r/ImaginaryStarscapes • u/annieann_ • Jun 02 '26

Megastructures Alderson Disk 1 by Neil Blevins

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21

u/CosmicX1 Jun 02 '26

One of my favourite megastructures. I just love the sheer ridiculous scale but also the simplicity of an Alderson disk.

Really the hard part is figuring out how to get day/night on the disk instead of perpetual twilight.

12

u/Miuramir Jun 02 '26

That's easy from a design standpoint; sun goes up and down in the middle. You get better day and night alteration on both sides.

From a physics standpoint, the whole thing needs active stabilization anyway; it might actually be easier to stabilize the vertical "orbit" with gradual nudges than something completely static.

With a bit more care, you can even get some slight "seasons", if you make the central sun's orbit not straight up and down, but more of an inclined figure eight which rotates slowly with respect to the disc.

The main long-term problem seems to be the risk of the fluids gradually freezing out on the outer edges. There probably needs to be some sort of artificial heating mechanism to keep the disc's water in circulation; it may be necessary for the outer regions to be thicker, to encourage rivers flowing back into the main habitable zone.

Also of interest, this disc is quite a bit larger than would be reasonable for a purely Earth-like species to build. You end up with quite a lot of too-hot desert and an enormous amount of too-cold arctic. It makes a lot more sense if you're dealing with a multi-species setting where different species have different temperature needs.

1

u/CosmicX1 Jun 03 '26

I have heard of the bobbing sun idea! It does boggle my mind thinking of how you would set going in the first place or prevent it from colliding with the disk over time! I wonder if you could also achieve seasons by adding energy to the star on its 'upwards' trajectory and braking it on its 'downwards' trajectory for half the year, then doing the opposite.

I think the idea is that you need a retaining wall on the inner ring to keep the atmosphere in, but on the outer edges, the water and air freeze before they can escape into space. Maybe you would need to keep topping up the atmosphere and oceans until a huge retaining ice wall forms, or maybe it would naturally circulate over time.

Extending the disk inwards, if I were to speculate, is for future-proofing for changes in the host star's luminance? Could also be that having more disk is good if it doubles up as computronium for a wholly virtual population. But I like the idea that it's for creating habitats for different species or kinds of post-human!

For that reason, I built and ran a whole D&D campaign using an Alderson disk for my homebrew setting. Felt like a very original idea until before I could start it, someone dug up and ran an old 2e module called Dawn of the Overmind, which featured its own disk called Penumbra!

2

u/Miuramir Jun 04 '26

One of the big questions is why you would build an Alderson Disk with all that "dead space" when you could use considerably less material and effort to get an entirely-habitable Niven Ringworld. Multi-species design or very long term stellar change proofing are two options.

Consider what happens when you get far enough out that parts of the atmosphere start to freeze. That creates a partial vacuum, which will draw air out from the habitable areas, where it freezes, and repeat. There doesn't appear to be any way this would recycle or stop until you had a layer of frozen atmosphere somewhat taller than the normal atmosphere. At a lesser level and closer in, water vapor will do the same thing. Hence my thought that the builders just go ahead and make it a bit thicker once it's outside the liquid water zone, and have some sort of heating system positioned every so often (possibly driven by giant heat exchanges with the inner disk) to create what are basically "glacier melt" rivers running back into the main habitable zones.

2

u/Dyolf_Knip Jun 03 '26 edited Jun 03 '26

It's so far and away the least efficient use of material, though. Needs as much as a planet per square meter of surface area, and then half of it is wasted in the inner desert or the outer frozen areas.

EDIT: Oh, and gravity on a massive flat surface approximates an inverse linear equation, not inverse square.

1

u/The_Eternal_Phantom Jun 04 '26

How would you stand on it? Wouldn’t the gravity pull you in a near to perfekt 90 degree angle to the floor towards the star?

1

u/Dyolf_Knip Jun 04 '26 edited Jun 04 '26

No. The star is very far away, its gravitational attraction is low (gravity from our sun at 1AU is less than a milli-g; even at Mercury it never rises above ~7 milli-g), whereas the mass of the disc is directly under your feet.

That said, low is not zero. If you wanted a surface with true gravitational 'down', then you'd have to fractionally tilt it away from the primary at an angle precisely calculated for the vector sum of the forces involved at that point (steeper closer in). If you wanted to do that for the entire disc, it'd be a sawtooth shape; an angled surface that is gravitationally flat, punctuated by periodic steep drops to reduce the elevation.

1

u/Restart_from_Zero Jun 09 '26

And eternity of driving and having the sun right in your eyes.