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What causes gravity? Why is there attraction between masses? Is it due to time or space distortion?

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I suggest that you start with the Wikipedia article (en.wikipedia.org/wiki/Gravitation) and then come back with more specific questions. –  gigacyan Feb 8 '11 at 7:46
    
I agree with @gigacyan. As it stands, it's a very confused question, which is clear to from the last sentence "Is it due to time or space distortion?". This site isn't meant to teach you something, but we can onl(y answer your questions . . . ) –  Dimensio1n0 Aug 28 '13 at 14:59
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I think he asked what is the mechanism behind gravity... –  Arafat Aug 28 '13 at 15:11
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3 Answers

I would say, it is rather vague to ask this "why" questions in physics when you are talking about fundamental things. We may say that gravity is a consequence of a deep symmetry of nature, called general covariance, which says that every reference frame should be equally suitable for the description of the laws of physics and it follows that the geometry of the spacetime is affected by the distribution of energy which is described by the Einstein's equations in simplest way. Particles follow straightest line called geodesic in this curved geometry. However this is actually "how" gravity works. What causes gravity may still be a vague question since gravity, as per current wisdom, is a fundamental interaction in nature. You just can not "explain" it from other simpler facts. Some recent attempts try to project it as less fundamental (like gravity is an entropic force). But that is very much controversial.

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Um, no, gravity is not a consequence of general covariance. Even if you have your space-time described by a manifold (even pseudo-Riemannian) that still doesn't tell you anything about interaction of the space-time and matter. To arrive at Einstein's equations you need lots of additional assumptions. There exist other theories of gravitation (even consistent with Einstein's in certain limit). en.wikipedia.org/wiki/Alternatives_to_general_relativity –  Marek Feb 8 '11 at 8:25
    
Dear @Marek, your complaint in this case seems to be missing logic. You don't need Einstein's equations to get gravity. SB1 only claims that gravity is a consequence of general covariance and it is true. General covariance requires a gauge field to exist, and this gauge field inevitably contributes terms to the action such as the Einstein-Hilbert action - which inevitably cause gravity between matter (whose terms in the action existed before we considered general covariance). You don't disprove this statement by sb1 by inserting new unnecessary intermediate links to the implication. –  Luboš Motl Feb 8 '11 at 8:40
    
Moreover, Marek, the page you linked to is very bizarre. All the entries are theories claiming to respect general covariance, and except for LQG, they actually do. So even if we could assume that these alternatives were right, it wouldn't disprove the implication "general covariance implies gravity". Moreover, none of them except for the first ones - Brans-Dicke - may be understood as "alternatives to GR". They're extensions of GR that also solve other things, but the gravitational portion of them is exactly GR. E.g. Kaluza-Klein theory is "more GR" than GR plus Maxwell's system. –  Luboš Motl Feb 8 '11 at 8:42
    
@Luboš: you've completely missed the point. Note that sb1 mentions Einstein's equations in his answer as well ;) Unless I am misreading, he's saying that Einstein's equations are a consequence of general covariance which is simply not true and it's this thing that I reacted to. –  Marek Feb 8 '11 at 9:18
    
@MareK: I thought by writing "described by Einstein's equations in simplest way" I will get away with this criticism. I guess my English is not good enough. I am very much aware that general covariance does not lead by itself to Einstein's equations. There are many other alternatives. But Einstein's equations are simplest possible. That's all I wanted to say. –  user1355 Feb 8 '11 at 9:37
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Hypothetically, Gravitons could also be attributable to why gravity exists. An explanation of gravitons would be too extensive for this answer box. You can read more on the Gravitons wiki page or google for more reference on the topic.

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Gravity is something mankind has some experience of, eg self-harvesting apples. Gravitons may be likely, but not prooven. So, anwering the (layman's) "why" by gravitons does not help. Maybe the questioner is impressed by another buzzword, but he does not know more than before. –  Georg Feb 8 '11 at 9:56
    
Completely agree. It was just another explanation. Also forgot to mention they were theoretical. –  thunderror Feb 8 '11 at 14:44
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"Gravity" does not exist. What exists is a behavior called "gravitational attraction" in which particles attract other particles in proportion to the product of their masses and in inverse proportion to the square of the distance between particles. To my knowledge, we have no current significant understanding of WHY this behavior exists.

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protected by Qmechanic Aug 28 '13 at 15:32

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