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According to Newton, gravity is the pulling - or in fact the attracting - force of any heavenly body towards any object to its center. But, to the contrary, Einstein once said that the four dimensions of space and time push the object downwards.

So can which one of them is correct?

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  • $\begingroup$ It's a mistake to think that Newton identifed gravity as a pull: see quotations from Newton in my answer below. $\endgroup$ – terry-s Apr 6 '18 at 13:45
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    $\begingroup$ In physics there is no such thing as a distinction between pulling or pushing forces. Forces are acting on something, this action can be expressed in terms of equations, that's it. $\endgroup$ – AtmosphericPrisonEscape Apr 9 '18 at 13:49
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In general relativity, gravity neither pushes nor pulls. To explain why ball travels in an arc you note the start and end points of the throw in 4d space time (3 space co-ordinates and 1 time coordinate) You then find the shortest path between these two 4d points in the curved spacetime surrounding the Earth. This shortest path is the path in spacetime that the ball travels.

So in General relativity, gravity is not seen as being a force, instead it is the result objects travelling in the most direct way in a region of curved spacetime.

However for nearly all practical purposes, the effect of gravity in relativity is amost identical to that produced by an attractive force, as supposed by Newton.

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    $\begingroup$ In Lorentzian spacetime, geodesics are locally longest paths, rather than shortest as in Riemannian space. This reversal is due the different signature of spacetime metric compared to a purely spatial one. $\endgroup$ – Stan Liou Nov 5 '15 at 20:59
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I'm guessing that this misunderstanding is a result of the oft-used rubber sheet analogy. The rubber sheet analogy says that, according to general relativity, mass curves space-time like a heavy bowling ball on a near-taut blanket (or rubber sheet) curves the blanket/sheet. This resulting curve makes other bits of matter/energy move in different ways. I'm guessing that this is your confusion.

The rubber sheet analogy fails massively in one area, any demonstration of it involves gravity on Earth. If I use a bowling ball to deform a sheet, and then role a golf ball along the sheet nearby, the golf ball will move a bit towards the bowling ball because of the force of gravity around me - not "gravity" in the simulation. It thus makes it seem like gravity pulls the golf ball "down" because the bowling ball pulls the rubber sheet "down". This is the result of using a two-dimensional analogy of a three-dimensional universe.

The point is, there is no "pushing" going on in the general relativistic model of gravity. Gravity is attractive (so long as the strong energy conditions holds for the object in question), just like Newton postulated.

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    $\begingroup$ Nitpick: whether or not gravity is attractive or repulsive in GTR depends on whether strong energy condition holds for the matter that generates it. The accelerating cosmic expansion is an example of gravitational repulsion, as dark energy does not satisfy that condition. $\endgroup$ – Stan Liou Nov 5 '15 at 21:01
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Earlier answers pointed out that this question is based on a mistake about general relativity, in which gravity neither pushes nor pulls: but it is also based on a mistake about Newton's view and about what he wrote. Newton was emphatic in refusing to commit himself to identify gravitational attraction either as a push or as a pull. He said he was concerned only with the mathematical quantities and relations of the forces, etc. He wrote that he used the words 'attraction' and 'impulse' indifferently, 'one for another' in respect of the general propensity of massive bodies to approach each other, and he expressly abstained from speculation about their physical nature or cause (see quotations below).

Accordingly, just as the other answers point out that GRT does not specify gravitation as either a push or a pull, neither did Newton's physics. Whether in GRT or in Newtonian physics, the answer 'push' or 'pull' is unnecessary to an account of the physics. The question in effect poses a false antithesis.

Here is the basis so far as what Newton wrote. In the opening parts of the 'Principia', in Definition VIII, Newton wrote (quoted here from the 1729 English translation of his original Latin, available online at https://books.google.com/books?id=Tm0FAAAAQAAJ):

"I ... use the words Attraction, Impulse or Propensity of any sort towards a centre, promiscuously, and indifferently, one for another; considering those forces not Physically but Mathematically : Wherefore, the reader is not to imagine, that by those words, I any where take upon me to define the kind, or the manner of any Action, the causes or the physical reason thereof, or that I attribute Forces, in a true and Physical sense, to certain centres (which are only Mathematical points) ..."

In the same vein, his Definition V stated that "a centripetal force is that by which bodies are drawn or impelled, or any way tend, towards a point as to a centre".

Later on he emphasised yet again the same abstention from speculation (Principia, Book 1, Section XI, Scholium following Proposition 69):

"I here use the word attraction in general for any endeavour, of what kind soever, made by bodies to approach to each other; whether that endeavour arise from the action of the bodies themselves as tending mutually to, or agitating each other by spirits emitted; or whether it arises from the action of the aether or of the air, or of any medium whatsoever, whether corporeal or incorporeal, any how impelling bodies placed therein towards each other. In the same general sense I use the word impulse, not defining in this treatise the species or physical qualities of forces, but investigating the quantities and mathematical proportions of them; as I observed before in the definitions."

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Newton sees gravity as an attractive force. Einstein sees gravity has nothing to do with force, but warps spacetime.

Stronger gravity dilates time more. From the observer frame the object takes longer time traveling under stronger gravitational potential, than under weaker one. Therefore, in the gravity as a force idea, given that the object still travels from AB regardless of its trajectory, it seems like the object is decelerated, aka pushed, under stronger gravity.

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How about thinking of it as pressure. If you imagine space as a big tank of water in zero gravity. Normally bubbles float up but in a zero gravity environment the idea of floating up gose out the window. The bubble would instead flow to the lower density areas rather then higher density areas. So if you can think of matter displacing space and creating a low density area, then matter will 'float' up towards it. Dose that make sense?

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    $\begingroup$ But bubbles don't do that. $\endgroup$ – Hohmannfan Oct 3 at 7:06
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    $\begingroup$ Your water tank analogy really doesn't illustrate what general relativity says about gravity. $\endgroup$ – PM 2Ring Oct 3 at 7:45

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