[cletus]

> How do we know an advanced technology would absolutely use Dyson spheres?

This depends on what question you're trying to answer.

The Fermi Paradox is a good one because you don't need to ask "would a civilization always use Dyson spheres?" It takes just one to use them within a billion years to be a sufficient counterexample. Would they be universally used? Who knows?

Dyson spheres are such an attractive idea because there's no new physics required here (like negative mass for wormholes and warp drives). It's largely just an engineering problem. Now it does require a fairly economical method of getting off-world but all of this seems relatively likely within the next 100-200 years.

So it's (relatively) low tech and attractive in terms of providing living area for unit mass (many, many orders of magnitudes better than living on planets).

It's worth noting that you don't even need nuclear fusion to make this all work (although that makes it much easier) and it's not a given that we'll have practical nuclear fusion.

If you don't have nuclear fusion, what is your energy source? The alternatives other than harnessing solar output are much, much higher technologies like using black holes (which is also theorized about as a starship drive).

[wongarsu]

If the goal is living area and capturing solar energy, wouldn't most civilisations start out with a dyson ring instead of a full sphere? Making a ring seems easier structurally (everything is in a very similar orbit, so less forces on the structure) and requires vastly less material, so it's much easier to get started. And once the ring is a few thousand kilometers wide the civilisation might not need more space (either because population and energy needs level off or because other solar systems are more enticing for expansion, or simply because the civilisation collapes eventually).

To me it seems that if you take an infinite number of civilisations, you should find a lot of rings and barely any full spheres. But a ring is a lot harder to detect: if it blocks the star from our point of view it's as obvious as a dyson sphere, but in most orientations it would be seen as a very thin band that radiates much less energy than the parent star, making it basically impossible to detect with current technology (none of our methods of finding exoplanets seems applicable, and emissions would be too low to be seen directly)

[cletus]

As I replied in another thread, a Dyson sphere in its original intent was a swarm of habitats, not a rigid shell. No known material is strong enough to support that.

What you're talking about I think is a ringworld, popularized by Larry Niven's "Ringworld" series. They have the same problem a Dyson shell does: the centrifugal force would tear the ring apart and there's no known material that could handle that.

A Dyson swarm has basically all the advantages of living area a shell or ring does with none of the material problems. It can also be built incrementally, one habitat at a time. And that too is important.

This is why a Dyson swarm is seen by many futurists as near inevitable:

- Can be built out of modern materials like stainless steel

- Can be built incrementally, one habitat at a time

- Is orders of magnitude more efficient in terms of living area per unit mass than planets

- It avoids large gravity wells, which are a problem for getting off planets

- It can take advantage of the full energy output of a star

[wongarsu]

At any fixed distance from the sun there's only one speed where you have a stable spherical orbit. Since actual spheres have poles that don't move much at all, any dyson swarm that approaches a sphere has to consist of parts in a number of different orbits. That's inconvinient for a whole host of reasons (sun often occluded as segments move below you, movement between segments is difficult etc.). In comparison a swarm that looks like a narrow ring has no such problem: everything is approximately at the same speed while traveling in the same direction. For that reason alone rings are the superior choice regardless if your structure is solid or a swarm of independend objects.

Maybe multiple rings would form for political reasons, but that just makes the individual rings proportionally thinner.

[logfromblammo]

I was under the impression that Dyson sphere participants would be mainly heliostats, whose station would be kept by balancing their inward gravitational attraction against the outward pressure from reflected or decelerated solar wind.

Whenever an object would intersect the sphere, the nearest heliostats alter the angle of their mirrors/sails to drift away and make a hole. Then they drift back to close it after it passes.

There's nothing to say that they can't also have an orbital velocity component, as it takes quite a lot of delta-v to decelerate from a near-circular solar orbit, and orbital velocity can make up for lack of sail area.

[db48x]

Dyson Spheres are not solid objects; they're swarms. https://www.youtube.com/watch?v=pP44EPBMb8A&t=331s

[amelius]

> Now it does require a fairly economical method of getting off-world but all of this seems relatively likely within the next 100-200 years.

Not if our brightest minds are trying to make people click more ads.

I think I finally know the answer to the Fermi Paradox.

[jessaustin]

...no new physics required...

Is this true? The mass required would generate forces that would rip apart any materials we've encountered. A Dyson belt could just be in some unstable sort of orbit, but a Dyson sphere has to be strong enough to hold itself in shape.

[cletus]

"Dyson sphere" is a misnomer in that at some point this was conflated to mean a shell than physically encompasses a star. That was never the original meaning or intent, which is why some people (including Isaac Arthur) prefer the term "Dyson swarm" as being true to the original idea and clear in intent.

A Dyson sphere/swarm is simple a sufficient cloud of habitats orbiting the star as to essentially block out the vast majority of its light, kind of like how droplets of water block light in a fog.

[semi-extrinsic]

This feels like it would violate the hairy ball theorem, no?

https://en.wikipedia.org/wiki/Hairy_ball_theorem

[MertsA]

No, for one you can only orbit in an ellipse around the center of mass. You can only "comb" in a direction that takes you around the middle of the ball. The major difference though, is that you can have overlapping orbits. There is no way to cover the entire surface without overlapping because of this, but you can still cover everything if you're willing to pay a price in efficiency by having portions of the constellation shade one another.

[semi-extrinsic]

Hmm, but it feels like a good tradeoff in marginal increase in total energy caught versus marginal decrease in efficiency would get you pretty far from 100% coverage.

[adrianN]

The satellites of the swarm don't have be at the same distance from the star and the orbits can overlap.

[chrischen]

Flying cars were in our imagination but never materialized (and probably never will).

[colordrops]

Why would an advanced alien civilization be bound to the limits 21st century human engineering and physics?

[gdpgreg]

They wouldn't be. We limit ourselves to current or near current technology for these analysis or you can easily get into the realms of hypotheticals very easily. If you say that the laws of thermodynamics don't apply to aliens because they have more advanced physics then the sky becomes the limit. So you have to stick by your own rule book when you imagine, so it'd be more appropriate to say that there is unlikely to be Dyson swarms near us that were built by aliens with a similar understanding of physics. For all we know the popular way that they gather energy is harvesting photons from within the star itself, stick a giant straw into a star and just drink away the photons.

[colordrops]

I'm not suggesting that we speculate into high fantasy. What I'm getting at is that we can't come to a conclusion about whether advance civilizations exist or what they might look like to us. So quite the opposite of realms of hypotheticals really.

[llbowers]

That's very clear. Thank you for your response.