When we harness tidal energy from the moon, what energy source is depleted?

According to M theory, from another universe. Since gravity is “leaking” into our universe I would think the source universe would feel the effects of thermodynamics.
But I could be wrong. Great Question.

How on earth are we gonna do that?!!

Wouldn’t gravity, along with the other three forces in nature be subject to entropy? Wouldn’t that dictate total decay and chaos in the distant future?

Great question Ben, that’s why I think the collective should sign this!

Lunar gravity moves the water. Moving water spins a turbine (or something). The energy from the water would move into the turbine, yes? It would lose inertia, since friction acted upon the moving water, “stealing” its energy.

I’m no scientist, just using the two brain cells I have to rub together and seeing what heat I can make with them. :)

Too funny aanuszeki1.
I have no idea. I think Ben is just trying to raise the bar on the quality of content here.

That link reminds me of this skit from Mr. Show.

Dammit. I can’t get the link to work. Thought I typed in everything correctly. Wish the iPhone had cut and paste.
Just go to YouTube and do a search for “america blows up the moon” and maybe somebody can put a link up for me.

It seems to me that laureth is correct. Astrochuck is imaginative but this question doesn’t require that kind of imagination, if one is satisfied that the energy that put the matter and energy into the conditions that resulted in the solar system, don’t need to be explained.

That is, the energy already is there in the form of matter, motion, and the forces that keep matter together and in the case of gravitational forces, gravitating towards each other. So the moon tugs on things on Earth, and we can build devices that generate electricity by resisting that motion. What is lost, is whatever other forces would have applied to that matter if we hadn’t built it into such a machine, or at least, the component of energy that we extract and that we convert into other kinds of energy (probably friction).

Here you go AC

Thanks. Lurve to you.

I have to disagree, Zaku. Tidal forces are caused by gravity. The friction is a result of the force of gravity. Thus the energy being depleted must come from gravity.

If there is a bowling ball on top of the Leaning Tower of Piza, it contains potential energy. If the bowling ball falls down, it has kinetic energy. When the ball hits the ground, that kinetic energy leaves the ball and escapes into the ground.

What energy source is depleted here? The kinetic energy of the ball. Even though it gets the energy from gravity pulling it toward the Earth, the energy is still in the ball (as it falls), not in gravity (which is not a thing that can store energy).

Same with the tides. The energy depleted is the kinetic energy of the water.

I’m not sure that the question is well-formed. The energy we take away from the tides is the kinetic energy of the water as we reduce its movement. The law of conservation is preserved.

If the question is about where the water’s kinetic energy is coming from, then I don’t think tidal power is a necessary part of the explanation. In that case, the question is basically how does gravity work? That is, the problem is that gravitation is a force that seems to create energy in the form of tides. We know energy can’t be created, so when water gains energy in the form of tides, what is losing energy?

That’s an interesting problem, and here‘s a physics forum dealing with it in ways that I don’t quite understand. Their answer is that due to gravity, “Earth’s rotation is slowing, and the moon is actually being raised into a higher orbit around the earth” (over a very, very long period of time). That means that the moon’s orbit around the earth transforms (via gravity) the kinetic energy of the Earth’s rotation into tidal movement.

However, if we harness tidal energy, we’re not increasing the amount of gravitation between the moon and the earth, so we won’t slow the earth any more quickly. Gravity is going to exist regardless; using tidal energy will just mean that on our end the energy created by gravitation will be used as electricity rather than water movement.

Well, I can’t argue with that, you guys. Still, the energy created by gravity has to deplete the source, doesn’t it? Are gravity, electro-magnetism, and the two sub-atomic forces immune to thermodynamic law?

AstroChuck: Humble mailman by day, Astrophysicist by night!
Now I finally understand what the “astro” in your username signifies

Now we know AstroChuck is not just a pretty face.

Hardly. You give me too much credit.

Gravity isn’t an object which gets depleted – it’s an observed behavior of matter. It’s how matter and energy behave over time. Matter and energy interact in certain ways and change form and location over time depending on how they are arranged. Energy is not created by gravity, as you say. Matter and energy simply change form as they interact with each other. There is potential energy in a situation, in that matter can interact in a way that the type of energy can move or change. In the case of gravity, the bodies are attracted to each other, which changes their state of motion over time, and a clever ape can build a machine to convert some of that into electricity and heat if he wants to, which will result in the motion being less than it otherwise would be, in general.

Note that I suppose it is theoretically possible that gravity may wear down with “use”, but if so, it’s so minute that the rate has not been observable. It’s just as possible that it increases with use, or increases or decreases over time, or the farther we get from Andromeda, or whatever, but it seems to be mostly constant.

By that reasoning you’ve just made it possible to create perpetual motion.

You mean like an orbit? ;-)

Oh snap!

I don’t see the comparison. An orbit is not an example of perpetual motion. It’s driven by a gravity well. Over time, albeit a long, long, long time, the well has to give up its energy. Are you saying the four forces in nature are eternal or am I missing what you are trying to say? I am just a postal worker, after all.

AstroChuck, aren’t you talking about cosmological theory at a time scale so extreme that you are talking about an unmeasurable effect that will only be relevant after the sun has transitioned to anther stage, and involves something like M theory’s other universe?

I have to oversimplify theory (ADD, sorry), so I put together this:

The moon is roughly 1/4 of the size of earth, big enough to be considered a “sister planet.” The orbit of the two planetary bodies is creating an orbital/rotational “gearing” effect, simultaneously speeding up the orbital/rotational velocity of the moon and slowing the earth’s rotation down. Of course, this happens on e minute, fractional (cosmically speaking) level, with any effective result visible after eons of activity.

So in a nutshell; harnessing of tidal energy, as a byproduct of a massive cosmic energy transfer, has the minute fractional end-result of slowing the earth’s orbit to a standstill (and most likely destroying all life as we know it in the process).

But as a glimmer of hope; the moon is getting farther and farther away (about 3 and a half centimeters per year) so maybe it’ll be too far away to have any effect at that time. AND, as we’ll have evolved into something else by then, perhaps we won’t even care.

My brilliant physics consultant says Knotmyday has got it. The water does work on a turbine affixed to the earth, which exerts a torque opposing the direction of the earth’s rotation, slowing the rotation just a bit.

Does it have to be the moon which receeds from the earth, and the earth which falls towards the sun? Could it be the other way around? Or does this depend on which object is larger (or smaller) than the other?

The moon, orbiting earth, would fall towards the sun, as well, even though it’s orbit around the earth was getting ever higher. Would there be a point at which the attraction between the earth and moon was no longer strong enough to keep the moon in orbit? Then which way would the moon fly away? Would it be the opposite direction from the son (where sun-assisted gravity pull is the least)?

And while were at it. Why do dust clouds around a celestial object always accrete into a single plane?

Okay, but if we didn’t harvest the tidal energy and just let it play out, would the end result be any different? Wouldn’t the energy just disperse other ways?

At first, I thought he was talking about more local, environmental consequences. Like the tides being lower, and fish habitats changing, etc., etc.

Sigh. You won’t slow down the planet by generating some electricity from tides, unless you only do it in one direction. To slow down planetary rotation, try a gyroscopic generator, and good luck measuring the effect. These are not going to do anything measurable to planetary rotation.

Juicing up on the tide slows down the tide a weeee bit. Good luck stopping it. I tried that when I was 11, by having me and a friend throw rocks at incoming waves. Annoyed the folks who had built the breakwater, but the tide remains unharmed. Also, even if you built a Death Star with a tractor beam big enough to stop the tide entirely, as soon as you stopped doing that, the tide would start up again, due to gravity, which is not going to run out even before the copyright on Star Wars does. If you want to cause environmental damage by messing with the tide, fire the main battery of the Death Star at the moon, and turn it into sparkle effects. That’ll stop the tide for you, assuming it’s vaporized or the debris has escape velocity on its own mass.

We are talking about the Conservation of Angular Momentum.

my first thought was the power of cheese…

pardon, i’m a silly goose today ;p

I think it would ultimately come from the energy contained in the moon’s inertia. The moon has SO MUCH inertia, though, that I think it would be the equivelant of putting a feather in front of a charging rhino.

Anyone agree?

@aanuszek1 ROFL!! I signed it! (I didn’t know the moon was gay!!)

@ben I agree and have the same points of reasoning as @laureth and @Zaku
finally, we are harvesting the kinetic energy of the water! its really that simple and clear, isn’t it?