Explanation of Paradox

Davel, TVF says gravity has waves that travel at light speed and some other details that excape me at this time(so I can't comment on that) but, why not have a model where the gravity field is stationary? It seems as good as anything else being kicked around. Maybe there is a good reason for assuming gravity maves at some speed or other but not to be stationary. Do you know if that is so?

Jim,

Gravity has to be dynamic, like other forces, otherwise objects wouldn't stay in orbit around each other as they moved. For example, the moon wouldn't stay in orbit around the earth as the earth orbited around the sun, unless the earth's gravity somehow moves with the earth.

So, if the attraction has to move with the source, that begs the question of exactly how in detail it moves. Usually, in physics, it takes time for effects to propagate. Its very well supported by observation that all electromagnetic long-range effects propagate with the speed of light. There is much less measured with respect to gravitational effects, because the measurements are more difficult. Personally, I don't at the moment understand the equations of general relativity (the most widely-accepted theory of gravity) well enough to have an opinion based on them about how fast either the gravitational force or gravitational radiation propagate. However, if it were analagous to electromagnetism in this area (as Prof Steven Carlip asserts per one of the links above), I am confident that a time-advanced force component would be generally considered much more plausible than an instantaneous effect.

As Feynman points out in his speech I linked to, the time-advanced parts are actually in the theory and generally ignored, rather than being an unnecessary addition.

Also, these would certainly not cancel tidal forces in general, to the extent that tidal forces are due to the variation of gravitational attraction with range effect on extended bodies. That is, the water on the side of the earth towards the moon is more attracted to the moon than the water on the far side. This is the m ain cause of a tidal budge, though probably not the complete story, but that is beyond my present ken. The range variation is still true under my hypothesis, just as it is under TVF's of instantaneous propagation.

I would agree that it should be assessed quantitatively what if any are the differences in observable effects of retarded versus instantaneous versus retarded plus advanced force. This assessment has to be done with a full mathematical formalism rather than just with hand-wavy arguments. I am not yet convinced that there is anything wrong with the current consensus that gravity effects are retarded at the speed of light, although I do find the hand-wavy argument very convincing. Until one can fully understand arguments to the contrary from such as Carlip, I would think it would be rash to dismiss them out of hand. However, if these arguments are flawed, and propagation delays do cause forces to become non-central, then restoring centrality via time-advanced effects would seem to me at least equally plausible as instantaneous propagation. So it would come down to what are the differences in observable effects in the two theories.

So far I have had two possible differences suggested. One above is tidal forces, but I can definitely argue that tidal forces are preserved under my proposal and I would think they would be more similar to the instantaneous-propagation model than to the retarded-only-force model. If somebody can explain how they would be different I would be fascinated to hear it.

The other proposed discriminant was suggested to me by TVF in an email, that as described in his speed-of-gravity paper there are measurements that show the gravity vector from earth points to the present position of the sun rather than the past position where light comes from. That contradicts the hand-wavy model of retarded effects but is equally consistent with instantaneous and hand-wavy retarded plus advanced forces. Carlip, however, says it does not actually contradict a rigorous treatment in GR with presumably retarded-only forces.

It will be interesting to see how this plays out but it will take a while. I do intend to build a model of the process using GR. Actually I already have one that implements retarded-potential electromagnetic forces and I intend to put Carlip's assertions about that to the test of simulation.

DaveL, You seem to be very deep into this topic and the assumptions you make should be studied in more depth too. You say the moon would not orbit the Earth if a gravity field(or a wave of the hand)model was used. Why would you assume the moon is orbiting the Earth? Why not assume the moon's orbit is perturbed by the Earth? Wouldn't the moon orbit the sun much as it does now if the Earth did not perturb the moon? You think the moon would fly off somewhere or what?

Jim, I'll reply to your post after I get done posting a couple of recent thoughts I've had on this general subject.

The first thought is a possible response to Carlip's critique (linked to above in the thread) that gravity has to be angular momentum conserving because there is a mathematical proof of it (I believe it is supposed to follow from Emmy Noether's famous theorem).

My conjectured response is that the proof applies only when all of the forces in the theory are included. The advanced forces are on an exactly equal footing and so cannot be discarded without potentially obviating the theorem. It might be that both the advanced and retarded parts of the force are separately conservative, or it might not. It seems to me that if the retarded parts of gravity alone are not conservative, then if including the advanced part makes for a conservative system, this would be a very powerful argument for the reality of time-advanced forces.

This statement is not that different than what I said above in the thread, I suppose, but it is a bit more precise and perhaps more useful if it suggests a direct line of theorectical investigation.

My second thought is that what I proposed does not appear to be quite the same as Wheeler-Feynman Absorber theory. I am still waiting for a reprint of the original paper to arrive but the second-hand accounts of it I've found only mention the advanced action component of the "absorber" particle acting back on the "emitter" particle, which is the (electrically charged) particle being accelerated and experiencing the radiation resistance. I haven't heard mention of the emitting particle acting on the absorbing particle in the absorber particle's past. Now, it may be that in the case of radiation resistance this case has no significance, but in general between two bodies there are four cases to consider, and all of them on equal mathematical footing. For the case of an attractive force (as in gravity or between opposite charges):

1. Object A feels an attraction to where Object B was one light-travel time interval in the past

2. Object A feels an attraction to where Object B will be one light-travel time interval in the future

3. Object B feels an attraction to where Object A was one light-travel time interval in the past

4. Object B feels an attraction to where Object A will be one light-travel time interval in the future


It would seem to me that W-F absorber theory considers influences (the force components other than Coulomb attraction, i.e., the Faraday and magnetic EM force components) due to 3 and 2. Standard dynamics using time-retarded forces would assume 1 and 3. This would imply that in addition to the non-central force component due to delay on the pull on the earth, there is also a non-central force component of the earth's pull on the sun. This latter term I think is less consequential than the one on the earth, but it tends to exacerbate the situation rather than partially remedy it.

What I propose then is that all four cases are in effect and of equal weight. It takes all four to recover an angular-momentum conserving system.

Jim, it is in principle possible to reformulate the problem in terms of, say, the earth 'orbiting' around the moon and the sun orbiting around the earth. (Early Selenites probably believed this, as well as present-day Luddite Selenites). This idea is known as Mach's principle, and it was known to Einstein I believe and Einstein's theory of gravity does conform with it. The idea is, that it is all the other matter in the universe pushing back when we feel the resistance of an object against an applied force. So, if we choose to consider that the object is stationary and the universe moving around it generating the forces, that is an equally valid point of view. But it turns out to be more complicated mathematically then the conventional view, so the convential view that (say) the earth otbits around the sun rather than vice-versa has advantages.

It has just occurred to me that my conjecture has some interesting other consequences, one of which should be easy to design an experiment around.

But first, some background discussion is in order. I mentioned the Huw Price book in the thread above and provided a link to it. In his book Price discusses (among many other interesting topics) the practicalities of detecting time-advanced effects. For example, one of the things he proposes is looking for time-advanced stars which would appear as radiation sinks and cold spots in the sky. Generally, he argues I think from a point of view that directly detecting time-advanced effects is made difficult because of an expectation that time-advanced radiation will differ from time-retarded in that it will generally be incoherent while time-retarded effects are coherent when viewed from an emitting particle. It seems obvious to me, though, that if 2 & 4 per my previous post are true, then there is no basis for this "temporal bias" with regards to coherency. For example, from the point of view of 1 through 4 all being equally true, when I shine a laser beam I am shining coherent light equally into the past and the future. One might ask, then, what are the observable consequences of this? I think there would be obviously observable consequences.

If I shine a steady state laser beam at a sufficiently distant object, I should see beats as a function of range as the retarded and advanced beams interfere and reinforce with each other.

Alternatively, if I shine a short-pulse laser at a sufficiently distant detector, I should see two pulses instead of one, each of half amplitude.

Its a little too late for me to calculate the experimental parameters involved tonight, but I feel pretty confident that either of the experiments should be pretty amazingly easy to do.

Hi Dave, The several details you are working on would fill several volumes. To comment on the one I made above; the moon orbits the sun and the Earth orbits the sun-not what you are thinking at all. Look at the moon as orbiting the sun and not the Earth orbiting the moon or the moon orbiting the Earth. The angular momentum works out as expected with this correction. I don't know why angular momentum is conserved but it is observed and in most cases it is conserved. I don't see how angular momentum can be conserved in all situations or why it should be-do you?