Gravity in a void

[JonVisc]

Repost from the general thread. For some reason I didn't see the Physics or Math sections the first time I logged in, my apologizes.

 

 

I was reading The Fabric of the Cosmos (okay, listening to the book-on-tape during my drive home...) by Briane Greene.

 

He was explaining how Newton thought about space and absolute-space and brought up Mach's (I believe it was Mach anyway) example about being in a void.

 

Nothing around just a black void, and then you started spinning. Newton explained that once you started spinning your hands would want to move away from your body. Mach didn't think that would be the case, to him it was because if there wasn't anything in space there wouldn't be anything pulling your hands away from your body, no outside gravity.

 

Okay, I can accept that, you need objects to influence gravity, but going a step further (and if someone has read the book, please let me know because I heard that part yesterday and haven't listened to anymore yet so they may have explained it) if there was only one sun or planet or body in space, wouldn't all gravity work on that body alone? Meaning that if you were in a void you couldn't begin to raise your arms because the force of gravity on you would be to great. Would all gravity only be working exactly from your core pulling you in on yourself? Perhaps crushing you into a single point? Which from there maybe something happens and BANG(and its a big one ;-)

 

Because it seems to me that the less amount of stuff in the universe, the more gravity would work on all the remaining stuff. If there was a planet the size of earth and it was the only thing in the void, ALL the gravity that should be working on everything else (earth included) is now just working on the earth? Which would be such pressure and force it couldn't do anything but shrink til it is a single point. Then since gravity doesn't have anything to work on anymore the point would be allowed to grow then?

 

I seem to keep getting myself into this loop of what might happen and am wondering if anyone with more knowledge knows about this and the outcome.

[DQW]

Okay, I can accept that, you need objects to influence gravity, but going a step further (and if someone has read the book, please let me know because I heard that part yesterday and haven't listened to anymore yet so they may have explained it) if there was only one sun or planet or body in space, wouldn't all gravity work on that body alone?

Yes, but also, that body will be the only source of gravity. Remember, mass creates gravity.

 

Meaning that if you were in a void you couldn't begin to raise your arms because the force of gravity on you would be to great.

Not at all. The force of gravity on you would be completely unnoticeable.

 

Would all gravity only be working exactly from your core pulling you in on yourself?

Yes, but since "you" are the only source of gravity, this force will be small.

 

Perhaps crushing you into a single point? Which from there maybe something happens and BANG(and its a big one ;-)

That's like bad sci-fi ! <help>

 

Because it seems to me that the less amount of stuff in the universe, the more gravity would work on all the remaining stuff.

The less stuff there is in the universe, the less gravity there is in the universe.

 

If there was a planet the size of earth and it was the only thing in the void, ALL the gravity that should be working on everything else (earth included) is now just working on the earth?

1. No, that's not true. Gravity does not distribute itself depending on "how many people there are, to eat the pie". This is known as the superposition principle.

 

2. If you remove objects from the universe, you also remove gravity from the universe. Finally, you are left with the tiny bit of gravity caused by the last remaining body.

 

Which would be such pressure and force it couldn't do anything but shrink til it is a single point. Then since gravity doesn't have anything to work on anymore the point would be allowed to grow then?

Gravity does not follow the laws of supply and demand. It does not switch on and off based on need.

 

I seem to keep getting myself into this loop of what might happen and am wondering if anyone with more knowledge knows about this and the outcome.

Outcome ? Nothing fancy. You'd have a very boring universe.

[oatjay]

well put.

 

in the book, the main question at hand is whether or not you'd experience centripetal force if there is no frame of reference to base your spinning. i'm pretty sure the question had very little to do with the gravitational effects (or lack thereof) of other bodies in the universe.

[J.C.MacSwell]

One "machian" question would be whether in this "Earth only Universe" would it be much, much easier to shake your fist.

[MetaFrizzics]

Look at the overview:

 

(1) You are talking about Newtonian Gravity, not General Relativity.

 

(2) You are talking about the historical (semi-updated) reformulation of Newton's work.

 

(3) Whether talking about planets or people floating in space, you are talking about characterizing whole 'systems' , not point-particle behaviour.

 

(4) One key philosophical and 'causal' problem in Newtonian gravity is angular velocity and centrepital force.

 

Problem One: Measurement

Mach and other critics of Newton's version of gravity made the following argument: If all motion is only detectable relative to other bodies, then motion is relative. (true relativity, not to be confused with Einstein's theories). We should be able to express all laws of motion in a relative (inertial frame-free) and equally simple way.

Counter-problem: If rotation can be detected without reference to other bodies, simply by measuring centripital force (e.g., equatorial bulge), then rotation is absolute, not relative, and independant of other bodies.

Problem Two: the CAUSE of centrepital force

Because of Newton's own 'Sphere Theorem', he was forced to abandon an explicit physical 'cause' of rotational effects, and postulate 'Absolute Space', a unique frame of reference for rotation. Unfortunately his other laws allow an infinite number of 'inertial frames', all equally valid as the final unique frame of reference, for translational motion (straightline velocities).

Mach countered that the only possible explanation for rotational effects must come from 'outside' the object, i.e., rotational motion was relative to the rest of the mass of the universe (the backdrop of stars) and hence must be caused by interaction with that mass. Mach envisioned a reformulation of gravity theory that would explain rotational effects straightforwardly as a force due to the backdrop of stars.

 

Newton never explained the cause rotational effects, only described them by analogy to translational laws of motion in a way that could preserve the laws of action-reaction and the conservation of energy.

 

And Mach never found a way to reformulate gravity to account for rotational effects via the backdrop of stars. He left this to his student Albert Einstein. Unfortunately, when Einstein was done, it was eventually noticed that Einstein also failed to carry out Mach's program of explaining gravity, inertia and rotation relatively.

 

General Relativity (in spite of Einstein's muddled early claims) does not account for rotational effects via the remaining mass in the universe. In fact, in the Solution to Gen Rel for a single spinning star, there is still centripital force and effects, even in an otherwise completely empty universe.

 

Einstein's 'Field' has an existance all its own, and is no better a solution philosophically speaking than Newton's "Absolute Space", which also was an artificial immeasurable construct that had a life of its own.

 

It remains true that there is no non-circular way to measure or establish rotational motion and its effects other than relative to other objects and masses. Even the Field Equations of General Relativity require a mass distribution which defines the field.

[MetaFrizzics]

It is ironic however that Einstein's plan (SRT) to follow his mentor Mach's lead and eliminate Absolute Space later backfired in his face too, as his (GRT) requires Absolute Motion relative to the G-field. Mach died disappointed, and Einstein quietly sidestepped the issue of 'eliminating the aether', and admitted he had not adequately solved the 'bucket experiment'.

 

In fact, Einstein clarified the remaining dilemma: In his 1916 paper he explicitly reformulates the thought-experiment as two globes rotating relative to each other in empty space along their axis of connection:

 

 

In (Einstein 1916, pp112-113) Einstein, in conformity with his understanding of the law of causality, restates Mach's thesis that the only possible explanation for any difference in the 'bulging' between globe 1 and 2 must lie in their mutual motion to distant masses (background of stars).

 

Einstein merely echoes the relationist tradition that phenomenae of rotation (equatorial bulge) be explained purly in terms of relative motions of bodies. What is novel, ironic, and embarrassing, is Einstein's implicit claim that GTR actually conforms to his demand. There is no basis for this latter claim, as Einstein later quietly acknowledged at various times many years later.

[buzsaw]

Yes, but also, that body will be the only source of gravity. Remember, mass creates gravity.

 

I'm not educated in physics, but thinking logically, I agree. In a void, there would be nothing to draw ones arms down, or in any particular position as it's the earth's gravity which pulls them down. It seems that if one's total body began spinning, ones arms would simply position wherever one positioned them since there would be no gravity powerful enough to cause a significant gravitational effect.

[buzsaw]

If this question is ok for this thread I'd like to address the void.

 

1. If the earth were the only object in a void, I'd call the void "unbounded space." Do you agree? If not, why not?

 

2. I'd also say the void/space would have no properties capable of it's expansion. Yes or no and why or why not?

 

3. Would gravity be a factor in either of the above problems?

[MetaFrizzics]

(1) By thinking that in a void there would be no force suggests the corollary of Mach that 'substance' out in space (i.e., distant masses etc) *should* cause a force on the spinning object. It is in effect a bold claim to 'explain' centrepital forces and rotational effects by a hitherto unconfirmed cause, based upon the appearances of physical phenomenae which we can't explain now.

 

(2) An account of forces within and across any system of masses must still be made, with or without a void. (i.e., is there a force of gravity acting between particles in the local system? and what kind of force is it that will explain predicted effects?)

[buzsaw]

(1) By thinking that in a void there would be no force suggests the corollary of Mach that 'substance' out in space (i.e.' date=' distant masses etc) *should* cause a force on the spinning object. It is in effect a bold claim to 'explain' centrepital forces and rotational effects by a hitherto unconfirmed cause, based upon the appearances of physical phenomenae which we can't explain now.

 

(2) An account of forces within and across any system of masses must still be made, with or without a void. (i.e., is there a force of gravity acting between particles in the local system? and what kind of force is it that will explain predicted effects?)[/quote']

 

In the physics classroom, does a void, by definition mean there would be no properties, such as particles, et al?

[MetaFrizzics]

In standard Newtonian Physics, the 'void' of space has no properties other than the 'axiom' concerning absolute rotation and a single unique 'rest frame'.

 

In various formulations of Quantum Mechanics, the 'void' is more complex, allowing instantaneous particle generation and annilation for very short periods of time. (its popping like popcorn, and the trails vanish like playful ghosts.)

 

Some of this 'randomness' built into QM 'space' allows quantum tunnelling of particles and energy out of 'black holes', while other particle pair formations allow extremely improbable events to happen. However, even in the 'loose cannon' spacetime of QM, there are rules about preservation and destruction of energy and information based upon symmetry...

[buzsaw]

In standard Newtonian Physics' date=' the 'void' of space has no properties other than the 'axiom' concerning absolute rotation and a single unique 'rest frame'.

 

In various formulations of Quantum Mechanics, the 'void' is more complex, allowing instantaneous particle generation and annilation for very short periods of time. (its popping like popcorn, and the trails vanish like playful ghosts.)

 

Some of this 'randomness' built into QM 'space' allows quantum tunnelling of particles and energy out of 'black holes', while other particle pair formations allow extremely improbable events to happen. However, even in the 'loose cannon' spacetime of QM, there are rules about preservation and destruction of energy and information based upon symmetry...[/quote']

 

My question pertained specifically to the meaning of the word "void," that is a total void. Doesn't that mean void of any existing thing, including particles and black holes?

[MetaFrizzics]

I think in that case a philosopher would want to 'predefine' the word before use.

The 'classical' or common-sense definition you are connecting to 'void' is probably based on a combination of modern usage by non-scientists and dictionary attempts.

 

I would imagine that by etymology it is an imported word, possibly Latin or Greek in origin, and borrowed into English by philosophers for the express purpose of starting with a 'clean' word they could define themselves, as opposed to say 'empty space'. But it might have originally been a religious term to do with hell or purgatory coined first by theologians. Perhaps a history of English usage encyclopedia could help.

[buzsaw]

I think in that case a philosopher would want to 'predefine' the word before use.

The 'classical' or common-sense definition you are connecting to 'void' is probably based on a combination of modern usage by non-scientists and dictionary attempts.

 

I would imagine that by etymology it is an imported word' date=' possibly Latin or Greek in origin, and borrowed into English by philosophers for the express purpose of starting with a 'clean' word they could define themselves, as opposed to say 'empty space'. But it might have originally been a religious term to do with hell or purgatory coined first by theologians. Perhaps a history of English usage encyclopedia could help.[/quote']

 

It appears that mainline scientists of the majority opinion and with the great majority of thought advancement in their favor, take it upon themselves to allow themselves the priviledge of revising definitions to advance the thought which is prevalent in their interpretations of what is observed. This seems to leave us with no acceptable scientific term with which to communicate or describe the common usage of the word void, that is a term in which absolutely nothing would exist. If that's the case, it appears that I've said all that I'm able to say, for I see no way possible to communicate what I wish to communicate since there appears to be no acceptible terms for me to use for such communications.

[MetaFrizzics]

If that's the case, ... I see no way possible to communicate what I wish to communicate since there appears to be no acceptible terms for me to use...

 

All is not lost by any means. Every philosopher worth his salt who wants to discuss a concept simply offers a brief preamble clarifying any terms that he thinks might be misunderstood because of such circumstances as above.

 

Your original 4th question "in a physics classroom" is easy to answer:

In the physics classroom, does a void, by definition mean there would be no properties, such as particles, et al?

 

(a) In a *high-school* classroom where students are not expected to have formed strong careful opinions, the teacher would probably go out his way to indicate what exactly is meant. Then a discussion like the one we are having about advanced physics might follow, qualified by an informal approach without mathematical details, simply to introduce a few surprising concepts like 'particles springing out of nothing'. This would be more for the purpose of arousing interest and stretching the students' minds to consider ideas they may not have heard of before.

 

(2) In a more advanced class, like an introductory physics course in University or College, there would probably be much more careful discussion of definitions, especially in the context of problems handed out to the students which they are expected to solve properly. So here it would be more important to make clear what treatment was expected by the professor in order to achieve full marks.

 

Now to your original 3 questions:

1. If the earth were the only object in a void, I'd call the void "unbounded space." Do you agree? If not, why not?

 

2. I'd also say the void/space would have no properties capable of it's expansion. Yes or no and why or why not?

 

3. Would gravity be a factor in either of the above problems?

(1) This seems reasonable provided you are willing to commit to Newtonian Physics for your world-view for the purpose of discussion. By the way, there is no reason why you can't believe in Newtonian Mechanics instead of General Relativity. Many scientists do, and are skeptical of General Relativity for various reasons.

 

(If you embrace General Relativity, you appear to be committed to the physical existance of *not* space, but a 4-dimensional 'spacetime' which has real properties like a flexible geometry that is affected by the distribution of mass in it. In this case, you can't just pretend there is something called 'empty space'.)

 

(2) The issue of 'expansion' is an interesting one. For instance we again have to commit at least tentatively to a physical philosophy to help define our terms and describe 'acceptable' cases of experiments to verify our theoretical ideas. For instance, you might embrace the idea of Machian Relativity of Motion, in which case, if there were only two objects available, you might not be able to distinguish whether the two objects were expanding side by side, or simply moving toward each other. You'd need a third object to further clarify what was meant or expected.

 

On the other hand, you might feel that ideas like 'expansion' can only have physical coherent and reliable meaning when they are referenced to objects that have mass and extension in space. Then you might argue that there can be no discussion of 'expansion' of 'space' since this philosophical concept is just a meaningless abuse of a term.

 

(3) Yes gravity would be a factor in the above problems, because you'd have to decide or try out the question of whether gravity was a 'true force' in the Newtonian sense, or just an illusion caused by the curvature of spacetime etc. These are not easy problems to make up your mind on, but are deep philosophical questions at the very core of our understanding of the world around us and our ability to grasp it.

[buzsaw]

These are not easy problems to make up your mind on, but are deep philosophical questions at the very core of our understanding of the world around us and our ability to grasp it.

 

Thanks very much, MF, for going to the work and time to explain as you have in a kindly manner, especially your comments pertaining to my item one question. I do go on the old Newtonian ideology, but seem to get in hot water, using it in science forums.

 

Imo, modern mainline science uses at least some of the illogical relative physics like some of QM to tiptoe around the tulips in the garden of the scientific unknowns.