[evan_au]

In my simple understanding, Gravity is a field, which creates a force, with a direction which always seems to be attractive. (Electric charge produces another field which can be attractive or repulsive.)

You require energy to move an object against a field - or turn the field's potential energy into kinetic energy when you allow an object to "fall" in the field.

If we are interested in Energy (as per the original post), what matters is the potential difference between two points in the field. The absolute value of the field does not affect the Energy required to move between two points in the field with a given potential difference.

• With an electric field, because it is "bipolar" (attracts and repels), we have a "natural" zero potential, where all surrounding matter has no excess or shortage of electrons. We can easily create a uniform zero electric field in the laboratory, inside a Faraday cage.
  
• However as the gravitational field is "unipolar" (always attractive, as far as we know), has infinite range and cannot be shielded, there is nowhere in the reachable universe that we can use as a "natural" zero potential. So we must pick an arbitrary point to make our calculations easier. 

For myself, as a person who is unlikely to ever travel into space, I might choose my arbitrary zero potential as the surface of the Earth, knowing that if I am climbing a tree which is well above my zero potential, I am likely to break something if I fall. Similarly, if I am standing at the top of a mine, which ends well below my zero potential, I could do myself an injury if I slipped.

Of course, if I were plotting the orbit of Philae landing on a comet or the structure of the galaxy, it would be sensible to select a very different gravitational reference point!

Like all good conventions, they don't make the answer "right" or "wrong", but they do make some calculations easier (simplifying communication), and others more difficult (hindering communication), so by all means choose your convention with care and state which convention you are using - but be aware of its limitations.

[jeffreyH]

It is in the picking of an arbitrary point that we create difficulties. The universe contains a gradient of forces of varying magnitudes that overlap. The point we choose must equalize all these forces.

[jeffreyH]

Gravitation takes energy away from mass. Not only the kinetic energy but the energy of action. This not only slows the froward momentum but the rates of change. For the photon at the event horizon all its forward momentum has been transferred to the gravitational field which still travels at c when light itself comes to a stop. Only at infinity does the speed of the photon match the speed of gravity. This means that gravity is apparently superluminal right up to infinity due to its dilation effects on the action of energy. Gravitation robs mass of its forward momentum until the mass stops moving forward. It is only then that gravitation can be considered a negative force as it is now subtracting momentum and propelling the mass in an opposite direction. There is a balance in the system so energy is conserved. Before this is kicked into new theories give it a good deal of consideration.

[acsinuk]

Why are we considering gravity energy instead of vacuum energy?

[JohnDuffield]

Because people say gravitational energy is negative, and cancels out all the vacuum energy to leave a universe with a net energy of zero. The trouble with this is that it contradicts general relativity. 

Now I am going to put the cat right amongst the pigeons. Some of what John says above I agree with. Not in the way it is said but what it implies. So don't get excited John I am not exactly backing you up and I would appreciate you not promoting it that way.

No problem, like you said, energy is conserved. If you can find anybody who can actually explain why gravitational energy is negative energy and therefore Einstein was wrong, do let me know.

You require energy to move an object against a field - or turn the field's potential energy into kinetic energy when you allow an object to "fall" in the field.

The point to remember is that when you lift a brick you do work on the brick. You add energy to it. Then when you drop the brick some of its internal kinetic energy is converted into external kinetic energy. There's no magical mechanism by which kinetic energy somehow flows into the brick from the surrounding space. 

[chiralSPO]

My understanding of the "negative energy" phenomenon of gravity is something along these lines:

Imagine two photons of sufficiently high energy interact to produce an electron positron pair in some hypothetical otherwise empty universe. We can calculate the amount of energy the photons need to have for this to happen.

Now imagine the same reaction occurs very near to a massive object (in a large gravitational field). The energy required for this to happen is going to be slightly less than the energy require in the absence of this gravitational field.

Now, instead of having a pre-existing massive object, let us consider forming two electron-positron pairs simultaneously. The energy required for this is going to be (slightly) less the closer the two pairs are because of their gravitational interaction with each other.

Finally let us ask the question: how close must these pairs be for the energy requirement to be zero? An overly simplified equation might read something like: E=m*c²–g*m²/(4*r) = 0; solve for r given a fixed m, or solve analytically for r as a function of m--I get r = m*g/(4*c²). If I take the mass of matter in the observable universe as 10⁵³ kg, then r ≈ 2*10²⁵ meters (2 billion light years), which is obviously wrong, but it shows that such a concept is, in principle, possible to consider, just with less simplistic models...

[jeffreyH]

My understanding of the "negative energy" phenomenon of gravity is something along these lines:

Imagine two photons of sufficiently high energy interact to produce an electron positron pair in some hypothetical otherwise empty universe. We can calculate the amount of energy the photons need to have for this to happen.

Now imagine the same reaction occurs very near to a massive object (in a large gravitational field). The energy required for this to happen is going to be slightly less than the energy require in the absence of this gravitational field.

Now, instead of having a pre-existing massive object, let us consider forming two electron-positron pairs simultaneously. The energy required for this is going to be (slightly) less the closer the two pairs are because of their gravitational interaction with each other.

Finally let us ask the question: how close must these pairs be for the energy requirement to be zero? An overly simplified equation might read something like: E=m*c²–g*m²/(4*r) = 0; solve for r given a fixed m, or solve analytically for r as a function of m--I get r = m*g/(4*c²). If I take the mass of matter in the observable universe as 10⁵³ kg, then r ≈ 2*10²⁵ meters (2 billion light years), which is obviously wrong, but it shows that such a concept is, in principle, possible to consider, just with less simplistic models...

For a start you have no kinetic energy in your mass-energy term. You can't subtract from M*c^2 because then you are removing mass energy from the particle. Consider Ke to be the positive kinetic energy of the particle. Then -Ke is the gravitational energy (sorry John ). If -Ke falls off with the inverse square of the field the particle gradually loses all forward momentum away from the source. You then ultimately slow down the mass by more than 100% of its original velocity. This is not only the subtraction of forward momentum but also of internal kinetic energy. It can always be considered a process of slowing of momentum even when it causes an acceleration. However the acceleration is then a positive momentum in the negative direction. This does NOT mean that gravitation has negative energy. That one stumped Maxwell and others. We just don't know the mechanism of this action yet.

[acsinuk]

But surely gravitational force is insignificant compared to the even the weak electric force like 10^25 so why try and balance the vacuum of space with gravity. Look to the electric force or EMF differences between solar objects.  It can't be measured directly but surely you do not think the surface potential of Mars much less the sun is the same as ours on earth?

[JohnDuffield]

Imagine two photons of sufficiently high energy interact to produce an electron positron pair in some hypothetical otherwise empty universe. We can calculate the amount of energy the photons need to have for this to happen. Now imagine the same reaction occurs very near to a massive object (in a large gravitational field). The energy required for this to happen is going to be slightly less than the energy require in the absence of this gravitational field.

This is all fair enough. But note that it still takes positive energy to make the pair. Yes it's less positive energy, but it's still positive. There's no actual negative energy anywhere.

Now instead of having a pre-existing massive object, let us consider forming two electron-positron pairs simultaneously. The energy required for this is going to be (slightly) less the closer the two pairs are because of their gravitational interaction with each other.

But not so much as to be measurable. You need a whole planet's worth of matter for that.

Finally let us ask the question: how close must these pairs be for the energy requirement to be zero?

There is no point at which you can make electron-positron pairs out of no energy.

[JohnDuffield]

Consider Ke to be the positive kinetic energy of the particle. Then -Ke is the gravitational energy (sorry John )

The kinetic energy is positive. So if the energy you added to the brick when you lifted it. When you dropped it, some of the mass-energy of the brick, the internal kinetic energy, is converted into external kinetic energy. Again there's no actual negative energy anywhere.

If -Ke falls off with the inverse square of the field the particle gradually loses all forward momentum away from the source.

When you throw the brick up, external kinetic energy is converted into internal kinetic energy. Conservation of energy applies.

You then ultimately slow down the mass by more than 100% of its original velocity. This is not only the subtraction of forward momentum but also of internal kinetic energy. It can always be considered a process of slowing of momentum even when it causes an acceleration. However the acceleration is then a positive momentum in the negative direction. This does NOT mean that gravitation has negative energy. That one stumped Maxwell and others.

This isn't clear.

We just don't know the mechanism of this action yet.

I think we do actually. It's down to the way light bends, and the wave nature of matter. See post number 2 on this thread for my stab at explaining it.

[jeffreyH]

Consider Ke to be the positive kinetic energy of the particle. Then -Ke is the gravitational energy (sorry John )

The kinetic energy is positive. So if the energy you added to the brick when you lifted it. When you dropped it, some of the mass-energy of the brick, the internal kinetic energy, is converted into external kinetic energy. Again there's no actual negative energy anywhere.

None of the mass energy of the brick is lost at all. Its momentum changes and the rate of energy flux changes, that's all.

If -Ke falls off with the inverse square of the field the particle gradually loses all forward momentum away from the source.

When you throw the brick up, external kinetic energy is converted into internal kinetic energy. Conservation of energy applies.

What on earth is external kinetic energy? It has to have a source.

You then ultimately slow down the mass by more than 100% of its original velocity. This is not only the subtraction of forward momentum but also of internal kinetic energy. It can always be considered a process of slowing of momentum even when it causes an acceleration. However the acceleration is then a positive momentum in the negative direction. This does NOT mean that gravitation has negative energy. That one stumped Maxwell and others.

This isn't clear.

Mathematically it's very clear.

We just don't know the mechanism of this action yet.

I think we do actually. It's down to the way light bends, and the wave nature of matter. See post number 2 on this thread for my stab at explaining it.

How does gravity affect a wave in such a way that it reverses momentum. If you can demonstrate a proof of that then you've cracked it.

[JohnDuffield]

None of the mass energy of the brick is lost at all. Its momentum changes and the rate of energy flux changes, that's all.

Check out the mass deficit. The kinetic energy of the falling brick doesn't come from nowhere.

What on earth is external kinetic energy? It has to have a source.

Drop a brick on your toe to really appreciate what it is. And it does have a source. The brick. When you lifted that brick you did work on the brick. You added energy to the brick.   

How does gravity affect a wave in such a way that it reverses momentum. If you can demonstrate a proof of that then you've cracked it.

It just bends it. There's proof of light being bent by the Sun, and proof of the wave nature of matter, but there's no proof that I know about of gravity bending the electron wave like my depiction. That's an inference. An obvious one IMHO, but I can't explain why it hasn't been common knowledge for decades.

[yor_on]

John, been to the pub, so you better take this with a pinch of salt

though, I think you're missing something citing this. Stephen Hawking defined it this way. "Two pieces of matter that are close to each other have less [positive] energy than the same two pieces a long way apart, because you have to expend energy to separate them against the gravitational force that is pulling them together"

If you take it as stated there is nothing wrong with his reasoning. As a 'system' in where you have to expend energy to move pieces from each other. Potential energy is not as I think of it 'locally defined', so experimentally existing, unless one first define a 'container' of some sort, and conservation laws, that need this bookkeeping to present our equilibrium.

where I differ it is avoiding the 'container' but wanting to keep the conservation laws. To do that I presume that it is possible to build dimensions and a universe from locality, which it is, locally and experimentally.
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To me, if I now was unclear before (quite often I'm afraid) 'negative energy' is similar to that hole that needs to be filled in charge. There's something missing to get back to a balance, and the universe abhors imbalance.

[yor_on]

The difference between Hawking's example and 'potential energy' is that there is nothing 'potential' with the energy you have to expend to move those pieces, locally defined. potential energy is ideally a 'virtual' bookkeeping, defining our universe, and as I think at the moment, a result of conservation laws existing.

[jeffreyH]

On all the bending nonsense I will say this. You do not have to have a physical curvature of trajectory for gravity to operate. If an object is propelled exactly perpendicular to the surface generating the gravitational field it will go straight up, stop, and come straight back down. While over time it will describe a parabola in space it won't. Curvature is in the gradient of change and not merely a physical curvature of trajectory.

[jeffreyH]

But surely gravitational force is insignificant compared to the even the weak electric force like 10^25 so why try and balance the vacuum of space with gravity. Look to the electric force or EMF differences between solar objects.  It can't be measured directly but surely you do not think the surface potential of Mars much less the sun is the same as ours on earth?

I have considered the em field in various ways but here it is energy conservation. Since gravity reduces kinetic energy and angular momentum it deserves equal if not more attention than the em field.

[JohnDuffield]

though, I think you're missing something citing this. Stephen Hawking defined it this way. "Two pieces of matter that are close to each other have less [positive] energy than the same two pieces a long way apart, because you have to expend energy to separate them against the gravitational force that is pulling them together"

Nothing wrong with that.

If you take it as stated there is nothing wrong with his reasoning. As a 'system' in where you have to expend energy to move pieces from each other. Potential energy is not as I think of it 'locally defined', so experimentally existing, unless one first define a 'container' of some sort, and conservation laws, that need this bookkeeping to present our equilibrium.

It is. If you throw a brick upwards, you give the brick kinetic energy. The brick slows down and this kinetic energy is converted into potential energy. But if you threw it upwards at 11km/s it's got escape velocity, and it escapes the system, taking all that kinetic/potential energy with it.   

where I differ it is avoiding the 'container' but wanting to keep the conservation laws. To do that I presume that it is possible to build dimensions and a universe from locality, which it is, locally and experimentally.

Sorry yor_on, I don't know what you mean by that.

To me, if I now was unclear before (quite often I'm afraid) 'negative energy' is similar to that hole that needs to be filled in charge. There's something missing to get back to a balance, and the universe abhors imbalance.

There just isn't any negative energy. Everything that exists is made of positive energy. There's just nothing out there that you can combine with something else to be left with nothing.

[syhprum]

When you say that the gravitational field can not be shielded you must be implying that the graviton has zero mass as even the very low mass Neutrino can be shielded against in theory.

[jeffreyH]

When you say that the gravitational field can not be shielded you must be implying that the graviton has zero mass as even the very low mass Neutrino can be shielded against in theory.

Any theory of gravity shielding has to overcome time dilation. So in one discrete region time has to move differently from the immediate surroundings. Even correcting for nanoseconds is some feat.

[yor_on]

Yeah John.

"    where I differ it is avoiding the 'container' but wanting to keep the conservation laws. To do that I presume that it is possible to build dimensions and a universe from locality, which it is, locally and experimentally. "

Sorry yor_on, I don't know what you mean by that. " "

you're correct, I don't know either what I mean, I have a feeling for what I want to get to, and it has taken me years to recognize the importance of it. I started with wondering what 'frames of reference'  meant, and I'm still stuck on this one. But I have a hope that you guys will help me there, if you start to wonder about it.
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Spelling sux. the worst of it is that writing too much English, I've found myself just as bad in Swedish
Divine justice I guess?