[HN Gopher] Learning Physics With Ringworld (2010)
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       Learning Physics With Ringworld (2010)
        
       Author : optimalsolver
       Score  : 11 points
       Date   : 2023-01-15 18:59 UTC (2 days ago)
        
 (HTM) web link (www.tor.com)
 (TXT) w3m dump (www.tor.com)
        
       | greenbit wrote:
       | 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 wrote:
         | 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 wrote:
       | The scale of Ringworld:
       | 
       | https://www.youtube.com/watch?v=sR2296df-bc
        
       | jmyeet wrote:
       | 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).
        
         | WalterBright wrote:
         | > 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 wrote:
           | 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 wrote:
             | You're right, I missed that. Oops!
        
         | sliken wrote:
         | 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.
        
         | nosianu wrote:
         | 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)?
        
         | greenbit wrote:
         | Iirc the Ring was made of 'scrith', which was probably
         | Protector for what we now call unobtanium.
        
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