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?

[Queso]

'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]

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]

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 :friday:

[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]

'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.

[Robert Angstrom]

Gravity causes space to grow shorter in the direction of the gravitational field and it also causes clocks to run slower so gravity is shorter space and slower time. An object, when free to move within this space-time gradient, will move in the direction where it occupies the lowest energy level. That is the point where it has the shortest space and slowest time.

[Harry Costas]

Hello All

 

 

Time itself does not change.

 

What changes is the relative time. If light is a method of communication and if this light is slowed down by gravity or electromagnetic radiation than the time for this light to reach us is slowed down. The actual time does not slow down.

[Eclogite]

if this light is slowed down by gravity or electromagnetic radiation than the time for this light to reach us is slowed down.

You have lost me here. Since light is electromagnetic radiation are you saying it is slowed by itself?

[arkain101]

The Ball on the trampoline is a topology example. You can invision the trampoline in any direction you want, and it does not matter. The point is, in the mathamtical sense a curve of change happens as you get near mass. The effects of this curve (which in 3d can be imagined as density or such) create effects we call gravity. It may not be true, but the thing is, in a math like sense it works.

[IDMclean]

I think the name is the miss leading part of this all. Gravity in GR and SR is Space-Time, Mass-energy is the thing that bends Space-Time. This is shown in the Einstein Tensor equation in Quantum Mechanics. Time is Space and Space is Time.

 

When you reduce the distance you have to travel you reduce the TIME it takes to get there. You are then thereby traveling faster or slower in time relative to other people trying to get there along longer paths.

 

Remember in GR and SR when you are traveling at relativistic speeds you do not experience any kind of speed up or slow down. Your watch stays the same ticking at the same speed as always, to you that is. If you are watching other things that are not relative to you then they will be zipping on by or moving so slowly that it they may not appear to be moving at all.

 

This is the wonder of flexible Space-Time is that Distance between two masses is not absolute, therefore the TIME it takes to get between these two points is variable, hence GR and SR...

 

Does that make any sense?

[Tormod]

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

 

If there is no source of gravity then the ball will lie perfectly still in relation to it's surroundings. This is basic physics and is explained by Newton's three laws of motion. An outside force is needed to give the ball motion, and also to change the ball's direction of motion, accelerate and decelerate it.

 

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.

 

This is a misconception. Like someone else mentioned, an orbit is basically just the shortest path for the object to take. The object is traveling in a straight path but the object it orbits is curving that path and making it a closed loop (usually an ellipse). As you point out, the planets do not lie on a single plane. Similarly, artificial satellites orbit the Earth in various degrees of inclination - polar satellites orbit over the poles, while equatorial satellites orbit the Earth along the equator - and you can place a satellite in any orbit in between!

 

So there is no "plane" when we talk about gravity - the rubber sheet thingy is simply a model where one dimension is taken away for simplicity (because we cannot portray 4 dimensional figures).

 

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?

 

I lost you.

[Harry Costas]

Hello All

 

Light does not slow itself. It has a constant velocity.

Even when the speed of light is reduced to lets say zero. The actual velocity is still at the speed of light 300,000 kms per sec.

 

The vector force to hold light back at zero occurs at the event horizon of a ultra dense plasma matter called a Black Hole.

 

Lets say the speed of light is at 50 % when it is out of the hold of the Black Hole it will pick up speed and reach 300,000 kms per sec.

 

At all times it has a constant velocity of 300,000 although its speed can be reduced.

Its all about vector forces.