Thread: Gravity Propogation Delay Question

Suppose three objects are moving through space parallel to each other,. So, they're side by side horizontally, and all moving at the same speed, at some fraction of C. From a gravity perspective, wouldn't each object perceive the other two to be pulling on it from a direction slightly behind, because the of propagation delay? The last point at which a gravity wave would have been emitted in time to reach it was physically located a little behind where the objects are now.


And, if that were true, wouldn't the objects be constantly slowing down from each other's gravity? How does GR get around that?

Moving at some fraction of c relative to what? They aren't moving relative to each other, so from their points of view, none are behind any of the others.

Originally Posted by kojax

Suppose three objects are moving through space parallel to each other,. So, they're side by side horizontally, and all moving at the same speed, at some fraction of C. From a gravity perspective, wouldn't each object perceive the other two to be pulling on it from a direction slightly behind, because the of propagation delay? The last point at which a gravity wave would have been emitted in time to reach it was physically located a little behind where the objects are now.


And, if that were true, wouldn't the objects be constantly slowing down from each other's gravity? How does GR get around that?

thats a good question. If there is in fact a propigation delay in the first place.

Originally Posted by MagiMaster

Moving at some fraction of c relative to what? They aren't moving relative to each other, so from their points of view, none are behind any of the others.

I know that their perspective doesn't tell them that they're moving at a fraction of C, but there has to be some way of transforming every reference to match up with every other reference, and that means the direction from which gravitational forces are arriving has to be transformable as well. I guess what I'm really asking is: how do we transform it without contradictions?

I could ask the same question about light emitted by the objects, and it would be just as confusing. Does a beam of light literally follow a curved path at those speeds, so that it can be observed to arrive from the side of each of the objects? Or do the objects' perspectives on direction change?

From the perspective of a nearby stationary object, gravity/light is moving at C, and the objects are moving at some fraction of C. The light has to follow some path that both objects can agree to, even if they disagree about the length, or time. So maybe they also disagree about the angle and straightness of its path. Maybe the moving objects perceive the light to travel in a straight line, but the stationary objects perceives it to follow a curved path?

How else would you correspond their perspectives?

The angles can change, but light will always follow a straight path in any frame. From the ships, the light is perpendicular to their forward. For the outside observer, it probably isn't.

Originally Posted by kojax

Originally Posted by MagiMaster

Moving at some fraction of c relative to what? They aren't moving relative to each other, so from their points of view, none are behind any of the others.

Originally Posted by kojax

I know that their perspective doesn't tell them that they're moving at a fraction of C, but there has to be some way of transforming every reference to match up with every other reference, and that means the direction from which gravitational forces are arriving has to be transformable as well. I guess what I'm really asking is: how do we transform it without contradictions?

Search on "speed of gravity" you will get to Van Flandern and an intro into gravity forces moving at a speed Vg >2x10^10(c). If the forces moved at the SOL the delay would result as you surmised. Take the force from the sun that reaches Jupiter, that then reacts and where the one way tine re the sun and Jupiter is an hour or so, The sun and earth are 8 minutes apart by SOL. With all planets in the solar system gravity acting wrt each other, anything other than instantaneous would effectively negate the conservation of angular momentum which maintains a very strict equilibrium regarding inter-planetary force distribution and effect. Newton though likewise but he refused detailed comment on this spooky subject.

Originally Posted by kojax

I could ask the same question about light emitted by the objects, and it would be just as confusing. Does a beam of light literally follow a curved path at those speeds, so that it can be observed to arrive from the side of each of the objects? Or do the objects' perspectives on direction change?

Flandern gives experimental references that the light from the sun does not strike the earth radially due to the aberration regarding the delay.

Originally Posted by kojax

From the perspective of a nearby stationary object, gravity/light is moving at C, and the objects are moving at some fraction of C. The light has to follow some path that both objects can agree to, even if they disagree about the length, or time. So maybe they also disagree about the angle and straightness of its path. Maybe the moving objects perceive the light to travel in a straight line, but the stationary objects perceives it to follow a curved path?

How else would you correspond their perspectives?

They cannot disagree about the physical effects,which is not subject to any agreeing process.

Your scenario requires a slight adjustment. The gravity force from the outer masses is felt by the inner mass, then there is a reaction which gets transmitted back to the outer masses - the system you propose is a cyclic delayed action reaction process that if applied to the solar system the solar system would be unrecognizable wrt its present form. Flandern gives the challenge to use a SOL for the speed of gravity forces and see if you can predict proper positions of the planets where now the gravity force effectively assumed instantaneous.

It would seem that the center mass is always slightly ahead of the gravity force just leaving the outer masses (where an action-reaction is always present in a delayed cycle) heading toward the center mass, in a mode of aberration hence the force would be to slow the center mass down, that is if the force of gravity did move at the slow speed of light. If the gravity forces were instantaneous the outer masses would maintain an equal and opposite force on the center mass hence, no change in position of velocity would result.

Originally Posted by kojax

Suppose three objects are moving through space parallel to each other,. So, they're side by side horizontally, and all moving at the same speed, at some fraction of C. From a gravity perspective, wouldn't each object perceive the other two to be pulling on it from a direction slightly behind, because the of propagation delay? The last point at which a gravity wave would have been emitted in time to reach it was physically located a little behind where the objects are now.


And, if that were true, wouldn't the objects be constantly slowing down from each other's gravity? How does GR get around that?

Remember, as long as they have no relative motion with respect to each other, they behave no differently than they do than if they were "at rest"

The only situations where such an effect could be seen is when the gravitational sources have a relative motion with respect to each other.
However, it can be shown in GR that such "gravitational aberration" is canceled out by other velocity dependent interactions.

You can safely ignore anything that Van Flandern has to say on the subject, as he is not an expert on neither gravity or GR. While at one time he did make some contributions to astronomy, he then sadly descended to advocating crackpottery.

Originally Posted by Janus

Originally Posted by kojax

Suppose three objects are moving through space parallel to each other,. So, they're side by side horizontally, and all moving at the same speed, at some fraction of C. From a gravity perspective, wouldn't each object perceive the other two to be pulling on it from a direction slightly behind, because the of propagation delay? The last point at which a gravity wave would have been emitted in time to reach it was physically located a little behind where the objects are now.


And, if that were true, wouldn't the objects be constantly slowing down from each other's gravity? How does GR get around that?

Remember, as long as they have no relative motion with respect to each other, they behave no differently than they do than if they were "at rest"

The only situations where such an effect could be seen is when the gravitational sources have a relative motion with respect to each other.
However, it can be shown in GR that such "gravitational aberration" is canceled out by other velocity dependent interactions.

You can safely ignore anything that Van Flandern has to say on the subject, as he is not an expert on neither gravity or GR. While at one time he did make some contributions to astronomy, he then sadly descended to advocating crackpottery.

This is helpful, but it doesn't quite address the issue. Suppose we're talking about light instead of gravity for a moment. You're saying that the middle object perceives the light emitted by the two peripheral objects to be coming at it from the sides, at a 90 degree angle.

If there were a stationary dust cloud between the objects, to allow a stationary observer to see what direction the light was traveling in, they would perceive it to have struck the middle object from a slightly rearward direction. (Suppose it was something like a laser beam, which has a clear direction.)

The question is: how do we correspond these two perspectives?

Originally Posted by kojax

This is helpful, but it doesn't quite address the issue. Suppose we're talking about light instead of gravity for a moment. You're saying that the middle object perceives the light emitted by the two peripheral objects to be coming at it from the sides, at a 90 degree angle.

Yes

If there were a stationary dust cloud between the objects, to allow a stationary observer to see what direction the light was traveling in, they would perceive it to have struck the middle object from a slightly rearward direction. (Suppose it was something like a laser beam, which has a clear direction.)

The question is: how do we correspond these two perspectives?

Imagine you have a string stretched between the objects with a bead that slides back and forth between them.

From the perspective of the objects the bead just goes back and forth in a straight line. To someone for which the objects are moving, the bead travels in a zig-zag. It is no different for a laser bouncing back and forth between the two.

Originally Posted by Janus

Originally Posted by kojax

If there were a stationary dust cloud between the objects, to allow a stationary observer to see what direction the light was traveling in, they would perceive it to have struck the middle object from a slightly rearward direction. (Suppose it was something like a laser beam, which has a clear direction.)

The question is: how do we correspond these two perspectives?

Imagine you have a string stretched between the objects with a bead that slides back and forth between them.

From the perspective of the objects the bead just goes back and forth in a straight line. To someone for which the objects are moving, the bead travels in a zig-zag. It is no different for a laser bouncing back and forth between the two.

So how does a force acting at an angle that seems to be different when observed from two different reference frames still behave exactly the same?

Acting on an object in motion, the stationary observer sees it approaching from slightly behind, but the object responds to this force as if it had acted upon it exactly from the side. It then responds by exerting a force in a direction that seems to be slightly forward from its current position, but which it perceives to also be perfectly sideways. Its action-reaction behavior is at two different angles.

Maybe that's the key. If it pulls on an object that appears to be in front of it, and the object pulls on it from behind, then behind/in front would tend to cancel out. Think maybe that's how this resolves? Of course, objects of dissimilar mass would exhibit interesting behavior. Maybe a black hole at the center of a galaxy might do interesting things?

Such questions tend to have interesting answers that require a lot of math to resolve.

Do you think factoring in Graveto-magnetism will help?

Somehow I doubt it, but I don't really know enough to accurately answer that. Given the formulas I could probably work everything out, but I don't know the formulas myself, nor do I know which ones to look up.

Besides, a formula, all by itself won't tell me what I'm really after.

I can see how Janus's string metaphor would apply in Newtonian mechanics. If I threw a ball sideways to an object moving parallel to me, then the ball would appear to be following a zigzag course from a stationary observer's perspective, and it would still only apply force to its target in a horizontal direction, even though it seems to be hitting the target slightly from behind. (Its forward velocity is equal to the forward velocity of its target, so no exchange of momentum is happening in that direction.)


I'm just having a hard time understanding how that can work with gravity waves or light waves.