detectable honeypot decoy dyson spheres all over the place just as an intelligence gathering tool to see if anybody will actually come after them and with what.

Its an interesting question; if you're an aggressor and see a 'low tech' civilisation recklessly build an obvious Dyson sphere what probability do you put that its a honeypot from a real threat... and so on.

I personally find the dark forest interesting not because I think it is likely but because even giving it a very small probability of being true would be enough for near immortal high tech civilisations to move very cautiously.

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