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Why does mass cause gravity

  1. Mar 28, 2010 #1
    I've seen the images of a large mass creating a dent in the space-time fabric, and I've read the analogies as how the objects "fall in" towards the dent, causing gravity. But this is just an analogy, not an explanation. The objects would only fall in if there was gravity, so this cannot be an explanation; you can't use the term you're defining in the definition. So what actually causes the gravity?
  2. jcsd
  3. Mar 28, 2010 #2


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    Find out and you're in for a nobel prize.

    We can describe it very well mathematically, but as far as why is concerned... well that's a much more difficult question. We posit that mass causes gravity in a predictable way, and this appears to be very consistent with the universe we live in. For most physicists, I think, this is sufficient.
  4. Mar 28, 2010 #3


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    You are quite right -- using gravity to explain gravity makes no sense, so the "ball on a rubber sheet" picture fails to explain gravity. The problem is that that picture depicts space, not spacetime.

    For objects moving in one dimension of space, spacetime is just a graph of distance against time. An object in space becomes a line in spacetime. In the absence of gravity, a freely moving object is represented by a straight line drawn on a flat piece of paper. When there is gravity, we have to draw the line on curved paper. Two "straight" lines that start off as parallel will converge later on.

    For further information, try http://www.relativitet.se/spacetime1.html
  5. Mar 28, 2010 #4
    This is one of the questions that needs to change in such a way that one be really able to answer it! I mean that only given gravity as a fundamental force in nature (getting for one moment out of the explanation credited by GR about gravity) we can say why the objects fall in towards the dent. This has something to do with the fact that any question about the nature of gravitational force is an open problem and been largely dragged into consideration in the 1930s via introducing particles that mediate this force, the so-called gravitons. I.e. as an interacting force, gravity does welcome gravitons as the 'photons' of the gravitational interactions because they are postulated to be massless due to gravity having unlimited range. For a brief introduction to this subject, see http://en.wikipedia.org/wiki/Graviton

    Last edited: Mar 28, 2010
  6. Mar 29, 2010 #5
    Objects don't cause gravity as they "fall in" to a gravity well. Gravity is the distortion of spacetime which is caused by matter embedded in spacetime. This distortion creates a "gravity well" with the bottom of the well being the center of mass of the object creating the distortion.

    Here's a very simplified scenario. Say you have two objects, one of which is large and the other small. Now imagine the smaller object moving through flat spacetime and then encountering the curved spacetime created by the larger object's gravity well. The smaller object will merely travel along the line of least resistance...which would be the curved spacetime. The behaviour of the smaller object as it travels through this curved spacetime would be dependent on things such as its speed, its mass, the angle at which it enters the curved space, etc. The smaller object may fall down the gravity well until it strikes the larger mass...or it might be caught in orbit around the larger mass...or it may be moving fast enough to travel through the curved spacetime and leave the gravity well with a different trajectory.

    Given this, I think the analogy you speak of is a correct, though overly simplified, explanation of what is occurring.

    I think the big question is the mechanism behind the distortion. What is it about energy (matter being just another manifestation of energy) that distorts spacetime?
  7. Mar 29, 2010 #6
    Using a term to define itself, as you said, is circular reasoning. This is the very thing I've been struggling with in the forums over the past few weeks, although from a different angle. The conclusion I've come to is that gravity works because it is a FORCE. However, since gravity doesn't have to be viewed as a force in relativity theory, lots of people have decided to abandon the connection between them.

    I now believe that relativity theory is a symbolic language, in just the same way that math is. We use it as a tool to make predictions of objects in gravity. But it seems that people have taken that tool too seriously. We would be taking math too seriously to say that "math is geometry". Math is only symbolic of geometry. Likewise, I think we need to acknowledge that relativity theory is not gravity, it is symbolic of gravity. Gravity is best described as a force but relativity theory is the tool that we use to make predictions of that force. "Falling into the dent of spacetime fabric" is a symbolic representation that we should not take too literally.

    This is something that I think is very important and I take it very seriously. So, as long as gsingh2011 feels this is in line with his/her question and permisses me, then I ask anyone who disagrees with what I've said to speak up. You may regret it later if you don't because, unless I have reason not to, I'm moving forward with this.
  8. Mar 29, 2010 #7
    I understand Temporal's explanation about the line or least resistance (some of you may want to look up geodesics). Hoku, I believe what I said above is most likely the correct answer. Therefore, we don't just use relativity to make predictions about gravity, a separate force, the dent in spacetime actually causes the gravity. I hope I understood you correctly though...
  9. Mar 29, 2010 #8
    What is it exactly that causes you to discount the idea that gravity is simply the curvature of spacetime due to the presence of matter? I'm not speaking of what actually caused the curvature (matter's interaction with the fabric of spacetime), but rather the behaviour we can directly observe due to the curvature itself. Are you saying that spacetime is not curved by the existence of matter or are you saying that the curvature really plays no role in how matter moves through spacetime?
  10. Mar 29, 2010 #9
    If we take the image of "mass denting spacetime" literally, then we are making the statement that spacetime is a physical object that can be "dented". I am 90% certain the popular view is that spacetime is NOT a "thing", it is a "phenomena". A phenomena is useful and real, but that does not make it an "object". Does anyone disagree with this?
  11. Mar 29, 2010 #10
    The quantum gravity school have one idea and the general relativity school have another and I'm not sure how close they are to finding some common ground. I think the quantum gravity school get the most funding with their search for the Higgs boson.

    Have you Googled 'What causes gravity'? Other theories tend to fall into three or four camps -
    1) the expanding universe, where all matter including atoms are experiencing an accelerated expansion which provides our 1g.
    2) Wave theories, which I guess would include the graviton idea.
    3) Shadow effects, which can be similar to the previous idea but more particle based - Le Sage is one of the original proponents.
    4) Matter decay is another.

    take your pick ;-)
  12. Mar 29, 2010 #11


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    I disagree completely. A force is no more real or correct than a geometrical explanation of gravity is. After all, what is a force? I can't measure it directly. I have to use other notions in order to describe this mystical "action at a distance".

    Of course, perhaps you're referring to the fact that forces are traditionally represented by carrier particles, so we should view gravity as arising from gravitons. But this is still quite an unknown thing, and I don't think attributing gravity to tiny, mass-less particles is any more rewarding or true a description than the geometrical definition. In fact, I prefer the geometric one!

    It's similar to the discussion which arises in classical mechanics when only potentials, not forces, enter into the fundamental equations. In this sense, the potential really is what's fundamental, not the force. In my mind, this situation is completely analogous.

    Await your response!
  13. Mar 29, 2010 #12
    Isn't the theory of "Gravitational Lensing" based on curved spacetime? The more massive an object is, the greater the lensing effect? This would seem to directly corroborate "mass denting spacetime".
  14. Mar 29, 2010 #13
    The issue here is that "Why" is a philosophical or metaphysical question, not a question of Physics. Physics is all about "How", and "Why" is only asked of PHYSICAL PHENOMENA. "Why does the apple fall", not "Why is there gravity to make the apple fall (with continuing reducto...)". The former is a question for physics, the latter is metaphysics or philosphy.

    As for Relativity, most of it is useful in describing HOW things occur, such as the motion of planets and stars, and far more complex phenomena. It is recent that access to some modicum of this knowledge was made so publicly available; without it seems, a similar focus on what is science, and what is not.

    I'm not saying that "Why?" is not a good question to ask... just that if you're looking for something more than physical theories, you're barking up the wrong tree!
  15. Mar 29, 2010 #14
    I think what you're saying here is that they are essentially interchangable ideas. If so, I'm inclined to disagree. Gravity as a force is very different from gravity as geometry. This can be understood by looking at a definition of a force found here, http://www.uoregon.edu/~struct/cours..._lecture4.html: [Broken]

    "A "force" is an action that changes, or tends to change, the state of motion of the body upon which it acts."

    What this is saying, is that a "force" has energy that does work to an object. So, when gravity is a force, it acts upon objects. Contrairily, when we look at gravity as geometry, it's no longer something that has energy or does work. Instead, it simply becomes a "path" that an object with it's own energy follows. In one scenario, gravity is active. In the other it is passive. There is a very big difference between these.

    The geometrical explanation of gravity still leaves "mystical" loopholes. Here's a link (courtesy of another user from another thread) that describes, very simply, how gravity works. http://www.adamtoons.de/physics/gravitation.swf . If you click on the "help" button at the bottom of the page and then the "examples" header at the top of the next page, you find this quote:

    "When you push play, you see in the 3-D view, how the object is moving along its world line. The world line changes it's direction in regard to the dimensions because it is taking the straightest possible way."

    This is the geometric view of gravity. It states that objects return to Earth because "it is the straightest possible way". However, there are countless geodesics or "straightest ways" that an object can follow depending on where it is going. So the "mystical" loophole that we can't avoid is, where is the object trying to go?

    Think about this:
    "The world line changes it's direction...because it is taking the straightest possible way."
    So if we ask, "The straightest possible way to where?" the answer is Earth. So if we ask, "Why is it's energy directed at the Earth?"
    Then, the we are brought back to the initial statement, "because it's the straightest possible way."
    This is more circular reasoning!

    I think that gravitons are an attempt to explain the force of gravity (I'm not partial to gravitons one way or the other). Unfortunately, there is still something that even gravitons can't explain - basic, pure energy.
    Last edited by a moderator: May 4, 2017
  16. Mar 29, 2010 #15


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    How do you scientifically define "thing" and "phenomena"? What experiment can you perform to distinguish "things" from "phenomena"?
    PS I was hoping that you would come back to the question of why doesn't a planet fall straight into the sun along a straight line geodesic.
  17. Mar 29, 2010 #16


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    The laws of physics divide into dynamical laws and initial conditions. It is the initial conditions which determine if a test particle takes a spacetime geodesic that is a closed periodic orbit, or whether it falls radially. You can call the initial conditions "mystical" if you like. But it is not any different in Newtonian mechanics.
  18. Mar 30, 2010 #17


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    Echoing atty's post, what do you find mystifying? Geodesics are given by:
    [tex]\frac{d^2 x^\alpha}{d \tau^2} = -\Gamma ^\alpha _{\beta \delta} u^\beta u^\delta[/tex]
    So with initial conditions, namely an initial 4-position and an initial 4-velocity, the geodesic should be determined on a given Riemann manifold, no?
  19. Mar 30, 2010 #18
    I don't think this statement conflicts with anything I've said. Nabeshin was giving one reason why the "geometric" view of gravity is better than the "force" view of it. The reason he gave was that geometry can be explained and understood whereas a "force" necessitates some "mystical" unknown. I think this is a fine idea, however, even the geometric view - and the graviton view - can't avoid any "initial conditions" that may forever remain mysterious. In all three cases, the "mystical" unknown that Nabeshin is trying to avoid are exactly the same thing - energy that moves something. Consequently, I think this reason for choosing geometry over force is a little weak.

    You're right! "Things" and "phenomena" are both nouns. However, I think we can differentiate "tangible" nouns from "intangible" ones. An "idea" is a noun but I wouldn't expect to float a planet on it or move an apple with it because it's an "intangible phenomena".

    I'm not suggesting we try to answer the "unanswerable", only that we resolve whether gravity is an active force or a passive geometry. The "unanswerable" will follow us wherever we go, so it's just not a good reason to choose one over the other.
  20. Mar 30, 2010 #19
    This may be terribly naive of me, but couldn't the notion of spacetime curvature (which seems pretty clearly demonstrated) as gravity and the notion of gravity as a force (with a QFT to describe it) doesn't seem to be a contradiction. They could be two accurate ways of describing the same phenomena, but in different conditions (not entirely unlike Newtonian vs. Relativity's predictions for orbits).

    Gravity is ALREADY anamolous in all sorts of ways (range, strength, ALWAYS 'attracting' vs other forces), in ways that are as yet explicable only in conjecture or theory. Perhaps gravity is the equivalent of Electroweak at our energy levels, or perhaps it's a force field, but unlike others it distorts spacetime, while following its geometry.

    If this were something which could be answered with confidence, there would be no Large Hadron Collider.

    @Hoku: Mysterious is different from Mystical. We may never understand initial conditions, but they might be (in a global sense) very bland. Hell, maybe the Brane cosmologists are right, and we'e just the product of a collision between membranes. The point is, if you mean "mystery, that's one thing, but Mystical = magic, religion, supernatural. A mystery is accepted in science... a mystic should be shot on sight, then asked questions. :wink:
  21. Mar 30, 2010 #20
    The trouble is with this definition it doesn't necessarily follow if you change it around i.e

    " if the state of motion of a body changes a force is acting upon it."

    The reason I say this is that if this were true then you would actually feel an acceleration in free fall and you don't. So gravity can't really be considered as a force.
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