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CosmicUnderstanding
2011-Jan-16, 08:40 PM
After browsing through some threads here, I see we have some formula(s) for determining how much influence one body has on another depending upon the size, mass, distance etc. Now I know the farther away from a planet or galaxy one gets, the less gravitational pull one will feel.

My question is - Does this force ever drop off entirely across vast expanses, or, for example could a cluster of galaxies tens of billions of light years away still have *some* very very small influence on us from that far away? Perhaps not even enough to detect with our instruments, but what does the math say about all of it?

caveman1917
2011-Jan-16, 09:18 PM
As distance increases it approaches zero but never quite reaches it, so it will always have some very small effect. But in a universe with accelerating expansion there will be places that never have, and never will have an influence on us, but that is due to the acceleration of expansion, not the laws of gravity per se.

a1call
2011-Jan-16, 09:26 PM
I think part of the your question can be rephrased as:
*- is there a gravity quantum (aka graviton) below which no gravitational effects can exist?
That is an open question but there are indirect evidences that gravitons do exist.
Then again even if they do exist and they are anything like photons then their minimum quantity would be a factor of frequency and since frequency is unbounded then there is no absolute minimum quantity. IE there is only a minimum quantity per frequency.
Another limiting factor for gravity is what limits the furthest light sources aka the limited age of the universe. Since gravitational pull propagates at the speed of light there are distance beyond which the pull can not affect us since there has not been enough time/age to the universe for the effect to reach us.
Then again I started an IMHO unresolved thread where I asked that, if we could be affected indirectly by objects further than the age of the universe would permit by observing their effects on objects in between us.

Perhaps someone else can clarify things further.

neilzero
2011-Jan-16, 11:34 PM
As the other posts imply, gravity and light red shift to zero frequency/equals zero at about 13.7 billion light years away as we are quite sure the speed at which gravity propagates is the speed of light.
Assuming gravitons, at some distance only one graviton per square kilometer per minute arrives. At a greater distance only one graviton per century arrives, so we are unlikely to detect that one, nor determine it's source if we do detect it.
Over distances of several billion light years both affects are moderately applicable.
Since the distance to most everything is changing as Earth orbits the Sun and our Sun orbits the galaxy, it is difficult to ascertain which object caused the thousandth weaker gravity. Earth first, Moon second, Sun third, Jupiter 4 th. Saturn, Venus and Mars likely take turns for 5 th strongest gravity near Earth's surface. Neil

CosmicUnderstanding
2011-Jan-17, 12:24 AM
Thanks for the replies, very useful info there.

forrest noble
2011-Jan-17, 12:29 AM
.....Does this force ever drop off entirely across vast expanses, or, for example could a cluster of galaxies tens of billions of light years away still have *some* very very small influence on us from that far away? Perhaps not even enough to detect with our instruments, but what does the math say about all of it? According to the interpretation of GR that I am aware of gravity moves at the speed of light and its effects can be distorted by intervening bodies, accordingly moving with the curvatures of spacetime as does EM radiation. Given enough time gravity might have no direction at all to it based upon fractionating and bending. The entire universe is 13.7 billion years old according to the BB model, so the oldest and therefore farthest gravitational influences so far would be confined to a radius of ~13.7 light years. If it can be seen by our telescopes then accordingly it could have gravitational influence of us according to GR.

a1call
2011-Jan-17, 12:45 AM
It's a terrible shame that Grant is not on board any more.
He could explain why the actual cosmological event horizon is greater than 13.7 billion. But you can read his explanation here:
http://www.bautforum.com/showthread.php/108258-Hey-look!-It%E2%80%99s-a-bird-it%E2%80%99s-a-plane%E2%80%A6-no-it%E2%80%99s-the-unobservable-universe.?p=1800021#post1800021

His Suggested article is here:
http://arxiv.org/abs/astro-ph/0310808

undidly
2011-Jan-17, 02:48 AM
If the universe is a hypersphere then gravity from one mass will attract any other mass from two directions,front and back.(allowing time of
couse for gravity to get there).

If the second mass is exactly on the opposite side of the universe then gravitational effects from the first mass will cancel.

The effect is that gravity falls to zero then reverses (if only one direction is measured).

caveman1917
2011-Jan-17, 09:42 AM
Then again I started an IMHO unresolved thread where I asked that, if we could be affected indirectly by objects further than the age of the universe would permit by observing their effects on objects in between us.

No. Suppose we have some photon from a far away object that hits the intermediate object. The question is then, can we observe this effect? That would mean this intermediate object then sends a photon to us about this reception of a photon from the far away object. But this scenario is exactly the same as if the intermediate object wasn't there in the first place. Wether you capture a photon and then send out a new one, or just let the original photon pass by unaffected is the same thing. Putting intermediate objects around doesn't change the mechanics of the situation.

Spaceman Spiff
2011-Jan-17, 02:53 PM
Whatever else you take away from this, know that there is nothing special about some distance of 13.7 billion light years. This is a common misconception concerning the big bang model. Read here (http://en.wikipedia.org/wiki/Observable_universe) and here (http://en.wikipedia.org/wiki/Distance_measures_%28cosmology%29), for example.

Ask yourself -- 13.7 billion light years -- what distance is this in an expanding universe? It is neither a spatial cosmic gravitational horizon nor a spatial photon-related horizon. 13.7 billion years is merely a time horizon representing how long the energy density in our universe has been diminishing.

What we call gravity is "simply" curvature in space-time.

a1call
2011-Jan-17, 04:11 PM
No. Suppose we have some photon from a far away object that hits the intermediate object. The question is then, can we observe this effect? That would mean this intermediate object then sends a photon to us about this reception of a photon from the far away object. But this scenario is exactly the same as if the intermediate object wasn't there in the first place. Wether you capture a photon and then send out a new one, or just let the original photon pass by unaffected is the same thing. Putting intermediate objects around doesn't change the mechanics of the situation.
Rather than hijacking this thread I will continue this discussion here:
http://www.bautforum.com/showthread.php/108258-Hey-look!-It%E2%80%99s-a-bird-it%E2%80%99s-a-plane%E2%80%A6-no-it%E2%80%99s-the-unobservable-universe.?p=1841384#post1841384

undidly
2011-Jan-17, 11:06 PM
Whatever else you take away from this, know that there is nothing special about some distance of 13.7 billion light years. This is a common misconception concerning the big bang model. Read here (http://en.wikipedia.org/wiki/Observable_universe) and here (http://en.wikipedia.org/wiki/Distance_measures_%28cosmology%29), for example.

Ask yourself -- 13.7 billion light years -- what distance is this in an expanding universe? It is neither a spatial cosmic gravitational horizon nor a spatial photon-related horizon. 13.7 billion years is merely a time horizon representing how long the energy density in our universe has been diminishing.

What we call gravity is "simply" curvature in space-time.

Why would "curvature" cause mass to accelerate?.

I ride my bicycle on a nearby horizontal road which curves left then right.
I observe no acceleration on these parts of the road.

Further along the road curves down then curves up.
I accelerate then decelerate.

Why the difference.

So "What we call gravity is "simply" curvature in space-time." is curvature in a specific direction?.

What direction is that?

caveman1917
2011-Jan-17, 11:26 PM
Why would "curvature" cause mass to accelerate?.

I ride my bicycle on a nearby horizontal road which curves left then right.
I observe no acceleration on these parts of the road.

Yes you do. Note that velocity is a vector, it includes direction. Changing direction of motion (left or right) amounts to acceleration. You'll feel a pseudoforce, just try and turn left on your bike really fast, you'll fall off unless you compensate.

Not that this is relevant to gravity. Curvature doesn't cause mass to accelerate. Even though it may seem like it from an external perspective, someone in orbit will not feel a (pseudo)force, hence no acceleration.

Spaceman Spiff
2011-Jan-17, 11:58 PM
Why would "curvature" cause mass to accelerate?.

I ride my bicycle on a nearby horizontal road which curves left then right.
I observe no acceleration on these parts of the road.

Further along the road curves down then curves up.
I accelerate then decelerate.

Why the difference.

So "What we call gravity is "simply" curvature in space-time." is curvature in a specific direction?.

What direction is that?

I specifically referred to curvature in space-time.

There is an old saying from Kip Thorne:

"Matter/energy tells space-time how to curve, and curved space-time tells matter/energy how to move."

Acceleration is what stuff does that isn't following its local geodesic in curved space-time. You'll need to jettison Newtonian notions of gravity, if interested in what General Relativity has to say about the question in the OP. Here (http://en.wikipedia.org/wiki/Introduction_to_general_relativity) is a basic introduction to GR.

undidly
2011-Jan-18, 12:02 AM
Yes you do. Note that velocity is a vector, it includes direction. Changing direction of motion (left or right) amounts to acceleration. You'll feel a pseudoforce, just try and turn left on your bike really fast, you'll fall off unless you compensate.

Not that this is relevant to gravity. Curvature doesn't cause mass to accelerate. Even though it may seem like it from an external perspective, someone in orbit will not feel a (pseudo)force, hence no acceleration.

When it turn left or right the wheels on my bicycle rotate at the same rate as before.
When the road curves down or up the wheels turn at a rate that is different.

" Curvature doesn't cause mass to accelerate. Even though it may seem like it from an external perspective"

I am interested in the external perspective.
From my external perspective mass accelerates toward another mass.
Where does curvature fit in here.
What is the direction of the curvature from my external perspective.

caveman1917
2011-Jan-18, 12:10 AM
When it turn left or right the wheels on my bicycle rotate at the same rate as before.
When the road curves down or up the wheels turn at a rate that is different.

And once again, acceleration is a change in velocity, not just speed.


From my external perspective mass accelerates toward another mass.
Where does curvature fit in here.

It doesn't. Because when you are choosing to take the perspective that gravity makes mass accelerate, that means you are using the newtonian approach, which in turn means curvature has nothing to do with it.

ETA: suppose you drop a ball from some high building. The thing that is accelerating is the surface of the earth towards the ball, the ball itself is not accelerating. You can easily see this because we on the surface of the earth feel a pseudoforce (what we perceive as weight), while when we are in freefall (such as the ball) we don't (hence the weightlessness thing).

grapes
2011-Jan-18, 12:34 AM
ETA: suppose you drop a ball from some high building. The thing that is accelerating is the surface of the earth towards the ball, the ball itself is not accelerating. You can easily see this because we on the surface of the earth feel a pseudoforce (what we perceive as weight), while when we are in freefall (such as the ball) we don't (hence the weightlessness thing).I'm scratching my head over this--why would the earth accelerate, but not the ball? They're both in freefall, in that example.

caveman1917
2011-Jan-18, 12:39 AM
I'm scratching my head over this--why would the earth accelerate, but not the ball? They're both in freefall, in that example.

Could it be that you read over the "surface of the earth"? The center of mass of the earth follows an inertial trajectory, but the surface doesn't (it accelerates away from the center - to 'meet' the ball at some time later).

CaptainToonces
2011-Jan-19, 05:12 AM
suppose you drop a ball from some high building. The thing that is accelerating is the surface of the earth towards the ball, the ball itself is not accelerating. You can easily see this because we on the surface of the earth feel a pseudoforce (what we perceive as weight), while when we are in freefall (such as the ball) we don't (hence the weightlessness thing).
Hmmm. Both the Earth and the ball are in freefall. The perception of weight for a person standing on the ground is caused by the degeneracy pressure of atoms in the ground pushing back against gravity. The ball experiences the feeling of accelleration as it falls in a gravitational field.

caveman1917
2011-Jan-19, 01:46 PM
Both the Earth and the ball are in freefall.

Only the center of mass of the earth is in freefall. Technically the same would hold for the ball but we can safely ignore that one. Whenever we say "the earth is in freefall" or "the moon is in freefall", what is meant is that the center of mass is in freefall. This is not true for the surface (or any other interior part other than the center).


The perception of weight for a person standing on the ground is caused by the degeneracy pressure of atoms in the ground pushing back against gravity.

Yes, though i don't think it's degeneracy pressure (that's the thing that holds neutron stars up). This means that the surface of the earth is accelerating. Just always keep track of the forces in a specific frame, whenever you have a pseudoforce (such as weight), it means the frame is accelerating, such as the surface of the earth (or any other rigid body strictly speaking).


The ball experiences the feeling of accelleration as it falls in a gravitational field.

Actually it doesn't (ignoring air friction). Ask yourself this: do the astronauts in the ISS experience the feeling of acceleration?

What you feel when you fall of a building or something, is in fact the absence of acceleration. We have gotten so used to being in constant acceleration that our minds have grown to intuitively think of this as the 'null baseline', but our minds have actually got it backwards. The ball doesn't feel anything (it is on an inertial trajectory), it is the surface of the earth that is under constant acceleration (just keep track of any pseudoforces in both frames).

Spaceman Spiff
2011-Jan-19, 08:41 PM
I'll second and add to caveman1917's nice description.

One way to think about the situation concerning a ball in "free fall" is to interpret that phrase in its strictest sense: it is falling freely -- no forces are acting on it, and in GR this means that the ball (or at least its center) is moving along its local geodesic (http://en.wikipedia.org/wiki/Geodesic). It is doing what it is supposed to be doing :dance:, given the presence of matter-energy in its local region of space-time. Technically, a tidal force is present on the ball because the quarks and electrons that compose it are not free to move along their individual geodesics due to the presence of nuclear and electromagnetic forces. But these tidal forces are small as long as the ball is small compared to the local curvature in space-time (speaking loosely). If not for the electromagnetic and nuclear forces, the quarks(**) and electrons composing you and the Earth would all be moving on their geodesic paths through space-time which in turn depend on the matter-energy density.

Also -- the primary reason you are being "pushed up" by Earth's surface is due to electrical repulsion (like charges) between the particles making up the solid Earth and those in your feet. The electron degeneracy pressure between the electron clouds is acting, but unless one exerts an enormous amount of compression, this pressure isn't dominant. Earth's surface pushing up on you (and the occasional wind) is the only force you feel when simply standing on Earth's surface, and your acceleration is 1g.

And like caveman1917 said the sensation you feel when you are in (near) free-fall is present because your physiology, nervous systems and brain have adjusted to being stationary on an Earth's surface constantly pushing up on you. You would feel the same way (as in free fall) if somebody placed you in interstellar space.

If the maths weren't so danged difficult to learn and use, we'd not be teaching Newton, whose conception and formulation of gravity serve to confuse anyone who decides they want to understand gravity more deeply.


(**) ignoring the conditions under which free quarks are allowed to exist or not. ;)

Hornblower
2011-Jan-19, 11:29 PM
I'll second and add to caveman1917's nice description.

One way to think about the situation concerning a ball in "free fall" is to interpret that phrase in its strictest sense: it is falling freely -- no forces are acting on it, and in GR this means that the ball (or at least its center) is moving along its local geodesic (http://en.wikipedia.org/wiki/Geodesic). It is doing what it is supposed to be doing :dance:, given the presence of matter-energy in its local region of space-time. Technically, a tidal force is present on the ball because the quarks and electrons that compose it are not free to move along their individual geodesics due to the presence of nuclear and electromagnetic forces. But these tidal forces are small as long as the ball is small compared to the local curvature in space-time (speaking loosely). If not for the electromagnetic and nuclear forces, the quarks(**) and electrons composing you and the Earth would all be moving on their geodesic paths through space-time which in turn depend on the matter-energy density.

Also -- the primary reason you are being "pushed up" by Earth's surface is due to electrical repulsion (like charges) between the particles making up the solid Earth and those in your feet. The electron degeneracy pressure between the electron clouds is acting, but unless one exerts an enormous amount of compression, this pressure isn't dominant. Earth's surface pushing up on you (and the occasional wind) is the only force you feel when simply standing on Earth's surface, and your acceleration is 1g.

And like caveman1917 said the sensation you feel when you are in (near) free-fall is present because your physiology, nervous systems and brain have adjusted to being stationary on an Earth's surface constantly pushing up on you. You would feel the same way (as in free fall) if somebody placed you in interstellar space.

If the maths weren't so danged difficult to learn and use, we'd not be teaching Newton, whose conception and formulation of gravity serve to confuse anyone who decides they want to understand gravity more deeply.
(**) ignoring the conditions under which free quarks are allowed to exist or not. ;)

My bold. I disagree, for the simple reason that I cannot visualize the GR geometrical construct.

Let's look at the Earth from afar. We see unsupported objects falling toward its center from all directions, until forcibly restrained by contact with the ground. To visualize the state of the latter as the equivalent acceleration in the GR construct, I would be imagining the Earth as expanding at an ever increasing rate, which it clearly does not do in any sense that our everyday experience can comprehend.

Does this mean I don't trust the GR 4-dimensional spacetime construct? Not at all, because I understand how it explains the extreme cases better than does the Newtonian construct, which would need some ad hoc fudge factors. It resolves Newton's fundamental misgivings about his own work by modeling gravitational motion as unforced motion. I just think it would be foolish to plunge novices into this visually unsupported mathematical work without first giving them plenty of training in advanced math exercises in visually supported Newtonian models.

I would say that GR does not contradict the Newtonian model, but transcends it. So far that is my best attempt at choosing a word to sum it up.

antoniseb
2011-Jan-20, 12:14 AM
I personally do not know how gravity's strength is altered by the expansion of the universe... which is one of the topics touched on above. Once we start detecting waves, we'll start to be able to make more definite statements about these things.

caveman1917
2011-Jan-20, 12:43 AM
My bold. I disagree, for the simple reason that I cannot visualize the GR geometrical construct.

There is one visualisation that i have found quite good. It should not be mistaken for the real thing, but it's close enough to be decent.

Instead of thinking of space as stationary, consider the view where you think of space itself as constantly 'falling' towards masses. Somewhat like putting a ball on a table, and then pulling the tablecloth towards yourself. The ball will be stationary relative to its piece of tablecloth (hence inertial), but you can easily see how it gets "gravitationally attracted" to you. Thinking of it this way, you can see how the ball moving towards earth is on an inertial trajectory, but the surface of the earth is accelerating outwards (it 'moves over' the space that's falling towards the center).
If you want to be really strict with this visualisation, instead of space itself falling towards masses with an increasing speed as it gets closer, you'd have to think of it as having an increasing acceleration (that's the relevant parameter, not the velocity). But this gets hard to visualise, so just sticking with space falling towards masses with increasing speed as it gets closer seems the best.

Hornblower
2011-Jan-20, 12:58 AM
There is one visualisation that i have found quite good. It should not be mistaken for the real thing, but it's close enough to be decent.

Instead of thinking of space as stationary, consider the view where you think of space itself as constantly 'falling' towards masses. Somewhat like putting a ball on a table, and then pulling the tablecloth towards yourself. The ball will be stationary relative to its piece of tablecloth (hence inertial), but you can easily see how it gets "gravitationally attracted" to you. Thinking of it this way, you can see how the ball moving towards earth is on an inertial trajectory, but the surface of the earth is accelerating outwards (it 'moves over' the space that's falling towards the center).
If you want to be really strict with this visualisation, instead of space itself falling towards masses with an increasing speed as it gets closer, you'd have to think of it as having an increasing acceleration (that's the relevant parameter, not the velocity). But this gets hard to visualise, so just sticking with space falling towards masses with increasing speed as it gets closer seems the best.

Thanks for the tip.

CaptainToonces
2011-Jan-20, 07:31 AM
Also -- the primary reason you are being "pushed up" by Earth's surface is due to electrical repulsion (like charges) between the particles making up the solid Earth and those in your feet. The electron degeneracy pressure between the electron clouds is acting, but unless one exerts an enormous amount of compression, this pressure isn't dominant.
This is from Wikipedia article on Pauli Exclusion, perhaps the Wiki article is wrong?

It has been shown that the Pauli exclusion principle is responsible for the fact that ordinary bulk matter is stable and occupies volume. This suggestion was first made in 1931 by Paul Ehrenfest, who pointed out that the electrons of each atom cannot all fall into the lowest-energy orbital and must occupy successively larger shells. Atoms therefore occupy a volume and cannot be squeezed too closely together.[5]

A more rigorous proof was provided in 1967 by Freeman Dyson and Andrew Lenard, who considered the balance of attractive (electron-nuclear) and repulsive (electron-electron and nuclear-nuclear) forces and showed that ordinary matter would collapse and occupy a much smaller volume without the Pauli principle.[6] The consequence of the Pauli principle here is that electrons of the same spin are kept apart by a repulsive exchange interaction, which is a short-range effect complemented by the long-range electrostatic or coulombic force. This effect is therefore partly responsible for the everyday observation in the macroscopic world that two solid objects cannot be in the same place in the same time.

Spaceman Spiff
2011-Jan-20, 07:09 PM
@CaptainToonces:
No, the Wikipedia article you quote isn't wrong. This quantum mechanical "pressure" is what gives size to individual atoms. However, the structure of a solid is usually dominated by inter-electrical forces (unless the compressions and so densities become very large).


@Hornblower:
As with all modern scientific theories -- more successful ones incorporate and subsume previous ones and go on to describe the behavior of nature more generally. I am not suggesting that Newton's theory of gravity and notions of space and time are useless. They are useful because they are easier to utilize computationally -- even for the experts. The problem can arise in which fictions(**) we must tell non-expert students (of various levels of non-expertness), some of which can eventually serve to confuse rather than illuminate. Consider the example of "free-fall" in its many manifestations, e.g., an astronaut falling around the Earth, an astronaut falling off a ladder on Earth's surface, and an astronaut in interstellar space.


(**without getting into a deep philosophical discussion of what is meant by a fiction in science)

CosmicUnderstanding
2011-Jan-21, 06:53 AM
This is why I love this board. I ask a question and get an answer, or many great answers as the case may be, and then continue to learn as the thread marches on with further detail and explanations. I just wish I had the right biological equipment to properly learn and absorb the math equations and formulas related to this field of study.

Hornblower
2011-Jan-22, 11:30 PM
There is one visualisation that i have found quite good. It should not be mistaken for the real thing, but it's close enough to be decent.

Instead of thinking of space as stationary, consider the view where you think of space itself as constantly 'falling' towards masses. Somewhat like putting a ball on a table, and then pulling the tablecloth towards yourself. The ball will be stationary relative to its piece of tablecloth (hence inertial), but you can easily see how it gets "gravitationally attracted" to you. Thinking of it this way, you can see how the ball moving towards earth is on an inertial trajectory, but the surface of the earth is accelerating outwards (it 'moves over' the space that's falling towards the center).
If you want to be really strict with this visualisation, instead of space itself falling towards masses with an increasing speed as it gets closer, you'd have to think of it as having an increasing acceleration (that's the relevant parameter, not the velocity). But this gets hard to visualise, so just sticking with space falling towards masses with increasing speed as it gets closer seems the best.

After thinking about it for a few days I have concluded that it is not close enough to be decent. Visualizing some sort of reference frame flowing into the planet at a steady rate is easy enough, but then when the ground is holding a ball that otherwise would fall in, it has the ball moving at a constant velocity with respect to the falling space rather than being accelerated. If this falling space is the no-drag equivalent of our familiar Newtonian model, then no force is needed to hold the ball in that position. I'm sorry, but I still find it impossible to create a visual image of the correct GR construct.


@Hornblower:
As with all modern scientific theories -- more successful ones incorporate and subsume previous ones and go on to describe the behavior of nature more generally. I am not suggesting that Newton's theory of gravity and notions of space and time are useless. They are useful because they are easier to utilize computationally -- even for the experts. The problem can arise in which fictions(**) we must tell non-expert students (of various levels of non-expertness), some of which can eventually serve to confuse rather than illuminate. Consider the example of "free-fall" in its many manifestations, e.g., an astronaut falling around the Earth, an astronaut falling off a ladder on Earth's surface, and an astronaut in interstellar space.


(**without getting into a deep philosophical discussion of what is meant by a fiction in science)
So you are saying that Newtonian mechanics is fiction, while GR is nonfiction, or closer to the truth, whatever the truth may be in a philosophical sense? I would say that neither one is fiction, and neither one is nonfiction. Both are mathematically logical constructs that were derived from rigorous study and analysis of observable phenomena at the respective times of their development. The mathematical details of GR are so much more difficult that I would be horrified at the prospect of skipping the act of training on advanced, multibody exercises in the visualizable and less demanding Newtonian model. Memories of the Newtonian way should not cause confusion in learning the methods of GR if we do each step in an orderly manner instead of trying to "wing it". If we already know how to do Newtonian analysis, then we can check our GR work to make sure its results converge properly with the Newtonian results at low speeds and energy levels.

Let's not forget that GR is not necessarily the last word. In fact we already know that it is incompatible with quantum mechanics in its present state. Physicists are resorting to 10 or 11 dimensional models in an attempt at developing a unified theory. If a usable theory emerges that unifies everything, it will make plain GR seem as easy as Newtonian mechanics by comparison. I would not want to attempt it without first training on the older, less demanding theories.

Spaceman Spiff
2011-Jan-23, 03:00 AM
After thinking about it for a few days I have concluded that it is not close enough to be decent. Visualizing some sort of reference frame flowing into the planet at a steady rate is easy enough, but then when the ground is holding a ball that otherwise would fall in, it has the ball moving at a constant velocity with respect to the falling space rather than being accelerated. If this falling space is the no-drag equivalent of our familiar Newtonian model, then no force is needed to hold the ball in that position. I'm sorry, but I still find it impossible to create a visual image of the correct GR construct.


So you are saying that Newtonian mechanics is fiction, while GR is nonfiction, or closer to the truth, whatever the truth may be in a philosophical sense? I would say that neither one is fiction, and neither one is nonfiction. Both are mathematically logical constructs that were derived from rigorous study and analysis of observable phenomena at the respective times of their development. The mathematical details of GR are so much more difficult that I would be horrified at the prospect of skipping the act of training on advanced, multibody exercises in the visualizable and less demanding Newtonian model. Memories of the Newtonian way should not cause confusion in learning the methods of GR if we do each step in an orderly manner instead of trying to "wing it". If we already know how to do Newtonian analysis, then we can check our GR work to make sure its results converge properly with the Newtonian results at low speeds and energy levels.

Let's not forget that GR is not necessarily the last word. In fact we already know that it is incompatible with quantum mechanics in its present state. Physicists are resorting to 10 or 11 dimensional models in an attempt at developing a unified theory. If a usable theory emerges that unifies everything, it will make plain GR seem as easy as Newtonian mechanics by comparison. I would not want to attempt it without first training on the older, less demanding theories.

Up above, I made the point that it was the difficult maths (even for the experts -- relatively few problems can be solved in detail in GR) which thusfar is the default excuse that prevents even the concepts of GR from being part of the physical science education.

And yes, GR is closer to the truth than Newtonian physics -- observations and experiment in the real world clearly demonstrate that. At the same time I don't argue with the fact that GR isn't the last word.

All I am suggesting is that the conceptual models and analogies we use to teach our students ought to illuminate more than they obscure what our best scientific models of nature have to say about he behavior of nature. And when explanations based on old ideas get in the way of understanding and cause confusion, then I am suggesting we ought to give second thought to just repeating the same old, same old.

Hornblower
2011-Jan-23, 03:23 AM
Up above, I made the point that it was the difficult maths (even for the experts -- relatively few problems can be solved in detail in GR) which thusfar is the default excuse that prevents even the concepts of GR from being part of the physical science education.

And yes, GR is closer to the truth than Newtonian physics -- observations and experiment in the real world clearly demonstrate that. At the same time I don't argue with the fact that GR isn't the last word.

All I am suggesting is that the conceptual models and analogies we use to teach our students ought to illuminate more than they obscure what our best scientific models of nature have to say about he behavior of nature. And when explanations based on old ideas get in the way of understanding and cause confusion, then I am suggesting we ought to give second thought to just repeating the same old, same old.Can you be more specific about what sort of confusion you are concerned about?

WayneFrancis
2011-Jan-23, 11:31 AM
According to the interpretation of GR that I am aware of gravity moves at the speed of light and its effects can be distorted by intervening bodies, accordingly moving with the curvatures of spacetime as does EM radiation. Given enough time gravity might have no direction at all to it based upon fractionating and bending. The entire universe is 13.7 billion years old according to the BB model, so the oldest and therefore farthest gravitational influences so far would be confined to a radius of ~13.7 light years. If it can be seen by our telescopes then accordingly it could have gravitational influence of us according to GR.


I haven't read the ahead yet but I'm pretty sure this isn't main stream. Just because gravity travels at the speed of light does not mean it acts like a photon. Infact we know it works like the other force carriers in that it isn't effected by things like...well gravity. Gravity escapes the event horizon of a black hole just fine because it is not a real photon. Just like the electromagnetic force can pass out of the event horizon. So I don't see how there can be any "scattering" effect to gravity over great distances. That said the further out an object is the more it blends into the background. IE the larger the scale we look at the more uniform the universe looks like so the pull from any object is more evenly matched by some other set of objects in the universe in the opposite direction. So not only does the effects go to zero but the net effect is more likely evenly balanced out.

caveman1917
2011-Jan-23, 02:08 PM
After thinking about it for a few days I have concluded that it is not close enough to be decent. Visualizing some sort of reference frame flowing into the planet at a steady rate is easy enough, but then when the ground is holding a ball that otherwise would fall in, it has the ball moving at a constant velocity with respect to the falling space rather than being accelerated. If this falling space is the no-drag equivalent of our familiar Newtonian model, then no force is needed to hold the ball in that position.

Yes indeed, that is the issue i hinted at when i said

If you want to be really strict with this visualisation, instead of space itself falling towards masses with an increasing speed as it gets closer, you'd have to think of it as having an increasing acceleration (that's the relevant parameter, not the velocity). But this gets hard to visualise, so just sticking with space falling towards masses with increasing speed as it gets closer seems the best.


I'm sorry, but I still find it impossible to create a visual image of the correct GR construct.

Sorry it didn't work out for you. It's the best i'm aware of, so if you encounter something better please share :). I wouldn't mind getting a better visualisation too.