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Vineeth T
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Can anyone give me a situation when the gravitational potential due is 0 but gravitational field is non-zero.
OP asked about a place where the gravitational field strength is non-zero.MikeGomez said:Maybe the center of the Earth?
MikeGomez said:If any of the good physicists whom I greatly admire on this forum see this I'll probably take a beating, a real bad beating for this. But then maybe I deserve it
Maybe the center of the Earth?
I don't think that it is necessarily true that the gravitational field does work in a similar manner. Even if it does, then you're saying that there is a gravitational field in the centre of the Earth in the same sense that there is a gravitational field at any arbitrary point in space, due to quantum fluctuations.MikeGomez said:Photons mediate the magnetic force. I interpret that to mean that the magnetic field is composed of photons, albeit virtual photons. If the gravitational force works in a similar manner, then I maintain my original argument that the center of the Earth has a non-zero gravitational field.
BruceW said:I don't think that it is necessarily true that the gravitational field does work in a similar manner.
BruceW said:there is a gravitational field at any arbitrary point in space, due to quantum fluctuations.
This is very much unlike the situation with gravitational orbits. Objects in gravitational orbit will not immediately slam into another object in gravitational orbit. This is because the effect of gravitational dipoles on orbit are much smaller that the effect of gravitational monopoles in orbit.MikeGomez said:It might be easier to explain in terms of the magnetic field, and then we can take the analogy back to gravity. Take two bar magnets. Position them such that they are fairly close to each other, both south poles face each other, and they are both aligned on an axis (say X axis), symmetrical and equidistant from some origin. I think you get the picture. If we place a piece of magnetically sensitive object exactly between them, say an iron BB, it will stay in the center as long as it is perfectly balanced.
Yes I know that in practice this is extremely difficult to do. Any variation in the parameters will offset the balance and the bb would immediately slam to one magnet or the other. Even approaching absolute zero temperature there remains some slight motion. But at least for some short period of time, at some exact center location, the bb can remain balanced.
The Superposition principle says nothing of the kind. The magnetic field strength inside a bar magnet is supposed to be strong according to mainstream physics.MikeGomez said:The Superposition Principle tells us that the net magnetic field strength felt at the center point is zero.
You got that right.MikeGomez said:It doesn’t say that there is no magnetic field there at all.
Which you got precisely backwards.MikeGomez said:There is a big difference!
This is an invalid thought experiment since it starts with hypotheses that have been experimentally refuted time after time. A thought experiment is a short cut in the mathematics, not in experimental physics.MikeGomez said:To continue with our thought experiment with magnets,
The motion of bar magnets is completely irrelevant to the motion of gravitational masses.Two monopoles of equal magnitude but opposite sign placed a small distance apart are equivalent to a pure dipole. Each magnet is equivalent to a North and South pole set a small distance apart. However, a gravitational dipole would have to be equivalent to a positive and negative mass located a small distance apart. Since there is no such thing as a negative mass, you can’t make a pure gravitational dipole. Scientists have looked for such things, but never found them. So your thought experiment has no significance.MikeGomez said:we now place two more magnets symmetrical from the origin along the Y axis, and two as such along the Z axis, and then more and more at many locations equidistant from the center such that we have an approximate sphere with the south pole point in the center and the north pole pointing out.
Your analogy is not complete because there is only one type of gravitational mass. Magnets always have two magnetic charges, North and South.MikeGomez said:Magnets have another difficulty in that they never come in monopoles, and in reality it is impossible to create our perfect magnetic sphere as we would like, but gravity doesn’t have this problem. So to complete the analogy we replace both ends of all the magnetic dipoles with a gravitational monopole (a piece of mass will do) and we state what we already know which is that is that unlike magnetism, gravity is an attractive force (like graviational charges attract instead of repel).
It doesn’t matter if gravity is mediated by gravitons, space-time, or leprechauns. The Earth may not have a gravitational field at it center. The mediating particle, or even the existence of a mediating particle, has nothing to do with whether a field is zero or not zero.MikeGomez said:Photons mediate the magnetic force. I interpret that to mean that the magnetic field is composed of photons, albeit virtual photons. If the gravitational force works in a similar manner, then I maintain my original argument that the center of the Earth has a non-zero gravitational field.
This is correct. Therefore, the value of the absolute potential has no physical significance. The choice of zero potential is arbitrary with respect to experiments.MikeGomez said:As for the potential, ModusPnwd is correct. It’s arbitrary and we can set zero to whatever we choose.
Darwin123 said:A thought experiment is a short cut in the mathematics, not in experimental physics.
Darwin123 said:This is very much unlike the situation with gravitational orbits. Objects in gravitational orbit will not immediately slam into another object in gravitational orbit.
Darwin123 said:the Earth has a very small gravitational dipole moment
I really know very little about the research into possible quantum gravity theories unfortunately. Just out of interest, what does the L stand for in QLG ?MikeGomez said:That’s o.k., there are many different views on how gravitation works as it is still an open question in physics. Your input on this is appreciated if you wish to elaborate a little, but if it has to do with QLG that is covered to a great extent in other sections on this forum.
I don't understand :( sorry. The divergence of what field? The electric field has zero divergence in any region which contains no charges. So I'm guessing you don't mean that. Do you mean the change in the electric field as you move from one antennae to the other? In other words, if we denote a coordinate which goes from one antennae to the other in a straight line as 's' then the partial derivative of the electric field with respect to 's', keeping other coordinates constant? So then it does make sense to me that this quantity (call it [itex]\partial \vec{E} [/itex] ) is zero at the middle point between the two antennae (just from symmetry).MikeGomez said:Now if you are at the center of two such antennae’s and they both get keyed, you will receive large doses of radiation at the center point. The radiation is coming from both directions at you, even if the divergence of the field there is zero.
BruceW said:Just out of interest, what does the L stand for in QLG ?
BruceW said:So then it does make sense to me that this quantity (call it [itex]\partial \vec{E} [/itex] ) is zero at the middle point between the two antennae (just from symmetry)
MikeGomez said:I don’t want to beat a dead horse, but I thought of a better (and shorter) analogy. Your boss tells you to go up to 50 kW transmitting antennae to make a repair. Some idiot keys the mic while you are up there. You’re going to be in a world of hurt.
Now if you are at the center of two such antennae’s and they both get keyed, you will receive large doses of radiation at the center point.
yes, I understand that bit. But I don't get where it makes an analogy to the situation of gravity at the centre of the earth.MikeGomez said:My point is simply that we will get fried by radiation if we are at the middle point (zero point) when someone keys the two mic's. Are we in agreement on that?
Any physicist, including me, can construct a hypothetical distribution of mass that has a nonzero gravitational monopole and a nonzero dipole. The nonzero dipole term exists even though mass can not be negative. However, a nonzero dipole has to be superimposed on a nonzero monopole contribution. The famous dictum "gravitational dipoles do not exist" only applies to isolated gravitational dipoles.MikeGomez said:First of all, wow, what a mean and negative post Darwin. I’m truly sorry that I pissed you off so badly. Let me try to be civil as I defend myself.
Really? You’re making me wrong about using the term thought experiment? Here is what Wikipedia says about it…
“A thought experiment or Gedankenexperiment (from German) considers some hypothesis, theory,[1] or principle for the purpose of thinking through its consequences.”
I never mentioned anything regarding gravitational orbits in any of my postings in this thread. Use the control ‘f’ key and search this page.
Regarding superposition, here is a quote from the Wikipedia article…
“the net response at a given place and time caused by two or more stimuli is the sum of the responses which would have been caused by each stimulus individually.”
I take that to mean that the net response of a field (any of the force fields) can be zero at some location and time, while at the same time having a non-zero response from each individual stimulus. That seems to me like mediators of a force occupying (or moving through) a region of zero response.
That is pure conjecture on your part! Now I think I see where you are coming from. My posts here are an honest attempt at understanding physics. If you wish to express new ideas, honestly I am all for that. Just please find a more appropriate way. When I’m wrong I say I’m wrong. Bust me on the facts as understood by the current state of the art in physics, not on the semantics of what a thought experiment is, or how my understanding doesn’t fit your personal theory.
BruceW said:yes, I understand that bit. But I don't get where it makes an analogy to the situation of gravity at the centre of the earth.
Darwin123 said:Any perturbation in density that the inversion operator changes can generate a gravitational dipole. However, one needs an asymmetric distribution of mass.
Nugatory said:You can use stationary electric charges to make good analogies with gravitation; they're both 1/r2 static forces. Moving charges and time-varying fields, not so much.
Dipoles are short range compared to monopoles. Therefore, dipoles have no significant effect on the "large scale". Monopoles have a large effect on the "large scale".MikeGomez said:Yes. I was thinking exactly the same thing right before your post came up. Using the magnetic field was a mistake. I am aware that the magnetic field is based on a time varying charge, that the magnetic flux lines will never be perfectly aligned and perfectly still, that even a bar magnet is only a magnet due the motion of it’s atomic components, etc. I was hoping to get a pass on all that, because (so I thought) that at a large scale those factors would be negligible.
I don't know what your mathematics level is. Mathematically, the inverse of a series of points is a series of points coordinates equal to the negative of the original coordinates.MikeGomez said:Darwin, I really don’t know much about inversion operators. With regards to gravitational dipoles, does this imply the existence of positive and negative gravitational charges at some elementary level? Or is it more of an effect such as you mentioned with the perturbation in density?
Regarding the aversion that some people have regarding gravitational dipoles, perhaps it is because they think it implies the possibility of negative mass. I can’t speak for anyone else, but I know that I personally have been guilty of having prejudices against the idea of negative mass.
There are leprechauns (i.e., virtual particles) jumping around everywhere. Even in a vacuum, in intergalactic space, there are virtual photons balancing each other out. Some names for virtual particles in an area with no measurable field is "vacuum state", "zero point energy", "quantum fluctuations", etc. They aren't a "field".MikeGomez said:If we were not speaking of radio antennae’s, but rather strong gravitational sources, would we not have a similar situation? I am assuming the gravitational equivalent of radiation. Perhaps that is the folly of my thinking.
Darwin says "it doesn’t matter if gravity is mediated by gravitons, space-time, or leprechauns. The Earth may not have a gravitational field at it’s center." I choose leprechauns (in good humor Darwin, I promise). The net effect of the leprechauns at the center of the Earth is zero. But that is because the leprechauns at the center of the Earth balance each other out. The leprechauns themselves still exist at the center of the earth, do they not?
MikeGomez said:Darwin says "it doesn’t matter if gravity is mediated by gravitons, space-time, or leprechauns. The Earth may not have a gravitational field at it’s center." I choose leprechauns (in good humor Darwin, I promise). The net effect of the leprechauns at the center of the Earth is zero. But that is because the leprechauns at the center of the Earth balance each other out. The leprechauns themselves still exist at the center of the earth, do they not?
With real antennae, the radiation will not be coherent. Therefore, the electromagnetic fields don't 'cancel out' and you will get fried by the radiation. At the centre of the Earth (assuming the Earth was spherically symmetric), the gravitational fields due to the mass of the Earth will all cancel out so that the gravitational field at the Earth's centre is zero.MikeGomez said:If we were not speaking of radio antennae’s, but rather strong gravitational sources, would we not have a similar situation? I am assuming the gravitational equivalent of radiation. Perhaps that is the folly of my thinking.
BruceW said:With real antennae, the radiation will not be coherent. Therefore, the electromagnetic fields don't 'cancel out' and you will get fried by the radiation. At the centre of the Earth (assuming the Earth was spherically symmetric), the gravitational fields due to the mass of the Earth will all cancel out so that the gravitational field at the Earth's centre is zero.
But conceptually, I can imagine a situation where coherent sources of electromagnetic radiation do cancel each other out (analogously to standing waves on a string). So that if you were at a specific point between the two (coherent) sources, then you would not get fried by the radiation. I think this is the kind of situation you were trying to describe.
BruceW said:Yeah, that is pretty interesting. I think maybe it is because of radio waves from one antennae getting absorbed by the other antennae, so they 'synchronise' in some way. (Again, I don't know much about antennae, but this seems plausible to me).
For certain, if there was only the radio waves in free space, then the radio waves must be coherent to be able to interfere with each other in a nice way. Perhaps this is what happens, since you were saying the frequencies need to be spot on. I think this is probably easier to do with radio waves than with most other forms of light simply because radio waves have a long wavelength, so you don't need to be as precise in the positioning of your detector to see where constructive and destructive interference occurs.
Edit: for clarity, I am not using the word 'coherent' in the QM context. I am using 'coherent' in the context of classical electromagnetism, i.e. same frequency and constant phase difference.
another Edit: I think we must also place restrictions on the polarisation, for coherence to occur. I would guess that for antennae, the geometry of the antennae sets the polarisation of the light given off in a particular direction from the antennae.
Anywhere you like. Zero in potential is arbitrary. You can take the sport where you sit and say that gravitational potential is precisely zero at that location. Feel gravity? Then your question is answered.Vineeth T said:Can anyone give me a situation when the gravitational potential due is 0 but gravitational field is non-zero.