Why Does Gravity Pull Objects Together?

[WannabeNewton]

"Why" is often a question about the physical cause; and that is a typical physics topic.

Couldn't agree more my friend.

 

[Dale]

According to our current understanding of Nature, there is nothing more fundamental than the explanation given by GR.

I think that this is a key point. In science there is always a fundamental explanation beyond which we can only say "because it fits the data", i.e. that's just the way things are. As you mention, that is the position held by GR. We can explain many less fundamental things in terms of GR's Einstein Field Equations, but if you ask "why the EFE" the only answer is that it fits the data.

 

[Dale]

Velocity is a distance per time in a certain a direction(vector quantity). Speed is a distance per time. Because Earth is orbiting a body which is in turn orbiting another body the speed will at some point have to change, and velocity is always changing. Earths speed is not a constant.

A circular orbit has no change in speed and therefore no change in KE. An elliptical orbit simply changes KE for PE, there is no energy transfer.

The only way to get an energy transfer is via a hyperbolic trajectory "gravitational slingshot" or via tidal effects. Neither of those are applicable to the Earth orbiting the sun.

 

[OnlyMe]

If conservation of energy is right that means every time Earth orbits the sun energy has to be moved from Earth to the sun and to the surrounding starts and Milky way galaxy and to the local cluster and then back to Earth to keep it in a stable orbit because of its change velocity.

No it doesn't. What would make you say that.

Actually Dale, would not that position be very Machian?
It is not phrased such directly, but for inertia to be as Mach believed, the essence is there. The inertia of each and every gravitational body must be accounted for in the essential nature of inertia itself.

How, that happens is yet to be discovered. Still it is consistent with how things appear to be.

For my part in the above exchange, which seems to have ignited some controversy, I can only say that my post was little more than a comment on what I saw as the larger context within CDCraig123's post. Generally I try to read posts, even published papers with an ear for the larger context. I should have probably explained my own context more fully at the time.

From the context of a discussion of gravity, the energy exchange between bodies suggested by CDCraig123's original post, can easily be associated with gravitational interaction, which does represent an exchange of energy, resulting in a balanced inertial system. Within that larger context, it just seemed that CDCraig123 was describing a Machian perspective of inertia. It was really initially no more than an observation, of the similarity. The energy transfer being the universal influence of gravitational interaction between all objects and the solar system model an inertial representation of that "exchange", locally.

While it is true, that to a significant extent Mach's views, as they relate to this topic, remain largely philosophical, Einstein was influenced sufficiently by Mach that he worked hard in an attempt, to try and incorporate Mach's ideas into his field equations.., unsuccessfully.

Even so, generally.., the concepts attributed to Mach involving the origin of inertia, continue to play a central role in many cosmological models involving inertia. And no we do not as yet have a definitive answer to its origin. Still it is difficult to deny the similarities and connections between what we experience as inertia and what we experience as the force of gravitation.

A circular orbit has no change in speed and therefore no change in KE. An elliptical orbit simply changes KE for PE, there is no energy transfer.

The only way to get an energy transfer is via a hyperbolic trajectory "gravitational slingshot" or via tidal effects. Neither of those are applicable to the Earth orbiting the sun.

This would seem to me, to assume that the involved gravitational center of mass is "at rest" or that at least the frame of reference from which it is true, is one at rest relative to the involved inertial system. The orbit of a planet, moon, star or even galaxy winds up more complex than the simple model suggested. While the Earth's motion/speed, in orbit relative to the sun appears to be constant, when compared to the galaxy as a whole it is constantly changing. Kinetic energy and speed are both relativistic and dependent upon one's frame of reference.

Though it was obviously not clear, my intent had not and is not to be contradicting the issues raised in Dale's posts. I only meant to be broadening the frame of reference.

With respect to the science vs philosophy argument, I don't believe it has any merit. When the discussion involves subjects or phenomena which are beyond the explanation of current scientific theory and/or models. The advancement of science in such cases has often evolved from a logical examination of a philosophical approach to those unanswered questions.

Science at present gives us no, unflawed explanation of inertia. Nether does Mach's ideas on the subject. On the other hand Mach's "philosophical" perspective does give science a valid underlying frame of reference from which to explore the subject.

 

[baywax]

I love how everyone seems to know so much, you ask a question and they throw you a rule of this or according to this it has to be this. This simple question needs to be used by everyone to keep in mind that we just don't know. We have been wrong fare more times then right about most things. At one point in our history we thought the world was flat, that the Earth was the center of all things. Almost nothing is truly known for a fact and can be proven when compared to the almost infinite unknowns. I just spent a lot money on a software program that was suppose to simulate how gravity works as NASA sees it, its suppose to be a copy of the program NASA uses to project orbits with some tweaks. I ran the pre loaded version of are solar system and let it run with in 3 years Venus was flying past Saturn with the latest RK4 (Runge Katta) calculations. I have been in my free time trying to find a simple formula or formulas to show how energy is transferred from different bodies in motion. If conservation of energy is right that means every time Earth orbits the sun energy has to be moved from Earth to the sun and to the surrounding starts and Milky way galaxy and to the local cluster and then back to Earth to keep it in a stable orbit because of its change velocity. What carries this energy? Look at the theory of expansion (assuming that the red shift we use is right) what we know about gravity tells us the universe should be slowing down not speeding up. If that is right where is all this energy coming from to expand everything. Sorry for being long winded.

In short I don't have a freaking clue why there is gravity, but I can come up with a lot of reasons there should be gravity.

It may be a little like saying, "why is the sky blue?" then answering "because it is." But, there is a "reason" why there is gravity and that is because there is mass.

 

[Drakkith]

From the context of a discussion of gravity, the energy exchange between bodies suggested by CDCraig123's original post, can easily be associated with gravitational interaction, which does represent an exchange of energy, resulting in a balanced inertial system. Within that larger context, it just seemed that CDCraig123 was describing a Machian perspective of inertia.

I disagree. I think it shows a lack of understanding of relativity, energy, and several other topics.

This would seem to me, to assume that the involved gravitational center of mass is "at rest" or that at least the frame of reference from which it is true, is one at rest relative to the involved inertial system. The orbit of a planet, moon, star or even galaxy winds up more complex than the simple model suggested. While the Earth's motion/speed, in orbit relative to the sun appears to be constant, when compared to the galaxy as a whole it is constantly changing. Kinetic energy and speed are both relativistic and dependent upon one's frame of reference.

The key here is that talking about an orbit necessitates identifying what is orbiting what. In Dalespams case it was the Earth orbiting the Sun. You cannot ignore that and still talk about the Earths kinetic energy because the Earth is not an inertial frame.

As you said, kinetic energy and speed are both relative to a frame. This is why it is so important to identify what frame you are observing from or comparing something to. CDCraig is NOT using the Sun as the frame for the Earths orbit, but is instead using multiple other frames. It is a given that according to SOME frame the Earth's speed is not constant.

 

[harrylin]

[..] In science there is always a fundamental explanation beyond which we can only say "because it fits the data", i.e. that's just the way things are. As you mention, that is the position held by GR. [..]

GR doesn't have such a position; if it did, then it would not be Einstein's theory as his position was different - see #28.

 

[Dale]

GR doesn't have such a position; if it did, then it would not be Einstein's theory as his position was different - see #28.

By "position" I meant that amongst the successful theories of gravity, GR is the fundamental theory. I.e. You can derive Newtonian gravity from GR, but not vice versa. Therefore, if you ask "why Newtons law of universal gravitation" you can answer in terms of the EFE, but is you ask "why the EFE" all you can do is answer in terms of experimental results. It has nothing to do with the Einstein's later thoughts on the aether.

 

[harrylin]

By "position" I meant that amongst the successful theories of gravity, GR is the fundamental theory. [..] is you ask "why the EFE" all you can do is answer in terms of experimental results. [..]

Thanks for the clarification; it sounded as if you meant the contrary!
Indeed, GR itself doesn't have the position that "there is nothing more fundamental than the explanation given by GR"; as a matter of fact, it doesn't even try to explain the "why" in the sense of the question.


Harald

 

[OnlyMe]

If conservation of energy is right that means every time Earth orbits the sun energy has to be moved from Earth to the sun and to the surrounding starts and Milky way galaxy and to the local cluster and then back to Earth to keep it in a stable orbit because of its change velocity. What carries this energy?

From the context of a discussion of gravity, the energy exchange between bodies suggested by CDCraig123's original post, can easily be associated with gravitational interaction, which does represent an exchange of energy, resulting in a balanced inertial system. Within that larger context, it just seemed that CDCraig123 was describing a Machian perspective of inertia.

I disagree. I think it shows a lack of understanding of relativity, energy, and several other topics.

If we assume that the solar system, our solar system, is an isolated gravitational system, when the Earth orbits the sun the Earth is accelerated toward the Sun and the Sun is accelerated toward the earth. The center of mass for the system as a whole is not the center of mass of the sun. It is the barycenter.

Are you saying that the kinetic changes in the motions of the planets and the Sun, which result from their gravitational interaction, do not represent an exchange of kinetic energy? And that CDCraig's post cannot be viewed as an exchange or transfer of kinetic energy resulting from the dynamic gravitational relationship involved?

 

[bill alsept]

There have been description that explain gravity. I'm not sure why they have not been studied more considering they are all that have been offered.

http://en.wikipedia.org/wiki/Le_Sage's_theory_of_gravitation

 

[OnlyMe]

There have been description that explain gravity. I'm not sure why they have not been studied more considering they are all that have been offered.

http://en.wikipedia.org/wiki/Le_Sage's_theory_of_gravitation

The Le Sage model of gravity has been examined and re-examined repeatedly since its first introduction in 1690. The Wiki article in the above link does a fair job of outlining the timeline and near its conclusion mentions Feynman's 1965 resurrection, even if only for reasons of example...

These kinetic gravity models, do a decent job of explaining the Newtonian concept of gravity. They fall short in some other respects, better explained by general relativity. However, there are, at least two significant flaws. The first is that to explain even the Newtonian model of gravitation, requires superluminal force carriers, which is not consistent with experience or experiment, in particle physics. The other involves the disipation of the kinetic energy required to effectively simulate gravitational forces. As I believe Feynmen pointed out, and I am paraphrasing, the kinetic energy would rapidly result in the vaporization of the atom and thus the universe as we know it.

 

[Delta2]

Well to try to answer this in a different manner, we can try to think what would happen if gravity didnt exist. Then some crucial macroscopic large scale phenomena of this universe wouldn't happen like the formation of stars and planets. Imagine a universe without stars! Gravity is the major player in the macroworld in cosmological scale.

 

[bill alsept]

[/QUOTE]These kinetic gravity models, do a decent job of explaining the Newtonian concept of gravity. They fall short in some other respects, better explained by general relativity.[/QUOTE]

In what way does it fall short? Could you be more specific? I would say GR falls short considering it never even attempted to explain how gravity works.


[/QUOTE]However, there are, at least two significant flaws. The first is that to explain even the Newtonian model of gravitation, requires superluminal force carriers, which is not consistent with experience or experiment, in particle physics.[/QUOTE]


Yes to explain the standard models of gravity one would need to come up with some unexplainable force like that. That is why models like or similar to Lasage's (I think it needs a few changes) should be looked at again. Kinetic models are the only models to even try to mechanically explain gravity. Kinetic energy needs no special or undetectable force carriers.



[/QUOTE]The other involves the disipation of the kinetic energy required to effectively simulate gravitational forces. As I believe Feynmen pointed out, and I am paraphrasing, the kinetic energy would rapidly result in the vaporization of the atom and thus the universe as we know it. [/QUOTE]

How?

This is what interests me about this subject. Everyone is willing to except gravity without physical explanation but dismiss kinetic gravity without specific or better explained reasons.

 

[bill alsept]

Well to try to answer this in a different manner, we can try to think what would happen if gravity didnt exist. Then some crucial macroscopic large scale phenomena of this universe wouldn't happen like the formation of stars and planets. Imagine a universe without stars! Gravity is the major player in the macroworld in cosmological scale.

There would still be the effect of gravity. Kinetics offer something to work with, an explanation that no one else has.

 

[cbConnect]

I believe that this is a rather interesting question. While it definitely does have its connections with philosophy, as i remember someone stating (and starting a mini-flamewar), the physics of gravity is a part of science i believe not to be fully understood. There is a lot of debate on the existence of gravitrons, which is quite the example of discord in the science community. I may be saying nothing new here, but i believe that while we do know how gravity works, we don't actually know why.

Though the reverse gravity concept Delta stated seems fun to imagine.

 

[OnlyMe]

These kinetic gravity models, do a decent job of explaining the Newtonian concept of gravity. They fall short in some other respects, better explained by general relativity.

In what way does it fall short? Could you be more specific? I would say GR falls short considering it never even attempted to explain how gravity works.

Newtonian gravity required gravity to function instantaneously between bodies. For a kinetic model as they have been presented to do this the force carriers have to have velocities many orders of magnitude greater than c. This was also noted below.

I have seen no kinetic model that explains either precession (a limitation of Newton's model also) or gravitational lensing.

However, there are, at least two significant flaws. The first is that to explain even the Newtonian model of gravitation, requires superluminal force carriers, which is not consistent with experience or experiment, in particle physics.

Yes to explain the standard models of gravity one would need to come up with some unexplainable force like that. That is why models like or similar to Lasage's (I think it needs a few changes) should be looked at again. Kinetic models are the only models to even try to mechanically explain gravity. Kinetic energy needs no special or undetectable force carriers.

Kinetic energy does in fact require a force carrier, either in the form of a super luminal particle as most LeSage style kinetic models involve or in the form of an ether like substance in space that transfers the kinetic energy as would a "liquid". The particle problem is covered below. The ether model has its own problems. There have been some ether models of GR proposed. I am unaware of any that have been successful in duplicating the success of GR. If an ether model were found that was consistent with GR, it might provide some basis for a kinetic model of gravity, of some sort.

The other involves the dissipation of the kinetic energy required to effectively simulate gravitational forces. As I believe Feynmen pointed out, and I am paraphrasing, the kinetic energy would rapidly result in the vaporization of the atom and thus the universe as we know it.

How?

When using a kinetic model for gravity that involves a particle as a force carrier, some portion of the kinetic energy exchange winds up as heat. There is a heat build up that exceeds any reasonable rate of dissipation, which results in the vaporization of atomic structures.

This is what interests me about this subject. Everyone is willing to except gravity without physical explanation but dismiss kinetic gravity without specific or better explained reasons.

I think that the kinetic models that have been proposed have been dismissed, because they have not duplicated the predictive success that GR has and in each case there have been some fatal flaw(s), as mentioned above.

Personally, I like the idea of a kinetic model, though I have not seen or come up with one that stands up to the issues raised above. Still, it seems that a kinetic model would provide a better opportunity to reconcile some of the problems that stand between GR and QM. That does not change the fact that there is a great deal of current experience and experimental evidence that does not seem explainable from within a kinetic model. In some sense quantum gravity and loop quantum gravity do begin to approach some of those issues from what could be described as involving a kinetic model. They also have not yet been entirely successful.

 

[DaveC426913]

QUOTE=OnlyMe-I have seen no kinetic model that explains either precession (a limitation of Newton's model also) or gravitational lensing. END QUOTE
yadda yadda

Unable to parse.

Is there something wrong that you can't use the existing quote feature?

 

[bill alsept]

Unable to parse.

Is there something wrong that you can't use the existing quote feature?

Yes, I am ignorant to how the function works for some reason when I use the multi quote I mess it up. Sorry

 

[DaveC426913]

Yes, I am ignorant to how the function works for some reason when I use the multi quote I mess it up. Sorry

Even if you can't get the Quote or Multi-quote buttons to work (and we can fix that), the advanced editor has a quote feature - and you can always manually add [ QUOTE ][ /QUOTE ] tags.

 

[bill alsept]

Thanks

 

[bill alsept]

Newtonian gravity required gravity to function instantaneously between bodies. For a kinetic model as they have been presented to do this the force carriers have to have velocities many orders of magnitude greater than c. This was also noted below.

I don't think Kinetic gravity is presented that way and it wouldn't need force carriers. The physics behind kinetic gravity are just as simple as two billard balls hitting each other or fields passing through each other causing drag. GR and Newtonian models on the other hand may need some kind of instantaneous and mystical force that can reach infinantly across the universe and grab hold of something and then pull it back.

I have seen no kinetic model that explains either precession (a limitation of Newton's model also) or gravitational lensing.

Why should it? Could you explain?

Kinetic energy does in fact require a force carrier, either in the form of a super luminal particle as most LeSage style kinetic models involve or in the form of an ether like substance in space that transfers the kinetic energy as would a "liquid". The particle problem is covered below. The ether model has its own problems. There have been some ether models of GR proposed. I am unaware of any that have been successful in duplicating the success of GR. If an ether model were found that was consistent with GR, it might provide some basis for a kinetic model of gravity, of some sort.

It would not be from ether it would be particles, most likely the smallest form of anything as in quanta. GR has never been successful at explaining how gravity works it only helps predict what it will do. GR never even attempts to explain it. As far as I know Kinetics have been the only idea offered to mechanically explain gravity.

When using a kinetic model for gravity that involves a particle as a force carrier, some portion of the kinetic energy exchange winds up as heat. There is a heat build up that exceeds any reasonable rate of dissipation, which results in the vaporization of atomic structures.

If this is the best argument against kinetic gravity then its still doing better than any other theory offered. I don't think the heat dissipation would be a problem besides it could never be as big a problem as the singularity that standard gravity causes and no one has a problem with that. As for heat there seems to be plenty at the center of dense planets.
One idea I never hear anyone speak of is the possibility of quanta being packed so dense at the center that they couldn't move anymore. Without movement there could be no heat and wouldn't that be completely cold? Wouldn't that be the perfect heat sink?

I think that the kinetic models that have been proposed have been dismissed, because they have not duplicated the predictive success that GR has and in each case there have been some fatal flaw(s), as mentioned above.

Kinetic models conform to the standard laws of motion and are completely predictable.

Personally, I like the idea of a kinetic model, though I have not seen or come up with one that stands up to the issues raised above. Still, it seems that a kinetic model would provide a better opportunity to reconcile some of the problems that stand between GR and QM. That does not change the fact that there is a great deal of current experience and experimental evidence that does not seem explainable from within a kinetic model. In some sense quantum gravity and loop quantum gravity do begin to approach some of those issues from what could be described as involving a kinetic model. They also have not yet been entirely successful.

Other than the issue of heat dissipation (which I don't see as a problem) what else is wrong with kinetic gravity?
Standard gravity has at least two problems much greater as in singularities and having to come up with a mystical force that can reach out billions of light years in every direction, grab hold of everything and then pull everything back and still no physical or mechanical description of how it does this.

 

[TGlad]

Kinetic gravity "Although it is not regarded as a viable theory within the mainstream scientific community, there are occasional attempts to re-habilitate the theory outside the mainstream"
:)

So the best explanation so far seems to be that gravity arises from the need for the laws of physics to be invariant under acceleration.
And the need for invariance can often be explained as simply massively bumping up the likelihood of us being in that system. Just as it is massively more likely that you will find yourself in an invariant orbit around a star than heading towards or away from one. Or massively more likely that your will be in a universe that conserves energy.

But that doesn't explain to me why we don't see invariance in higher derivatives of time, why aren't the laws invariant under a frame of reference that is jerking?

 

[bill alsept]

Kinetic gravity "Although it is not regarded as a viable theory within the mainstream scientific community, there are occasional attempts to re-habilitate the theory outside the mainstream"
:)

So the best explanation so far seems to be that gravity arises from the need for the laws of physics to be invariant under acceleration.
And the need for invariance can often be explained as simply massively bumping up the likelihood of us being in that system. Just as it is massively more likely that you will find yourself in an invariant orbit around a star than heading towards or away from one. Or massively more likely that your will be in a universe that conserves energy.

But that doesn't explain to me why we don't see invariance in higher derivatives of time, why aren't the laws invariant under a frame of reference that is jerking?

I'm not sure if your for or against kinetic gravity?

If you were explaining how gravity actually work in some way I didn't get it.

 

[TGlad]

Oh sorry Bill, I am against Kinetic gravity, but my own opinion isn't very important so that was a link to a wikipedia article with citations explaining why it isn't a viable theory.

The rest was referring to https://www.physicsforums.com/showpost.php?p=3441303&postcount=30".

 

[D H]

Yikes! This thread is a mess, particularly the last page or so. Rather than wasting my energy to clean it up, this thread is locked.

Final note: Le Sage's theory of gravitation is and always has been a crackpot theory. Discussions of fringy and crackpot notions are not condoned at this site. PhysicsForums is a site for discussions and questions regarding mainstream science.