A formal notification that condition "Argh!" exists

[james_davis_nicoll]

You cannot surround a star with a solid dyson shell and have people living on the inner surface/ Objects on the inner surface will have no gravitational attraction to the shell (all the forces from the outer shell cancel out) but they will have some measurable attraction to the star at the middle. This leads to a situtation technically known as "bad" as all the loose objects on the inner surface fall into the star.

Thank you.

Comments

[nexstarman.livejournal.com]

You can live on the outer surface, heated from the inside...

But building the polar regions of the sphere is the most daunting problem because you can't get them going at orbital speed like the equatorial area.

[james-nicoll.livejournal.com]

You can live on the outer surface, heated from the inside...

Sure, but aside from Fred Pohl and Jack Williamson 30-odd years ago, I can't think of anyone who ever took that route. Anyway, the specific example I have in mind used the inner surface.

But building the polar regions of the sphere is the most daunting problem because you can't get them going at orbital speed like the equatorial area.

Anyone who goes for a shell instead of a swarm clearly has impressive technology. For one thing, the star isn't gravitationally bound to the shell, so its position within the shell can drift. Something needs to correct that.

[mmcirvin.livejournal.com]

Stephen Baxter had people living on the outside, in at least one short story.

Don't most people who write about solid Dyson spheres put bogotech gravity generators on the inner surface?

[dewline.livejournal.com]

That's generally the rule. It's assumed that if you can build one of those blasted shells in the first place, you should've already planned for solving those headaches James mentions.

[mishalak.livejournal.com]

I thought that being a sphere made it gravitationally stable, unlike a ring. If the star moves from the center much there is so much more sphere on the more distant side that it pulls things back into balance. I suppose I need to go look up if that is true or not.

[blufive.livejournal.com]

IIRC from my uni days, the gravitational forces from the sphere itself balance out exactly at every point within the sphere. There may be more sphere on the more distant side, but it's also more distant, and those two effects cancel each other out. That means there's absolutely nothing to stop the star wandering about, without extensive quantities of handwavium.

[grimjim.livejournal.com]

A ring is just a 2D slice of a sphere, perpendicular to the axis of rotation. The ringworld vulnerability remains along that plane.

[nexstarman.livejournal.com]

Louver the walls?

Thank you! You'd be surprised at the number of scifi franchises that don't realize this simple fact.

[razorsmile.livejournal.com]

I don't know jack about physics (beyond my failing grade in high school and a fuckton of comics) and even I knew that.

[mmcirvin.livejournal.com]

Hollow-earth crazies tend not to know this either.

[lederhosen.livejournal.com]

Also, has anybody worked the numbers for the sorts of particles habitually emitted by a star, and made sure that catching them all in a shell wouldn't, say, cause a hydrogen buildup in the atmosphere? Maybe there's no problem there, but I'd feel happier if I knew somebody had checked it.

(Technically, you could have people on the inner surface as long as you spin the shell, but then the poles are uninhabitable, so why not just build a ring?)

[james-nicoll.livejournal.com]

It's not a problem for us at 1 AU from a big star.

[lederhosen.livejournal.com]

We don't have containment, though - to the contrary, we have a magnetosphere that actively deflects solar wind so it blows right on past us and outwards.

Though now I do the math, it doesn't look that bad. Wikipedia tells me that the Sun ejects about 1E9kg/sec material as solar wind, by mass three-quarters H⁺ and one-quarter He²⁺. For a 1-AU sphere, that makes an average concentration of roughly 8E-7 moles H₂ per cubic meter per billion years, so it shouldn't be an issue.

(I'm assuming that if one has the ability to build a Dyson sphere, one has the handwavium necessary to deal with the EM consequences of not having a magnetosphere. This may or may not be a sound assumption.)

[antikythera.livejournal.com]

Handwavium. That sounds like such a useful substance.

[llennhoff.livejournal.com]

And much easier to get than unobtanium.

[knotty-pine.livejournal.com]

It's closely related to unobtanium.

[knotty-pine.livejournal.com]

Scooped! That's what happens when you switch away from a tab for a while and don't refresh. Let this be a lesson to you.

[james-nicoll.livejournal.com]

why not just build a ring?

If the ring has a star at the center, it's actively unstable, as opposed to having nothing to stop slow drift.

A swarm made up of habitats, which could very well be rings, should be ok.

[redaxe.livejournal.com]

(seen via friendsfriends)

I'd always inferred (if not stated in the various works of fiction I've read that used Dyson spheres) that a culture that could (and needed to) build a DS would have artificial gravity or some equivalent that permitted them to live inside. Or that they had a double-layer shell, with the inner surface being completely photo-converters to capture the star's energy, and living space between the shells. In either event, they'd also have to have attitude jets of some sort, but again, a culture at that level of technology ought to be able to handle the problem pretty straightforwardly.

[badgerbag.livejournal.com]

Hellooooo! They'd totally be wearing magnet boots. And loose objects are so "outie".

[elynne.livejournal.com]

Loose object like... atmosphere?

Enh, in the future, we'll all wear magnet-boots and have respirators implanted into our faces. Yay! I'd ask if we could have jet-packs too, but what with the gravity pulling you up and all, maybe not such a great idea.

[badgerbag.livejournal.com]

Damn. Air, shmair. Maybe we could ... I dunno... learn to breathe iron filings.

[elynne.livejournal.com]

I am personally in favor of becoming a polyester-based lifeform. Mmmm, inhale that sweet sweet polyester...

[james-nicoll.livejournal.com]

Twice in one day, disco seems to be on topic on my LJ.

[elynne.livejournal.com]

It's a Discopalypse!

.... and now, I can't stop giggling.

[elynne.livejournal.com]

Ooh! You could make - giant mirror balls, which float around the sun in various (hopefully non-colliding) orbits, soaking up the solar energy, and also making entertaining light shows for the people magnetically stuck to the interior surface of the Dyson sphere... especially "entertaining" when several of the mirrors happen to focus on the same spot...

So what if a superintelligent, ridiculously technologically advanced species made this vast habitat, ostensibly for themselves, but in fact kidnapped entire populations of other planets and dumped them in there to make "reality TV shows" while watching the poor insects trying to survive? I know, I'm sure it's been written, probably several dozen times, but still.

[alexx-kay.livejournal.com]

So what if a superintelligent, ridiculously technologically advanced species made this vast habitat, ostensibly for themselves, but in fact kidnapped entire populations of other planets and dumped them in there to make "reality TV shows" while watching the poor insects trying to survive? I know, I'm sure it's been written, probably several dozen times, but still.

This isn't exactly the plot of Charles Stross' Missile Gap, but it is kinda close.

[elynne.livejournal.com]

Hm... sounds like something I'm going to have to add to my reading wishlist.

[alexx-kay.livejournal.com]

Currently available in the SFBD-published omnibus _One Million A.D._, though I believe a chapbook of the novella by itself will be coming out in a bit. Other than those options, you might have to wait until Charlie builds up enough short material for a new collection.

[elynne.livejournal.com]

Hmm... I added the book to my Amazon.com wish list before realizing that it hasn't yet been released. Ah well; I don't yet have any money. :P

[brooksmoses]

No, no, the jet packs are actually pretty important. If you make a mistake and jump, you need the jet pack in order to get back "down".

[elynne.livejournal.com]

Okay, good point. *nod*

[redbird]

Note that "loose objects" in this context includes the atmosphere.

[blackbirdcd.livejournal.com]

And wouldn't the inhabitants be forced to wear SPF 50,000,000 sunscreen?

I'd hate not having a night-time.

[james-nicoll.livejournal.com]

And wouldn't the inhabitants be forced to wear SPF 50,000,000 sunscreen?

I don't see why. The shell is the same distance from the sun as we are.

[(Anonymous)]

"The shell is the same distance from the sun as we are."

Actually it's twice as far away, if you want the same surface temperature.

A planet absords solar energy through its cross-sectional area, but radiates over its surface area, which is four times as large. A Dyson sphere absorbs over its inner surface and radiates over its outer surface (i.e. the same area), so compared to a planet, it only has 1/4 the radiating area, relative to absorbing area. (Energy radiated from the inner surface doesn't count, since it's intercepted again by the same surface.) So for the same surface temperature, you need 1/4 the incoming power per unit area, which means twice the distance. (With some adjustment for the albedo of the outer surface.)

-- Ross Smith

[(Anonymous)]

1) Gravity generators! So easy!

2) Spin the sphere. Yes, this only works around the equator. Yes, the rest of the sphere is totally wasted. Presumably you don't care.

3) Live on the outside. You probably want a smaller star for this, since the gravitational force of the Sun at Earth orbit is around 0.002 gravities.

[pulls chin thoughtfully] Okay, this really doesn't work for anything but a red dwarf. Assume a star with 0.2 solar masses. It'll have about 1/400 solar luminosity, so you'd build the Dyson sphere about 20 times closer. This would give you a roughly lunar surface gravity, around 0.16.

Of course, you'll need artificial lighting... little fusion plants in solar orbit, I suppose. Don't dig any deep wells.

Anyway.


Doug M.

[krfsm.livejournal.com]

The non-equatorial parts of the sphere could, one presumes, be put to various industrial uses.

[(Anonymous)]

Stephen Baxter used this (rotating sphere with only the equatorial belt habitable) in The Time Ships. This design was just fine for the Morlocks who built it, since they preferred to live underground anyway. The relatively small habitable area (still several zillion Earths, of course) was used as a sort of wildlife preserve for the surviving Homo sapiens.

-- Ross Smith

[(Anonymous)]

3) Live on the outside. You probably want a smaller star for this, since the gravitational force of the Sun at Earth orbit is around 0.002 gravities.

And what is the problem with this ? I recall an essay ("Bigger than Worlds" ?) with a ginormous Dyson shell with world sized habitats in the atmosphere.

rgl

spindrift

[(Anonymous)]

At least a spinning sphere will allow you stand on the inner surface and watch while your sphere falls* into the sun.

Why not make little balls of dirt, surround them with air and orbit them around a nice stable star? I bet that would work.

*falls: the only meta-stable position for a sphere enclosing a star is with the star dead center. Any eccentricity will be gravitationally encouraged. A gentle puff of solar wind and the sphere will "fall down".

Re: spindrift

[corruptedjasper.livejournal.com]

That's true for a ring, but it's not true for a sphere, I think. At least not with Newtonian physics.