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Nature of gravity

  1. Dec 9, 2008 #1
    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.
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  3. Dec 9, 2008 #2


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    Welcome to PF!

    Hi loafula! Welcome to PF! :smile:

    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 :smile:
  4. Dec 9, 2008 #3


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    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:
  5. Dec 10, 2008 #4
    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.
  6. Dec 10, 2008 #5
    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!!
  7. Dec 11, 2008 #6
    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?
    please help..
  8. Dec 11, 2008 #7


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    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.
  9. Dec 11, 2008 #8


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    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.
  10. Dec 11, 2008 #9
    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.
  11. Dec 13, 2008 #10
    That sounds like the Fatio-Lesage model.

    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.
  12. Dec 13, 2008 #11
    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.
    Last edited: Dec 13, 2008
  13. Dec 13, 2008 #12
    Inquisitive posted:
    You are correct....negative pressure in general relativity is repulsive....
    but this has little if anything to do with local planetary motion..
  14. Dec 15, 2008 #13
    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.
  15. Dec 15, 2008 #14
    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..?
  16. Dec 15, 2008 #15


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    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).
  17. Dec 15, 2008 #16
    thanx i somewhat got it right..
    it is the INERTIA that keeps them from fallin straight into d Sun
  18. Dec 17, 2008 #17
    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.
    Last edited: Dec 17, 2008
  19. Dec 17, 2008 #18
    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
  20. Dec 18, 2008 #19
    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..
  21. Dec 24, 2008 #20
    Marry Christmas to all the homies out there........
    may this christmas bring rejuvinating geodestic curves in your space-time..
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