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Disk Around a Star
February 1, 2014 9:55 AM   Subscribe

An Alderson Disk is a science fiction megastructure imagined first by scientist Dan Alderson. It's a solid disk that is thousands of kilometers thick, with a circumference equal to the orbit of Mars or Jupiter. The habitable zone would be on both sides of the disk and would be millions of times the surface area of the Earth. Not much theoretical work has been done on its feasibility, but some have tried. Missile Gap, by MeFi's own Charles Stross, which won the Locus readers' award for best novella of 2006, features a 1960s Earth transposed to an Alderson disk and is available for free on the publisher's website.
posted by Kattullus (70 comments total) 22 users marked this as a favorite

 
Maybe I'm not understanding the diagrams and description, but if the thing is solid, what is the benefit if the central star which it outweighs anyway? Everything will be in the shade, right? I think it might need one of those two sun systems that show up in bad sci fi.
posted by Dip Flash at 10:10 AM on February 1


For light, as far as I can understand. In Stross's story, the source of daylight is "polar flares from an accretion disk inside the axial hole."
posted by Kattullus at 10:16 AM on February 1


One answer to the shade problem is for the sun to oscillate through the center hole making day-night cycles but the whole idea seems much less practical than a ringworld or even better a Culture Orbital.
posted by localroger at 10:17 AM on February 1 [2 favorites]


(light and heat, of course)
posted by Kattullus at 10:17 AM on February 1


This seems like one of those interesting sci-fi ideas that ends up being a crutch on which you can write a horrible story of the form, "look at my cool technological setting, I will send some explorers to visit my setting, they will let me describe it in detail, then they will go home" which is what "Ringworld" was.
posted by tylerkaraszewski at 10:23 AM on February 1 [6 favorites]


Way to spoil my next three novels, Helixworld, Laceworld and Frothworld.
posted by George_Spiggott at 10:26 AM on February 1 [21 favorites]


Missile Gap creeped the hell out of me in a way Ringworld never did, FYI.
posted by PMdixon at 10:29 AM on February 1 [5 favorites]


Yeah, there are bits of Missile Gap that made me go "agh! ugh! eech!" while reading them. It's also fairly thought provoking. Everything ties together very neatly in a way that somehow manages to generate a whole new host of questions.
posted by Kattullus at 10:37 AM on February 1 [1 favorite]


but the whole idea seems much less practical than a ringworld


Less useful than a Dyson Sphere, More cumbersome than a ringworld, More wasteful than a trillion automobile economies... it's a construct, it's a flight of fancy, it's... an ALDERSON WHEEL!
posted by clarknova at 10:43 AM on February 1 [13 favorites]


features a 1960s Earth transposed to an Alderson disk

What a wonderful idea. It's like an episode of ST:TOS that never was:

"Spock! You're telling me that these people think they are living on Earth in the 1960's? That's impossible!

"Illogical, Captain, but not impossible."

posted by octobersurprise at 10:47 AM on February 1 [5 favorites]


Yeah, if the thing is "thousands of miles thick," the best light you'd get on its surface would be twilight. Even if the star oscillated back & forth through the hole, that oscillation would constantly decrease because of gravity.

Not a great idea.
posted by Kirth Gerson at 11:00 AM on February 1 [1 favorite]


Only slightly less practical than a Dyson Cube.
posted by blue_beetle at 11:08 AM on February 1 [9 favorites]


*pulls a Jason dinAlt on these worlds*
posted by infini at 11:10 AM on February 1 [1 favorite]


So I tried to think if you could get the whole day / night thing sorted by having a klemperer rosette of suns and discs orbiting at different rates, and then I realised that I know nothing of orbital mechanics and I know less mathematics and physics than I did at school and it wasn't much then. So I decided to just shrug and say 'Yeah, that'd probably work' and get on with the rest of my day.

I solved the problem! Yay!
posted by YAMWAK at 11:25 AM on February 1 [2 favorites]


"raw mass may need to be drawn off of entire stars" - That answers that question.
posted by Ardiril at 11:41 AM on February 1


Such an immense construct would likely out-mass its central star...
So where does this mass come from?
posted by MtDewd at 11:42 AM on February 1 [2 favorites]


Reminds me of the Agassian Plane -- a flat, rectangular surface that stretches roughly the diameter of Saturn's orbit with the star above it. Day and night are achieved by two very large tennis players that bat the ball back and forth across the surface. The only downside is the millions of square miles of conflagration that occurs upon each bounce...
posted by chasing at 11:45 AM on February 1 [5 favorites]


which is what "Ringworld" was.

Behold, there is a difference between a story by Larry Niven and a story by Charlie Stross. Who would have guessed?
posted by localroger at 11:51 AM on February 1 [6 favorites]


With regards to 'where does the mass come from', I don't know if this link to an old sci-fi show is scientifically accurate, but the technobabble sounds about right and who could ask for more than that?
posted by YAMWAK at 12:01 PM on February 1 [2 favorites]


(Uh, ignoring the handwaving involved in turning sub-atomic particles into useful self-replicating swarm components)
posted by YAMWAK at 12:02 PM on February 1


The Arkadians of Spartakus And The Sun Beneath The Sea lived on a structure very similar to an Alderson Disk.
posted by mediocre at 12:36 PM on February 1


Tech Level: 23
Not nearly enough techs.
posted by device55 at 12:41 PM on February 1 [2 favorites]


Not much theoretical work has been done on its feasibility

Let me see if I can take a crack at this feasibility thing.

Hmm. 2000km thick disc...
Radius of 228,000,000km...
squared... it, multiply by pi and then by the height....

Hmmm...is a solid with a volume of more than 80,000,000,000,000,000,000 kilometers "possible to do easily or conveniently?"


Nope. Not so much with the feasibility.

Hope this analysis helped. Looking forward to my Nobel.
posted by dersins at 1:19 PM on February 1 [2 favorites]


I think it might need one of those two sun systems that show up in bad sci fi.

Like this one?

posted by ActingTheGoat at 1:24 PM on February 1 [3 favorites]


There's an argument that the World of Warcraft may be an Alderson Disk, based on observations of the idiosyncratic movements of its sun and moon (both rise in the northwest, ascend to zenith and then return along the same path).
posted by Hogshead at 1:30 PM on February 1 [1 favorite]


If you really wanted to waste a lot of mass instead of building simple Orbitals, why not just chuck it straight into black holes instead of building... that?
posted by ROU_Xenophobe at 1:31 PM on February 1 [3 favorites]


This seems like one of those interesting sci-fi ideas that ends up being a crutch on which you can write a horrible story of the form, "look at my cool technological setting, I will send some explorers to visit my setting, they will let me describe it in detail, then they will go home" which is what "Ringworld" was.

Hey, I liked Rendezvous with Rama when I was a kid!

John Leonard, writing in The New York Times, finding Clarke "benignly indifferent to the niceties of characterization,"

posted by ActingTheGoat at 1:33 PM on February 1 [1 favorite]


The Flat Earthers are going to love it, though.
posted by Fnarf at 1:37 PM on February 1 [1 favorite]


Kirth: You might think that gravity would damp out such an oscillation, but in fact (assuming the disk is rigid) it would persist indefinitely. Basically, if you have an infinite sheet of matter, an object dropped from above the sheet will oscillate above and below it, following the same equations that govern a mass bouncing up and down on a spring -- but in this case there's no friction.

The Sun's orbit through the Milky Way oscillates above and below the plane of the Galactic disk for exactly this reason. This motion has a period of ~64 million years, distinct from the ~220 million year period for the Sun to complete an orbit around the Galaxy's center. In the case of stars in a galaxy, the amplitude of oscillation actually tends to increase with time, rather than decrease due to a friction-like effect, due to close encounters with more massive objects (probably large gas clouds) that tend to scatter things out of the plane.
posted by janewman at 1:49 PM on February 1 [3 favorites]


Figuring out the period of this motion is a classic homework problem for advanced physics/astro students -- it depends on a clever application of Gauss' Law, which relates the gravitational field on any imaginary closed surface to the amount of mass within it.
posted by janewman at 1:51 PM on February 1 [1 favorite]


instead of building simple Orbitals, why not just chuck it straight into black holes

Yeah it might be one indication that an idea is over the top when it makes you think of Culture Orbitals as "cheap and simple."

Then again, Iain M. Banks himself described Orbitals as "matter cheap" in one of his novels.
posted by localroger at 1:53 PM on February 1


The biggest problem with the Alderson Disk is that it really does make the engineering challenge of holding a ringworld or Orbital together look simple by comparison. The thing is far too heavy to hold its shape based on the rigidity of chemical solid bonds alone; in fact, if it's thick enough for normal gravity, it would probably be an Oreo of two crustal plates with a central liquid core. Without some fairly serious arbitrarily advanced tech the thing would inevitably collapse into a sphere, which would probably instantly supernova because it's way too big to be a stable star. And if you have that kind of tech you probably don't need all that mass to create gravity.
posted by localroger at 1:58 PM on February 1 [1 favorite]


Okay, ignorant question here. If you're generating day and night by bobbing a star up and down in the central hole, is there any danger of the star 'smearing'? i.e. would the least-energy solution be for it to elongate into a prolate spheroid and minimise its movement with respect to the disc?
posted by YAMWAK at 2:06 PM on February 1


You might think that gravity would damp out such an oscillation, but in fact (assuming the disk is rigid) it would persist indefinitely.

IANAPhysicist, but I would bet that tidal forces induced in the star would damp the oscillation.
posted by ROU_Xenophobe at 2:07 PM on February 1 [2 favorites]


a crutch on which you can write a horrible story of the form, "look at my cool technological setting, I will send some explorers to visit my setting, they will let me describe it in detail, then they will go home" which is what "Ringworld" was.

But in fairness, The Smoke Ring wasn't; the characters were all born there. Also I liked the fact that he didn't set it in the Known Space future either.
posted by George_Spiggott at 2:12 PM on February 1 [1 favorite]


I would bet that tidal forces induced in the star would damp the oscillation

Assuming the star is like Sol, its "surface" gravity is 27 times that of the Earth, so the gravitational gradient created by the Alderson disk is pretty trivial compared to its own gravity. The disk itself is of course much more massive but for practical purposes its gravity field is completely symmetrical at the center. The slight tidal friction induced by passing through the disk's plane might be noticeable over the course of hundreds of millions of years but probably not before the generators keeping the disk from collapsing in on itself fail.
posted by localroger at 2:17 PM on February 1 [1 favorite]


Missile Gap is super great fun. Ekranoplan!
posted by Artw at 2:33 PM on February 1


The disk itself is of course much more massive but for practical purposes its gravity field is completely symmetrical at the center.

It's not stable, though; the sun would have no tendency to return to the center if any impetus nudged it away. Quite the reverse: even a small perturbation would cause it to eventually collide with the inner edge of the disk. So while you're engineering this thing you have to make the sun use directed flares to keep itself centered.
posted by George_Spiggott at 3:14 PM on February 1 [1 favorite]


So, basically, if you had the technology and materials to build this, you probably wouldn't have to?
posted by GenjiandProust at 3:23 PM on February 1 [1 favorite]


You wouldn't waste time talking to ants like us about why you did it, for sure.
posted by Artw at 3:26 PM on February 1 [2 favorites]


No no no, we're the apes. Those other guys are the ants.

Whether the builders are either one, that's the question.
posted by Kattullus at 3:44 PM on February 1 [4 favorites]


It's not stable, though

Yeah, I think the math is actually exactly the same as for Niven's ringworld. I guess it's Orbitals all the way up.
posted by localroger at 3:48 PM on February 1 [1 favorite]


Bloody communists.
posted by Artw at 4:07 PM on February 1


I think you mean: Bloody eusocialists.
posted by Kattullus at 4:16 PM on February 1 [7 favorites]


So which side sits on the back of the elephants?
posted by arcticseal at 4:33 PM on February 1 [2 favorites]


Hi! I just noticed this -- been a few days since I checked MeFi due to an intercontinental flight.

If anyone wants to ask me any direct questions about "Missile Gap" I'd be happy to try to answer (but please bear in mind I wrote it in 2005, so some degree of brain fade has taken place since then) ...
posted by cstross at 5:00 PM on February 1 [10 favorites]


No no no, we're the apes. Those other guys are the ants.

Well, Italians often mistake bees for apes, so who knows what alien linguistics will lead to?
posted by GenjiandProust at 5:04 PM on February 1 [2 favorites]


If anyone wants to ask me any direct questions about "Missile Gap" I'd be happy to try to answer

Were the mock-termites imported from another planet or were they locally evolved? And why did they stay away from the coast?

I also didn't quite understand the significance of "Mount Bugmore." Was the implication that agents similar to Samsa and Brundle had infiltrated a copy of Earth and instituted a kind of hybrid government (which failed at some point, apparently) or what?
posted by jedicus at 5:33 PM on February 1


jedicus: The whole Alderson disk thing is a live-action ancestor simulation (that is: one running on a real platform rather than in a software sim) by an unimaginably distant-in-time Kardashev Type III civilization, who are probing questions about the origins of tool-using space-going civilization using the galaxy's largest petri dish. (One which is difficult for the cultures growing on it to escape, and which can conveniently be sterilized by triggering a nearby supernova if things show signs of running out of control.)

Of course, the implicit question is "which clade do the creators of the sim belong to?" (And humans, as always, tend to assume It's All About Us, even when it isn't.)
posted by cstross at 5:51 PM on February 1 [2 favorites]


One can only wonder at the objectives of reproducing snapshots of multiple (presumptively) evolved tool-using species into an environment which they A) immediately realize is synthetic, and B) can interact with each other. It actually makes me think of the Fredric Brown story "Arena" writ very large indeed.
posted by George_Spiggott at 6:00 PM on February 1


cstross: A few questions, if you'll answer them:

1) Did the builders chose some sort of interrupted equal-area scheme for projecting the earth's surface onto a flat plane? The continents didn't seem to have changed, but Europe was now much farther from the Americas, which is typical for that kind of projection. If so, did you consciously choose a known projection, or do we just assume it's one in that class which builders cooked up?

2) Was the celestial body in the center of the doughnut hole a black hole?

3) How did the builders engineer day/night cycles?
posted by clarknova at 6:16 PM on February 1


George_Spiggott: "But in fairness, The Smoke Ring wasn't; the characters were all born there. Also I liked the fact that he didn't set it in the Known Space future either."

Although it was set in his other future history timeline, The State.
posted by Chrysostom at 7:21 PM on February 1


I solved the problem! Yay!

Great! Now do Global Warming.
posted by Kirth Gerson at 7:56 PM on February 1 [1 favorite]


cstross: Is there any (author/publisher-blessed) way of getting Missile Gap as an EPUB or other format suitable for an ereader? Short of just copying-and-pasting it into a text editor and then into Calibre or something, I mean. Wouldn't mind paying for it, but it doesn't seem to be available through the usual channels.
posted by Kadin2048 at 8:42 PM on February 1


FWIW it is part of the short story collection Wireless.
posted by Artw at 9:02 PM on February 1 [3 favorites]


Clearly it won't work, there are no elephants or a turtle for it to ride on!
posted by usagizero at 11:25 PM on February 1 [2 favorites]


I liked Wireless. In particular, I enjoyed Stross's foreword, in which he explains why he writes short fiction.
posted by SPrintF at 11:52 PM on February 1


a crutch on which you can write a horrible story of the form, "look at my cool technological setting, I will send some explorers to visit my setting, they will let me describe it in detail, then they will go home" which is what "Ringworld" was.

Good thing a horrible story like that didn't win a Hugo and a Nebula, then.

(To be perfectly honest this summary is usable for a good swathe of Niven's early fiction, such as all the Beowulf Shaeffer stories, or Becalmed in Hell. A few of them omit the last act, e.g. Wait it Out. Then there are books like A World Out of Time where you can't get home and being a tourist is the only option. I don't think it's a bad model, and it beats the pulp approaches -- based largely on colonialism -- by astronomical units.)
posted by dhartung at 12:30 AM on February 2 [1 favorite]


cstross: And humans, as always, tend to assume It's All About Us, even when it isn't.

Does the end suggest that the ants also always tend to assume it's all about them?

And since I'm asking, is the pigeon poisoning that Gregor does meant to suggest that there are bird/dinosaur civilizations out there that the ants are also fighting? Perhaps those purple lizards?

Oh, and a thematic question. I'm assuming that the Cold War setting of Communists vs. Capitalists was supposed to mirror Ape vs. Ant. Were there other such Individual vs. Collective stories playing out? The bit where Maddy struggles against the sexism of her frontier society made me think this was probably the organizing principle for every story.
posted by Kattullus at 1:55 AM on February 2


If Space Tourism travelogues are wrong, I don't want to be right. Just call me Mary-Sue, kthxbye!
posted by mikelieman at 5:40 AM on February 2


So I went to the bookstore today to see if I could find cstross' short story collection, and its limited selections in the English language up here in Finland.

did pick up Videssos cycle's first and second by Turtledove though, they were all out of Modesty Blaise oddly enough
posted by infini at 6:22 AM on February 2


Cstross: I appreciated the reference to Hoyle's October the First is Too Late. That one terrified me a fair bit as a youngster and has stayed with me over the years. I almost never see it mentioned.
posted by bowline at 7:38 AM on February 2 [1 favorite]


clarknova: (a) Dymaxion projection, (b) yes, a black hole, (c) modulating the infall of material into the accretion disk to produce increasing and decreasing flares of the polar jets.
posted by cstross at 8:11 AM on February 2 [4 favorites]


Kattallus: Yes, thematic issues are writ both large and small; it's really about eusociality vs. individualism (hint: on a cosmic scale, the eusocials win -- at least in this story).
posted by cstross at 8:13 AM on February 2 [3 favorites]


What would it be like on the outside edge of an Alderson Disk? You'd have the mass of the ENTIRE DISK under your feet.
posted by codswallop at 11:07 AM on February 2


What would it be like on the outside edge of an Alderson Disk?

As you start to get closer to the outside edge than the inside, the force due to gravity would start to change direction and would no longer be directly perpendicular to the surface.

What I'd imagine this would feel like would be that the disk starts to slope upwards. I.e., as you'd continue to walk forwards towards the outside edge, you'd feel like you were walking up an incline of sorts, or as though the ground was starting to slope upwards beneath you without actually gaining any elevation. I'd imagine it would be disconcerting.

Of course, being on a flat disk the size of Saturn's orbit would be disconcerting for any number of reasons.

It's been a while since I've done the math — as others have mentioned, calculating the force of gravity on infinite planes of finite thickness, finite squares, disks, etc., are pretty common problems, and have solutions with similar form to common E&M problems — but I think that on a disk of finite radius with a hole in the center, there would be a circumferential band where the force due to gravity was perpendicular to the surface; it would be at some non-normal angle on either side of this band. However, I think if you had the capability of building an Alderson Disk you could also safely assume you'd have the ability to directly manipulate gravity.
posted by Kadin2048 at 12:23 PM on February 2


Kadin2048: I disagree somewhat on the gravity at the edge of the disk -- with a sufficiently thick disc, one should only notice the non-verticality of the gravity vector when you're fairly close to the edge. It would also slightly decrease, so I expect it would feel a bit like an incline, but an incline into decreasing gravity (as you walk outward). Still fairly disorienting I think.
posted by chimaera at 3:50 PM on February 2


The polar jet idea helps deflect the problem of the sunlight hitting at a shallow angle and the several problems that causes, such as atmospheric refraction and a general reduction in insolation/area. Some of which can be overcome by using a brighter, more energetic sun or moving your notion of the "habitable zone" inward.

The idea of a cycling sun that bobs up and down through the hole is interesting, but I can't believe you'd get a practical day-night cycle for earthlike ecosystems out of that -- the rate of oscillation for any useful angular change would surely be measurable in months, not days.

But for the disk as a whole there are still just a crazy number of interesting problems that the broadly-stated idea leaves you with. What kind of weather are you going to get with a common atmosphere over such a huge surface with that kind radial temperature gradient? How does rotation affect that? Are we talking about hurricanes ten million miles across? The story has some indications that the mapping of the Earth was fine tuned, e.g. the immense radiators for polar conditions, for example. But there'd definitely have to be way more stuff going on in order for life to continue even remotely as normal after the change; think of the annual rains in our agricultural areas. Much disruption to that and your experiment collapses almost immediately.
posted by George_Spiggott at 3:51 PM on February 2 [1 favorite]


A week later and I'm still thinking about this story. I think Niven considered that a marker of success in fiction writing.

The polar jet idea helps deflect the problem of the sunlight hitting at a shallow angle and the several problems that causes

One problem with relativistic jets is they emit a lot of synchrotron radiation, usually in the form of x-rays. Any surface that's being brightly illuminated by the incandescent gas is probably also being flooded with lethal radiation. But we can always assume the builders did some wand waving there.

Maybe the modulation of infalling gas is precise enough to ensure there isn't much kink instability in the jet's plasma, and most of the radiation is truly perpendicular to the 'flares'. Or maybe there's some other shielding we don't know about.

I wonder if the builders populated both sides of the disk. Would that require a second fail-safe supernova on the opposite side? Could cultures on one surface survive the nova sterilization of the other?

Well done, cstross.
posted by clarknova at 4:56 PM on February 6


MetaFilter: there isn't much kink instability
posted by Kirth Gerson at 5:16 PM on February 6


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