We harness solar energy with constant supply of light but not with gravity. Why? What are the latest developments that can harness gravity? Why do Perpetual machines fail?
closed as unclear what you're asking by user81619, Kyle Kanos, John Duffield, Carl Witthoft, ACuriousMind♦ Nov 5 '15 at 13:40
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We do. For instance in a hydrodynamic power plant we convert potential energy to kinetic and via a turbine to electric energy. Or what do you mean by harness gravity? It's not a radiation like light, where we use electromagnetical waves to produce energy. If you are asking why we don't do this with gravitational waves, the answer is simple: they are so weak that they aren't even detectet yet, but we are sure that we will do so soon. It won't be efficient as a power source although.
Solar energy and gravity are two very different things. We get solar energy in the form of photons which are essentially energy packets. You can convert them to other forms of energy and use it. And this flow of energy is coming constantly to earth from the Sun.
Gravitational potential energy is the energy associated with the system. It is not actually located right within the object - rather it is situated in fields. But still, for the near-earth objects, we can deal with the situation even if we consider a model in which the particular objects possess some gravitational potential energy in them. But the real problem is - once you use the gravitational potential energy of an object to do some task, there is no more gravitational potential energy in the object that you can use. You need to lift the body again to some level and then you can use the increased potential energy, but obviously, you provided the object some energy when you lifted it. On the other hand, Sun is continuously providing you some energy and you do not need to do anything in return to again get some energy from the Sun.
Gravity seems an infinite source of energy after a naive analysis because we think gravity is always there, why isn't it an everlasting source of energy? The reason is, as I mentioned, gravity is essentially a force continuously acting downwards. It will take energy from you while you are moving the object upwards and will return you the same energy when the object is moving down. If an object is already at some height, once you get some energy from it but then again you have to perform work to take it to some heights.
Gravity could have been a great source of energy if the situation would be that a huge bunch of matter is naturally situated at great heights and you make it fall and use the energy and yet there is much matter at heights which you can further use. And in a sense, that happens in the hydraulic power plants as Noidig has mentioned.
Edit This explanation covers only an explanation about the gravity of earth. And I think it essentially clears the things for OP. We can use lunar gravity (via tidal energy concept) to get some energy and it is a relatively long term energy source. (as already mentioned in one of the answers.)
You can indeed harness the energy of gravitation and it is done on a somewhat small scale, as the main method to do it is to use the tidal movements of water. Most places do not get particularly impressive tides, so it remains a rather minor method of producing energy.
The two biggest such plants are in Rance :
and the Sihwa lake :