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Conservation of energy says that any energy extracted from our ocean's tides would have to come from somewhere. Since these tides are a gravitational effect due to the moon, it would seem that the extracted energy's source would be that gravitational effect, hence any removal of energy from the tides would have an effect on the Earth/moon orbit's energy balance and alter that orbit. So the question arises, what would the long term effect of such energy extraction have over many, many millennia?

Edit: It's unfortunate that my earlier searches on this topic here did not produce the links that came up under "Related Links" after I posted this question as it seems this is almost a duplicate question. I'll say almost here because I did not include in the question my motivation for posting this question now. I was recently reading the paper Physics in the Real Universe: Time and Spacetime by George F R Ellis of Mathematics Department, University of Cape Town, (which is a very good read by the way), where he states

"But the whole point of this paper is that most models are not deterministic, irreversible unpredictable processes and emergent properties will take part in determining space time curvature; on relatively small scales, even human activity does so (when we move massive objects around)."

The Earth/Moon system itself is probably the most massive object that we could move around, and using tidal generators would do just that! Think about it. Changing the Earth/Orbit would change the local curvature of spacetime (as pointed out by Ellis), which will have an eventual effect on neighboring star systems. So humanity does influence other worlds in our galaxy, more so than just sending information out as radio/TV signals! It's a bit aw inspiring (and humbling) to realize that we can alter the curvature of spacetime in our galaxy, is it not?

Anyway, his mentioning "moving massive objects" brought to mind this question, so I posted it.

Edit 2:

It seems that I eliminated too much detail from my original (off line) writeup for this question before I posted it (there was originally over two pages of verbiage!). Here I will put some of it back, the most important of which is I'm looking for the long term (both global and local) effects from placing an electrical power generator in some specific location. For example:

The tides in the local vicinity of a generator would be effected by its presence, as the generator would extract some of the energy that would have kept the tide as it was before the generator (an effect I know first hand after a large reservoir was constructed upwind of my family's ranch. It reduced our annual rainfall by more than 40% as noted in over 60 years of records!). The generator could also extract energy at a higher rate (locally) than the original frictional losses. Does this altered flow affect the local gravitational interactions with the moon? What would be the long term effects?

While extracting power during tide flow, the generator would apply forces to the Earth where it was attached to the Earth. These would provide a push/pull force to any nearby geologic fault. If these 'tickling' forces induced a geologic event (an Earthquake or volcanic eruption for example) there could be large scale changes in the distribution of solid earth. What long term effect would this have on the Earth/moon orbit?

The controversies surrounding the "2 bulges" description has been a surprise to me, hence I find myself in "catch up" mode. My mental picture has been that put forward by the NOAA page here which is also reflected on Wikipedia page, Tide. The Wikipedia page Theory of Tides provides a good alternative description using harmonic analysis which includes several good external links which I've been trying to digest. For example, the paper Understanding Tides by Steacy Hicks provides a very readable identification of the primary frequencies involved in a harmonic analysis of the problem and A Manual of the Harmonic Analysis and Prediction of Tides by Paul Schureman provides an historic look at the application of harmonic analysis applied to tides.

I don't understand or agree with this controversy at all! The "2 bulges" picture is decidedly an idealized model not directly useable to predict tides. It is just that, an idealized model. It does, however, provide insight into many aspects of the problem, as well as providing actual numbers that are required for a harmonic analysis, specifically the dominant frequencies. This tells me that the "2 bulges" model is fundamentally required, hence it being implied that the "2 bulges" paradigm is not true is going too far.

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  • $\begingroup$ Check out the following link. large.stanford.edu/courses/2010/ph240/chenw1 $\endgroup$ – Bob D May 31 at 18:56
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    $\begingroup$ Various related links: physics.stackexchange.com/q/6400/520 physics.stackexchange.com/q/77606/520 physics.stackexchange.com/q/267550/520 (and other links therein). Not sure if these are close enough to qualify as duplicates, but they certainly touch on the same issues. $\endgroup$ – dmckee May 31 at 19:04
  • $\begingroup$ A closely related question: will extracting power from ocean tides affect the rotation rate of the earth? $\endgroup$ – David White May 31 at 19:44
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    $\begingroup$ Energy is already extracted from the tides, by nature. The tides cause currents, which lead to turbulence, which results in heating. And yes, it affects the Earth's rotation and the Moon's orbit. $\endgroup$ – S. McGrew May 31 at 20:43
  • $\begingroup$ @BobD Thanks for the link, an interesting paper, and the references it provided were also useful. $\endgroup$ – OneMug Jun 2 at 15:21
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The oceans tides are not a perpetual motion machine that can be "disrupted" by human intervention. In simplest terms, they are caused by the dragging of the Earth's rotation as it spins inside the gravitational field of the moon. Since the Earth rotates 30 times faster than the moon orbits, this means that the tides move across the earth in regular cycles.

enter image description here

Ultimately the energy for this comes from the rotation of the earth. Overtime the earth spins 17 microseconds slower per year while the moon gains orbital energy making it recede from the earth at 4cm per year. Given enough time, the Earth and the Moon will become tidally locked, and all tides will cease.

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  • $\begingroup$ Please don't use two tidal bulge image. It's not accurate. See this question $\endgroup$ – M. Enns Jun 1 at 2:39
  • $\begingroup$ I updated to use the force vector only image. $\endgroup$ – Benjamin Jun 1 at 16:48
  • $\begingroup$ Thanks for providing this answer. This question hinges on conservation of energy, so how does perpetual motion enter? However, I must take issue with your statement "Ultimately the energy for this comes from the rotation of the earth." The gravitational forces in the Earth/moon system must do work ($W=F\cdot d$) to maintain the tidal bulge to compensate for the work done by the frictional force between Earth/bulge as Earth's rotation tries to drag that bulge away, so ultimately, the energy comes from the gravitational field, not Earth's rotation. $\endgroup$ – OneMug Jun 2 at 14:58
  • $\begingroup$ Tides are not static. They go up and down twice a day meaning you can extract energy from them (energy otherwise lost to friction) That energy comes the rotation of the earth. $\endgroup$ – Benjamin Jun 2 at 16:18
  • $\begingroup$ I still have problems with your statement "you can extract energy from them ... That energy comes the rotation of the earth". Show me how this should be true. $\endgroup$ – OneMug Jun 3 at 14:20

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