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Suppose two parties $A$ and $B$, some distance apart and at rest relative to each other both had control of one massive local object each.

If party $A$ lifted their local object upward relative to $B$, then the total force of gravity on $B$'s mass would angle upwards slightly.

If B then measured this shift in gravitational pull over time a communication wave could be formed between $A$ and $B$.

Wouldn't this communication channel operate instantaneously? How does this not violate the laws of causality with respect to the speed of light?

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    $\begingroup$ Possible duplicate: physics.stackexchange.com/q/26742/2451 $\endgroup$
    – Qmechanic ♦
    Commented Sep 28, 2012 at 5:38
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    $\begingroup$ This was pretty much what got Einstein thinking about gravity and prompted him to come up with general relativity after he had developed his theory of special relativity. He realized that Newton's description of gravity was in conflict with the basis of his theory ($c$ is constant and 'the speed limit') so the description of gravity had to be adjusted. And boy did he adjust it. :) $\endgroup$
    – Wouter
    Commented Jan 19, 2013 at 20:30

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Alas, no, the communication would not be instantaneous. This is a valiant attempt, and it's been considered by many people before, since we're mostly all taught Newtonian gravity before we're taught about the speed of light, and the former is then never treated again prior to general relativity.

What happens is the shift in the mass of A causes a disturbance in spacetime (a small perturbation to the background metric) and this disturbance travels outward at a speed no greater than $c$. It's exactly the same as if you moved a charge collection and had someone far away with an electromagnetic receiver (well the GR equations are nonlinear and complicated, but the same principle is at work).

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    $\begingroup$ I had no idea the influence of gravity traveled at the speed of light. Thanks. Need to read GR I guess. $\endgroup$
    – Andrew Tomazos
    Commented Sep 28, 2012 at 3:41
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You won't be able to see the effect immediately, for example if Sun suddenly vanishes, earth will continue in it's orbit for 8 minutes and 12 seconds before it going straight. Gravity travels at the speed of light...

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  • $\begingroup$ See also this Phys.SE post. $\endgroup$
    – Qmechanic ♦
    Commented Sep 28, 2012 at 14:06
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Suppose two parties $A$ and $B$, some distance apart and at rest relative to each other [...] Wouldn't this communication channel operate instantaneously?

If the proposed communication channel were indeed found to mutually "operate instantaneously", as may well be found experimentally, then the two distinct parties $A$ and $B$ are thereby measured to have been co-located (coincident; not apart at all), instead of having been "some [non-zero] distance apart".

How does this not violate the laws of causality with respect to the speed of light?

The experimental determinations of the two parties $A$ and $B$ don't seem to "violate the laws of causality with respect to the speed of light" under any circumstance; regardless whether they find (mutually) "instantaneous communication", or not.

Also note that in (S)RT "the speed of light (in vacuum)" is primarily understood as the signal front speed of whichever communication channel happened to operate the quickest. This is readily identified as the familiar signal front speed of electro-magnetic signals if the parties involved, such as $A$ and $B$, are (thought of as being) constituted by electro-magnetic charges.

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  • $\begingroup$ Nice technically correct content, but you seem to have missed the forest for the trees $\endgroup$
    – Jim
    Commented Apr 24, 2014 at 0:04
  • $\begingroup$ @Jim: "Nice technically correct content," -- Thanks. "but you seem to have missed the forest for the trees" -- Hmm ... Well, in my humble opinion, when it comes to foundational issues of physics (i.e. geometry, RT) then arguably it is a gain of insight (and not a miss) to speak pointedly of $\frac{n (n - 1)}{2}$ pairs of neatly aligned cellulose fibers, instead roughly of $n$ neatly aligned cellulose fibers; for instance. $\endgroup$
    – user12262
    Commented Apr 24, 2014 at 5:48
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    $\begingroup$ "neatly aligned cellulose fibers" you're a punny guy. $\endgroup$
    – Jim
    Commented Apr 24, 2014 at 13:33
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It is my understanding that instantaneous "communication" between a and b can be possible. I believe this has to do with "time" which I understand as simply motion relative to motion. Therefore without motion there is no "time" as there can be no rate. This applies to distance similarly as you cannot have distance without two points of reference as the distance is the "space" relative to the size of the two points. For distance and time, the "rate" for each is itself. I believe a universal medium must exist that is of such a condensed nature that "signals" sent of it are able to be registered at all "points" throughout it at once. Similar to how gravity can be "felt" at all points within its field. An example would be air and sound. Air is the medium for sound. The factors deciding the time it takes for a sound wave to travel a distance consist of the distance between the particles of air, the energy applied, the energy required to move each particle the distance between particles into the next and the number of particles (units of energy transfer). This is why sound waves cannot travel indefinitely as their medium is restricted and there is energy required per particle to transfer the initiating force between particles. Should a medium have no distance between units of transfer, it would require neither time nor energy other than the initiating force to continue. If something meets no resistance (no energy is required to propagate it's movement) then it continues at the rate of the initiating force. Atoms bind together and therefore require energy to disband, but no energy is lost to the medium that they travel through (vacuum) when a force is applied. Therefore it is subject to resistance to change of force and travels at a rate measurable by its own parameters as a grouping. However, should a force be applied directly to the condensed medium, only that force would be expressed as there are no limiting factors.

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    $\begingroup$ Are you using a time-tunnelling browser to post this from the 19th Century? $\endgroup$
    – Oscar Bravo
    Commented May 5, 2015 at 8:15

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