Curved Space and Gravity?

Question

If you place a bowling ball and a marble in a bed, the bowling ball will curve the bed, and the marble will fall towards the bowlng ball. However, that only works because of Earth's gravity. If we do the exact same experiment in space, the bowling ball will not curve the bed. And even, if an external force was to push the ball, and curve the bed, the marble will not fall towards the bowling ball, instead, it will just remain floatin on the same spot it was.

So how exactly does space curvature relate to gravity? Is there a force in space that acts like Earth's gravity in the case of the marble and the bowling ball?

Should we stick with gravitons?

Right, I understand all that. However, when the objects curve space, why do they fall towards each other if there is no other force? A curvature without force wouldn't move stuff arround. if 'G' is indeed that force, what causes 'G'?

Not yet. The marble falls towards the bowling ball because the indentation of the bed gives the marble x and y components due to the force of Earth's gravity. But in space there is no such external force(unless G, but G is not a force), therefore any curving in space shouldn't cause objects to move at all. Objects in space are moving because the force of the Big Bang pushed them away, not because the space is curved. The only thing that the curvature is doing is goving the objects a direction to follow.

So say 2 planets appeared right next to each other out of nowhere in space. No matter how much they curve space, they will not accelerate towards each other unless there is a force. At most they would move towards each other at constant speed due to the inpulse given by the bending in space they just created.
hmmm, in fact, this last idea doesn't sound so bad. If the planets were to recive an inpulse for every inch they moved due to bending, they might actually start accelerating.

Answer 1

Your additional details is the hard part to answer.

The curvature is the force gravity. The amount of curvature is proportional to the force so where the space is very curved we feel much more gravity.
The gravity comes from the stress energy caused by the curved space around the planet, star, bowling ball.

Stress energy. I remember it is a tensor field but I really cant remember and I am not sure whether I ever really understood it.

This is a really hard question I thought I new the answer but now I am not sure. I'm going to think about it and try and come up with a way of explaining it.

Failing that I think we need a question on stress energy

addition:
http://en.wikipedia.org/wiki/Stress-energy_tensor

Your last idea is Good but it relies on there being an initial movement if there was no movement the 2 objects will feel no force. also where would the impulse come from?

the first answer is very good but doesn't explain why a stationary object feels the pull of another. Following the curvature of the space-time implies movement. we feel gravity all the time even at rest.

Answer 2

Yes there is a force in space that acts like Earth's gravity. It is called ... "Gravity". All objects exhibit gravity (in Newtonian physics) give by the formula F= G (m1m2/r^2) where the force is equal to the product of the two masses divided by the square of the distance between them, times the gravitational constant. However, in your experiment, the bed has more mass than either of the other two objects, and closer proximity. This would negate the gravitational effects of the two objects on each other. Take the bed away and create completely empty space, and the marble will eventually move to the bowling ball (provided no outside forces are exerted on either).

Answer 3

The curvature of spacetime in GR is NOT the curvature of a 3 space in 4 dimensional spacetime. It is the whole 4 dimensional spacetime that is curved.

Its a bit less misleading to think of it this way. You live on an Earth that looks flat locally to you. So you naturally draw a map that has staight lines for North-South and East-West. But actually the surface of the Earth is curved -so these straight lines will look less and less like reality the further you go. Your only option will be to curve them.

This is exactly what happens in GR. You try to fill the whole universe with straight lines of x, y, z and t. But it does not work - where there is gravity, for instance, the t stops matching reality (t passess more slowly where there is gravity compared to where there is not). So your straight lines are going to have to curve.

Answer 4

In space, the bowling ball would curve space, instead of the bed. And the marble would fall toward the bowling ball, following the curvature of space around the bowling ball.

The curving of the bed due the weight of the bowling ball is merely an illustration of how Einstein's General Relativity works. It's not meant to be literal, since the surface of the bed is only 2-dimensional, but the curving of space is curvature of a 3-dimensional surface in 4-dimensions.

Einstein showed us through General Relativity that gravity can be viewed NOT as a force, but as the distortion of the spacetime fabric due to mass and energy. The path of any object moving through space follows the curvature of spacetime created by any nearby massive objects. So you might say that the shortest path between two points in space is not a straight line, but the curve or distortion (called geodesic) created by matter around that space.

Gravitons are the force-exchange particles of the gravitational force in the context of the Standard Model of particle physics. The reason we believe it exists is because Einstein's General Relativity predicts the existence of gravitational waves, and we all know that if there is a wave, there is a corresponding particle (this is known as the particle-wave duality).

Another reason to believe that gravitons exist is that physicists today believe that even space and time is quantized (meaning that space and time come in discrete packets) just like matter comes into discrete pieces (electrons, protons, and neutrons), and just like energy comes in discrete packets (i.e. photons). As it turns out, if you quantize spacetime, the elemental discrete packet of spacetime is, you guess it, gravitons!

PS - objects moving through space are merely following the curvatures in spacetime created by other objects around them. There are NO other force acting on them. If you go back to the bowling ball on the bed example, you can visulaize what is going on more easily. The marble falls toward the bowling ball, because the bowling ball has created a large indentation on the bed, so if the marble trabels too close, it will become trapped by the bowling ball by falling into the indentation. The behavior of the marble is as if there is a force acting on it from a distance, puling it toward the bowling ball.

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PS - to the critics of my answer:

(1) Yes, it's both space and time being curved (so it's a 4-d curved surface), but our good host does NOT even understand the 2-d example with the bowling ball and the bed, so I saw no point in adding time to further complicate the explanation. I was just trying to draw a parallel between our 3-d space and the 2-d space of the bed.

(2) "the first answer is very good but doesn't explain why a stationary object feels the pull of another. Following the curvature of the space-time implies movement. we feel gravity all the time even at rest."

First, there are NO stationary objects in this Universe. All objects are in relative motion with respect to each other. If an object appears to be stationary, it's because you are moving together with the object. So for example, we are standing on the surface of the Earth, and the Earth is rotating about it's axis and rotating around the Sun and the Sun is rotating around the Milky Way. But we don't see any of this motion, and that's because we are ALL moving together.

Second, ALL objects in the Universe, including those that appears to be "stationary" relative to you, are subject to the gravitational force of other objects. And the way an object "feel" the pull of other objects is through the curvature of spacetime. All objects in motion must follow paths dictated by the curvature in spacetime.

So are we following the curvature of spacetime (or in motion) while we are at "rest" on the surface of the Earth? The answer is that we are NOT moving relative to Earth or the ground, but yes, we are indeed moving with the Earth and following the curvature of spacetime. And the reason we are stationary relative to Earth is because we can not fall through solid matter of the ground. There is electrostatic force keeping atoms apart. This is why solid objects feel solid. (Of course, this is actually much more complicated, but I think I leave the detailed explanation for another question in the future.)

Answer 5

they are actually not too particular what gravity is. they're hoping the LHC will demonstrate the Higgs Boson and that they're going to comprehend greater helpful. What they do comprehend is that the curvature of spacetime attracts gadgets at the same time in a predictable way. The top-rated concept is that gravitons distort the textile so gravity might distort spacetime (which might fairly be delicate). in the event that they're like photons, they might have twin particle/wave properties, or another unusual property (that's why we've not been waiting to stumble on them yet). additionally, check out LIGO - Laser Interferometer Gravitational Wave Observatory, for a manner they're attempting to work out the ripples brought about via huge explosions.

Answer 6

The example you refer to is just that, an example used to assist visualizing and understanding how space is bent by masses. The exact way that matter shapes and bends space is determined using Einstein's field equations, which are quite complex mathematical entities.

Answer 7

We should be researching things such as an anti-gravity device for the I.S.S, and one day, enter into a "star trek" era. I know there is some positive research being done on a cloaking device, and years ago, they were working on making forcefields, but there was a problem in containing anti-matter, and combining it with matter.

Answer 8

The mattress is a two dimensional model of a four dimensional effect. Space itself if warped by mass causing objects to be attracted to each other.

Answer 9

There is a theory going that dark matter exsists out there in space,and it consitutes 90 percent of the total matter in the universe,that's what's keeping it on even keel

Answer 10

gravitons are spin - 2 particles and you need to rank 2 tesor field to describe it, which means there is a coupling! (Gravity attracts itsef!)