[llbowers]

>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.

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

I'm not asking to challenge but in serious inquiry. Off the top of my head it just seems a highly advanced civilization might be able to come up with something completely different to meet their energy needs.

[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.

[p1mrx]

If a civilization has portable nuclear fusion (plug your spaceship into a suitcase for power), then there's no reason to hang out near a star. People living inland never think about how to obtain salt anymore.

[Cthulhu_]

Nuclear fusion is not magic infinite energy; the amount produced is limited by the amount of fuel being used. I mean how much power is generated from fusion anyway? Especially at the scale (suitcase) you mentioned?

I dunno where your remark about salt comes from either; a lot of salt intended for consumption is dug up from the ground (salt mines).

[dwaltrip]

If you want to use vastly more energy than a portable fusion reactor can provide, the nearest star is probably the best place to get it.

[saagarjha]

They’d have to collect fuel from somewhere…

[0-_-0]

From space for example:

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

[ben_w]

It would have to violate the laws of thermodynamics to not be visible. I’m not saying “no” (my brother has wondered if dark energy could be waste output of such civilisations, for example), but we have zero reason to treat the idea as anything more than the softest of science fiction.

[Pharmakon]

Of course if we’re talking about an FTL-capable civilization then our understanding of physics is out of the window to begin with. I don’t think it’s reasonable to expect a civilization with FTL to make Dyson spheres or swarms, and it may be thst even if FTL is impossible it could be I desirable to build them. If the need to hide distinct technosignatures for some reason (Dark Forest perhaps) then any civilization would avoid Dyson structures.

[ben_w]

FTL makes it worse. The only tech we need for us to Dyson up (95%?) of our entire visible light cone is self-replicating factories that work in a vacuum on (for example) Mercury: http://ukspace2015.co.uk/presentations/36

FTL makes that 100%, from beyond out cosmic horizons, even if it turns out that FTL isn’t automatically also a time machine like we currently think it is.

Pocket universes might help? I don’t know though, I’m saying that only because I’ve not seen them ruled out.

[lainga]

The GP didn't ask how you would make a Dyson sphere that was invisible, but why it was certain the Dyson sphere would be built at all.

[ben_w]

Ok, but the effect of the energy output of any civilisation is basically the same, and for the same reasons. It doesn’t matter if it’s a star or TARDIS whose inside is eternally expanding and you’re grabbing energy from its internal dark energy field, if you use that energy, you get hot, and that heat is visible.

Unless you can violate the laws of thermodynamics.

[tzs]

I wonder if advanced civilizations would use a lot of energy?

We always imagine they have very advanced physics and engineering, to do things like take apart planets to build mega-structures and things like that, but usually don't think about their other sciences.

My guess is that by the time they have gotten that far in physics, they have also gotten way ahead of us in biology. They'll have wiped out disease and illness, stopped aging, and only die by choice or accident. They'll have figured out geology and ecology and climatology and psychology.

I suspect that the final steady state for most civilizations that don't end up wiping themselves out by doing something stupid is a relatively small (by our standards) population of essentially immortal beings, living on a world they have restored to a largely pre-civilization state, using less energy by far that we are using but using it way more efficiently.

[bane]

Maybe collect the heat and release it as a laser towards very sparsely populated parts of the sky? Perhaps that helps stabilize the sphere in the orbit of its star?

[db48x]

A Dyson Sphere is not a solid object. Instead, it's many small objects each of which is in a stable orbit. https://www.youtube.com/watch?v=pP44EPBMb8A&t=331s

Also, a huge laser can certainly direct energy in a direction that nobody will notice. Unfortunately, creating the laser beam also creates waste heat, and that waste heat can be seen. Even collecting waste heat generates waste heat that you cannot collect.

On the other hand, if you want a huge laser for some other purpose (such as vaporizing distant planets), then a Dyson Sphere is the ideal way to create one.

https://groups.google.com/forum/#!msg/sci.space.tech/uh5iB9X...

[bane]

> A Dyson Sphere is not a solid object. Instead, it's many small objects each of which is in a stable orbit.

Actually, we've never seen one so we have no idea how one might be engineered! It could for example be a "fog" of worlds that extends out past several local AU and to us would look nothing more like a dust cloud obscuring their local star. There wouldn't be a telltale signal of a Dyson Sphere, just another star with a big dust cloud.

Imagine a civilization like this, those closer to the star get more power literally, and those on the outskirts and in the shadows of the other worlds become dependent on the more inner worlds to re-radiate their absorbed energy, or to condense and lase the energy outwards to the shadow worlds...at some cost that limitless free energy can't pay for.

Beyond some distance the worlds become so cold that the inhabitants freeze to death and exile of your entire world at the whims of the inners becomes a real punishment. Dead worlds are recycled for mass for the growing population of the inners, or repurposed for other things.

There are billions of such worlds. Perhaps they are customarily shaped as small ringworlds and rotate in a complex manner to produce gravity and a daynight cycle. Dead or frozen worlds may have a reflective sail hoisted along the inner opening and expeditionary generation ships are sent out to nearby stars powered by lasers collected from hundreds of inner worlds.

Successful colonies may start to immediate transform the mass of nearby planetary systems into new "dust" clouds rather than settle on the planetary surfaces. Many adjacent Dyson spheres may look to us like just interstellar gases between several stars containing an unusual amount of organic molecules but could be the exchange of trillions of generation ships moving mass and energy back and forth between stars.

Such a civilization could eventually become nomadic in a way, moving from star to star as they burn out, leaving behind frozen husks of trillions of dead ringworlds.

[i_feel_great]

There is Larry Niven's Ringworld [0], which proposes a ring instead of a sphere.

0.https://en.wikipedia.org/wiki/Ringworld

[amelius]

It also sounds logical: why would you waste energy by radiating it if you can retain it?

[krapp]

It's also a waste of energy to work to capture energy you don't intend to use. Using less energy more efficiently is also logical.

[amelius]

More energy is more computation.

[krapp]

That doesn't mean an advanced civilization would need to harness the output of an entire star for computational power.

Unless maybe it's storing everything on blockchain.