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I understand the “rubber sheet” model of Relativistic gravity is just an illustration, and beyond the initial issues of mixing three-dimensional objects with a two-dimensional representation of 3D space/time, the basic concept is easy enough to grasp… The billiard ball “falls” into the depression of warped space/time created by the heavier, denser bowling ball, and is captured in orbit.

Let’s suppose, on the other hand, that rather than two dissimilar “balls”, we place two identical, inert, non-radiant particles (dark matter?) into the model, let’s say 100,00 light years separating them, and get rid of the bowling and billiard balls altogether. Each particle warps the spacetime fabric equally, and since we imparted neither directional motion nor rotational spin to either particle, and there is no emission of energy/light to create pressure, it would seem that, according to Einstein, neither particle would move from the spot we placed it, as there is no “slope” to “fall” along when both particles are deforming the sheet equally.

But according to Newton/Galileo, both particles would instantly, (not 100,000 years from that point), be drawn towards each other by the force of gravity, slowly at first, but with ever increasing acceleration. So, do the particles move towards each other instantly or don’t they? Who is right, E or N? Why?

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    $\begingroup$ After 100,000 years, the particles begin moving toward each other, because those are the geodesic paths, given the geometry that they are forcing on the spacetime. I have no idea why you say "there is no slope to fall along", or even quite what this might mean. $\endgroup$ – WillO Oct 28 '15 at 3:45
  • $\begingroup$ In other words, creating mass-energy out of nowhere, which is impossible due to conservation laws, would trigger spherical gravitational waves (which for the mentioned reason don't exist). $\endgroup$ – CuriousOne Oct 28 '15 at 4:03
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    $\begingroup$ Welcome to Physics Stack Exchange. Please read our FAQ on writing useful question titles. $\endgroup$ – DanielSank Oct 28 '15 at 4:51
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    $\begingroup$ @hep yes, that is what I meant. Keep in mind, however, as seen in the answers below, the rubber sheet analogy is not so powerful. $\endgroup$ – Declan Oct 28 '15 at 6:50
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    $\begingroup$ @Hep you may be confusing static with dynamic field behavior. While the Earth certainly orbits the position of the sun where it is NOW (and not ~8 minutes ago, where we see it when we look up in the sky) this is because all motion in the system is non-accelerating. If the sun were to disappear, the Earth's orbit would not be altered for ~8 minutes. There is no contradiction here. Static fields may appear to act instantaneously, but any change in a field must abide by the speed of light. $\endgroup$ – JPattarini Feb 23 '16 at 14:25
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Each particle warps the space/time fabric equally, and since we imparted neither directional motion nor rotational spin to either particle, and there is no emission of energy/light to create pressure, it would seem that, according to Einstein, neither particle would move from the spot we placed it, as there is no “slope” to “fall” along when both particles are deforming the sheet equally.

The image of the rubber surface and distortions applies to the case of a much larger mass in the center, and a small one around it:

the Schwarzschild solution, it is assumed that the larger mass M is stationary and it alone determines the gravitational field (i.e., the geometry of space-time) and, hence, the lesser mass m follows a geodesic path through that fixed space-time. This is a reasonable approximation for photons and the orbit of Mercury, which is roughly 6 million times lighter than the Sun. However, it is inadequate for binary stars, in which the masses may be of similar magnitude.

The metric for the case of two comparable masses cannot be solved in closed form and therefore one has to resort to approximation techniques such as the post-Newtonian approximation or numerical approximations.

so it is not that simple to visualize:

The post-Newtonian expansion is a calculational method that provides a series of ever more accurate solutions to a given problem. The method is iterative; an initial solution for particle motions is used to calculate the gravitational fields; from these derived fields, new particle motions can be calculated, from which even more accurate estimates of the fields can be computed, and so on. This approach is called "post-Newtonian" because the Newtonian solution for the particle orbits is often used as the initial solution.

When this method is applied to the two-body problem without restriction on their masses, the result is remarkably simple. To the lowest order, the relative motion of the two particles is equivalent to the motion of an infinitesimal particle in the field of their combined masses. In other words, the Schwarzschild solution can be applied, provided that the M + m is used in place of M in the formulae for the Schwarzschild radius rs and the precession angle per revolution δφ.

It can be shown that Newtonian physics emerges at the limit of validity of General Relativity,

You state:

But according to Newton/Galileo, both particles would instantly, (not 100,000 years from that point),

General Relativity was partially invented to reconcile the velocity of light as a limit in special relativity and this assumption of Newtonian physics , which is wrong.

From the link:

That means that Newton's theory depends upon a notion of absolute simultaneity. A change there is felt here at the same moment. However Einstein's 1905 theory had banished absolute simultaneity from physics. Different observers would judge different pairs of events to be simultaneous. Newton's theory had to be adjusted to accommodate this new relativity.

The change needed was, apparently, straightforward. In the revised theory, a change in the sun should not be felt here on earth instantly, but only after a time lag of around 8 1/3 minutes, the approximate time light takes to propagate from the sun to the earth. Then absolute simultaneity would no longer be needed in the theory.

This meant that Newton's theory needed to be adjusted to look more like electrodynamics. In the latter theory, effects do not propagate instantly in the electromagnetic field; they propagate in waves at the speed of light.

etc in the link.

You state:

be drawn towards each other by the force of gravity, slowly at first, but with ever increasing acceleration. So, do the particles move towards each other or don’t they? Who is right, E or N? Why?

As it has been shown that Newtonian gravity emerges from the limiting case of General Relativity I trust that the numerical approximations needed for the setup you have visualized will give as a solution the correct Newtonian attraction between two massive bodies.

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  • $\begingroup$ I appreciate your detailed response, though I take issue with this: "General Relativity was invented to reconcile the velocity of light as a limit in special relativity and this assumption of Newtonian physics , which is wrong." The speed of light is not the fastest pace at which things occur in the universe, unless quantum entanglement, which has been established, is disregarded. It follows that if E was wrong about this "spooky motion at a distance", that GR might still need some tweaking, and gravity could very well operate "instantly", contrary to GR's speed limit. $\endgroup$ – Hep Oct 28 '15 at 5:50
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    $\begingroup$ I am sorry , but you are wrong. Quantum entanglement does not transmit information or anything not before established. The speed of light is the absolute limit in our present mathematical models of physics for any transfer of force or information $\endgroup$ – anna v Oct 28 '15 at 5:55
  • $\begingroup$ @Hep: The speed of light is a constant in every coordinate system. That has absolutely nothing to do with the commonly referenced but patently false idea that the speed of light is the cosmic speed limit. Entanglement, on the other hand, has also absolutely nothing to do with a speed limit or a breaking thereof. Entanglement and the common misunderstandings of what it means are simply the talk flavor of the day among the physically semi-educated. That's not a personal attack on you, that's just a fact of amateur science appreciation. We are trying to turn that around over here. $\endgroup$ – CuriousOne Oct 28 '15 at 5:57
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    $\begingroup$ So? Nothing faster than light there. Entanglement just means that there exists a quantum mechanical solution for the system under consideration. Nothing more spooky than the magica of sleight of hand magicians. $\endgroup$ – anna v Oct 28 '15 at 6:45
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    $\begingroup$ That was an experimental error. en.wikipedia.org/wiki/Faster-than-light_neutrino_anomaly $\endgroup$ – anna v Oct 28 '15 at 8:55
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Assuming we are able to create the two bodies out of no where, and in the state that you described. (even though, I do not think it is possible to do so).

Not only this way, but it is next to impossible to measure speed of gravity in any way. We have been able to measure speed of gravitational waves, but not that of gravity. Doing that, just eludes scientists due to us not being able to create stuff out of nothing, or make it disappear totally. I am not aware of any experiment confirming speed of gravity (which can be different from speed of gravitational waves).

Therefore, I can not say who is right Newton, or Einstein. However, quantitatively, GR is much more accurate in pretty much any circumstances. Neither of them proves which one is right when it comes to speed of gravity. GR comes close, at least it predicted speed of GW and they have been confirmed.

To your slope part, per GR -

  1. As soon as the particles came into being, the space, and the particles will start interacting due to certain properties of space.

  2. Due to this interaction, the space will start to curve. Rubber sheet analogy, is not accurate, but just ok for describing..

  3. This curving effect will spread in spherical directions (3D) at the speed of light. It will spread in symmetric manner. i.e. no slope. You can imagine it as a dip of space is spreading from the particles.

  4. After 50K years, the two curving effects (dips) will meet one another, making it a slope. Because, now the curving effect starts to add here on and is no more symmetric for individual bodies.

  5. After another 50K years, both the bodies will come to know that there is a slope, and will start moving towards one another.

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