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Is Gravity a force, or not a force?

  1. Aug 12, 2010 #1
    I am not a student and am simply interested in physics and the like, do studying on my own and just trying to learn about this fascinating subject. While reading or watching videos i get 1000's of questions but i have nobody to ask so i figured id join here.... i hope my questions aren't too stupid lol.

    Anyways, my question is, so is gravity a FORCE, or is it a cause from an effect?

    I thought that there was no 'invisible force' called gravity like what has been believed for so long, and that what was actually going on was that things were merely following a straight line through a curved space/time fabric.... which would cause an object to 'attract' to another due to its curving of the fabric.

    But in the lecture I'm watching, he says that Gravity IS a force, and its message carrier is the Graviton, and that obviously its one of the major forces of our universe.

    So which one is right or are they both right? I know the space/time fabric is right, but is gravity then still a force?
  2. jcsd
  3. Aug 12, 2010 #2


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    A force is something which results in acceleration. However, due to the equivalence principle this idea for gravity doesn't make sense anymore.

    See for instance Carroll's treatment


    and particularly
    I think this section is not very technical, so with a little background you should be able to follow ;)
  4. Aug 12, 2010 #3


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    If a test particle is released from rest ( wrt to the source of the field), it experiences an acceleration that can easily be defined.
  5. Aug 12, 2010 #4

    It depends on what theory you're using.
    General Relativity doesn't consider gravity as a force. Massive objects create a space-time geometry with a "curvature" described by the curvature tensor. There is no actual "force" of gravity, just the curvature of space-time that is created by gravity.

    The idea of a graviton comes from quantum field theory. In QFT, the "force" of gravity is mediated by the graviton with the stress-energy tensor as the source.

    If the lectures are for Newtonian gravity, then gravity is also considered a force.

    GR considers gravity as the "cause." The "effect" is the geometry of space-time.
    I suppose you could say that GR presents gravity as mass. Anything with mass will cause the curved geometry of space-time.

    It is this mass (energy) that generates the curvature of space, which causes other objects to accelerate in response to the curvature. When a human accelerates from this curvature, it feels like there is a force that is causing him or her to accelerate. Being vocal creatures, we gave this sensation the name "gravity."
  6. Aug 12, 2010 #5


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    What if the field has multiple sources, all in different states of motion? What if you have a cosmology that's a vacuum solution to the field equations, so there are all kinds of gravitational waves running around, but they were never created by sources? Acceleration is easy to define, but in GR that definition is completely dependent on the choice of coordinates, so the definition is arbitrary.
  7. Aug 12, 2010 #6


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    I regret saying 'wrt to the source' now. We can define sets of worldlines in any spacetime and define frames where coordinate velocity is zero, which is what I mean (probably) 'from rest'.

    The definition I had in mind depends on the Christoffel symbols ( I didn't say it was coordinate independent ) and as long as they exist the details of the solution don't matter.

    A vacuum solution may still have sources (?). Presumably a spacetime with gravitational waves will still have EOMs. Please put me right if this is incorrect.

    For example, a test body released in a standard FLRW expanding dust cosmology feels nothing. The same thing in a Schwarzschild spacetime will begin moving towards the source. The 'sticky-bead' argument says that a test body moves in a gravitational wave.

    I need to redefine what I was thinking of in manifold concepts.
  8. Aug 18, 2010 #7
    Thank you all for your responses.... its refreshing actually being able to discuss such things with someone.

    Is it understandable that this bothers me?
    This puts the two theories against eachother and one of them must be flat out wrong in this case....
    I rented the 'great courses' lectures on String Theory by Jim Gates, and whole lectures pertain to particle physics and the forces of the universe, in which the graviton gets considerable time, and also on how string theory actually predicts these message carriers to be.

    It bothers me then, that this would be in direct opposition to G.R......

    At least now i know that its for sure that these two are in opposition and that i wasnt just totally confused.

    Thank you for your answer
  9. Aug 18, 2010 #8
    I think it is mostly a terminology issue, it depends on how you define a force.

    Gravitation causes inertial accelerations and rotations, from that perspective you can say it is not a force in the traditional sense, however gravitation also causes tidal effects and these effects certainly look like a force in the traditional sense.
  10. Aug 18, 2010 #9

    My view is that Science is not about truth, it is about building a model to better understand how the universe works. While in Newtonian description it is considered a force, in GR it is the curvature of spacetime, and so on in other theories, they are all different ways to look at things as mathematical models, but they don't really say much about what gravity *really* is.
  11. Aug 18, 2010 #10

    It probably should bother you that the major theories of the world have definite areas where they can't agree.
    But don't let the word "force" be too much of a factor in that.
    Force is just the word given to describe a particular mathematical act.

    One theory has gravity being sent out as a force (by gravitons) that act on other objects.
    The other theory has gravity changing the geometry of the space which those other objects "live." This change in geometry makes those objects feel something like a force.

    Whether the earth is sending gravitons to "force" you from floating away or the earth is changing the geometry of space in order to "force" you from floating away.....you're still stuck on earth.
    Just be glad that the atmosphere is feeling the same thing.....It's hard to think about physics when your brain isn't getting oxygen.
  12. Jul 24, 2012 #11
    I think gravity cannot be a force in a traditional sense. Namely, it could not affect light if it was a force, as light (photons) have no mass. Gravity bends space-time around it so light, choosing the shortest route (in space-time!), seemingly bends around mass (gravitational lensing).
    Another thing. Great Feynman, in one of his lectures has drawn attention to an astounding fact and that is that the gravity pull beween two elctrones is 1/ (4.17 *10^42) in comaprison to the intensity (force) of their electromagnetic repulsion! In other words the two "forces" are a whole universe away!. If you put 10^42 protons in the universe next to one another they would add up to a diameter of the known universe!!!!
    So, I think that gravity is in no way connected as a "force" to electromagnetic, strong or weak nuclear force. Gravity is an inherent characteristic of the structure and the fabric of the universe and it has yet to be understood. I don't beleive in the "unified" theory at all and it is inherently impossible to find a magic mathematical formula to unite all "four forces".
    Last edited: Jul 24, 2012
  13. Jul 24, 2012 #12


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  14. Jul 24, 2012 #13


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    Why? The centrifugal force, that appears in a rotating frame, also causes non-uniform acceleration. But it's still just an inertial force.
  15. Jul 24, 2012 #14
    "Namely, it could not affect light if it was a force, as light (photons) have no mass."

    Photons do not have rest mass, but they do have mass. Anything with energy has mass--you just divide the energy by the square of c to calculate that mass. It might seem that I am engaging in a semantical quibble but I am not--photons generate gravitational fields, a characteristic of things with "mass".

    "I think gravity cannot be a force in a traditional sense"

    Even if photons did not have mass, their deviation in a gravitational field in General Relativity is not suggestive that gravity in General Relativity is not a force. For example, in the Newtonian regime the force on a body is mg where g is the local gravitational field strength, and the acceleration is thus (mg)/m. For the case of m going to zero, (mg)/m does not at all vanish!
  16. Jul 24, 2012 #15
    "Newton's model : Gravity is an interaction force
    Einsteins's model : Gravity is an inertial force"

    Actually, the Einstein perspective is that gravity is an interaction force--he certainly believed the Sun exerted a force on the Earth-- but that it is also an inertial force. That was the beauty of General Relativity!

    It is important to remember that General Relativity is a physical theory. It most certainly was about "forces".
  17. Jul 24, 2012 #16
    The theories are different but both are approximations rather than 'flat out wrong'. All our
    models are approximations. Some to twenty or more decimal places, others not so good! Quantum gravity is the incomplete theory, a work in progress, that it is hoped may someday be completed and combine quantum gravity and general relativity.

    We thought we were so 'smart' about mass and energy, and gravity, in the universe until maybe 20 or 25 years ago, I think when Vera Rubin's observations about galaxy rotations led to more widespread acceptance of dark matter [and dark energy] that Fritz Zwicky had postulated in the 1930's. Suddenly all we 'understood' was about 4% of the mass and energy in the universe instead of the virtually 100% previously thought!

    Dark energy supposedly has negative pressure, a form of repulsive gravity I think comes from general relativity; anybody know if there is a corresponding view from quantum mechanics??
  18. Jul 24, 2012 #17


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    Inertial forces are per definition not exerted by some source object. That's what sets them apart from interaction forces.
  19. Jul 24, 2012 #18


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    In Newtonian physics, how much force does it take to produce a finite acceleration in an object with 0 mass?
  20. Jul 24, 2012 #19
    "Dark energy supposedly has negative pressure, a form of repulsive gravity I think comes from general relativity"

    It does indeed come from GR. There are two major cosmological equations. One indicates that the deceleration of the expansion is proportional to the mass densitiity plus 3 times the pressure. If, as the observational data is, as believed to indicate, thatt the Universe's expansion is accelerating then either the mass density or the pressure (or both) must be negative. But the other equation implies the mass density is positive. So we are stuck with the strange implication of negative pressure if the Universe's expansion really is accelerating.
  21. Jul 24, 2012 #20
    "Inertial forces are per definition not exerted by some source object. That's what sets them apart from interaction forces"

    Earlier you wrote:

    "Newton's model : Gravity is an interaction force
    Einsteins's model : Gravity is an inertial force"

    Taking both statements together you seem to be implying things like that the Sun is not a source of a force exerted on the Earth. I don't think you really mean to imply that.
  22. Jul 24, 2012 #21


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    The acceleration caused by the traditional Newtonian force of gravity does not depend on the mass of the accelerated object.
  23. Jul 24, 2012 #22


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    I do mean to imply that. In GR there is no direct force interaction between Sun and Earth. They interact only indirectly via space-time geometry. The locally experienced force of gravity is an inertial force, due to using an accelerating reference frame.

    In GR the Sun is a source of "gravitation", but it's not a source of "the force of gravity". The "force of gravity" in GR is caused by the coordinate choice, not by the Sun.
    Last edited: Jul 24, 2012
  24. Jul 24, 2012 #23
    "In GR there is no direct force interaction between Sun and Earth."

    Well, yeah, it is action at a distance. In electomagnetism a charge interacts with another charge not "directly" but through a field, too.

    " They interact only indirectly via space-time geometry."

    But a quantity in that geometry, the affine connection, is the forces. You have forces--they are geometric quantities. The two are not mutually exclusive. GRn is a physics theory about physical quantities.

    "In GR the Sun is a source of 'gravitation', but it's not a source of 'the force of gravity'." The "force of gravity" in GR is caused by the coordinate choice, not by the Sun."

    Obviously coordinate choices can turn gravitational fields on and off locally, but that misses the big picture--the Sun really is acting on the Earth. If the Earth were made to stand still, and then released, it would fall into the Sun and crash into it. The Earth would melt. If the Sun had no mass, no coordinate choice could cause that event to occur.
  25. Jul 24, 2012 #24
    Me: "The Earth would melt."

    Well, actually, more than just melt, of course.
  26. Jul 24, 2012 #25


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    "Field" is too vague. The topic is "interaction force". The only way to create a "force of gravity" in GR is to chose a non-inertial reference frame. But that is an "inertial force" then, not an "interaction force".
    "Acting" is to vague. The topic is "interaction force". To clarify what I mean:

    "interaction force" : causes proper acceleration
    "inertial force" : causes only coordinate acceleration

    Based on this definitions there is no "interaction force of gravity" in GR. Just an "inertial force of gravity".
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