It is said that there is no such thing as gravity. Mass/energy curves spacetime and a body follows this curvature so gravity is basically the geometry of spacetime. If there is no force of gravity why does a mass move at all along the curvature of spacetime and what determines which way it will move. E.G. If I drop an apple, if there is no gravitation attraction to the earth why does it move towards the earth, why doesn't it move upwards instead?
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3Related: http://physics.stackexchange.com/q/219306/50583 and its linked questions. There's a difference between saying "gravity isn't a force" and "there is no gravitational attraction"; I'm not sure why you think the two are equivalent. – ACuriousMind Feb 05 '17 at 00:22
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2It is said that there is no such thing as gravity. Mass/energy curves spacetime and a body follows this curvature so gravity is basically the geometry of spacetime. Can you see the contradiction in this statement? Would there be a problem if, instead of saying "matter follows the geometry of spacetime", we just used the word "gravity" to mean the same thing? – Feb 05 '17 at 00:23
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As you move along the apple's worldline in one direction, you get closer to the earth; as you move along the apple's worldline in the opposite direction, you get farther from the earth. The reason the apple moves toward the earth instead of away from it is that you chose to trace out the apple's worldline in one direction rather than the other, and you then chose to label that direction "forward in time". – WillO Feb 05 '17 at 01:27
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Gravity is a force. It is exerted by curvature of space, which in turn, is caused by mass of earth. So, it is roundabout way of saying that earth exerts a force on the apple. This is per GR which so far, is the most accurate way for quantitative description of the force/phenomena.
It causes acceleration, it requires another force to counter it, it imparts momentum, and kinetic energy, it follows inverse square law, similar to electromagnetic force .. It has all the characteristics of a force.
Curvature of space describes how the action/force at a distance is realized. It does not say that the action/force does not exist.
kpv
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1Nowadays we don't think of gravity as a force, but as the curvature of spacetime. And yes that causes acceleration. When the curvature is very weak you can approximate it as a force. It only goes as 1/ r squared in that limit. General Relativity does not generally go that way. It is not similar to the electromagnetic force, there are plenty differences. By the way the nuclear force also causes acceleration, requires another force to counter it, imparts momentum and kinetic energy, and is also not similar to the electromagnetic force. – Bob Bee Feb 05 '17 at 05:16
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@BobBee: Yes there are differences. Main one being that gravity is only attractive, electromagnetic is both, attractive and repulsive. Yes, inverse square is an approximation. Still looking at the effects of the curvature of space on mass, I do not think there is anything wrong in considering it a force which is accurately (in quantitative sense) described as curvature of space, especially if someone having difficulty in grasping the effects in absence of a force. Moreover, It can be considered a force as a consequence of the equivalence principle. $a$ = $g$. Same units, similar effects ... – kpv Feb 05 '17 at 19:51
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'Nothing wrong'. 'It can be considered a force as a consequence of the equivalence principle'. You're misstating things, and deciding what is wrong or not, on things that have already been decided. Gravity is not a force because of the equivalence principle. That led Einstein to understand that it was an effect of spacetime, not a force. Further, you're confusing the OP, who is trying to understand how it can work with spacetime; your answer is think of it like the electromagnetic force with a few differences. Things would have been a lot easier for physics if what you said had 'nothing wrong' – Bob Bee Feb 06 '17 at 01:25
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@kpv Mass curves spacetime. For an object with mass to move though time in a straight line, it must follow the curvature of spacetime. Try making two ribbons lie flat along the surface of a sphere, they will come together even if they started apart. This is because the straight line on a curve causes things to come together. So when zero forces are acting on an object with mass, movement through time over the curvature of space causes objects to come together. – Velocibadgery Mar 12 '18 at 02:36
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If you drop an apple it does not accelerate. Weird, huh, but fundamental. To elaborate, if I was in the apple's frame of reference I would feel no acceleration. But normally there I am standing on the ground by the apple, and I do feel acceleration. But I'm not speeding up. In fact I experience a force resisting the natural motion that free objects will take in a gravity well.
Anyway in the way I described above you can intuitively feel the truth - that gravity is different from other forces.
JMLCarter
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All the answers are perfect. I am telling the same in a simpler version. The idea is, consider mass as a deformation, or for convenience, conceptually, a drawing in space-time. Seeing it from a particle physics level, the drawings consist of certain jigzo puzzle like pieces(again conceptually). They cannot be altered or deformed but, together a few such pieces make the drawing. The fundamental particles are these jigzos. When fundamental particles get into a structure, with stability, the nucleus, the atom and everything, they form the matter that we see. Now, to explain gravity, it will be too complex to think in this particle sense. But, since the deformation doesn't really have a definite frame of reference, we can treat gravity like we do to isometric factors. We can see the earth as a big piece of mass, that is a big drawing in space time. But, these drawings or deformations have a tendency to get back to nothing. This cause a force to decrease the deformation. Two such deformations, as they get closer, the sum of their deformations will get lesser. In order to attain such a partial stability by reducing deformations, masses tend to come closer. Call it gravity, if interested. Thank you.
