Let's say an intelligent civilization of beings capable of building planetary superstructures created an artificial planet with a solid core capable of generating a gravity well and covered the surface with a combination of hydraulic turbines facing the same direction across the planet along certain paths and transmission towers capable of far-field electromagnetic radiation. They cover the surface in a liquid up to the turbines and place small, super dense satellites with their own gravity wells in low, stable orbits mimicking the paths of the turbines. For every satellite travelling East to West, there is another travelling West to East at a slightly different inclination to prevent rotation of the parent planet ever matching satellite orbit velocity. The satellites are also organized so they will never collide with one another or enter each others' gravity wells. The satellites cause waves which turn the turbines which generate electricity which is transmitted off the planet by the transmission towers. Assume that-

A: The machinery is maintained by autonomous drones capable of self service.

B: There is no cataclysmic interaction with extraterrestrial bodies.

C: All the construction materials and liquids involved don't chemically react to any of the others, regardless of state of matter.

D: The drones' power use does not exceed the power generated by the planet (derived from the fact that our hydraulic turbines and generators do not consume more resources than they produce).

Does this create a cycle of infinite energy generation that would eventually exceed the resources used to create it, or would the inertia of the satellites eventually be reduced by the opposing forces of the waves they pull, resulting in their deceleration and destabilization, causing them to crash down? Would the friction and solar radiation be able to exceed the liquid's heat loss from thermal radiation, causing it to eventually evaporate and gain enough energy as a gas to escape the parent planet's gravity well? Is this scenario truly a closed cycle with exception to energy output, or could there be other forces acting on it, even in unobservable magnitudes over millennia, from outside the bounds of the parent planet's gravity well? Is this a macro scale example of Newton's Third Law being broken, as has already been observed at the quantum scale in lasers?

  • $\begingroup$ Would Worldbuilding be a better home for this question? $\endgroup$ – Qmechanic Dec 1 '17 at 17:17
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    $\begingroup$ Perpetual motion machines are an epistemic impossibility. That is to say they absolutely cannot occur within our current formulation of the physical laws. If you want to read more about the subject I suggest reading the article about them on Wikipedia. If you want an explanation for why this particular machine is impossible that's a question about physics not worldbuilding. $\endgroup$ – sphennings Dec 1 '17 at 19:28
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    $\begingroup$ This looks like a network of tidal energy power stations. Tidal energy comes from planet + satellite rotational energy. $\endgroup$ – Alexander Dec 1 '17 at 19:28
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    $\begingroup$ are you aware that "gravity wells" do not have borders, right? Your statement "their gravity wells won't interact" is true only if they are at infinite distance... $\endgroup$ – L.Dutch - Reinstate Monica Dec 1 '17 at 19:30
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    $\begingroup$ @user177182 I don't know. I'd suggest reaching out to a moderator on Physics to find out. $\endgroup$ – sphennings Dec 1 '17 at 19:54

would the inertia of the satellites eventually be reduced by the opposing forces of the waves they pull, resulting in their deceleration and destabilization, causing them to crash down?

This one. Even though there's may be no atmospheric friction slowing down the satellites, they're still losing energy as they orbit the planet.

Tidal heating (also known as tidal working or tidal flexing) occurs through the tidal friction processes: orbital energy is dissipated as heat in either the surface ocean or interior of a planet or satellite.


Of course, if you have a planet and a moon that are completely tidally locked, like Pluto and Charon, they don't lose any momentum in this way. The same side of Pluto always faces Charon, and vice versa, so they act like a single connected object spinning in space. Of course, they aren't producing any tidal heating or other harvestable energy either.


Check out Tidal Acceleration/Deceleration.

In real life, the moon is getting further away from Earth by tidal acceleration. This is because the Earth is rotating faster than the moon spins around the Earth. Some of the energy of the Earth's rotation is transferred to the moon through it's gravitational effect on the oceans. We can (and do) get power from the tides, as in your proposal, but it's not free energy. We're actually stealing some of the Earth's rotation!

Now, in at least one direction, your satellites will rotate faster than the spin of the planet. In that case, they won't experience tidal acceleration, but deceleration. It makes sense -- the satellites can't move the oceans for free. The energy to move the oceans is stolen from the satellite's momentum. Their orbit will get lower and lower until they crash into the surface of the planet.

So, no, this isn't a way to create perpetual motion. However, tidal power is a way to convert some of the energy of a planet or moon's rotation to a more usable form, and that's pretty cool.

How much power can we get? You could imagine a hyper-advanced culture trying to use tidal power to adjust the rotation of a planet. Say one day we have oceans on Mars and want to adjust it to have an exactly 24 hour day. That requires stealing energy from its moons to speed up the rotation of the planet.

We can calculate the difference in energy of a hypothetical Mars with a 24 hour period vs the current Mars.

KE = Iw^2, where I is the moment of inertia, and w is the angular velocity
I = fmr^2, where f is the moment of inertia factor, m is mass, and r is radius
m = 6.39 x 10^23 kg
r = 3.389 x 10^6 m
f = 0.3662
I = 2.69 x 10^36 kg m^2
w_now = 1.1514 x 10^-5 per second
KE_now = 3.56 x 10^26 J

w_target = 1.1574 x 10^-5 per second
KE_target = 3.60 x 10^26 J

So we need to give Mars 4 x 10^24 J of energy by stealing energy from the moons through tides. Unfortunately, Phobos, the larger moon, only has about 1.24 x 10^21 J of energy in its rotation. For comparison, the annual global energy consumption is only about 5 x 10^20 J.


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