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We can 'tap energy' from what gravity does to mass, as using a waterfall to drive a wheel. But let us discuss it from redshift. Imagine yourself far outside a gravity well measuring light falling towards a event horizon. Will it redshift to you? Where did it lose the 'energy' you measure it to have? To what?A immense gravity right? Now assume a waterfall again and the wheel spinning just above the Event horizon, and we will ignore tidal forces. Would you expect the waterfalls water to have more energy as it hits the wheel?Let's go back to that light quanta, and assume that you instead observe it infalling towards you at the event horizon. Do you expect it to have more energy, becoming blue shifted, as you now observe it from the even horizon.Are we observing the same light quanta? You can assume that it is, and we don't need to define a simultaneous 'instant' for our observations.And what would that light quanta's 'energy' be intrinsically, changing depending on observer, or of one magnitude?]We can 'tap energy' from what gravity does to mass, as using a waterfall to drive a wheel. But let us discuss it from redshift. Imagine yourself far outside a gravity well measuring light falling towards a event horizon. Will it redshift to you? Where did it lose the 'energy' you measure it to have? To what?A immense gravity right? Now assume a waterfall again and the wheel spinning just above the Event horizon, and we will ignore tidal forces. Would you expect the waterfalls water to have more energy as it hits the wheel?Let's go back to that light quanta, and assume that you instead observe it infalling towards you at the event horizon. Do you expect it to have more energy, becoming blue shifted, as you now observe it from the even horizon.Are we observing the same light quanta? You can assume that it is, and we don't need to define a simultaneous 'instant' for our observations.And what would that light quanta's 'energy' be intrinsically, changing depending on observer, or of one magnitude?
Gravity neither pull or push. It's just a preferred direction. The proof for that is any 'free fall' and measuring the forces acting on the object locally. As Pete has explained, you can transform away all gravity there is, just by changing coordinate system. You standing on a tower will feel gravity acting on you, but falling from it there will be no gravity at all (ignoring tidal forces). Just think of yourself inside a black box free falling, how will you prove the existence of a gravity?=To be strict we should define it as 'geodesics' instead of 'free falls', but geodesics are much more non intuitive beings than a free fall to me. If we take a bullet shoot in deep space, then as long as it is accelerating it will not be in a geodesic as I think, but as soon as the acceleration stops the geodesic will come into existence, defining its further path. (although the barrel will constrict it, so we better consider it after leaving that:)
There is one view though from where it make sense. And that is relativity, in there you always need to compare to define a 'energy' as I'm thinking. If we now assume that any 'energy' to be measurable need a comparison between 'frames of reference' then 'energy' is a expression of your comparison between frames. But then there is accelerations? And 'gravity', locally perceived through it. What frame of reference am I comparing that 'local gravity' too? And if we use black boxes to define what is correct experimentally, then that gravity is just as real as the absence of gravity in ones fall from that tower.==The point to it is that as different motion exist there should be difference's in energy to them, but locally that won't be measurable. Although, as soon as you set two such objects in motion against each other you will find that kinetic energy existing, in a collision depending on 'motion', not just 'relative' but very real and differing with what 'real motion' we find them to have relative each other. And that I presume to be explained through the stress energy tensor, although I still have to see where I should place the energy physically.
Another disturbing fact is that using 'energy' to define costs for motion etc becomes quite strange in relativity. Assume you're near the speed of light, The universe you exist in will then have shrunk physically from your frame of existence. The force you expended getting to that velocity can in no way relate to a whole universe shrinking in the direction of your motion. (Meaning that if we treat it as 'forces', and 'energy expended, what would the energy needed be for compressing our known universe one light year?) And it will stay shrunk after you stopped accelerating too, and in a uniform motion you expend no energy at all.But there must still be a relation between that velocity, and energy expended locally, and the universe you observe.
Hmm.I like relativity myself, although I doubt I understand it all, what I think I get still makes a lot of sense to me. And it turns a lot of old definitions & expressions upside down, well sort of We made the science we have from using a inertial point (Earth) and then define what forces we saw acting. That's from where push and pull comes too as I see it. And physics is more about vectors and magnitudes than that, as I read it. But wondering about 'flows' in non linear systems, and what makes them may be fruitful, that's what chaos mathematics is about.
Why potential energy of spring has mass, but gravitational potential energy has no mass?
Quote from: simplified on 28/02/2013 13:34:17Why potential energy of spring has mass, but gravitational potential energy has no mass?Gravitational energy does have mass. Who said it doesn't? It's for that reason that Einstein's Field Equations are nonlinear.It's also for this reason that it is sometimes said that "gravity gravitates."
Quote from: Pmb on 01/03/2013 00:16:33Quote from: simplified on 28/02/2013 13:34:17Why potential energy of spring has mass, but gravitational potential energy has no mass?Gravitational energy does have mass. Who said it doesn't? It's for that reason that Einstein's Field Equations are nonlinear.It's also for this reason that it is sometimes said that "gravity gravitates."Let's consider weight on spring.Counteracting forces are equal.If you think that energy of the spring creates gravitational field then you have left nothing for the weight and its force.
Quote from: simplified on 01/03/2013 17:59:43Quote from: Pmb on 01/03/2013 00:16:33Quote from: simplified on 28/02/2013 13:34:17Why potential energy of spring has mass, but gravitational potential energy has no mass?Gravitational energy does have mass. Who said it doesn't? It's for that reason that Einstein's Field Equations are nonlinear.It's also for this reason that it is sometimes said that "gravity gravitates."Let's consider weight on spring.Counteracting forces are equal.If you think that energy of the spring creates gravitational field then you have left nothing for the weight and its force. I don't understand your reasoning. It's a fact that a compressed spring has more mass than when the spring is not compressed and as such it weighs more. The energy in the spring also creates a gravitational field. So what do you mean "you have left nothing for the weight and its force" That has no meaning for me.
The spring mass is interesting to this thread. A push force is allowed to flow into the spring as energy, and sort of fill up the electrons faster, and therefore add mass. That is different to the current description of energy to mass.
So here again seems to be another change in a theory based on a pull force physical interaction, compared with a flow force physical interaction. Both have a mass increase, but again the flow force seems much simpler to understand.