# Nature of gravity

#### loafula

I will start by saying I am not a physicist and what I am posting here is pure thought experiment on my part.
I was wondering about the nature of gravity, and trying to determine how the force could act to pull two objects together. What physical interaction causes two objects to accelerate towards each other? I came up with the idea that gravity is not a property of mass, and it does not cause objects to pull each other. Instead, what if it is a property of an expanding universe that pushes objects together, and is only affected by mass?
For example, take an object in empty space. Imagine that gravity affects this object by pushing equally on all sides. It acts as an equal force coming in from all directions. Take another object and place it near the first. Again, this object experiences gravity as a force coming in from all directions. With two objects, though, the pushing force acting on the faces of the objects that are pointing towards each other will be weaker compared to the forces acting on the rest of the objects, because the mass of each object absorbs some of this force- essentially "blocking" it from reaching the other object. The result here is that the two objects would be pushed together.

Related Special and General Relativity News on Phys.org

#### tiny-tim

Homework Helper
Welcome to PF!

Hi loafula! Welcome to PF!

The problem with your theory is that it depends on the size of each object …

an object with large diameter will block more than an object of the same mass with small diameter

physics theory has to agree with experimentally observed results, and experiment tells us that gravity (from a sphere, anyway) depends on mass, not on size

experiment also tells us that the gravitational force of object A on object B is the same as of B on A … which also wouldn't work on your theory

#### JesseM

In Feynman's book The Character Law he has a good section on pushing gravity, and on the general futility of trying to "explain" mathematical laws in terms of mechanical models...I copied it out in another post, so I'll just repost it here:
On the other hand, take Newton's law for gravitation, which has the aspects I discussed last time. I gave you the equation:

F=Gmm'/r^2

just to impress you with the speed with which mathematical symbols can convey information. I said that the force was proportional to the product of the masses of two objects, and inversely as the square of the distance between them, and also that bodies react to forces by changing their speeds, or changing their motions, in the direction of the force by amounts proportional to the force and inversely proportional to their masses. Those are words all right, and I did not necessarily have to write the equation. Nevertheless it is kind of mathematical, and we wonder how this can be a fundamental law. What does the planet do? Does it look at the sun, see how far away it is, and decide to calculate on its internal adding machine the inverse of the square of the distance, which tells it how much to move? This is certainly no explanation of the machinery of gravitation! You might want to look further, and various people have tried to look further. Newton was originally asked about his theory--'But it doesn't mean anything--it doesn't tell us anything'. He said, 'It tells you how it moves. That should be enough. I have told you how it moves, not why.' But people are often unsatisfied without a mechanism, and I would like to describe one theory which has been invented, among others, of the type you migh want. This theory suggests that this effect is the result of large numbers of actions, which would explain why it is mathematical.

Suppose that in the world everywhere there are a lot of particles, flying through us at very high speed. They come equally in all directions--just shooting by--and once in a while they hit us in a bombardment. We, and the sun, are practically transparent for them, practically but not completely, and some of them hit. ... If the sun were not there, particles would be bombarding the earth from all sides, giving little impuleses by the rattle, bang, bang of the few that hit. This will not shake the earth in any particular direction, because there are as many coming from one side as from the other, from top as from bottom. However, when the sun is there the particles which are coming from that direction are partially absorbed by the sun, because some of them hit the sun and do not go through. Therefore the number coming from the sun's direction towards the earth is less than the number coming from the other sides, because they meet an obstacle, the sun. It is easy to see that the farther the sun is away, of all the possible directions in which particles can come, a smaller proportion of the particles are being taken out. The sun will appear smaller--in fact inversely as the square of the distance. Therefore there will be an impulse on the earth towards the sun that varies inversely as the square of the distance. And this will be the result of a large number of very simple operations, just hits, one after the other, from all directions. Therefore the strangeness of the mathematical relation will be very much reduced, because the fundamental operation is much simpler than calculating the inverse of the square of the distance. This design, with the particles bouncing, does the calculation.

The only trouble with this scheme is that it does not work, for other reasons. Every theory that you make up has to be analysed against all possible consequences, to see if it predicts anything else. And this does predict something else. If the earth is moving, more particles will hit it from in front than from behind. (If you are running in the rain, more rain hits you in the front of the face than in the back of the head, because you are running into the rain.) So, if the earth is moving it is running into the particles coming towards it and away from the ones that are chasing it from behind. So more particles will hit it from the front than from the back, and there will be a force opposing any motion. This force would slow the earth up in its orbit, and it certainly would not have lasted the three of four billion years (at least) that it has been going around the sun. So that is the end of that theory. 'Well,' you say, 'it was a good one, and I got rid of the mathematics for a while. Maybe I could invent a better one.' Maybe you can, because nobody knows the ultimate. But up to today, from the time of Newton, no one has invented another theoretical description of the mathematical machinery behind this law which does not either say the same thing over again, or make the mathematics harder, or predict some wrong phenomena. So there is no model of the theory of gravity today, other than the mathematical form.

If this were the only law of this character it would be interesting and rather annoying. But what turns out to be true is that the more we investigate, the more laws we find, and the deeper we penetrate nature, the more this disease persists. Every one of our laws is a purely mathematical statement in rather complex and abstruse mathematics.

...[A] question is whether, when trying to guess new laws, we should use seat-of-the-pants feelings and philosophical principles--'I don't like the minimum principle', or 'I do like the minimum principle', 'I don't like action at a distance', or 'I do like action at a distance'. To what extent do models help? It is interesting that very often models do help, and most physics teachers try to teach how to use models and to get a good physical feel for how things are going to work out. But it always turns out that the greatest discoveries abstract away from the model and the model never does any good. Maxwell's discovery of electrodynamics was made with a lot of imaginary wheels and idlers in space. But when you get rid of all the idlers and things in space the thing is O.K. Dirac discovered the correct laws for relativity quantum mechanics simply by guessing the equation. The method of guessing the equation seems to be a pretty effective way of guessing new laws. This shows again that mathematics is a deep way of expressing nature, and any attempt to express nature in philosophical principles, or in seat-of-the-pants mechanical feelings, is not an efficient way.

It always bothers me that, according to the laws as we understand them today, it takes a computing machine an infinite number of logical operations to figure out what goes on in no matter how tiny a region of space, and no matter how tiny a region of time. How can all that be going on in that tiny space? Why should it take an infinite amount of logic to figure out what one tiny piece of space/time is going to do? So I have often made the hypothesis that ultimately physics will not require a mathematical statement, that in the end the machinery will be revealed, and the laws will turn out to be simple, like the chequer board with all its apparent complexities. But this speculation is of the same nature as those other people make--'I like it', 'I don't like it',--and it is not good to be too prejudiced about these things.

#### loafula

Thanks for the feedback guys! I guess I just have trouble trying to visualize a pulling force- it would seem to me that whatever force is doing the pulling would have to wrap around or hook into whatever object it is actually pulling (or be very sticky:).
Also, I wasn't thinking the the force would be blocked by the size of an object, but by it's mass.

#### Naty1

Think more carefully about a "pushing" (repulsive) force....if you really do have a tight visual analogy you are a lot further along than I!!!

If you want to continue your thought experiment, you can also consider electric charge attraction as an analogy. Gravitational force attraction GMm/r^2 "looks" a lot like electric point charge attraction kQq/r^2.

You can think of gravitational attraction in at least three ways: a Newtonian force via a classical gravitational field; an Einstein curvature of space (in which spacetime becomes the "field") or a quantum exchange of attrractive particles (gravitons).

Have fun!!

#### inquisitive_i

hi guys..
i'm new at this forum....... hope u'll not hav trouble detangling my silly silly querries..
without beating around the bush anymore.. my doubt is::
gravitational force that holds the planetary motion is only attractive by nature.
In our Solar system..its only our Sun that hold planets to their orbits and effect of distant stars is negligibe.. than what force keep these planets from fallin into d sun in spirals.. planets hav elliptical path due to which they loose energy continuously(being under acceleration cntinuously), gravitational pull of d sun on the other hand can b consider constant.. yet d planets are fixed to their orbits?

#### HallsofIvy

Homework Helper
One of the things Newton developed the calculus to do was to show that if a central force depends on 1/r2 then the orbits must be conic sections (hyperbolas, ellipses, parabolas, etc.) not spirals.

#### JesseM

than what force keep these planets from fallin into d sun in spirals.. planets hav elliptical path due to which they loose energy continuously(being under acceleration cntinuously), gravitational pull of d sun on the other hand can b consider constant.. yet d planets are fixed to their orbits?
Being under continuous acceleration does not mean they lose energy continuously. Ignoring gravitational waves and just looking at Newtonian gravity, the sum of potential energy + kinetic energy is always conserved--when the planet is closer to the sun it has less potential energy and more kinetic energy, when the planet is farther from the sun it has more potential energy and less kinetic energy. No additional force is needed to keep the planet orbiting the same way forever in Newtonian gravity.

#### Phrak

It acts as an equal force coming in from all directions...The result here is that the two objects would be pushed together.
As it turns out, modeling gravity as a flux having equal momentum from all directions can have an inverse square law just as Newtonian gravity.

I'm not saying that this is how gravity works. It's an interesting idea.

One way to do this is to propose a small absorbtion coofficient. If it is small, then most of the flux passes through a planet or star. The larger the absorbtion coefficient, the further this model will deviate from inverse-square of masses.

#### Histspec

As it turns out, modeling gravity as a flux having equal momentum from all directions can have an inverse square law just as Newtonian gravity.
That sounds like the Fatio-Lesage model.
http://en.wikipedia.org/wiki/Le_Sage's_theory_of_gravitation

However, as Maxwell and Poincaré calculated, to produce attraction one has to assume that the particles/waves were absorbed, and that leads to an enormous production of heat. Also (as mentioned in the Feynman citation above) moving bodies must experience some resistance in the direction of motion, which is not observed.

#### Phrak

That sounds like the Fatio-Lesage model.
http://en.wikipedia.org/wiki/Le_Sage's_theory_of_gravitation

However, as Maxwell and Poincaré calculated, to produce attraction one has to assume that the particles/waves were absorbed, and that leads to an enormous production of heat. Also (as mentioned in the Feynman citation above) moving bodies must experience some resistance in the direction of motion, which is not observed.
I see that Le Sage concludes that a fully elastic scattering model, where energy is not absorbed, will not result in a gravitational-like force. So he proposes a method to modify elastic collisions by his "ultramundane corpuscles" in such a way to recover a gravitational force.

I was messing with this idea when I was like, fifteen (a few hundred years late, it seems). Growing highly suspicious of attractive forces, I wanted to know if neutrinos could be the source of gravity. I was also uneasy about absorbed energy, but lost interest before examining scattering.

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#### Naty1

Inquisitive posted:
my doubt is::
gravitational force that holds the planetary motion is only attractive by nature.
You are correct....negative pressure in general relativity is repulsive....
but this has little if anything to do with local planetary motion..

#### loafula

hi guys..
i'm new at this forum....... hope u'll not hav trouble detangling my silly silly querries..
without beating around the bush anymore.. my doubt is::
gravitational force that holds the planetary motion is only attractive by nature.
In our Solar system..its only our Sun that hold planets to their orbits and effect of distant stars is negligibe.. than what force keep these planets from fallin into d sun in spirals.. planets hav elliptical path due to which they loose energy continuously(being under acceleration cntinuously), gravitational pull of d sun on the other hand can b consider constant.. yet d planets are fixed to their orbits?
In order for something to orbit, it has to be moving in a path perpendicular to the force of gravity (I.E. sideways). So imagine a satellite way up in the sky. It is moving sideways relative to the ground. The Earth is pulling the satellite downwards, though and that satellite is falling. It stays in orbit, though, because it is moving sideways and essentially misses the earth as it falls past it.

#### inquisitive_i

In order for something to orbit, it has to be moving in a path perpendicular to the force of gravity (I.E. sideways). So imagine a satellite way up in the sky. It is moving sideways relative to the ground. The Earth is pulling the satellite downwards, though and that satellite is falling. It stays in orbit, though, because it is moving sideways and essentially misses the earth as it falls past it.
but regardless of its inclination.. the orbiting body will always move sideways as the source of gravity is spherical..! so how can we say that the motion of planets is perpendicular to Sun's gravitational field..?

#### JesseM

but regardless of its inclination.. the orbiting body will always move sideways as the source of gravity is spherical..! so how can we say that the motion of planets is perpendicular to Sun's gravitational field..?
The acceleration of the planet is always straight towards the sun, but don't forget that planets have inertia so they "want" to keep going in the direction they're going, tangential to the orbit. An object moving in a circle is in fact accelerating in the direction of the center at all times, if the force from the center was suddenly turned off it would fly off on a straight line that'd be tangential to the curve of the orbit, because of its own inertia (Newton's first law: 'an object in motion tends to stay in motion', i.e. if no external forces are accelerating a moving object, it'll move in a straight line at constant speed).

#### inquisitive_i

thanx i somewhat got it right..
it is the INERTIA that keeps them from fallin straight into d Sun

#### Nickelodeon

You can think of gravitational attraction in at least three ways: a Newtonian force via a classical gravitational field; an Einstein curvature of space (in which spacetime becomes the "field") or a quantum exchange of attrractive particles (gravitons).

Have fun!!
Einstein's curvature of space idea was based on the Equivalence principle which equates the notion that gravity at the earth surface, for example, is a similar experience to being accelerated at 1g in a craft of some sort. Einstein then asked himself "if these two experiences are in fact the same, ie caused by linear acceleration, then what properties of space do we have to modify in order for the maths to work?" Einstein's model works better than a Newtonian model but I'm not sure the underlying assumptions need necessarily be correct.

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#### yogi

Newton took the position that masses act directly upon one another at a distance, and presumably instaneously. Einstien looked at the problem more from the perspective of Faraday - specifically that an isolated mass conditions the space around it in such a way that another mass placed within the influence of the conditioned space is acted upon - not directly by the first mass, but rather by the local properties of space in the neighborhood of the second mass

The question yet to be answered is why does mass condition space - Einstein postulated that matter induced a static curvature affect upon space and time ...this gives a good result for all tests that have been conducted - but neither Einstein's Theory of General Relativity nor Newton's theory of absolute space + action at a distance, offered any explanation for why the Gravitational constant has the value we measure - in both theories G must be put in by hand.

Therefore, we do not have a complete theory - in fact it would seem that the equivalence principle could shed some light if the force acting upon a static mass is related to cosmic expansion as suggested by loafula in his original post

#### inquisitive_i

Newton took the position that masses act directly upon one another at a distance, and presumably instaneously. Einstien looked at the problem more from the perspective of Faraday - specifically that an isolated mass conditions the space around it in such a way that another mass placed within the influence of the conditioned space is acted upon - not directly by the first mass, but rather by the local properties of space in the neighborhood of the second mass

The question yet to be answered is why does mass condition space - Einstein postulated that matter induced a static curvature affect upon space and time ...this gives a good result for all tests that have been conducted - but neither Einstein's Theory of General Relativity nor Newton's theory of absolute space + action at a distance, offered any explanation for why the Gravitational constant has the value we measure - in both theories G must be put in by hand.

Therefore, we do not have a complete theory - in fact it would seem that the equivalence principle could shed some light if the force acting upon a static mass is related to cosmic expansion as suggested by loafula in his original post
i believe Einstein theory has an upper hand over Newtonian version as it proficiently explains the bending of light in cosmos where Newton's fell short..

#### inquisitive_i

Marry Christmas to all the homies out there........
may this christmas bring rejuvinating geodestic curves in your space-time..

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