# If we can convert rad. from the sun to useable energy, why can't we use grav rad.

1. Feb 11, 2009

### Mirza90

In the world we live in, we can convert and use energy in the form of radiation and turn it into a significant power source. Gravitatoinal energy, which exists throughout the whole universe could be used someday as a power source. Think of it. A power source abundant throughtout the universe.

I need some feed back if I'm just spit balling or any helpful research would be greatly appriciated.

2. Feb 11, 2009

### ray b

tidal flow is driven by gravity
and fairly eazy to capture [in some places]

water dams use gravity too in rivers

so we useing some gravity to make power now

3. Feb 11, 2009

### Staff: Mentor

Welcome to PF. I think you have a fundamental misunderstanding of what gravity and gravitational energy are. Gravity is not radiated energy, it is an attraction between two objects due to their mass bending space. A book sitting on a table is affected by gravity forever, with a constant force, but no energy is exchanged between the book and table/earth.

Gravitational energy is a form of potential energy. A book sitting on a table has more gravitational potential energy than a book sitting on the floor below the table. But the only way to use this energy is to move the book from one location to to the other. And obviously, once you've moved the book from the table to the floor, you've used up the difference in potential energy. To use it again, you have to give back the energy you gained to lift the book back up to where it started. In this way, gravitational energy is conserved.

So gravity itself is not and cannot be a continuous/perpetual power source any more than a spring or a battery can be a continuous/perpetual power source. Power plants that use differences in gravitational potential energy - ie, hydroelectric dams - are not really powered by gravity, they are powered by something else, typically the sun.

Note: ray_b's example of tidal power isn't even truly gravity power: While tidal power uses the tides as a driver, the energy really comes from slowing down the earth's rotation.

4. Feb 12, 2009

### Chalnoth

Well, the fundamental thing that needs to be understood for making use of energy is thermodynamics: whenever the entropy of a system increases, it is possible to extract some energy out of that system to do directed work. Naturally, gravitational systems offer the capacity to help in this regard. The primary problem is that we will probably have to move outside our own solar system to use a gravitational system in this fashion.

One pipe dream, for example, is making use of a black hole: if you drop matter into a spinning black hole, a portion of that matter is shot at very close to the speed of light out the poles of the black hole. Imagine the uses: a society that lives around a black hole might dump their waste products into the black hole, and extract energy from the high-velocity stream of subatomic particles that results. And the energy in that stream is nearly equivalent to the mass-energy of the matter that was dropped into the black hole!

Of course, it won't last forever. The matter that is dropped in takes away from the angular momentum of the black hole. Eventually it will stop spinning, and no more useful energy can be extracted.