# Gravity

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1) What is holding us against the Earth?

2) Why is it able to do so?

3) What interaction is there between matter and space?

4) How is it relevant to what is keeping us from being flung from the Earth?

5) What exactly is gravity?

6) Why is G found is both Einstein and Newtons equations?

(curvature of space-time) = (mass-energy density) * 8
pi G
/
c
4

Fg=Gm1m2/d^2

These are just a few questions to get the discussion going. I think this topic will clarify a lot of what was said in a different topic called "do objects fall at the same speed? no" or something close to that. The arguments mainly revolved around gravity so here is a chance to explain your understanding of what it actually is.

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Some of the anwers will have a different flavor if cast in terms of Newtonian gravity vs general relativity. You really should pick one or the other for consistency of discussion.

6) Why is G found is both Einstein and Newtons equations?

(curvature of space-time) = (mass-energy density) * 8
pi G
/
c
4

Fg=Gm1m2/d^2

It would be very suspect if it didn't, since GR reduces to Newtonian gravity in the limit of weak effects. G is proportional to the coupling strength of gravity.

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• 3 weeks later...

coupling strength

is gravity a "counter" influence to the tendency of a system to move from order to disorder?

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Not at all.

It's just that you have to be quite specific as to what you mean by order and disorder. This is why we use words like entropy and free energy which have very specific meanings (whereas, in a non physics context order and disorder mean subtly different things).

Our solar system (when taken along with the gas and photons that left it over the past billions of years) is a higher entropy state than the gas cloud it evolved from, even if it is more orderly when you look at it on a large scale.

Further thoughts:

If you look at what remains in the sun and planets it might be more ordered than a hydrogen cloud of equal mass. I do not know one way or the other, but it would not surprise me. Systems can have localised pockets of low entropy (such as life, or the LCD monitor I am staring at right now), this is quite permissable (and even expected -- by the same logic that entropy increases, there will be more ways for the system to increase in entropy non-uniformly and uniformly so we expect any given system to have pockets of low entropy) as long as the whole system increases in entropy.

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coupling strength

is gravity a "counter" influence to the tendency of a system to move from order to disorder?

I have an intuitive feeling that gravity is a counter force against a destructive force—the force that works on macroscopic materials so as to disassemble them.

That is simply my intuition. Thus, I cannot tell the mechanism in detail.

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Schrödinger's hat,

Well I will admit a lack of complete understanding of the term entropy, but it does seem to rely on the definition of the "system" you are taking into consideration. In some senses, the gas and photons that have "left" the system are not really part of the system anymore. That leaves the entities that are still around, to consider. And the Sun seems to provide us with some ordered energy. Enough to power the ordered subsystems of life on our planet.

When asking the question as to whether gravity could be a counter influence to entropy, I was particularly thinking of the Sun, collecting itself together (with the influence of "gravity" playing a large role) from a gas cloud, to pack all that hydrogen close enough together for all the fussion that goes on, that provides all the photons, that power the biosystem on Earth.

My thinking was that if there were no counter influence to entropy, even if in only forming "pockets" of low entropy, then the universe would be, or might have to have been pretty much "averaged out" by now. All points riding around about the same temperature, with no differentials through which heat could flow from high to low.

Regards, TAR2

Reminds me of a thought I had about 15 years ago when I was reading several books on physics (QED,quantum mechanics, relativity and the like, but I couldn't grasp the math, so I wandered away)

I was thinking that every atom seems to want to get rid of all its energy and come to rest or absolute zero, but it cannot, because although it continually releases photons, it keeps absorbing photons released from all the other atoms in the universe.

Related to this topic, it's even harder to get rid of all your energy (if you are an atom) if you have a bunch of other atoms trying to do the same, in close proximity.

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I have an intuitive feeling that gravity is a counter force against a destructive force—the force that works on macroscopic materials so as to disassemble them.

That is simply my intuition. Thus, I cannot tell the mechanism in detail.

Gravity is neither destructive nor counter to destruction.

We often refer to entropy as something that 'forces' things to become more disordered, but in this context it does not have the same meaning as 'a force'. The former just meaning a tendancy towards disorder, the latter is a precise physical meaning about how it changes momentum/potential energy.

Schrödinger's hat,

Well I will admit a lack of complete understanding of the term entropy, but it does seem to rely on the definition of the "system" you are taking into consideration. In some senses, the gas and photons that have "left" the system are not really part of the system anymore. That leaves the entities that are still around, to consider. And the Sun seems to provide us with some ordered energy. Enough to power the ordered subsystems of life on our planet.

Entropy goes up if you consider a closed isolated system (which the solar system along with all the matter it ejected is a reasonable approximation of, the gas that left the sun is mostly just sitting around in interstellar space and even if this is not good enough you can consider the galaxy closed for almost all purposes).

At any rate, even if we only consider the solar system as it is now (whether or not we exported enough entropy at some time in the past to be more ordered than the original gas cloud), it's still becoming more disordered (the sun and planets are slowly coming into equilibrium with space around us).

The sun is becoming more disordered as fusion occurs, we can lower our local entropy because we have the benefit of sitting in an energy gradient (with the still hot sun on one side and cold space on the other).

When asking the question as to whether gravity could be a counter influence to entropy, I was particularly thinking of the Sun, collecting itself together (with the influence of "gravity" playing a large role) from a gas cloud, to pack all that hydrogen close enough together for all the fussion that goes on, that provides all the photons, that power the biosystem on Earth.

This still increased the overall entropy, even if we wound up with a local pocket of low entropy (I still don't know if this is the case and I'm not quite sure how to go about setting up the calculation).

My thinking was that if there were no counter influence to entropy, even if in only forming "pockets" of low entropy, then the universe would be, or might have to have been pretty much "averaged out" by now. All points riding around about the same temperature, with no differentials through which heat could flow from high to low.

Regards, TAR2

Reminds me of a thought I had about 15 years ago when I was reading several books on physics (QED,quantum mechanics, relativity and the like, but I couldn't grasp the math, so I wandered away)

I was thinking that every atom seems to want to get rid of all its energy and come to rest or absolute zero, but it cannot, because although it continually releases photons, it keeps absorbing photons released from all the other atoms in the universe.

Related to this topic, it's even harder to get rid of all your energy (if you are an atom) if you have a bunch of other atoms trying to do the same, in close proximity.

Yes, this is pretty much the concept of entropy.

If you have one hot atom and a bunch of warm ones, the hot one tries to get rid of its energy, heats up a warm one and so on until they are all at the same temperature.

Thermodynamics doesn't say that everything will cool down, just that it'll all come to the _same_ temperature (whatever that may be).

Once you include some stuff from cosmology (including expansion) you figure out what that temperature is (if expansion is accelerating then it will tend to 0 given infinite time).

As to why it hasn't smoothed out yet.

I think the anthropic principle sums it up fairly nicely.

If the universe were a lot hotter (/less spread out) life (like us at least) couldn't exist.

If the universe were a lot colder (/more even temperature) life (like us at least) couldn't exist.

Therefore the universe must be about this smoothed out (ie. cooled down somewhat but not too much) if we are to exist.

Even if we aren't the only way life can exist in the universe, and life that did exist in the early universe would be very different and would have the same two conditions as true (and thus come to the same conclusion).

Same goes for any life that may form later after all/most of the stars burn out.

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As to why it hasn't smoothed out yet.

I think the anthropic principle sums it up fairly nicely.

If the universe were a lot hotter (/less spread out) life (like us at least) couldn't exist.

If the universe were a lot colder (/more even temperature) life (like us at least) couldn't exist.

Therefore the universe must be about this smoothed out (ie. cooled down somewhat but not too much) if we are to exist.

Schrödinger's hat,

Well certainly I can go along with that principle, except "here" is not the same temperature and gradient condition as "everywhere". And "now" is not the same temperature and gradient condition as "always".

It does not seem to me that one should consider the conditions of a particular place at a particular time as the "same" conditions that exist always and everywhere. If there were only "one" condition, already, then where is the possibility of a "particular" condition going to come from?

For instance, the conditions available in the void between strings of galaxies, are different than those inside the string. And one could use the anthropic principle to state that since we arose inside a string of galaxies, the conditions inside a string of galaxies provide a temperature gradient sufficient for the task, which may not be available in the void between galaxies. It (the principle) does not necessarily apply to the complete universe, always.

Besides, even if the void and string contain different conditions, and are taken as composing together, one system, the "timing" of any change in conditions, or averaging out that may be in process, is rather extensive and important. The conditions will not average out instantly, but instead will attempt to do so at a rather creeping (at that scale) speed of light.

Here, always can be understood as one thing. Everywhere now can be understood as one thing.

But everywhere always is everything. And I am not sure there is a way to write that equation. You have to take a perspective and hold something stationary for anything to make sense.

So what is the difference between void and string? More Matter in the string. More "gravity" in the string. More gradients in the string.

What is possible in the string might well not be in the void.

So "collecting matter together" may be important for life. Regardless of the average temperature of the universe. at first, now or later.

Regards, TAR

Edited by tar
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• 2 months later...

Good morning.

If the material things ("bodies") consisted by really points (with no dimensions, each one of them), then the total gravity would be a sum of zero-factors, and thus a total zero?

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• 1 month later...

Good morning.

If the material things ("bodies") consisted by really points (with no dimensions, each one of them), then the total gravity would be a sum of zero-factors, and thus a total zero?

Total gravity would be a positive value if going scalar. Total gravity of a "perfect" system would be 0 vector-wise. (i am being brief)

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In low friction or frictionless systems it is easy to see an alternative to the standard attractive model of gravity that negates neither Newton's nor Einstein's observations and may prove fruitful in reassessing problems of micro gravitational anomalies and sub atomic behaviour. We see in such systems a propulsive rather than an attractive model. For a crude macro example, two ships will tend to drift together in open water, not because of the attraction of their general mass, but because in the general maelstrom of forces, a lull develops in the lee of the vessels, (the interstice between the vessels), that effectively constitutes a "downslope" attraction - so they drift together. (They do not pull each other, they are "pushed" together by the slight variation in the net propulsive force to the "weather" side of the vessel). The same is, of course, observable in satellites.

Both Newton and Einstein effectively describe and model the behaviours of material, without necessarily understanding the underlying principles. We are obliged therefore to take the forces that have no rational explanation as axiomatic. In reality there are very few axioms. Radiation is well defined and perceived experimentally. Attractive forces are accurately observed but are not convincingly demonstrated, and do not seem well understood. You should perhaps spend more time reassessing your axioms than developing ever more ludicrous Ptolemaic epicycles.

Unfortunately the conception of "attractive" forces (the only truly attractive forces are vacuum based, as in EM systems) is so deeply entrenched in highschool physics that it is unlikely that a net propulsive force will be seriously considered by the current generation.

Edited by justlookingin
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In low friction or frictionless systems it is easy to see an alternative to the standard attractive model of gravity that negates neither Newton's nor Einstein's observations and may prove fruitful in reassessing problems of micro gravitational anomalies and sub atomic behaviour. We see in such systems a propulsive rather than an attractive model. For a crude macro example, two ships will tend to drift together in open water, not because of the attraction of their general mass, but because in the general maelstrom of forces, a lull develops in the lee of the vessels, (the interstice between the vessels), that effectively constitutes a "downslope" attraction - so they drift together. The same is, of course, observable in satellites.

Both Newton and Einstein effectively describe and model the behaviours of material, without necessarily understanding the underlying principles. We are obliged therefore to take the forces that have no rational explanation as axiomatic. In reality there are very few axioms. Radiation is well defined and perceived experimentally. Attractive forces are accurately observed but are not convincingly demonstrated, and do not seem well understood. You should perhaps spend more time reassessing your axioms than developing ever more ludicrous Ptolemaic epicycles.

Unfortunately the conception of "attractive" forces (the only truly attractive forces are vacuum based, as in EM systems) is so deeply entrenched in highschool physics that it is unlikely that a net propulsive force will be seriously considered by the current generation.

nonsense

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nonsense

A prompt, but not terribly informative response. Which words did you find particular difficulty with?

I like the username. It has been my regretted privilege to have been a "rocket scientist".

[i see that you may have pre-empted a clarificatory edit: (They do not pull each other, they are "pushed" together by the slight variation in the net propulsive force to the "weather" side of the vessel)). Perhaps that helps.]

PS. While Feynman may have been a wonderful teacher of the physics of his time, neither etymology nor ornithology were among his great achievments. The name of anything, in any language, should tell you something about the entity, and the more languages that you use, the broader the picture that you will develop. Granted, the words may tell you more about the people that name the bird than the bird itself, but nevertheless a certain amount of information regarding observed characteristics and behaviours is likely to be communicated.

Edited by justlookingin
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A prompt, but not terribly informative response. Which words did you find particular difficulty with?

I like the username. It has been my regretted privilege to have been a "rocket scientist".

I am not surprised that you don't understand.

Repulsive models for gravity have been proposed, evaluated, and debunked ad nauseaum. Yours is neither the first nor, by a long shot, the most novel. Nevertheless, it remains nonsense.

I share your regret that you might ever have been considered a rocket scientist.

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nonsense

You should have replaced n with W, o with e, n with l, s with c, e with o, n with m, s with e, and deleted the last e.

"Welcome."

Edited by michel123456
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You should have replaced n with W, o with e, n with l, s with c, e with o, n with m, s with e, and deleted the last e.

"Welcome."

Thank you very much. The spell checker must have missed that.

That negative rep from you means as much to me as a positive rep from Terry Tao.

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Thank you very much. The spell checker must have missed that.

That negative rep from you means as much to me as a positive rep from Terry Tao.

Don't take that as personnal.

Neg. rep was about the content of your post: the way a very knowledgeable person engages conversation with a brand new member.

I doubt justlookingin will come back after such an experience.

Not only that, justlookingin (which is also a real person in the real world) will spread everywhere the way he was treated on this forum. So IMHO you made something bad to SFN.

So I voted neg.

Note: I remember having stated boldly some time ago that I never vote neg. I changed my mind.

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!

Moderator Note

Do try and keep things civil and on topic.
Michel, take it to a PM or report it to the mods if you do not like DrRocket's behavior, arguing in the middle of a thread is bad for the discussion.

DrRocket, some links to (or explanations of) said models along with a slightly friendlier attitude would probably be more effective for helping Justlookingin understand his/her misconceptions.

Justlookingin, please post any speculatory or non-mainstream explanations and ideas in the speculations forum.

Edited by Schrödinger's hat
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1) What is holding us against the Earth?

2) Why is it able to do so?

3) What interaction is there between matter and space?

4) How is it relevant to what is keeping us from being flung from the Earth?

5) What exactly is gravity?

6) Why is G found is both Einstein and Newtons equations?

(curvature of space-time) = (mass-energy density) * 8
pi G
/
c
4

Fg=Gm1m2/d^2

These are just a few questions to get the discussion going. I think this topic will clarify a lot of what was said in a different topic called "do objects fall at the same speed? no" or something close to that. The arguments mainly revolved around gravity so here is a chance to explain your understanding of what it actually is.

I suppose my first question to see where minds are at would be "How may are familiar with the theories, discarded but occassionally revisited, of LeSage and Fatio?"

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• 2 weeks later...

Even if we actually get to a point where the Higgs boson is conclusively discovered, will that conclusively and finally answer the question?

I understand why a lot of the members here try to seperate the metaphysical from the pure, hard physics this forum has a legitimate claim to. I mean, we can all postulate our own metaphysical theories and ideas without fear of reprehence (because opinions are never wrong) but scientific 'facts' are binary, yes or no, 1 or 0. They either are or aren't.

So, if we find the Higgs, if we detect the Higgs field in an alternate dimension or it is all linked through some other intangible force or force carrier - will that still satisfy the question? I'm focusing purely on 'what is gravity?' here, because that is the question that has been asked for hundreds of years and as far as I know - we still don't have a concrete answer to.

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I must have missed the revolution in Physics since I was under the impression that we had a concrete answer to "what is gravity ?"

Gravity is nothing more than the intrinsic curvature of space-time in response to a presence of mass/energy. The mass/energy then follows this intrinsic curvature in its motion along lines called geodesics, unless of course another force is acting on that mass/energy.

Anything else implies action at a distance, which I and a lot of others are unconfortable with, and is also the case with Newton's gravity, and though Newton's gravity is fairly accurate ( it put people on the moon, did it not ? ), he never could explain what it was. We had to wait for Einstein's GR for an answer.

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in my opinion gravity is the resultant of forces upon objects on earth due to the spinning...

but the angular momentum of earth remains constant and so we have objectss weight...

counter reaction of objects to this spinning is the friction...

by extrapolation came G,who is an imitation of g and is calculated by g...

so G formula and value are very doubtful...

you big dusty savants look like in the moliere plays,a bunch of doctors disputing at a sick mans head...

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in my opinion gravity is the resultant of forces upon objects on earth due to the spinning...

No. The spinning actually results in the net force on you being smaller, depending on your latitude, but it is not the source of the gravitational force.

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• 1 month later...

Hi

Not sure if this is still on.

If I would say that I'm working on Origin of Gravity, probably I will be penalised.

I would like to say that Origin of Gravity, Energy and Mass are the most important part of our understanding of physics and our universe. I agree with p-cunfused that pure hard physics should be applied here. But look, how one could be so sure what he/she is talking about. If you (who claim that you are the most educated here) think as you were thought in school what is right and what is wrong, please give some freedom to all others. Looks like even most educated or experianced physicist can not be 100% right. If you are, than please tell us the origin of Gravity so others wont be confused.

With all respect to everybody who is using this forum.

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If I would say that I'm working on Origin of Gravity, probably I will be penalised.

If so I myself would stick up for you since this seems to be about a searc of knowledge, not a new theory about gravity.

I would like to say that Origin of Gravity,..

There is now known mechanism of gravity. Same with EM either. There is deeper and more precise knowledge of EM too but no known mechanics of it.

Energy and Mass are the most important part of our understanding of physics and our universe.

I agree with p-cunfused...

But look, how one could be so sure what he/she is talking about.

Experience with the theory and the relevant experiments. Anynbody who believes that they know everything 100% is ignorant!

If you (who claim that you are the most educated here) ...

I find that comment offensive. Therefore this is where I end my part in this thread.

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