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What am I missing here?

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What am I missing here?

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Ibix

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WannabeNewton

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The diagrams which try to model space-time curvature as "dents" in space-time are not accurate because we are actually talking about curvature in 4 dimensions which is extremely difficult to picture so one must settle for so-called 'embedding diagrams'.

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Ibix

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If you could make a perfectly frictionless bowl, the marble would carry on "orbiting" forever (assuming you've picked your bowl very carefully to be shaped such that stable orbits are possible). It would speed up as it went down into the bowl, and slow down by the same amount as it climbed back out. But it will not spiral inwards.

In reality, you cannot make a frictionless bowl. Friction inevitably leads to the marble slowing down and spiralling in. But this is due to friction sapping energy, not an effect of gravity.

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This straight line you refer to, does it exist in the 4th dimension (time)?

The diagrams which try to model space-time curvature as "dents" in space-time are not accurate because we are actually talking about curvature in 4 dimensions which is extremely difficult to picture so one must settle for so-called 'embedding diagrams'.

- #7

WannabeNewton

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Yes these paths are paths in space-time. This is one of the most elegant aspects of GR!This straight line you refer to, does it exist in the 4th dimension (time)?

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I'll have to learn more about friction then, but thanks.

If you could make a perfectly frictionless bowl, the marble would carry on "orbiting" forever (assuming you've picked your bowl very carefully to be shaped such that stable orbits are possible). It would speed up as it went down into the bowl, and slow down by the same amount as it climbed back out. But it will not spiral inwards.

In reality, you cannot make a frictionless bowl. Friction inevitably leads to the marble slowing down and spiralling in. But this is due to friction sapping energy, not an effect of gravity.

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Interesting. Do space-time geodesic's reduce entropy in 4-dimensional space?Yes these paths are paths in space-time. This is one of the most elegant aspects of GR!

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Nugatory

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No, they have nothing to do with entropy.Interesting. Do space-time geodesic's reduce entropy in 4-dimensional space?

Although we cannot (well, I cannot!) visualize a four-dimensional space, you can learn a far amount about space-time works by learning to draw and understand two-dimensional space-time diagrams. There are many examples in various posts in this forum.

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WannabeNewton

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I'm not sure I understand the question. Entropy is a state function of a given system involving the number of microstates associated with a macrostate. What system are we looking at here and what is it being compared to when asked if it is "reduced"? A thermally isolated system at equilibrium has maximized its entropy. By the way, if you have questions on entropy in GR, you should create a new thread on it since it is not related to your original question.Interesting. Do space-time geodesic's reduce entropy in 4-dimensional space?

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http://en.wikipedia.org/wiki/Entropic_gravity

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From the same source.Based on the work of Jacobson [1–6], Padmanabhan [7– 12], and others, there are also good reasons to suspect a thermodynamic interpretation of the fully relativistic Einstein equations might be possible.

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Thanks for the advice. I'm currently self-teaching myself the subject along with thermodynamics and some of my own work.No, they have nothing to do with entropy.

Although we cannot (well, I cannot!) visualize a four-dimensional space, you can learn a far amount about space-time works by learning to draw and understand two-dimensional space-time diagrams. There are many examples in various posts in this forum.

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WannabeNewton

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All Ibix is saying is that in the absence of friction, a particle can stay in a certain "orbit" for all time if the system is let to evolve on its own. The bowl analogy is not a good one for another reason: there are constraint forces on the particles so they are not in free fall. Their motion isn't geodesic so it isn't really representative of the motion of a freely falling particle in space-time. As long as the earth is in free fall around the Sun, there is nothing perturbing it significantly from its observed path around the Sun so as to make it spiral inwards (it has a stable orbit).I'll have to learn more about friction then, but thanks.

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- #16

Nugatory

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You might want to try reading the "Talk" page for that article as well.gravityarises from the effects ofentropy.

http://en.wikipedia.org/wiki/Entropic_gravity

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Alright, thanks.

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Read about SR to learn about the dimensions & intervals. Those are great concepts to know if poking about GR stuff, specifically geodesic paths, acceleration. That helped me know what gravity is a bit better, far from "understanding" though, you need to be able to speak math . Light decreasing in gravitational potential is a good scenario / thought experiment.gravityarises from the effects ofentropy.

http://en.wikipedia.org/wiki/Entropic_gravity

You maybe asking why energy density can be used to calculate gravity so accurately, I don't think the How part is known.

Stress-Energy Tensor

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Cool. As I said, I'm self-teaching myself this stuff. It's getting intense.

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Be careful though, there are plenty of ideas about how gravity works, but you should reallyCool. As I said, I'm self-teaching myself this stuff. It's getting intense.

http://johncarlosbaez.wordpress.com/2012/02/01/entropic-forces/

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I will check out your link.