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madsquid

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In summary, the conversation discusses a flawed model of gravity represented by a rubber sheet analogy. The model is criticized for not accurately representing the 4-dimensional nature of gravity and for oversimplifying the concept. The conversation also touches on the concept of space-time being bent and distorted by mass, and the idea that gravity acts in all directions rather than just in a dip on a 2-dimensional surface.

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madsquid

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xAxis

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I don't think you can say that objects are pushing outward on space-time. Where did you get that?

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Nugatory

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madsquid said:you are probobaly aware of a paradigm that allows us to visualise exactly how gravity works. the precise nature of this concept varies but it goes along the lines of this. if four people held up a sheet, and a fith then places a basketball (representing a planet or star) in the center of the sheet, it will create a dip in the sheet. if the fith person places a marble (representing say, an asteroid) on the sheet, it will roll toward the ball

That "paradigm" is a deeply misleading model that has nothing to do with any past or current theory of gravitation. It's one of those things that just isn't true, but that the non-technical press just won't give up.

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A.T.

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Then you have encountered a problem your logic, not with gravity.madsquid said:i have encountered a problem with gravity, acording to logic gravity should not be able to work...

What you describe is the flawed marble-on-rubber-sheet-model of General Relativity. There are many threads here explaining what is wrong with it.madsquid said:you are probobaly aware of a paradigm that allows us to visualise exactly how gravity works.

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sophiecentaur

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Damo ET

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Your argument would be like me saying that I have discovered a flaw with maps, the fact that they don't represent what is actually on the ground when you are standing on the top of a mountain (or any other place). A map is a 2d representation of a 3d surface.

Everything with mass in the universe sits inside the space/time of the universe, and that space/time IS bent and distorted by the mass. I don't understand your take on mass pushing against space/time?

Also, you can't use a bed sheet for your explanation, as gravity doesn't increase or decrease at a linear amount from any mass, the square of the distance is used, which means even the 2d gravity analogy needs a 'rubber sheet' to mimic this.Damo

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A.T.

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Gravity in GR is 4d, not 3d. And the 4th dimension (time) is crucial to the model. But the sheet analogy just shows 2 space dimensions, which doesn't explain anything. You can provide much better analogies by using 1 space and the time dimension:Damo ET said:It is a 2d representation (loosely) of 3d gravity.

http://www.relativitet.se/spacetime1.html

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Damo ET

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A.T. said:Gravity in GR is 4d, not 3d. And the 4th dimension (time) is crucial to the model. But the sheet analogy just shows 2 space dimensions, which doesn't explain anything. You can provide much better analogies by using 1 space and the time dimension:

http://www.relativitet.se/spacetime1.html

I understand that, but I wasn't talking of gravity in GR, I was talking of gravity in the sheet model that the OP suggested, which is still a 2d model of how gravity effects 3d space.

Damo

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madsquid

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Nugatory

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madsquid said:

Yes, but so what? The sheet model and the entire concept of a "dip" has very little to do with gravity.

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sophiecentaur

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Nugatory said:Yes, but so what? The sheet model and the entire concept of a "dip" has very little to do with gravity.

Exactly. The force that pulls marbles down into dips in a rubber sheet is only symbolic / analogous. The dip in the sheet is a graph of potential and not a distortion of space time which is what GR is about. Making the model in 3D, with some contrived central force law would not make it more real; in fact, it would be more confusing because of the apparent parallel when there is none.

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Low-Q

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That is correct. However, in addition to be a 2D model it also has another "flaw". If you push a small "planet" towards the "sun" in the middle of the rubber sheet, the deflection in the sheet which represent the gravitational force from the sun let's the small "planet" also deflect as it approach the "sun", and finally hit the "sun" at the botton rather than at the "equator". In a 4d universe the "planet" would more likely move towards the "sun" in a straight line.madsquid said:

An important thing to notice with the rubber sheet, is that the deflection is increasing closer to the "sun", representing the increased gravity that does not increase linear, but exponential to the distance. A useful experiment to understant the basics, even if it will for most people confuse more than educate if not explained "with a tweezer".

Vidar

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madsquid said:

Please

You only saw the shadow of the animal. You haven't seen the animal itself.

Zz.

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jbriggs444

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Low-Q said:That is correct. However, in addition to be a 2D model it also has another "flaw". If you push a small "planet" towards the "sun" in the middle of the rubber sheet, the deflection in the sheet which represent the gravitational force from the sun let's the small "planet" also deflect as it approach the "sun", and finally hit the "sun" at the botton rather than at the "equator". In a 4d universe the "planet" would more likely move towards the "sun" in a straight line.

Ummm. A 2d model will not show a planet hitting the "bottom" of the sun. It's a 2d model. It does not model "top" or "bottom". Or equator.

An important thing to notice with the rubber sheet, is that the deflection is increasing closer to the "sun", representing the increased gravity that does not increase linear, but exponential to the distance.

Actual gravity is inverse square, not linear. I believe that gravity as modeled with a rubber sheet is, to a first approximation, inverse square.

The Three Dimensional Gravity Paradox is a theoretical problem in physics that arises from the mismatch between the number of dimensions in our observable universe (3) and the predicted number of dimensions in the theories that govern gravity (10 or 11 in string theory).

The paradox highlights the limitations of our current understanding of gravity and the need for a more comprehensive theory that can reconcile the mismatch between the observed and predicted dimensions. It also raises questions about the true nature of gravity and its relationship with the other fundamental forces.

There are several proposed solutions to the paradox, including the theory of brane cosmology, which suggests that our observable universe is a three-dimensional "brane" embedded in a higher-dimensional space. Another proposed solution is the holographic principle, which suggests that the information in a higher-dimensional space can be projected onto a lower-dimensional space.

If a solution to the paradox is found, it could lead to a deeper understanding of gravity and the unification of the four fundamental forces of nature. It could also have practical applications, such as advancements in space travel and the development of new technologies.

Currently, there is no direct experimental evidence for the existence of extra dimensions. However, some theories that propose solutions to the Three Dimensional Gravity Paradox, such as string theory, make testable predictions that could potentially provide evidence for the existence of extra dimensions.

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