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An O'Neill Cylinder (as defined by O'Neill, per Wikipedia) has two cylinders which are both "5 miles (8.0 km) in diameter and 20 miles (32 km) long, connected at each end by a rod via a bearing system".

Ignoring the financial cost, assume the thing got built with materials that can be created today and got launched into an extremely high orbit of Earth (say, Earth/Moon's L5 point). It spins up, gravity (1g) is felt by all (on the surface), and people living on it for 100 years or so (as self-sufficiently as possible).

Then, something bad happens, resulting in the station's permanently losing the ability to fire/power/run the engines that keep the cylinders spinning.

The bearing system connecting the cylinders is as friction-less as can be made usable in space long-term today, and the engines in their nonfunctional state don't add any friction between the cylinders.

On the one hand, it seems like the station shouldn't slow down provided that there was no net movement of mass (and that the movement was "balanced" between spinward and antispinward movement - if the trains all move antispinward, spin would increase). On the other hand, that sounds like a perpetual motion machine, since the force pushing people "down" has to come from the spin of the station (compared to Earth's gravity where "down" is caused by deformations in space, which doesn't happen on the station to an appreciable extent). One hand must be wrong, right?

How long would the inhabitants feel 1g, more than 0.9g, and more than 0.166g (the moon's gravity, again, per Wikipedia)? Or, would the station spin forever, aside from minor tidal forces and friction on the bearing (and micrometeorite impacts, etc., etc.)?

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closed as unclear what you're asking by Gert, Jon Custer, hft, Dvij Mankad, Kyle Kanos May 14 at 13:01

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ It doesn’t take an engine to keep the Earth spinning, so.... $\endgroup$ – G. Smith May 11 at 1:49
  • $\begingroup$ I don't know how to start approaching this question other than "on the one hand, what's stopping the spinning other than the bearing system; on the other hand, there's "stuff" that's feeling a force which suggests an energy expenditure that has to come from somewhere". $\endgroup$ – minnmass May 11 at 3:15
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    $\begingroup$ The Earth does not expend any energy keeping you on its surface with its gravitational force, so why do you think that forces require an energy expenditure? $\endgroup$ – G. Smith May 11 at 3:28
  • $\begingroup$ I'm looking for how to clarify my question; some thoughts: on the one hand, it seems like the station shouldn't slow down provided that there was no net movement of mass (and that the movement was "balanced" between spinward and antispinward movement - if the trains all move antispinward, spin would increase). On the other hand, that sounds like a perpetual motion machine, since the force pushing people "down" has to come from the spin of the station. Compared to Earth's gravity where "down" is caused by deformations in space, which doesn't happen on the station. One hand must be wrong, right? $\endgroup$ – minnmass May 16 at 17:21
  • $\begingroup$ It’s “perpetual motion” in the same sense that Newton’s First Law is perpetual motion. Without friction or other forces, objects simply continue moving in the future the way they were moving in the past. When people talk about perpetual motion machines, they really mean a machine from which an unlimited amount of energy can be extracted, which is something quite different and impossible. The bad terminology is unfortunate. $\endgroup$ – G. Smith May 16 at 17:33
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Energy needs to be expended in order to get the two cylinders spinning. After they have reached their design speed, energy needs to be provided only to compensate for any friction that occurs in the bearings. The rotational kinetic energy of the cylinders would be enormous. The efficiency of the bearings could be very, very good if, for example, they were magnetic bearings and the two cylinders were perfectly balanced. While conventional magnetic bearings have a small but significant amount of energy loss, superconductive bearings have essentially no energy loss other than the energy required to keep them cold enough to remain superconductive. In space, with a background temperature of a few degrees K, this energy loss should be almost completely avoidable.

So, once the O'Neill cylinders have been spun up, they should continue spinning at the same speed essentially forever (assuming no motion of the masses - like people living there - and no changes in the total masses of the cylinders).

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