Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question on gravity and mass

  1. Dec 23, 2009 #1
    I am having difficulty understanding the statement "gravity propagates through mass without loosing power". I asked a friend about this and he replied that gravity is not a "force" that acts on objects. Its more like a curving of space. A rock rolls down a hill towards the "deeper" part of the hill, but the hill remains the same gradient, its effect on the rock doesn't change the hills shape.

    Sadly I am still not grasping either statement. Surely gravity is a force, a free body force diagram of objects of rest even indicate gravity as a force. And while I do understand what gravity can act on objects without loosing power, I cannot fathom the mechanics behind it.
  2. jcsd
  3. Dec 23, 2009 #2
    Yes. Your friend is right. According to the Einstein's theory, matter (and energy in general) curve spacetime. To understand the meaning of these words you really need to look at the math behind them.

    Yes. This statement is correct, but in the classical Newtonian mechanics.
  4. Dec 23, 2009 #3
    Could you perhaps expand on this further?. What does it really mean when someone says "gravity propagates through mass without loosing power?". If there is maths behind it could you explain how this conclusion is drawn or perhaps a link to where I can better understand the principle.
  5. Dec 23, 2009 #4
    I think it means that if you jump out of an airplane, you will accelerate just as fast towards the ground, whether there is a lot of mass between you and it, or not. So, if you're falling towards a building or a mountain, their mass won't somehow "shield" you against gravity, and you're still pretty screwed....

    Not sure if that's what you were lookign for.
  6. Dec 23, 2009 #5
    I think they mean that superposition applies, and there is no shielding. Suppose there's a wall in front of you, and behind the wall there's an elephant. The gravitational force that they together exert on you is the sum of two terms: the force exerted on you by the wall if the elephant weren't there, plus the force exerted on you by the elephant if the wall weren't there. Being able to add them in this way is called linear superposition. The wall doesn't shield you from the effect of the elephant. There are some things in physics that shield you from fields, for example, a sheet of metal would block an electric field, but there's nothing that can block gravity.

    By the way, it's spelled "losing", not "loosing."
  7. Dec 23, 2009 #6

    I see. But what if for example, I take a bucket of water and spin it around vertically. Doesn't the water will move and change shape at the bottom of the bucket as you spin it. So I don't see how "Gravity is not a "force" that acts on objects" is valid.
  8. Dec 23, 2009 #7
    In the method of Newton, gravity is visualized as an "action at a distance." The mass over here somehow knows that there's another mass out there. It's a force because one mass pulls on the other.

    In the method of Einstein, gravity is a local effect. The existence of a mass causes a disturbance in the geometry of space and time, and that disturbance is spread outward to distant places. The mass that feels the presence of another mass does so because it's responding to the characteristics of spacetime in it's own local vicinity. There's no assumption about a force from the first mass reaching across a distance to pull on the second mass, so many people say that gravity isn't a force.

    Gravity is still a force in the sense that a force is whatever causes a mass to accelerate. It's not a force in the classical idea of an invisible line that spans a distance and then pushes or pulls on something.
  9. Dec 23, 2009 #8
    Thank you for clearing that up for me. It was very helpful.
  10. Dec 23, 2009 #9


    User Avatar
    Gold Member

    Let me elaborate on this. (Mikelepore, I'm not commenting on you, please consider this posting an independent elaboration.)

    It's interesting to consider why the Coulomb force is referred to as a force both in terms of classical physics and quantum physics, whereas in the case of gravitation it is pointed out that in terms of GR gravitation is not a force in the classical sense.

    Let's see what the Coulomb force and gravitation have in common:

    - Originally the Coulomb force was thought of as instantaneous action at a distance. Later there was a shift towards thinking of the Coulomb force as being mediated by a field, the electromagnetic field, and a change in one location propagates as a change of the field with a finite velocity: the speed of light.
    - Prior to the introduction of relativistic physics gravitation was thought of as instantaneous action at a distance.

    After the introduction of relativistic physics both the Coulomb interaction and gravitational interaction are thought of as being mediated by a field.
    - For the Coulomb force: charged particles are not thought of as interacting directly with each other, instead each charged particle is thought of as the source of an electromagnetic field that extends in spacetime, and other particles interact with the local field. (The local field is thought of as the superposition of all fields that extend to that local region.)
    - For gravitation: Inertial mass is thought of as affecting its local spacetime, and gravitational effects extend away from a source because spacetime warps adjacent spacetime, thus extending curvature over stretches of spacetime. In other words: the spacetime curvature acts as mediator of gravitational interaction between gravitational sources.

    I hope I've made the similarities clear. So why is the Coulomb force still considered a force, both in terms of classical physics and quantum physics, while at the same time it is argued that in terms of GR gravitation is not a force in the classical sense.

    The difference - the only difference - is that the Coulomb force causes acceleration with respect to the local inertal frame of reference. That is why it's still qualified as a force.

    The gravitation from the Sun causes the planets to orbit the Sun, but the acceleration that the Sun causes is not acceleration with respect to the planet's local inertial frame. Instead the local inertial frame of a celestial body is an inertial frame of reference itself.
    (More precisely, the local frame that is co-moving with the center of mass of a celestial body does feature tidal effects, and in that sense it's not an inertial frame of reference, but the tidal effects are comparatively small. To a good approximation the local frame is an inertial frame of reference.)

  11. Dec 23, 2009 #10


    User Avatar
    Science Advisor

    Gravity propagates through mass without losing power - maybe unlike electromagnetism? I can make sense of such a statement in the sense that there are TWO sorts of electric charges, and UNLIKE charges attract, clump together and neutralize each other to extremely good approximation at large distances. Gravity has only ONE sort of charge and LIKE charges attract, clump together and don't neutralize each other at large distances.

    Gravity is a force, just like electromagnetism - both are classical fields, as well as quantum fields. In Newtonian physics, gravity is a force. In full general relativity, gravity is a field, just like the electromagnetic field, and reality is a simultaneous solution of the combined equations for both fields. In the ray limit of general relativity, gravity is not a force in the sense that test electrically uncharged particles follow geodesics of curved spacetime.
    Last edited: Dec 23, 2009
  12. Dec 23, 2009 #11
    What's being said is that there are no gravity insulators. Gravity cannot be blocked. If you put an elephant in the way, the apple doesn't feel anything differently.

    Gravity can be described as a force, but it also can be described as just an intrinsic parameter in the current spacetime, like the slope of the hill. It's like particles and waves.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook