# Is gravity an infinite source of energy?

1. Aug 15, 2010

### Xtensity

I was thinking about Hydroelectric generator turbines which are built underwater to utilize the energy of the waves/current.

I was thinking that if the waves are generated by the moons gravitational pull..... would the presence of the moon cause the waves to continually being made, resulting in a seemingly infinite source of energy?

Perhaps this is the wrong section of the forum, but is gravity an infinite force since it is created by the presence of Mass? Whether it's gravitons doing it or simply bends in the Space-Time Continuum.

Ideas???

2. Aug 16, 2010

### Staff: Mentor

No, gravity is not an infinite source of energy.

TANSTAAFL

Using tidal waves for electricity generation slows down the Earth rotation.

3. Aug 16, 2010

### Xtensity

You did not address the question though.

I have discussed the idea of robbing the earths kinetic energy as a source of energy many times, and yes it would slow the earths rotation.

I said nothing about tidal waves because they're utilizing energy from tectonic plate movement, which if we used that for electrical energy the earths rotation would slow. I am talking about normal waves. Waves not generated by tectonic plate movement.

Waves generated as a direct result from wind AND the gravitation pull of the moon.

Does some of the energy in waves not come from the moons gravitational pull? If not then why do waves get larger and more powerful(aka more energy in them) during full moons or at night.

Meaning if gravity can increase the energy of the tides, then why can the gravitational energy of the moon not be utilized to generate some kind of energy?

I actually am thinking you didn't even read the thread because based upon your response, it seems you formed your response after seeing the words "infinite", "Gravity", and "wave".

Can you explain why gravity is not infinite.

If gravity is a force, and energy is not infinite, would an object loose its gravitational pull over time despite still having the mass which originally produced the gravity?

Whether that gravity is caused by gravitons being created by the presence of mass, or simply bends in Space-Time forcing objects to roll towards massive enough objects.

4. Aug 16, 2010

### n.karthick

If the above statement is true, then if I jump up and down it would disturb earth's rotation.

5. Aug 16, 2010

### Xtensity

Yes, but by a negligible amount.

When the tidal waves hit a Turbine, the turbines kinetic energy is turned into electrical energy which is stored......

If the turbine is never hit then the energy simply goes back into the earth/ocean as it smashes down.

Though I do not know of any turbine that can withstand a tidal wave and even if we somehow did absorb all of the energy from a tidal wave, the earths rotation would not slow by any noticeable amount unless we did this for a very long time with every tidal wave that occurs. The same goes with leeching off the earths rotational energy, it slows it down, but it may take quite a bit of down for a noticeable difference.

6. Aug 16, 2010

### Staff: Mentor

Not exactly - part of the energy is dispersed as a heat, so the slowing takes place whether we harvest the energy or not. See below.

The idea is to find a narrow bay, then to close it with a dam, so that tidal wave can be trapped. Then you use normal turbines.

Process is slow, but observable, especially in long periods of time. Paleontological evidence suggests Devonian year was around 400 days, which means Earth was rotating substantially faster.

7. Aug 16, 2010

### Xtensity

All of that is true, I wasn't thinking about the heat aspect.

Though you still never addressed what I asked about gravitation effects on the waves, and if gravity will ever actually run out assuming the object producing the gravity remains.

8. Aug 16, 2010

### sophiecentaur

You state the question in a strange way. "Gravity" is a form of Potential Energy. The energy is 'there' because of the separation of two objects. As they get closer together, the potential gets lower and lower. It's only gone when they are resting right next to each other.
There is another factor, of course, and that is that Orbital Energy consists of a combination of Kinetic and Potential energy. Only in cases like Hydro Power, on the surface of the Earth, are we just talking in terms of stored Potential Energy. Mostly, we are dealing with KE and PE.

9. Aug 16, 2010

### Xtensity

How can energy/pull be "Gone" if they are resting right next to eachother? When they are resting right next to eachother, there is still an attraction between the 2 objects.

As far as I am aware, gravity has been theorized as bends in Space-Time, which results in objects rolling along the fabric of space-time via the Curvature of the bend which the massive object created.

Then there's the theoretical particle gravitons which are massless and generated by the simple presence of mass. They travel at infinite speed due to having no mass so their location can not be measured. An object simply existing it is producing "Gravitons" in all direction at infinite speed. Gravitons pass through all matter and because of their spin they create the force which seemingly pulls objects in whichever direction the graviton came from.

Whichever of these theories of gravity are true, neither of them contradict because they both have the same properties. Instantaneous effect in all directions.

Regardless of all this.... Can you explain why tides and waves are strengthened during heightened lunar activity(at night, etc). If the waves are becoming larger, then this must mean more energy is entering the wave from somewhere.... which it has been said is the graviational pull.

What mistake in my thinking am I making, since based on what I observe to be the moons gravitational effects on the waves, this is what is happening. (sorry if this last sentence is foggy, I am having a hard time putting that into words)

Last edited by a moderator: Apr 25, 2017
10. Aug 16, 2010

### n.karthick

As far as I know from physics, nothing can travel faster than light. Infinite speed for anything is not possible at all. Even effect of gravity can travel at light speed only. That is what Einstein theory of relativity says. If sun disappears suddenly we cannot know it (including its gravitational effect) for 8 minutes (approx). I am confused with your saying that gravitons travel at infinite speed, thier instantaneous effect .

11. Aug 16, 2010

### Staff: Mentor

Gravity is not instantaneous, it travels at c.

12. Aug 16, 2010

### Staff: Mentor

Actually, he did, but you're confusing two separate issues here, which is why you aren't seeing it. "Gravity" is not an energy source, it is a field or a static force. So it is meaningless to ask if it will "run out". A book sitting on a table will always have about the same force pulling it to the table and that force will never "run out", but there is no energy usage associated with it.

Tidal power plants don't steal "gravity" energy, they steal rotational kinetic energy. A changing gravitational field is just how the rotational energy is transported. This energy source will "run out" when the earth and moon become tidally locked.

Similarly, a hydroelectric dam is ultimately powered by the sun. Gravity's involvement is just in transporting the water up and down. This is also similar to a typical steam engine, where a pump transports the steam, but the boiler is what supplies the energy to run the turbine.
Well the moon can be in the sky at any time, not just at night, but this is also a peculiar question. The phrase "lunar activity" has no meaning that I can discern.

Tides are on a daily cycle (with two peaks) that changes very little from day to day. The tides get stronger when heavenly bodies align and when the distance changes by a significant amount. Both of these occur on nearly monthly cycles (again, with two peaks).

Last edited: Aug 16, 2010
13. Aug 16, 2010

### Xtensity

Regardless, is energy being put in the waves as a result of the existence of gravity? If gravity is not influencing energy over the waves, then in theory a wave should have no physical impact on anything.(which we know is not true).

If the moon influences a wave to rise, and a turbine is under that wave, then that turbine rotates and generates energy as a direct result of the wave hitting it, what is the original source of energy?

Also let me ask this. Would there be such a thing as "Waves" if the earth did not rotate or orbit around the sun. Putting aside the fact that it would freeze then burn on one side, if somehow the climate was maintained, would there be waves? and if so were would the energy be coming from.

I'm pretty sure this is a heavily debated issue
[off topic portion deleted]

Last edited by a moderator: Aug 16, 2010
14. Aug 16, 2010

### Staff: Mentor

No. Energy is being put into waves due to the rotation of the earth. The tidal force creates a bulge, but that bulge would be stationary if the Earth was not rotating. The rotation of the Earth causes it to move, moving water around the earth in a large wave, with countless secondary effects.
The rotation of the earth. Unless the moon appeared and disappeared repeatedly in the same spot, what you describe could not happen without the rotation of the earth. There would be no rising and falling.
Yes, certainly. Most waves (on water) are caused by wind, not the tides.
No, it isn't. At least, not among real scientists. It is a component of Einstein's General theory of Relativity. Virtually everything you have said about its implications is wrong. Read the wiki on "speed of gravity" and ask any questions you still have in the Relativity forum. It isn't relevant to this topic. For that matter, this topic isn't engineering, it is astronomy. So I'm moving the thread to astronomy.

15. Aug 17, 2010

### Chronos

Ocean waves slow the earth's rotation over time regardless of whether we harvest their energy, or not. In that sense it is 'free' energy. Please note the term 'tidal waves', as used by SA's in this thread, are not the same thing as tsunami. The two are totally unrelated. 'Tidal waves' are generated by the gravity of the moon and sun, tsunami are generated by plate tectonics and colossal landslides.

16. Aug 17, 2010

### sophiecentaur

Of course gravity is relevant to the phenomenon of tides and tidal energy (the forces involved are gravitational) but it is not the 'source' of energy any more than the gearbox of a car is the source of energy to drive it forward.
Your statements seem to be somewhat 'tangential' to the actual Science involved. If you use the accepted terminology then you may find the subject more accessible.

17. Aug 17, 2010

### zeromodz

He is right, gravity does travel at c. No information can travel faster than light. The graviton would need imaginary mass to travel faster than c.

18. Aug 17, 2010

### Staff: Mentor

Let's try to circle back around the question in the OP. The OP asked about hydroelectric turbines using underwater to generate power. AFAIK, there are three related types:

1. Turbines at the bottom of a river.
2. Turbines in an ocean current such as the Gulf Stream.
3. Turbines at the mouth of a large bay, generating power as the tides cause the bay to fill and empty.

All three can generate power for a very long period of time. The first is powered by the sun transporting water from the ocean to land through the atmosphere. The second may be powered by a combination of the sun and the rotation of the earth due to the coriolis effect, depending on the particular current. The third is powered by the rotation of the earth moving the tidal bulge around the earth.

The OP also mentions waves. None of the above devies generates energy from regular ocean waves. Ocean waves do not involve any mass transport and therefore to harness them, you have to harness the oscillation itself: you have to use some kind of bobbing device (or tiny turbine) on the surface. However it is done, normal waves are powered primarily by the wind, which is powered by the sun. And they are in most cases not really related to the tides. In other words, it is not really correct to call motion of water due to the tides a "wave": the term "tidal wave" is a colloquial term, not a scientific one.

I don't see how terms like "seemingly infinite" in the OP are useful to discuss, much less argue about. It should be clear to everyone that there are no truly infinite sources of energy available to us, but there are sources of energy that have such longevity that they will certainly outlive us. For practical purposes, that's all that is required.

But the clarification made several times above is important to understand: "gravity" is not an energy source, infinite or otherwise.

19. Aug 17, 2010

### sophiecentaur

I sail a lot in the Solent. There are regular (even during neap tides) fast tidal currents there (four times a day) and I'm convinced that a set of turbines under the water could produce oodles of energy. Of course, there's the problem of shipping (me included).
It seems to me that there are many advantages; they are not an eyesore, they are protected from the ravages of the weather, there is no problem of silting which happens with 'estuary / gravitational potential energy' systems and I can't imagine any other serious environmental issues. There are some systems for yachts that use water turbines rather than wind turbines and they work well (afaik) although they are more expensive than the wind variety.
Edit - second thoughts on the silting - there can be some surprising effects on sandbanks but these things could be re-sited if necessary (?).

Last edited: Aug 17, 2010
20. Aug 17, 2010

### stewartcs

Ocean waves do involve some mass transport IIRC (e.g. Stokes drift).

CS