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Post by Progenitor A on Oct 12, 2012 8:30:09 GMT 1
Does gravity break the sacred law of conservation of energy? (That law is nonsense philosphically anyway)
For gravity is a store of energy that is apparently inexhaustable. A massive body entring the earth's gravitational field gains an enormour amount of energy, but the gravitational field strength of the earth does not decrease to compensate for this energy gain (in fact the gravitational field strength increases!)
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Post by nickrr on Oct 13, 2012 17:51:23 GMT 1
Why?
Such a body simply loses potential energy and gains an equal amount of kinetic energy. There is no net gain or loss of energy in the system as a whole.
The "gravitational field strength of the earth" is not energy so does not come into the energy equation. It remains constant, although of course the force it exerts on the massive body will increase as the two bodies get closer.
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Post by fascinating on Oct 13, 2012 19:31:49 GMT 1
I don't agree with that nick. The falling object has potential energy, but loses that as it gains kinetic energy. But the question is, where does the energy (kinetic or potential) come from?
Think of an asteroid in space on a path that takes it several million miles from Earth, at the closest. It will continue on its way and out of the Solar System, but suppose someone comes along and pushes it in the direction of Earth. For a small input of energy, that asteroid is now placed on a path that makes it spiral into the Earth. When it hits the Earth a huge amount of energy is released, much more than the energy that was input to knock the asteroid from its original path. So where has the extra energy come from?
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Post by Progenitor A on Oct 14, 2012 8:46:29 GMT 1
We will come back to this Nick once we have sorted out the practicalities Such a body simply loses potential energy and gains an equal amount of kinetic energy. This is true, but the body has no potentail energy in its own right - that energy is bestowed upon it by the gravitational field. The fact that the gravitational field can bestow either (or both of) potential and kinetic energy does not detract from the central fact that that energy exists soley due to the gravitational field. The falling body is actually gaining kinetic energy as it falls, and that energy is provided by the gravitational field. Ther is no corresponding weakening of the gravitational field so there is simply a gain in energy at no 'cost' There is no net gain or loss of energy in the system as a whole. Simply not true. The falling body has a kinetic energy that it would not have without the gravitational field. The "gravitational field strength of the earth" is not energy so does not come into the energy equation. Disagree. Energy is the capacity to do work, and the gravitational field has plenty of that! Rather like the electric field of a charged battery has the capacity do do work |
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Post by nickrr on Oct 14, 2012 13:32:03 GMT 1
To me you provide the answer in your question "The falling object has potential energy, but loses that as it gains kinetic energy".
Are you asking where that initial potential energy comes from? If so it comes from the energy input into the system to separate the massive body and the earth. Ultimately this leads us all the way back to the big bang which caused the initial separation of matter in the first place.
Again, the kinetic energy comes from the gravitational potential energy. These are both forms of energy - one is just converted into another so there is no net gain or loss of energy. Of course it's correct that the body would not have the kinetic energy without the gravitational field, but without the gravitational field there would also be no potential energy so nothing would happen.
A more mundane scenario. You carry a heavy rock up a hill. That takes energy so the rock now has increased potential energy. You let the rock roll down the hill. It gains kinetic energy at the expense of the potential energy you expended taking it up the hill. There's no creation of energy here, just changing from one form to another. The kinetic energy of the rock as it rolls down the hill comes from the energy you expended in taking it up the hill.
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Post by mrsonde on Oct 14, 2012 16:56:19 GMT 1
Why? Such a body simply loses potential energy and gains an equal amount of kinetic energy. There is no net gain or loss of energy in the system as a whole. The "gravitational field strength of the earth" is not energy so does not come into the energy equation. It remains constant, although of course the force it exerts on the massive body will increase as the two bodies get closer. Well, philosophically, as Nay correctly puts it, the deeper unanswered question behind this equation is why does mass create a gravitational field, or vice versa, in the first place? As Einstein pointed out, that's the ultimate question. |
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Post by buckleymanor1 on Oct 14, 2012 18:20:03 GMT 1
Not so sure a gravity field in isolation can create mass on it's own. It would have to act on an object to create some extra mass by say increasing it's temperature as the object fell through the atmosphere.
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Post by mrsonde on Oct 14, 2012 18:37:36 GMT 1
A gravity field in isolation? Is there such a thing?
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Post by Progenitor A on Oct 14, 2012 19:43:24 GMT 1
To me you provide the answer in your question "The falling object has potential energy, but loses that as it gains kinetic energy". Are you asking where that initial potential energy comes from? If so it comes from the energy input into the system to separate the massive body and the earth. Ultimately this leads us all the way back to the big bang which caused the initial separation of matter in the first place. Yes there's the nub The potential energy was caused , ultimately by the BB. Where the energy came from to create a massive body of plasma then expand it outward is of course unanswerable That's where the 'philosophical' bit comes in. If as some physicists maintain, there was nothing before the BB, then that must mean tha the energy was created at the BB (doesn't it?). That means of course that the law of conservation of energy was flouted Not surprising as all known laws of physics seem to have been flouted at the moment of creation |
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Post by buckleymanor1 on Oct 14, 2012 20:36:48 GMT 1
A gravity field in isolation? Is there such a thing? No there is all ways some mass present.The presence of mass produces the field I don't see how a Field can produce mass or exist without mass.Sometimes you get particles that jump into existence, then vanish, in effect they exist for a short period of time. |
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Post by mrsonde on Oct 14, 2012 21:31:31 GMT 1
Well, Einstein said it was at this stage of our understanding an unreslovable question, whether mass produces a gravitational field, or whether a gravitational field (the distortion of the space-time metric) produces energy, which can then become "looped" into matter. As you say, one seemingly cannot exist without the other.
What seems philosophically most economic is that first comes the field - the Aether, if you like. It's this that gives us space-time. It can be distorted from its metric of least resistance - this gives us energy. It has the topological properties that enable these distortions to circularise, to persist, and this gives us matter. Thus the Field exists first.
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Post by nickrr on Oct 17, 2012 21:39:05 GMT 1
Current theories suggest that because of the inflationary period very shortly after the big bang the positive energy of matter is balanced by the negative gravitational energy caused by inflation. The total energy in the universe is therefore zero. I don't pretend to understand the details but you can find more here: en.wikipedia.org/wiki/Zero-energy_universeOf course this isn't in any way proven but personally I find it a persuasive idea. It's much easier to imagine how something with zero energy came into existence spontaneously. |
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Post by Progenitor A on Oct 18, 2012 16:07:56 GMT 1
Current theories suggest that because of the inflationary period very shortly after the big bang the positive energy of matter is balanced by the negative gravitational energy caused by inflation. The total energy in the universe is therefore zero. I don't pretend to understand the details but you can find more here: en.wikipedia.org/wiki/Zero-energy_universeOf course this isn't in any way proven but personally I find it a persuasive idea. It's much easier to imagine how something with zero energy came into existence spontaneously. What is meant by the 'positive energy of matter'? Does it imply the e=mc 2 thingy? If so gravitational energy (called the weak energy) is no match for the energy of matter |
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Post by mrsonde on Oct 18, 2012 16:29:50 GMT 1
These fantasies of cosmologists are entertaining, and sometimes intriguing, especially when the numbers seem to unexpectedly come out right, as with Guth's inflation theory. But that's all - speculations like this must be based in observational evidence, else they're not "scientific" at all. Cosmological Physics has now become so divorced from observation, so rarefied and rooted solely in mathematical constructs (themselves empirically ungrounded), that it's hardly surprising that most of them seriously believe that "only the mathematics are real". They need more Samuel Johnsons in their ranks, who responded to Berkeley's similar assertions by kicking a rock and declaring, "I refute you thus, sir!"
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Post by nickrr on Oct 19, 2012 12:22:20 GMT 1
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