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Learning Physics With Ringworld (2010)

25 points by optimalsolver 4y ago

jmyeet 4y ago
Weird. The key physics you should learn is that Niven’s Ringworld would require a material so strong we can’t even theorize about it. A Ringworld around our Sun at Earth’s distance with Earthlike gravity would rotate at (IIRC) 1.5 million kmh. The centrifugal forces would tear our apart. A Dyson Swarm remains the most promising megastructure for creating efficient living area (per unit mass).
- nosianu 4y ago
  It's also terrible for failure modes. Planets are independent of one another, this ring is one fragile entity. The largest destruction of life by a single unfortunate event in the entire universe made possible by one design decision. We know it's not indestructible (and can't be unless it's a fantasy novel with "magic"), I only read the first book and there were two large holes even in the tiny section they explored. Whatever happens to that hole in the ring that they find when they first encounter and land - crash - on the ring? Air is constantly lost through it. Not the one with the ultra-high "mountain" near where they crashed, the other one further along their route with the weather pattern caused by this air-loss storm. How is that sustainable - given the ultra-long time horizon, surely billions of years, that such a structure should be able to support life for (if a planet can do that much)?
  - geon 4y ago
    The lost air through the hole was not sustainable. The entire ring had fallen into disrepair. That was a major plot point. The terrible failure mode was typical for the extreme arrogance of the ringworld engineers.
  - public_defender 4y ago
    They blow up an orbital ring structure in Consider Phlebas and it definitely leaves one wondering who thought it was a good idea to build it in the first place.
- greenbit 4y ago
  Iirc the Ring was made of 'scrith', which was probably Protector for what we now call unobtanium.
  - BizarroLand 4y ago
    To be fair, materials sciences are still advancing to this day, so even though no base element contains a measurable fragment of scrith's material properties, it's not entirely infeasible that no alloy in the universe could approach it.
- WalterBright 4y ago
  > The centrifugal forces would tear our apart. I'm not so sure about that. Consider slicing the ring into sections. If the ring is at the same radius and velocity of an object in orbit, then the slices will (mostly) stay put. They aren't going to fly off radially. So connecting them will not put undue stress on them. I say mostly because they'll be tugs from the other planets, etc.
  - sliken 4y ago
    I think you are missing that the gravity comes from the centrifugal forces. Said gravity is for humans to be able to walk around, as well as keeping air and water from being lost to space. So sure you could make a non-spinning ring that minimizes the tension, velocity, and centrifugal force, but it would not provide a large living area like the ringworld does.
    - WalterBright 4y ago
      You're right, I missed that. Oops!
    - jojobas 4y ago
      I don't get it. There should be 1g of centrifugal force. (This requires kilometers of atmospheric altitude to keep the air, but that's beside the point). What would tear us apart?
      - consilient 4y ago
        Hoop stress scales with the radius too, since there's more material pulling on itself. You're looking at many trillions of pascals for an Earth-equivalent ringworld.
        moloch-hai 4y ago
        Way, way more than that.
      - sliken 4y ago
- sliken 4y ago
  So maybe it would work with a red dwarf with much less gravity and brightness. As a bonus the star would last 10-1000 times longer.
  - deleted 4y ago
    [deleted]
greenbit 4y ago
Wondering about the calculation for stability, whether solar radiation pressure was factored in. Surely something of that scale would behave partly like a solar sail. If the center was off the sun by, say, half an AU it'd seem like gravitational effects would have to dominate, but what about if the center were off by only some small amount? There could be a threshold under which radiation pressure would dominate and tend to stabilize the ring. But then I suppose even if that were the case, it'd only take a well aimed flare to put a wrench in that.
- sliken 4y ago
  Sure, it might stay centered on the star if it started centered. Any mild disturbances would cause the closer parts of the ring to receive more solar wind and light pressure while the more distant parts of the ring would receive less. However I expect that the momentum in the ring is many orders of magnitude higher than the light pressure, so it might well just destabilize anyways. However it's a 3D system, what if the plane of the ring didn't perfectly bisect the center of gravity of the star? The feedback loop would cause things to get worse, since there's no counter pressure.
optimalsolver 4y ago
The scale of Ringworld: https://www.youtube.com/watch?v=sR2296df-bc https://www.youtube.com/watch?v=sR2296df-bc
kordlessagain 4y ago
If anything, this book seated the idea in my mind that "pseudo gravity" is the exact same thing as "gravity". See Newton's bucket argument... https://en.wikipedia.org/wiki/Bucket_argument#:~:text=Newton%20discusses%20a%20bucket%20(Latin,to%20the%20water%20it%20contains https://en.wikipedia.org/wiki/Bucket_argument#:~:text=Newton....
moloch-hai 4y ago
I would rather treat Orbitals, sort of like in The Culture but sans doubletalk magic. I once calculated a ring (IIRC) a half-million km in radius, orbiting the sun in place of Earth (or opposite it, if you like), could spin at 1G in 24 hours, but would also need to be made of scrith. The interesting result was that the tension would be the same as supporting a quarter-length at the target acceleration, hanging from one end. So, for mine you need a material that can support a straight length cable 0.5M*pi/2 km long hanging in a uniform 1G field, or (equivalently) while accelerating it at 1G pulling on that end. Of course the cross-sectional area cancels out, so you can just talk about how strong the material is. Steel can support quite a few km. Kevlar can support hundreds of km. Idealized carbon fiber might support, what, ten thousand km? A ten-thousand km orbital would need to spin too fast to give you a 24-hour day, and could not keep the air in without a lid, but ought to be interesting enough to work on, in-principle buildable. Someday.

25 points by optimalsolver 4y ago

Weird. The key physics you should learn is that Niven’s Ringworld would require a material so strong we can’t even theorize about it. A Ringworld around our Sun at Earth’s distance with Earthlike gravity would rotate at (IIRC) 1.5 million kmh. The centrifugal forces would tear our apart. A Dyson Swarm remains the most promising megastructure for creating efficient living area (per unit mass).

nosianu 4y ago
It's also terrible for failure modes. Planets are independent of one another, this ring is one fragile entity. The largest destruction of life by a single unfortunate event in the entire universe made possible by one design decision. We know it's not indestructible (and can't be unless it's a fantasy novel with "magic"), I only read the first book and there were two large holes even in the tiny section they explored. Whatever happens to that hole in the ring that they find when they first encounter and land - crash - on the ring? Air is constantly lost through it. Not the one with the ultra-high "mountain" near where they crashed, the other one further along their route with the weather pattern caused by this air-loss storm. How is that sustainable - given the ultra-long time horizon, surely billions of years, that such a structure should be able to support life for (if a planet can do that much)?
- geon 4y ago
  The lost air through the hole was not sustainable. The entire ring had fallen into disrepair. That was a major plot point. The terrible failure mode was typical for the extreme arrogance of the ringworld engineers.
- public_defender 4y ago
  They blow up an orbital ring structure in Consider Phlebas and it definitely leaves one wondering who thought it was a good idea to build it in the first place.
greenbit 4y ago
Iirc the Ring was made of 'scrith', which was probably Protector for what we now call unobtanium.
- BizarroLand 4y ago
  To be fair, materials sciences are still advancing to this day, so even though no base element contains a measurable fragment of scrith's material properties, it's not entirely infeasible that no alloy in the universe could approach it.
WalterBright 4y ago
> The centrifugal forces would tear our apart. I'm not so sure about that. Consider slicing the ring into sections. If the ring is at the same radius and velocity of an object in orbit, then the slices will (mostly) stay put. They aren't going to fly off radially. So connecting them will not put undue stress on them. I say mostly because they'll be tugs from the other planets, etc.
- sliken 4y ago
  I think you are missing that the gravity comes from the centrifugal forces. Said gravity is for humans to be able to walk around, as well as keeping air and water from being lost to space. So sure you could make a non-spinning ring that minimizes the tension, velocity, and centrifugal force, but it would not provide a large living area like the ringworld does.
  - WalterBright 4y ago
    You're right, I missed that. Oops!
  - jojobas 4y ago
    I don't get it. There should be 1g of centrifugal force. (This requires kilometers of atmospheric altitude to keep the air, but that's beside the point). What would tear us apart?
    - consilient 4y ago
      Hoop stress scales with the radius too, since there's more material pulling on itself. You're looking at many trillions of pascals for an Earth-equivalent ringworld.
      - moloch-hai 4y ago
        Way, way more than that.
    - sliken 4y ago
sliken 4y ago
So maybe it would work with a red dwarf with much less gravity and brightness. As a bonus the star would last 10-1000 times longer.
- deleted 4y ago
  [deleted]

greenbit 4y ago

Wondering about the calculation for stability, whether solar radiation pressure was factored in. Surely something of that scale would behave partly like a solar sail. If the center was off the sun by, say, half an AU it'd seem like gravitational effects would have to dominate, but what about if the center were off by only some small amount? There could be a threshold under which radiation pressure would dominate and tend to stabilize the ring. But then I suppose even if that were the case, it'd only take a well aimed flare to put a wrench in that.

sliken 4y ago
Sure, it might stay centered on the star if it started centered. Any mild disturbances would cause the closer parts of the ring to receive more solar wind and light pressure while the more distant parts of the ring would receive less. However I expect that the momentum in the ring is many orders of magnitude higher than the light pressure, so it might well just destabilize anyways. However it's a 3D system, what if the plane of the ring didn't perfectly bisect the center of gravity of the star? The feedback loop would cause things to get worse, since there's no counter pressure.

optimalsolver 4y ago

The scale of Ringworld: https://www.youtube.com/watch?v=sR2296df-bc https://www.youtube.com/watch?v=sR2296df-bc

kordlessagain 4y ago

If anything, this book seated the idea in my mind that "pseudo gravity" is the exact same thing as "gravity". See Newton's bucket argument... https://en.wikipedia.org/wiki/Bucket_argument#:~:text=Newton%20discusses%20a%20bucket%20(Latin,to%20the%20water%20it%20contains https://en.wikipedia.org/wiki/Bucket_argument#:~:text=Newton....

moloch-hai 4y ago

I would rather treat Orbitals, sort of like in The Culture but sans doubletalk magic. I once calculated a ring (IIRC) a half-million km in radius, orbiting the sun in place of Earth (or opposite it, if you like), could spin at 1G in 24 hours, but would also need to be made of scrith. The interesting result was that the tension would be the same as supporting a quarter-length at the target acceleration, hanging from one end. So, for mine you need a material that can support a straight length cable 0.5M*pi/2 km long hanging in a uniform 1G field, or (equivalently) while accelerating it at 1G pulling on that end. Of course the cross-sectional area cancels out, so you can just talk about how strong the material is. Steel can support quite a few km. Kevlar can support hundreds of km. Idealized carbon fiber might support, what, ten thousand km? A ten-thousand km orbital would need to spin too fast to give you a 24-hour day, and could not keep the air in without a lid, but ought to be interesting enough to work on, in-principle buildable. Someday.