A Speculative Question About the Nature of Gravity

[bundil]

This seemed like the best place to put something as vague as the question I have, though I will try to make it as clear as possible with my relatively limited knowledge. As far as I'm aware, it's mostly lower-level info, but if a source is needed I will find one. Also I hope this doesn't leave a bad impression as a first post 

As an author, I tend to spend a lot of time thinking, and occasionally odd propositions are distilled from a few dots being drawn quite by accident. It's hard to detail exactly what basis I have for this, but I will try to encompass those points as briefly as I can, as I understand them. If they are wrong or flawed, please tell me, I haven't looked in a physics book since IPC in 2013.

My question is this:

What causes gravity?

My information (epistemology prevents me from confirming any of this is knowledge until verified by a reliable source, but it is as reliable as I can manage) follows.

To whit(ish):

• Gravity is considered a property of matter, directly correlated with mass, with a limited range (in that it has that inverse square relationship with distance).
• Gravity is a force, acting constantly on all things across the universe, but again only intensely and, for the most part, noticeably, near bodies of great mass. Planets, stars, black holes, even dense planetoids and moons, with greater gravitational force from higher mass bodies.
• Gravity from a small, dense object is much more noticeable than gravity from a large, spread-out(?) objects. This is because the bodies upon which the gravity is acting are relatively closer or further away (respective to size) to all of the matter which makes up the object in question. This is also related to the inverse square law of distance, though on a much more local scale than, say, a solar system.
• As a force, gravity is constantly conferring energy to every object in range, directing it towards the source of that gravity and its center of mass. Gravitational potential energy is evidence of this, letting us sleep in one spot without drifting off the planet's surface, or pour a cup of coffee rather than needing a sealed bag to contain the bubble of hot bean juice.
• Energy can be neither created nor destroyed (f...first law of thermodynamics?), only transferred or converted from one form to another.
• Matter can be conferred energy to such a point that it becomes energy, to our understanding, in much the same way energy can compound to become matter (see: stars).
• Energy has mass, enough that a hot cup of coffee is measurably heavier (in Earth-normal gravity) than the same cup after it's cooled to room temperature. It's incredibly less than that of matter, but why else would it be affected by gravity, or display the properties of particles?
• Magnetic fields are not, certainly not directly, altered, augmented, or diminished by gravity, which means they are fundamentally different from electromagnetic radiation and, frankly, much more similar to a gravitational pull.
• Magnetic fields have a direction, which gives the polarization and, of course, the 'north' and 'south' poles of a magnet. However, when acting upon an object that is magnetic but does not possess its own magnetic field, the object is always attracted to the magnet, regardless of polarization.
• Magnetic fields confer kinetic energy to objects that are magnetized to them, or to themselves in the case of fridge letters and the like, to allow this attraction to result in contact. This is again irrespective of gravity, though obviously a gravitational field strong enough to overcome the magnetic attraction would counteract it.
• Magnetic fields also work over a limited range, similar to gravity but (as far as I can find) not per the inverse square law as light and gravity. I don't know if there's an inverse cube law but it sounds right (mathematically speaking, since magnets have two points of force, as opposed to gravity and light which can and typically have a single source for a given instance).
• Energy, electrical and electromagnetic, are affected by magnetic fields and gravity both. It takes considerably more effort to produce a gravitational field as strong as a magnetic one, but it does occur enough that we have math to figure out where behind a gravity field a star's light is coming from.
• Gravity seems caught between the likes of energy (monopolar, inverse square law) and magnetic fields (relentless attraction/conferring of energy to all things subject and proximal to the field, caused by the arrangement of matter rather than the release of energy).
• The Earth's outer core is the source of the geomagnetic field, and the center of the inner core is the centerpoint of gravity on this planet.

For answers:

Before pointing at mass or quantity of mass, which I already listed above, consider that magnetic fields are caused by an alignment of ions either by nature or by force, and electromagnetic energy is expelled by either a conversion from another energy type (electrical, frictional, etc.) or from a reaction (sun, campfires, etc.)

Here's the messy bit, please bear with me:

It seems to me that studying gravity alone is like studying magnetic fields but not magnets. I am not certain where to start to find evidence to support this, but I would hypothesize that gravity is not only a property of matter, and that it is instead caused by some interaction in the same way that magnetic fields, light, radiation, and even the wind is caused by an interaction.

Gravity is a universal force, of course, but that it shares properties with energy and with magnetism leads me to believe (or, at least, to want belief) that it is related to the actual shift between matter and energy. Rather, an interaction similar to what creates the geomagnetic field (geodynamo?) causing gravity, as opposed to it being locally correlated with mass.

Another way to look at my question, I suppose, would be this: bonds between atoms, the ionic/covalent(?) sort (chemistry was not my strong suit, sorry) are a property of matter, but they are caused by interactions between atoms, I think the sharing of electrons or... however the other kind works? Regardless, this fundamental property of matter (bonding) is what allows the most basic structures (molecules) to form. Gravity is what allows an atmosphere and an ocean and a species to cling to the surface of a rock spinning like a madman's top as it orbits an honestly pretty small star. So, to reiterate and (finally, and sorry if there are loose statements) end this tirade:

What causes gravity?

Edited April 18, 2018 by bundil

[beecee]

1 hour ago, bundil said:

What causes gravity?

Gravity manifests itself when mass/energy curves/warps/twists, or acts in anyway on the geometry of flat  spacetime.  That's all we can say.

1 hour ago, bundil said:

• Gravity is considered a property of matter, directly correlated with mass, with a limited range (in that it has that inverse square relationship with distance).

More a property of spacetime in the presence of mass/energy.

[bundil]

25 minutes ago, beecee said:

Gravity manifests itself when mass/energy curves/warps/twists, or acts in anyway on the geometry of flat  spacetime.  That's all we can say.

Where can I read more on/who is preeminent in the particular field of study? I know magnetism is/was considered to be caused at the quantum level but all I can find about the guys is kind of rough to understand.

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More a property of spacetime in the presence of mass/energy.

Okay, so that's not really clear to me, though as I said my understanding is gravity associated with mass. If it's a property of spacetime, why does it need mass/energy to... be, I guess? How does it confer energy, though? Or I suppose the better way to put that is where is the energy with which gravity acts coming from? Without an opposing force, gravity accelerates infinitely to the limit of its center. We rely on the ground to counteract it, and our physiology has evolved directly in relation to it, because it is infinitely relentless.

So, how? What is the active source of gravity? Is that a question still being figured out/mulled over?

34 minutes ago, bundil said:

So, how? What is the active source of gravity? Is that a question still being figured out/mulled over?

...Am I even asking the right question?

[Strange]

1 hour ago, bundil said:

Gravity is considered a property of matter, directly correlated with mass, with a limited range (in that it has that inverse square relationship with distance).

This is the Newtonian view: that gravity is a force (inherently) associated with mass, in the same way the electric/magnetic forces are associated with charge.

(We have an alternative model now, which I will talk about in a moment)

Also, note that an inverse square law means that gravity extends for infinity (but with a decreasing effect). This means that the mass of a single star, for example, becomes insignificant at some distance but the mass of a whole galaxy of stars might still have a measurable effect.

2 hours ago, bundil said:

Gravity from a small, dense object is much more noticeable than gravity from a large, spread-out(?) objects. This is because the bodies upon which the gravity is acting are relatively closer or further away (respective to size) to all of the matter which makes up the object in question. This is also related to the inverse square law of distance, though on a much more local scale than, say, a solar system.

That is sort of correct but could be misinterpreted.

The difference is that you can get closer if the radius is smaller. There is no difference in the actual gravity from two objects of the same mass.

So if you had two objects with the same mass but different densities, then at the same distance the gravity would be the same. For example, if the Sun suddenly turned into a black hole, it would have no effect on the Earth's orbit. (But it would get very cold and dark!)

2 hours ago, bundil said:

As a force, gravity is constantly conferring energy to every object in range

There is, as you say, potential energy inherent in the gravitational field (because energy needs to be expended to move something higher) but there is no transfer of transmission of energy between objects (I don't think that is what you meant but, as lawyers say: "for the avoidance of doubt").

2 hours ago, bundil said:

Matter can be conferred energy to such a point that it becomes energy, to our understanding, in much the same way energy can compound to become matter (see: stars).

Yes. But the occasions when matter is converted to energy (e.g. matter-antimatter annihilation) are pretty rare and insignificant on a grand scale. And the conversion of energy to matter (e.g. pair production) is even rarer. 

The conversion between mass and energy is more common; e.g. fusion in stars.

2 hours ago, bundil said:

Energy has mass, enough that a hot cup of coffee is measurably heavier

True, but I doubt this is measurable (off the top of my head - perhaps we should work it out ...)

2 hours ago, bundil said:

Magnetic fields are not, certainly not directly, altered, augmented, or diminished by gravity, which means they are fundamentally different from electromagnetic radiation and, frankly, much more similar to a gravitational pull.

Electric and magnetic fields are just two aspects of the more general electromagnetic field. Electromagnetic radiation is the propagation of changes in the field. The reason why light, for example, is affected by gravity and a magnetic field isn't, is because it is moving. (A better explanation of this is possible with the "other" explanation of gravity I mentioned before...)

2 hours ago, bundil said:

Magnetic fields also work over a limited range, similar to gravity but (as far as I can find) not per the inverse square law as light and gravity. I don't know if there's an inverse cube law but it sounds right (mathematically speaking, since magnets have two points of force, as opposed to gravity and light which can and typically have a single source for a given instance).

Magnetism is due to the movement of electric charges. The force between two electric charges follows an inverse square law (like gravity) and so extends forever.

As you say, because a magnet is a dipole, the force follows an inverse cube law (and also extends forever). If you could have a magnetic monopole, it would have an inverse square law.

2 hours ago, bundil said:

Energy, electrical and electromagnetic, are affected by magnetic fields and gravity both.

I think that statement is too general to be useful. Energy is not a thing that exists by itself, it is a property of things. 

2 hours ago, bundil said:

Gravity seems caught between the likes of energy (monopolar, inverse square law) and magnetic fields (relentless attraction/conferring of energy to all things subject and proximal to the field, caused by the arrangement of matter rather than the release of energy).

I'm not sure what you mean by "energy" in this statement. It sounds like you might be thinking of electric charge?

2 hours ago, bundil said:

The Earth's outer core is the source of the geomagnetic field, and the center of the inner core is the centerpoint of gravity on this planet.

The gravity of a spherically symmetrical object (a good approximation for the Earth) can be considered to come from the centre. So if the all the mass of the Earth were at the centre, or all concentrated in a thin shell at the surface, we would not see any difference. 

2 hours ago, bundil said:

What causes gravity?

So the modern view (if 100 years counts as "modern") is that gravity is caused by the curvature of the geometry of space-time. And that curvature is caused by the presence of mass (or energy). 

Lets try an analogy. Imagine two people who start walking parallel to one another but a few feet apart. If they are on a flat plane, then there will be no change in the distance between them. For the sake of this analogy, lets call the distance between them "space" and the direction they are travelling "time" (so they are walking into the future at a steady pace).

Now, instead of walking on a flat plane, they are on the surface of the Earth, in the Arctic, walking towards the North Pole. As they move towards the pole (into the future) they get closer together. There is not force pushing them together, it is just the nature of the curved geometry of the surface of the Earth.

Now, simple extend that analogy to four dimensions and it will all make sense!

And this is why light, for example, is affected by gravity: it is travelling in the equivalent of a straight line in curved space-time. 

You might ask, "why does mass cause the geometry of space-time to curve?" And I guess the only answer is, "that is the definition of mass." Science doesn't really deal with the ultimate "why" questions. Newton refused to guess what might cause the force between two masses; it is just there.

That will probably just raise more questions but I hope it helps...

1 hour ago, bundil said:

Or I suppose the better way to put that is where is the energy with which gravity acts coming from?

The energy to accelerate something to the ground, comes from the fact you have to use energy to lift it up in the first place.

So, being on the ground is a lower energy state but there is no energy transfer due to gravity itself.

It doesn't require a source of energy to maintain a static gravitational field. In the same way, it doesn't require any energy to maintain a static magnetic field. A magnet doesn't use up its "power" by being attracted to things.

[swansont]

A better analogy to the Newtonian relation between mass and gravity is charge and electric field, as they are both 1/r^2 dependencies. I would say that our understanding (or lack thereof) of why charge causes an electric field is similar to our understanding of why mass causes gravity. But people focus on gravity as being the mystical one, for some reason.

[bundil]

8 hours ago, Strange said:

This is the Newtonian view: that gravity is a force (inherently) associated with mass, in the same way the electric/magnetic forces are associated with charge.

(We have an alternative model now, which I will talk about in a moment)

Also, note that an inverse square law means that gravity extends for infinity (but with a decreasing effect). This means that the mass of a single star, for example, becomes insignificant at some distance but the mass of a whole galaxy of stars might still have a measurable effect.

That is my understanding of it. Something likely to be repeated quite a bit: sorry for poor wording.

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That is sort of correct but could be misinterpreted.

The difference is that you can get closer if the radius is smaller. There is no difference in the actual gravity from two objects of the same mass.

So if you had two objects with the same mass but different densities, then at the same distance the gravity would be the same. For example, if the Sun suddenly turned into a black hole, it would have no effect on the Earth's orbit. (But it would get very cold and dark!)

So acceleration due to gravity (and the extent of its effective reach before being relegated to "negligible") in the presence of a consistent mass will only change due to distance from the whole object, rather than with density?

(I'd also hope significantly that the Sun doesn't turn into a black hole. Isn't it pretty far below the minimum mass anyways?)

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There is, as you say, potential energy inherent in the gravitational field (because energy needs to be expended to move something higher) but there is no transfer of transmission of energy between objects (I don't think that is what you meant but, as lawyers say: "for the avoidance of doubt").

You are correct, I mean more that I don't understand how gravity applies a force in much the same way as friction, but only with proximity? 

Maybe an example of my confusion is that, say, wind is a property of atmospheric pressure and gasses moving, but everything from the density of the air to the gaseous molecules themselves have energy differences and, typically, confer energy in some way or another when interacting with other objects, say, a kite.

Gravity, however, doesn't seem to use particles, certainly not the way wind or light does, while it must constantly be worked against. Getting out of your chair is an exhibition of force applied against gravity. It doesn't seem sensible that there is nothing about gravity which demonstrates the need to expend energy against even just to walk across the street, though that's obviously less energy needed than to, say, do a pull-up, or freeclimb a cliff face. 

How about this: when I lift a rock from the ground and hold it in front of me, I expended energy of my own to counteract gravity, and so long as I hold the stone it has potential energy because of gravity.

What's the difference in the rock's potential energy from on the ground to where I'm holding it? Nature could open up a sinkhole right under that rock and drop it another six hundred feet, or it could sit by a riverbed until erosion turns it to sand. Does it not still have potential energy due to gravity, which is acting on it at all times?

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Yes. But the occasions when matter is converted to energy (e.g. matter-antimatter annihilation) are pretty rare and insignificant on a grand scale. And the conversion of energy to matter (e.g. pair production) is even rarer. 

The conversion between mass and energy is more common; e.g. fusion in stars.

I'm sorry, I'm not understanding your distinction between matter and mass in this. Is mass not a property? Or maybe I'm misunderstanding the relationship between matter and mass?

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True, but I doubt this is measurable (off the top of my head - perhaps we should work it out ...)

I'm fairly certain it's negligible until you get to very large quantities of coffee, and even Starbucks won't make that much for one order.

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Electric and magnetic fields are just two aspects of the more general electromagnetic field. Electromagnetic radiation is the propagation of changes in the field. The reason why light, for example, is affected by gravity and a magnetic field isn't, is because it is moving. (A better explanation of this is possible with the "other" explanation of gravity I mentioned before...)

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Magnetism is due to the movement of electric charges.

I am sure I don't understand, can you restate or clarify these?

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I think that statement is too general to be useful. Energy is not a thing that exists by itself, it is a property of things. 

My intent was "electromagnetic and electrical energy (light, & energy transferred along power lines, e.g.)".

Most of my knowledge is too general to be useful. 

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I'm not sure what you mean by "energy" in this statement. It sounds like you might be thinking of electric charge?

Back to the "too general for use" bit, I was trying to describe gravity as sharing properties with magnetic fields and with electric fields. It's not the deepest observation, I know, but nature likes its patterns and it seems... relevant?

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The gravity of a spherically symmetrical object (a good approximation for the Earth) can be considered to come from the centre. So if the all the mass of the Earth were at the centre, or all concentrated in a thin shell at the surface, we would not see any difference. 

Okay, here's that bit from before with the mass/density relationship to gravity, and I probably made a mess of that statement so I'll try again:

Our understanding of Earth's gravity is at its surface, and has an acceleration effect of 9.8m/s². If all the Earth's mass were condensed into a ball the size of, say, the outer and inner core, acceleration due to gravity would still be 9.8m/s² at the distance from the center that the surface was prior to that.

So, would the acceleration due to gravity not increase were you to measure from the new surface of the condensed planet, which has a considerably higher density and the same mass? Increasing density while maintaining mass necessitates reducing the volume, and therefore the radius, right?

 

Similarly, if we had that hollow shell planet where all of the mass of the Earth became focused at the crust, what would gravity be like inside that shell? Would you be pulled/flung into the "ceiling" by gravity/inertia (the planet is still rotating in this hypothetical), or would you be drawn towards the center void, which is still the "center" of the planet's mass (in a sphereically symmetrical object as described)? Sorry if this is kind of a wandering question I'm in the dark with a box of matches.

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So the modern view (if 100 years counts as "modern") is that gravity is caused by the curvature of the geometry of space-time. And that curvature is caused by the presence of mass (or energy). 

I think it's the contemporary view?

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You might ask, "why does mass cause the geometry of space-time to curve?" And I guess the only answer is, "that is the definition of mass." Science doesn't really deal with the ultimate "why" questions.

I would actually ask how, but that's close enough for a question without an answer (yet) that I'll concede it's the limit of the conversation.

As for the analogy, I guess the need the would be a cause for why warping spacetime doesn't seem to affect magnetic fields? Or... how does a magnetic field ignore spacetime? I feel like I'm closer to the right question

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The energy to accelerate something to the ground, comes from the fact you have to use energy to lift it up in the first place.

So, being on the ground is a lower energy state but there is no energy transfer due to gravity itself.

It doesn't require a source of energy to maintain a static gravitational field. In the same way, it doesn't require any energy to maintain a static magnetic field. A magnet doesn't use up its "power" by being attracted to things.

I don't understand what you are saying, though I suppose the bit above regarding potential energy will leak into this topic as well so I'll let you decide if you want to merge them.

Thanks a ton, Strange, seriously. Not a tonne, though.

 

7 hours ago, swansont said:

A better analogy to the Newtonian relation between mass and gravity is charge and electric field, as they are both 1/r^2 dependencies. I would say that our understanding (or lack thereof) of why charge causes an electric field is similar to our understanding of why mass causes gravity. But people focus on gravity as being the mystical one, for some reason.

I see. I'll read into electric fields more, then.

 

D'you think that's because we feel like humanity has grasped electricity as a tool, as rudimentary as it may end up being in the grand scheme of things we don't know, while gravity remains squarely in the realm of "we know what it does, but not enough about how to affect change"?

[Strange]

7 minutes ago, bundil said:

So acceleration due to gravity (and the extent of its effective reach before being relegated to "negligible") in the presence of a consistent mass will only change due to distance from the whole object, rather than with density?

Correct.

7 minutes ago, bundil said:

I'd also hope significantly that the Sun doesn't turn into a black hole. Isn't it pretty far below the minimum mass anyways?

Yep. We absolutely don't need to worry abut that happening. It is just a nice example.

8 minutes ago, bundil said:

I am sure I don't understand, can you restate or clarify these?

Electric charge is the fundamental property. Individual particles (e.g. electrons) can have a charge. If that charged particle moves, this generates a magnetic field (this can be explained by special relativity, but we probably don't need to get into that).

This all goes back to Faraday's experiments where he found that an electric current in a wire generates a magnetic field. And a changing magnetic field can generate an electric current in a wire. This was then turned into a mathematical description by Maxwell (and others) which is when it was realised that this described how changing electric and magnetic fields could generate a wave that travels through space. This turned out to be light.

22 minutes ago, bundil said:

I mean more that I don't understand how gravity applies a force in much the same way as friction

There is a big difference here. Friction will only oppose a force you apply. It isn't a force in itself.

The force between two masses is, as swansont says, is equivalent to the force between two charges. You can think of them both as being caused by a "disturbance in the field" (in the first case the field is space-time, in the second it is the electromagnetic field).

There are no particles involved in either case. (In quantum theory, the force is described in terms "virtual particles" but they are not really particles.)

30 minutes ago, bundil said:

Gravity, however, doesn't seem to use particles, certainly not the way wind or light does, while it must constantly be worked against. Getting out of your chair is an exhibition of force applied against gravity.

Getting out of your chair requires energy. But just sitting there doesn't. So energy is only expended when you lift yourself against gravity (i.e. add to your potential energy).

32 minutes ago, bundil said:

What's the difference in the rock's potential energy from on the ground to where I'm holding it? Nature could open up a sinkhole right under that rock and drop it another six hundred feet, or it could sit by a riverbed until erosion turns it to sand. Does it not still have potential energy due to gravity, which is acting on it at all times?

The potential energy is given by m x g x h (mass of the rock times the acceleration due to gravity times the height you lift it). We are only ever concerned with differences in potential energy so if a hole opens up underneath you, that doesn't change the energy you have given it.

34 minutes ago, bundil said:

Similarly, if we had that hollow shell planet where all of the mass of the Earth became focused at the crust, what would gravity be like inside that shell?

There wouldn't be any! It would all cancel out. You would be close to a small amount of the shell on one side but there would be a larger amount of the shell on the other side pulling on you from further away. This all balances.

Newton's Shell Theorem: https://en.wikipedia.org/wiki/Shell_theorem

36 minutes ago, bundil said:

As for the analogy, I guess the need the would be a cause for why warping spacetime doesn't seem to affect magnetic fields? Or... how does a magnetic field ignore spacetime? I feel like I'm closer to the right question

To be honest, I am not sure how electromagnetism works in curved space-time. So I will refrain from commenting further!

 

[swansont]

1 hour ago, bundil said:

 D'you think that's because we feel like humanity has grasped electricity as a tool, as rudimentary as it may end up being in the grand scheme of things we don't know, while gravity remains squarely in the realm of "we know what it does, but not enough about how to affect change"?

That's probably part of it. Electrostatics allows for cancellation and shielding, which are not possible for gravity, so the ability to control these aspects may add to the illusion that we understand the basics better.

And as some anecdotal support for this idea, there have been a number of people who have posted here, insisting that if we understood gravity better we could become more adept at manipulating it (leading to shielding or even antigravity). I disagree with that conclusion, but the sentiment surely exists. 

[beecee]

13 hours ago, bundil said:

Where can I read more on/who is preeminent in the particular field of study? I know magnetism is/was considered to be caused at the quantum level but all I can find about the guys is kind of rough to understand.

The following video is around 8 minutes long, and gives a nice simplistic picture of the history of Newtonian and Einsteinian gravity and how each describe with varying degrees of accuracies, how gravity operates.

 

Edited April 18, 2018 by beecee

[bundil]

1 hour ago, Strange said:

Electric charge is the fundamental property. Individual particles (e.g. electrons) can have a charge. If that charged particle moves, this generates a magnetic field (this can be explained by special relativity, but we probably don't need to get into that).

This all goes back to Faraday's experiments where he found that an electric current in a wire generates a magnetic field. And a changing magnetic field can generate an electric current in a wire. This was then turned into a mathematical description by Maxwell (and others) which is when it was realised that this described how changing electric and magnetic fields could generate a wave that travels through space. This turned out to be light.

Okay, so the same thing that determines ions is what generates a magnetic field, regardless if it's due to a ferromagnetic substance or an electromagnet (such as what's used at scrapyards)? That's... interesting, but I'm not sure what to do with the info. Maybe it'll come up in a year when I'm staring at my ceiling again? 

1 hour ago, Strange said:

There is a big difference here. Friction will only oppose a force you apply. It isn't a force in itself.

The force between two masses is, as swansont says, is equivalent to the force between two charges. You can think of them both as being caused by a "disturbance in the field" (in the first case the field is space-time, in the second it is the electromagnetic field).

There are no particles involved in either case. (In quantum theory, the force is described in terms "virtual particles" but they are not really particles.)

Okay, so friction is limited extremely to only appearing in opposition to an applied force, and is... just is? Is this the real energy tax?

I guess I'm asking a bunk question if it's "how does this field exist and exert itself", huh?

2 hours ago, Strange said:

The potential energy is given by m x g x h (mass of the rock times the acceleration due to gravity times the height you lift it). We are only ever concerned with differences in potential energy so if a hole opens up underneath you, that doesn't change the energy you have given it.

Ah, so it's the wrong tree to bark up. Makes sense.

2 hours ago, Strange said:

There wouldn't be any! It would all cancel out. You would be close to a small amount of the shell on one side but there would be a larger amount of the shell on the other side pulling on you from further away. This all balances.

Newton's Shell Theorem: https://en.wikipedia.org/wiki/Shell_theorem

That page makes me wish I had taken more math in college. Yeesh.  

That in mind, you say the interior of the shell would have a net gravitational acceleration of zero, but would the spin of the planet not still apply an inertial force? I would think that the planet spins fast enough, but I've been wrong before.

2 hours ago, Strange said:

To be honest, I am not sure how electromagnetism works in curved space-time. So I will refrain from commenting further!

Oh, in that case don't feel too bad! Any advice on papers or publications to read on that subject?

 

1 hour ago, swansont said:

That's probably part of it. Electrostatics allows for cancellation and shielding, which are not possible for gravity, so the ability to control these aspects may add to the illusion that we understand the basics better.

Something something, 'the arrogance of man'?

1 hour ago, swansont said:

And as some anecdotal support for this idea, there have been a number of people who have posted here, insisting that if we understood gravity better we could become more adept at manipulating it (leading to shielding or even antigravity). I disagree with that conclusion, but the sentiment surely exists. 

S-something someth-thing, a-arrogance of man??

More seriously, I can understand the sentiment of excitement but it seems to be very much the unknown and I would predict propulsion induced by electromagnetic systems long before a grasp of gravity.

 

1 hour ago, beecee said:

The following video is around 8 minutes long, and gives a nice simplistic picture of the history of Newtonian and Einsteinian gravity and how each describe with varying degrees of accuracies, how gravity operates.

That was informative! I find it a bit difficult imagining three-dimensional space like what we live in as a flat plane but then they had the guy standing in the cube grid and it got way easier. Makes me wish we could still use the term aether instead of "void" or "vacuum of space". 

[Mordred]

The field of study suitable to understand how Electromagnetism and gravity work together in the same region of spacetime is gravitoelectromagnetism. Though it is very math heavy the relevant field equations are in this paper.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://arxiv.org/pdf/gr-qc/0011014&ved=2ahUKEwihkI3rgsXaAhWJqVQKHRS2Bm0QFjAAegQICRAB&usg=AOvVaw1-oqasirIHBCIGCsn4AyiX

Its an arxiv paper working from phone

[beecee]

3 hours ago, bundil said:

Makes me wish we could still use the term aether instead of "void" or "vacuum of space". 

I'm  ready to be corrected here, but the ether, as in the stuff Michelson and Morley invalidated, was thought to be a "physical substance"....Spacetime on the other hand, though certainly seen as real, is a non physical entity, in which we  locate events and describe them in terms of spatial coordinates and time. The concept of spacetime follows from the observation that the speed of light is constant and invariant, and in which light always follows geodesics. Whereas prior to M+M, most thought light needed this "physical substance"which we called the ether, in which to traverse.

Edited April 19, 2018 by beecee

[Mordred]

Fairly accurate, it waa thought to be a type of matter field as opposed to a force field ie electromagnetic. The term field is acceptable and no one will question it.

Edited April 19, 2018 by Mordred

[bundil]

2 hours ago, Mordred said:

The field of study suitable to understand how Electromagnetism and gravity work together in the same region of spacetime is gravitoelectromagnetism. Though it is very math heavy the relevant field equations are in this paper.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://arxiv.org/pdf/gr-qc/0011014&ved=2ahUKEwihkI3rgsXaAhWJqVQKHRS2Bm0QFjAAegQICRAB&usg=AOvVaw1-oqasirIHBCIGCsn4AyiX

Its an arxiv paper working from phone

I will read it, thanks!

26 minutes ago, beecee said:

I'm  ready to be corrected here, but the ether, as in the stuff Michelson and Morley invalidated, was thought to be a "physical substance"....Spacetime on the other hand, though certainly seen as real, is a non physical entity, in which we  locate events and describe them in terms of spatial coordinates and time. The concept of spacetime follows from the observation that the speed of light is constant and invariant, and in which light always follows geodesics. Whereas prior to M+M, most thought light needed this "physical substance"which we called the ether, in which to traverse.

Yes, though I more meant appropriate it for a new definition that does not necessitate the presence of matter. It's not like words get new literal definitions as common use demands or anything. I dunno, maybe I just prefer a warmer word than "void". 

[Mordred]

I forgot to add another related study is Kaluza Klien gravity which couples gravity to electromagnetism using a mathematical 5th dimension.

(dimension means any independant variable that can change without affecting any other variable)

Here is an arxiv for that.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://arxiv.org/abs/gr-qc/9805018&ved=2ahUKEwihqezcqMXaAhVJsVQKHRqMDnIQFjAHegQIBhAB&usg=AOvVaw070frg753xw0_wjdOr61HV

 This theory has historical importance as it is one of the earlier theories towards unification. The techniques towards unification today arise from this theory. (in the lessons learned)  It is part of the post graduate curriculum in cosmology to unification development. An interesting tidbit Klein imposed a condition in that the 5th dimension at each coordinate is a circle represented today by the U(1) Lie symmetry group. This circle being too small to be measurable. This is in some literature referred to as cylindricity  This 5tth dimension key feature is that it is unobservable. U(1) is oft referred to as the circle group for this reason. This will provide hints on the mathematics (antisymmetric) for the electromagnetic field.

Edited April 19, 2018 by Mordred

[swansont]

11 hours ago, bundil said:

 Something something, 'the arrogance of man'?

S-something someth-thing, a-arrogance of man??

Sure. Except in many cases we now know this better as the Dunning-Kruger effect.

[bundil]

8 hours ago, swansont said:

Sure. Except in many cases we now know this better as the Dunning-Kruger effect.

I've always felt that one is a bit of a double-edged sword. After all, if it turns out you really don't know what you're on about, but you brought up Dunning-Kruger...

 

15 hours ago, Mordred said:

I forgot to add another related study is Kaluza Klien gravity which couples gravity to electromagnetism using a mathematical 5th dimension.

(dimension means any independant variable that can change without affecting any other variable)

Here is an arxiv for that.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://arxiv.org/abs/gr-qc/9805018&ved=2ahUKEwihqezcqMXaAhVJsVQKHRqMDnIQFjAHegQIBhAB&usg=AOvVaw070frg753xw0_wjdOr61HV

 This theory has historical importance as it is one of the earlier theories towards unification. The techniques towards unification today arise from this theory. (in the lessons learned)  It is part of the post graduate curriculum in cosmology to unification development. An interesting tidbit Klein imposed a condition in that the 5th dimension at each coordinate is a circle represented today by the U(1) Lie symmetry group. This circle being too small to be measurable. This is in some literature referred to as cylindricity  This 5tth dimension key feature is that it is unobservable. U(1) is oft referred to as the circle group for this reason. This will provide hints on the mathematics (antisymmetric) for the electromagnetic field.

Well, I'll certainly give it a read, but I can't promise solid comprehension of post-graduate topics when (as stated before) I don't have the pre-graduate background. Thanks again, though!