Violent Messiah

I've just thought an idea that could destroy the essence of General Relativity (Hehe, of course i'm dreaming)

Seriously...

General relativity postulates that there is no such thing as "Force of Gravity". Matter just bends the fabric of space-time. So far right?

A better understandable picture would be the "Trampoline Universe". Wherein a big bowling ball is placed in a certain location. If you place a tennis ball near it, it would roll towards the bowling ball. If the tennis ball is far enough - the effect of the bending of the space-time continuum (trampoline) would be insignificant and the tennis ball would seemingly remain at its position. Basically, the bowling ball exerts no force whatsoever to the tennis ball to make it come towards it. The surface is sagged in such a way that the tennis ball will roll towards it.

I propose to move this system into space - the tennis ball would definitely not roll towards the bowling ball.

In the first place why would it necessarily mean that the tennis ball would come towards the bowling ball just because the surfcace is bended.

Gravity would pull the tennis ball towards the bowling ball.

There is still a force that will make the t ball roll towards the b ball.

Kindly point outn the mistake i committed in my assumptions.

Jesse

The trampoline thing is just a metaphor, and it is a bit misleading for the reason you say. In the mental picture you actually shouldn't worry about whether masses cause depressions or humps, the curvature is all that matters, not the orientation of the bulges in some external space. General relativity just says that mass curves spacetime, and that objects in the absence of other forces will take an extremal path (usually the shortest possible path) between points in curved spacetime, a path known as a "geodesic." An analogy I like better is with airplane flight lines projected onto a map. The shortest path between two points on the globe is always going to be an arc of the "great circle" that includes those two points, ie a circle whose center is the center of the earth, like the equator or the lines of longitude. But if you project this path onto, say, a mercator map, the path will look like a curve rather than a straight line.

This analogy isn't perfect either...one of the key reasons it's hard to have good intuitions about curved lines being the shortest distance between points in curved spacetime is that we're talking about curved spacetime rather than just curved space. In flat space the distance between points is given by the 3-D version of the pythagorean theorem, the square root of (+x^2 + y^2 + z^2), which will be the same regardless of how you orient your cartesian coordinate system. But in flat spacetime "distance" is measured in terms of something called the "proper time" between those points, which is given by the square root of (+t^2 - x^2 - y^2 - z^2), which also has the feature of being independent of your coordinate system (in contrast to distance and time on their own, which depend on which observer is measuring them in relativity). In general relativity you're looking at the proper time along a path between two events (you have to do an integral over the curve of some kind), and trying to find the path with the extremal (usually the minimal) proper time.

You might want to look at these newsgroup posts which makes a similar point:

rubber-sheet analogy for general relativity considered harmfull

Re: General Relativity

Here's another analogy, courtesy of Misner, Thorne and Wheeler's Gravitation, which doesn't depend on the orientation of the curves in some imaginary external space:

The Parable of the Apple

fando

Just to add to the exposition, it is wise to not get too absorbed in the metaphors because the subjet they treat is usually limited in scope. Going beyond the scope leads to trouble and confusion. In this case, the question arises: "Wait a second, in the trampoline metaphor, you still need Gravity, the Force, to pull objects downwards!" The problem with that statement is that it's taking the metaphor as the essence of gravity under General Relativity rather than the intended purpose as a visualization tool and nothing more.

To further illuminate the connection between spacetime geometry and the gravitational force, think of it in purely mathematical terms. Here you have a force that moves matter around, and matter which creates the force. Now, the genius of General Relativity is to frame that relationship in a geometrical fashion--to imagine the force and force generaor behaving as shapes and surfaces instead of arithmetically as points, distances and the inverse square law. The geometrical treatment is somewhat more flexible than the analytical treatment under Newton, a quality which allows for an enhanced ability to treat complex "shapes" and situations in the gravitational field. To think that spacetime is actually a rubber sheet is taking things too far... the rubber sheet thing is just a metaphor for a mathematical modelling of the gravitational force. There are no shapes, except in our minds.

Baptist Vine

Maybe this is part of the answer.

I think the natural state of things is still motion, like Newton postulated in his first law. But the fabric upon which things move is spacetime, and in the case the phenomenon we call gravity, objects move in a geodesic path because of the warping of spacetime caused by matter.

Peter Soderqvist

Objects of mater in the universe exert gravitational pull on each other, by exchanging gravitons! Gravitons are the message to objects to reach each other, and its general outcome; is the curvature of space-time!

Jesse

As I understand it the relationship between theories of gravity based on the exchange of "gravitons" and the theory of general relativity is not so clear. From this FAQ on virtual particles in quantum field theory:

Jabu Khan

gravitons have never been detected the particles responsible for gravity are unknown. However gravitational fields are detectable this is one of the problems with what is called "The theory of everything" you can't merge the strong, weak, and electromagnetic forces( which have all been reconciled with classical physics already )with gravity if you know next to nothing about what causes gravity(physically).

Gravity is such a pain in the butt because with most of the other forces you are dealing with two elementary particles and a third particle that forms from radiation background due to their proximity. This formed particle carries the force and is detectable during particle collisions. Gravity does not have a known carrier and can not be reduced to the interactions of specific elementary particles (not yet anyways). Gravity may be an amplifying of the effects of these other carriers when many of them are near each other wich means a very complex situation to reduce to the level of quanta ( elementary particles).

This makes it hard to discuss general relativity like you would discuss electromotive force or magnetism. There are no clearly defined parts to discuss only the overall field created.

DBrant

Einstein's space-time continuum is four-dimensional. Since our brains have never encountered anything in four dimensions, we physically can't visualize anything of the sort. To make matters worse, it is precisely in the fourth dimension that our space-time is curved.

So the standard metaphor for "visualizing" this scenario is with a rubber sheet (2D) curved in space (3D), which represent the observable universe (3D) curved in space-time (4D). As was pointed out already, this metaphor itself is a bit misleading, since we can't make such a direct transition from 3D space to 4D space-time.

Ice

The general theory of relativity relies on the fact that nothing can exceed the speed of light. Scientists at Princeton have used an exceptionally small particle to break the speed of light.

Jabu Khan

General relativity relies on the fact that light can only travel at one speed in a vaccuum or any other medium not that the speed is the same through all mediums Einstien knew why a straw looks displaced when you set it in a glass of water.