# Why Doesn't Tidal Energy Violate the Laws of Thermodynamics? **URL:** \ **Category:** Factual Questions\ **Created:** [December 16, 2006, 1:48am UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358 "2006-12-16T01:48:51Z")\ **Posts on this page:** 14\ **Page:** 2
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [December 16, 2006, 7:34pm UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/21 "2006-12-16T19:34:10Z")
[QUOTE=Cheesesteak] So, when I jump off the ladder, energy is created? [/QUOTE] No, it’s being _released_. You stored that energy in your body when you climbed up the ladder in the first place. The energy comes from your muscles, which get their energy from your food (which gets it from the Sun, which gets it…blah blah blah, _ad nauseum_), not from the Earth’s gravity. Gravity is the force which accelerates your body towards the center of the Earth, turning potential energy into kinetic energy. ---
**Author:** ![DanBlather](https://avatars.discourse-cdn.com/v4/letter/d/f4b2a3/32.png) [@DanBlather](https://boards.straightdope.com/u/DanBlather)\ **Post date:** [December 16, 2006, 7:40pm UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/22 "2006-12-16T19:40:33Z")
Interesting thread. I’m not sure about rotation being the source of tidal energy. If neither the earth nor the moon were rotating, but the moon still orbited the earth, then there would still be tides. Indeed the only way there cold be no tides is if the earth were to rotate at the same rate that the moon orbited the earth. In that case you could say that rotation \*prevented \*tidal energy. One could argue that the earth rotating once per-day increases the rate of tides from every two weeks to twice a day, and that effects the tidal energy. I’m not sure if that increases the total amount of energy or just changes its nature. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [December 16, 2006, 8:55pm UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/23 "2006-12-16T20:55:53Z")
[QUOTE=DanBlather] Interesting thread. I’m not sure about rotation being the source of tidal energy. If neither the earth nor the moon were rotating, but the moon still orbited the earth, then there would still be tides. Indeed the only way there cold be no tides is if the earth were to rotate at the same rate that the moon orbited the earth. In that case you could say that rotation \*prevented \*tidal energy. [/QUOTE] The energy that generates the ocean tides that flow into and out of coastal areas and harbors is due to the difference between the rotation of the Earth and the orbital period of the Moon (and to a lesser extent, the Sun). If the Earth were not rotating at all (tidally locked to the Sun), there would still be the orbital energy (although much less) between the Moon and the Earth. Given sufficient time, the Moon would lose the kinetic energy to hysteresis in the ocean and crust, slowing down (and via the perverse laws of orbital mechanics, speeding up), spiraling inward and eventually crashing into the Earth, though over a timespan that is much longer than the predicted lifetime of the Universe. If the Earth and Moon were tidally locked (always facing each other) there would be no tidal movement between the Earth and the Moon (though there would still be tides from gravity gradient and the velocity differential, just as the centrifugal force on someone on the outside of a merry-go-round is greater than someone on the inside). Every time a tide comes in, the Earth loses rotational momentum, and thus energy. (You can see this in the sound of the ocean crashing, the erosion of beaches and piers, and surfers who find the Big Wave.) Fortunately, the Earth has rotational momentum to spare. As **Q.E.D.** and **brossa** have already correctly and repeatedly noted, gravity itself is not “energy”; it is a force. Forces are, of course, the result of directed energy at some level (in the case of magnetism, by moving electrons, or in a steam engine, by the Van der Waals forces which cause the molecules to repell one another when heated). Where does this energy come from? It comes from distortions in the plenum of spacetime itself due to the presence of mass (per General Relativity). However gravity is conserved–that is, without actually disposing of mass, the total gravitational energy of a system can’t change; this is a direct result from applying Newton’s Laws of Motion. So the energy can’t come “from” gravity; gravitational attraction is merely the mechanism, the lever by which rotational energy of the Earth is extracted. Stranger ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [December 17, 2006, 1:25am UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/24 "2006-12-17T01:25:47Z")
[QUOTE=Bryan Ekers] …I was trying to be polite and avoid calling you completely out to lunch on this subject… [/QUOTE] You were saying…? ---
**Author:** ![DanBlather](https://avatars.discourse-cdn.com/v4/letter/d/f4b2a3/32.png) [@DanBlather](https://boards.straightdope.com/u/DanBlather)\ **Post date:** [December 17, 2006, 5:52am UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/25 "2006-12-17T05:52:45Z")
[QUOTE=Stranger On A Train] The energy that generates the ocean tides that flow into and out of coastal areas and harbors is due to the difference between the rotation of the Earth and the orbital period of the Moon [/QUOTE] Good explanation ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [December 17, 2006, 4:49pm UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/26 "2006-12-17T16:49:17Z")
[QUOTE=Q.E.D.] You were saying…? [/QUOTE] I stand by my earlier statements, including an effort at politeness that was specifically not returned. The electromagnetic analogy is problematic because while opposite charges have a gravity-like attraction, the equally important like-charges _repulsion_ has no analogous gravitational effect. Besides, you seemed to think I was calling gravity energy. I said _potential_ energy way back in my first post to this thread which you later supported with the ladder-climbing analogy. Cute alternate analogy: stirring cake batter in a large bowl. The batter is viscous and as the mixing spoon makes rapid circuits, the bowl, if not secured, will start to rotate in the direction of the stirring. Eventually, the bowl and the spoon will be rotating at the same rate, achieving “tidal lock”. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [December 17, 2006, 5:37pm UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/27 "2006-12-17T17:37:13Z")
[QUOTE=Bryan Ekers] I stand by my earlier statements, including an effort at politeness that was specifically not returned. [/QUOTE] And yet you managed to call me “completely out to lunch” while at the same time claiming you were be polite and avoid just that. Sorry, but that’s bullshit. Nowhere have I been impolite to you; if you perceived rudeness in my posts, that’s your problem, not mine. [QUOTE=Bryan Ekers] Besides, you seemed to think I was calling gravity energy. [/quote] You said tidal energy was “all gravity” way back in your first post, did you not? That statement is not correct, and is the statement I was responding to. In any case, I introduced the magnetism analogy as a way to illustrate the difference between a force and energy; in that respect, the analogy is an apt one. Naturally, if you try to carry the analogy too far, as you have done, it will break. All analogies will. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [December 17, 2006, 5:50pm UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/28 "2006-12-17T17:50:36Z")
[QUOTE=Bryan Ekers] I stand by my earlier statements, including an effort at politeness that was specifically not returned. The electromagnetic analogy is problematic because while opposite charges have a gravity-like attraction, the equally important like-charges _repulsion_ has no analogous gravitational effect. [/QUOTE] No, it’s not really problematic; while it’s true (as far as we have observed, although the principle of symmetry suggests otherwise) that there is no repulsive form of gravity; however whether the force is attractive or repulsive is not at issue. The energy stored in the field of a permanent magnet is conserved; that is to say, without breaking up the magnet there is no change in the respulsive or attractive forces delivered by the magnet. This is an exact analogy to mass and gravity. The energy that causes ocean tides (not the equatorial bulge) and a variety of other visible tidal effects comes from the rotation of the Earth, due to which any individual plot of land sees a varying gravitational attraction to the Moon throughout the day. If the Earth and Moon were tidally locked, so that the Earth showed no rotation with respect to the Moon, there would be no tides; just an egg-shaped cross-section with the peaks nearest and opposite the Moon. (Also, real estate on the Luna-facing side would be way more pricey than that on the non-facing side.) Whether the energy comes from the Earth’s (or Moon’s) gravity or whether is extracted from the rotation of the Earth via a change in rotational momentum isn’t a semantic argument; it’s a fundamental one. Gravity is conservative; in order to extract gravitational energy out of a planet, you’d have to break up and distribute the mass. Since that isn’t happening with Lunar tides, the stored energy comes from somewhere else; specifically the rotation of the planet with respect to the Moon. Stranger ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [December 17, 2006, 10:41pm UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/29 "2006-12-17T22:41:05Z")
Just wanted to pop in and point out that since the Moon is receeding from the Earth, the gravitational potential energy of the Earth-Moon system is _increasing_, not decreasing, so it can’t possibly be supplying the energy that drives the tides. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [December 17, 2006, 11:28pm UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/30 "2006-12-17T23:28:32Z")
[QUOTE=Q.E.D.] And yet you managed to call me “completely out to lunch” while at the same time claiming you were be polite and avoid just that. [/quote] Well, that you missed my earlier attempt to disengage with no loss of face on either side (“I’m willing to concede, however, that this comes down to how certain terms are defined”) surprises me not at all. > [@](#): > > You said tidal energy was “all gravity” way back in your first post, did you not? That statement is not correct, and is the statement I was responding to. Then we’re even because it was the off-the-cuff remark in _your_ first post (misleading or, to be charitable, grossly incomplete) that got this started. Ta. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [December 17, 2006, 11:43pm UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/31 "2006-12-17T23:43:44Z")
[QUOTE=Bryan Ekers] Well, that you missed my earlier attempt to disengage with no loss of face on either side (“I’m willing to concede, however, that this comes down to how certain terms are defined”) surprises me not at all. [/QUOTE] Well, not to carry this too far afield, but I had to address that earlier because it’s not a correct statement, either. You can’t pick and choose your definitions in science; if everyone is going to work together, we all have to agree on what things mean and how we define the terms we use to describe things. I’m sorry you don’t like my definitions, nevertheless, most scientists use these terms just as I have here and would agree with me that the source of energy was kinetic rotational energy, not gravity. I don’t know what else to tell you, but I’m sorry if somehow I’ve offended you. You seem to be focused on me, even though others are saying the same thing I did. Fine, I give up. It’s really not worth getting all worked up about. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [December 18, 2006, 12:43am UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/32 "2006-12-18T00:43:06Z")
[QUOTE=ZenBeam] Just wanted to pop in and point out that since the Moon is receeding from the Earth, the gravitational potential energy of the Earth-Moon system is _increasing_, not decreasing, so it can’t possibly be supplying the energy that drives the tides. [/QUOTE] Actually, the energy for this comes from the Earth’s rotation, too. See [thread=5780813]this thread[/thread] for more explanation and links. Stranger ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [December 18, 2006, 3:16am UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/33 "2006-12-18T03:16:30Z")
[QUOTE=Stranger On A Train] Actually, the energy for this comes from the Earth’s rotation, too. See [thread=5780813]this thread[/thread] for more explanation and links. Stranger [/QUOTE] Yes, the extra potential energy for the moon comes from the decreasing rotational energy of the earth. I always thought the tidal energy came from the moon’s gravitation. When the tidal bulge resulting from the moon’s gravity passes my location I trap some of the high water in a basin. Then when low water comes I let it out through a turbine and recover the energy supplied by the moon. ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [December 18, 2006, 3:53am UTC](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358/34 "2006-12-18T03:53:33Z")
If the Earth, and the Moon were truly not rotating, nor orbiting about each other’s mutual center of gravity there would be no tides. If they had been formed that way, and never rotated at all, but each remained fixed in orientation with respect to the universe at large, (an impossible occurrence, by the way) They might have tidal bulges as antique characteristics due to tidal forces on the coalescing gasses and liquids that formed them. But that dynamic would be long ago stabilized. No tides. Yet still there would be gravity. In fact, eventually that gravity would cause the three to collide, releasing a large amount of energy created by the application of the force of gravity upon mass, turning it into kinetic energy. So, gravity, without tides. Now suppose that there were rotation, and orbiting, but no gravity. (Yes, yes, this too is impossible) There would be tides! Nothing like the ones we have now, but something has to make the solid mass of earth turn, and something has to make it bend in its orbit around the sun. The sea will slosh around some, as will the atmosphere. Just plain old inertia would be the source of tides in this case, since there is no gravity, but it would be because of the rotational motion, and whatever mysterious force caused it. Since gravitation inevitably causes orbits, and rotations in bodies of astronomic size, gravity causes tides. It causes them by causing rotation. But it isn’t the force of gravity, it is the mathematical consequence of the fact that missing what you are falling towards is much more common than hitting it. It is inevitably more common, of course, so pretty much everything spins. Every thing falls, most things miss, so it all orbits. And it all has tides. Tris [Previous page](https://boards.straightdope.com/t/why-doesnt-tidal-energy-violate-the-laws-of-thermodynamics/384358.md?page=1)