Don't quite understand how gravity affects space/time

[Guido]

I've seen a million times the example of a ball on the trampoline and how it deflects the fabric of space. I understand this perfectly, but only because it is happening in an environment with gravity: ie. the ball "falls" into the fabric, stretching it. But what happens when there is no source of gravity (besides the ball)? The ball "falling" doesn't make sense. Also, there must be more than one deflection or dimple in the fabric of space/time because orbiting objects do not need to follow the same plane - so basically there are an infinite amount of deflections at all angles. When I try to imagine this I picture a sphere surrounding the ball which doesn't make sense either.

Here is a related question: If the fabric of space/time deflects like a ball on a trampoline, then if you were to shine a light directly towards the center of the ball, won't the light curve towards the edge of the ball as it enters the deflection?

[orbsycli]

'Cause Time's a dimension,

The super intense gravity of a black hole will actually cause time to HAULT on it's surface!

Just like light,
except not.

[InfiniteNow]

Don't feel bad, Guido. Gravity is not yet very well understood. My first thought upon reading the subject of your thread was, "Yeah, you and me both." However, there are a few here on Hypography who might be able to offer us some insights into your questions, and I too look forward to what they have to share.


Cheers.

[infamous]

Originally Posted by Guido

the ball "falls" into the fabric, stretching it. But what happens when there is no source of gravity (besides the ball)? The ball "falling" doesn't make sense.

Actually the ball is not falling Guido; The mass energy of the ball is warping the fabric of space. Any object traveling thru space along or parallel to the lines will find instead of moving in a straight course, they will follow the bent lines toward the mass causing the warpage. Physics states that all objects will move in a straight line unless acted upon by an external force. For the objects following the curved lines of gravitational warpage, they appear to be the shortest line between two points. Ofcourse, when we think about the shortest line between two points we automatically think about a straight line. Where gravity is concerned, this is never really the case. BTW, Welcome to Hypography Guido, enjoy the discussions...........................Infy

[Jay-qu]

what your thinking with your question is the result of using a poor analogy - it wont work the way your thinking because in the analogy there is a two dimensional surface representing a three dimensional area..

[Qfwfq]

The example of a ball on the trampoline is indeed a vague analogy and doesn't really describe the matter properly, let alone prove anything.

In order to understand GR properly you need to first master a bit of differential geometry. It's a fairly advanced topic. No wonder people are a bit perplexed by it.

[Guido]

Originally Posted by Qfwfq

The example of a ball on the trampoline is indeed a vague analogy and doesn't really describe the matter properly, let alone prove anything.

Does anyone know of a better example to describe it more accurately. I understand it won't be precise, but at least more precise than the trampoline which I think just confuses things.

I picture how black holes are depicted in books - a cone with matter falling down into it. But since there is no up or down in space, then matter can fall into it from any angle. So then I imagine a point (the center of the black hole) with a sphere around it (the event horizon). I think this more accurately depics the gravitational field of a star for example. And with smaller radius spheres, the gravitational force increases. And where spheres interact between masses you get warped lines taking into effect the strength of the spheres.
Every object is surrounded by these spheres and all object are affected by these spheres.

This is the way I picture it. Which to me makes more sense than comparing gravity to a fabric (or trampoline) that doesn't physically exist.

[Jay-qu]

a more correct analogy, you have to think of a 3-dimensional grid, and then warp all the points inwards towards the center of an object, as i gets closer the grid gets warped even more.. sorry its not easy to explain, try googling for pics

[Qfwfq]

Actually, a 4 dimensional grid, unless you only want to get the picture for a plane comprising the star/BH. It's actually a curvature of space-time. A plane is of course certainly good enough for understanding the orbit of a single object, subject only to the gravitational field, so you could work that as a 2 + 1 = 3 dimensional manifold.

However, it isn't easy to give a translation of the differential geometry into an intuitive illustration that holds up fully. In precise terms, the non-zero curvature has do do with the connection not being an exact differential form, although it is defined over a simply connected domain. Curiously enough, this goes for any field that can be described by a gauge group, and hence for the other three forces, the odd thing in the case of gravity is that it involves the metric of space-time itself.

[ldsoftwaresteve]

Originally Posted by orbsycli

'Cause Time's a dimension,

The super intense gravity of a black hole will actually cause time to HAULT on it's surface!

Just like light,
except not.

uh, I don't know about that orb. There was an article posted by c1ay recently, I think, that talked about the discovery of stars that were within the event horizon of a supermassive black hole. This, to me at least, was an indication that something about our understanding of this is flawed. Also, it is well known that matter is ejected from black holes so .... with that in mind, I'm of the opinion we don't know the actual nature of a black hole well enough to make any kind of inference with any kind of certainty.