# How long would “artificial gravity” remain on an O'Neill cylinder without power? [closed]

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.)?

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

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• It doesn’t take an engine to keep the Earth spinning, so.... – G. Smith May 11 at 1:49
• 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". – minnmass May 11 at 3:15
• 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? – G. Smith May 11 at 3:28
• 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? – minnmass May 16 at 17:21
• 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. – G. Smith May 16 at 17:33