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DaveL
USA
15 Posts |
 Posted - 02 Apr 2005 : 15:16:17
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The paradox being, if the speed of gravity is the speed of light, how does the earth stay in orbit?
Here is my proposed solution:
Gravity travels at the speed of light, but both time advanced and time retarded forces are in action.
Its easy to draw the same diagram that shows the paradox and see that a linear combination of equal-size time-retarded and time-advanced forces exactly cancels any non-radial component of the force.
Also, the idea of time-advanced forces is considered plausible. Many people will know, Feynman and Wheeler wrote a famous paper about it, and Huw Price's book, "Time's Arrow and Archimedes' Point" is all about it and how it might account for the apparent non-locality of quantum theory.
I did a search on "time advanced" and couldn't find a match on this. I am interested if anybody else has considered this.
It seems possible to me, that the fact that the earth stays in orbit may be a first demonstration that time-advanced effects exist. Thew Wheeler-Feynman paper explained something with it too but that is not the only explanation for what they explained, of course.
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Larry Burford
USA
1355 Posts |
Posted - 03 Apr 2005 : 16:45:13
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DaveL,
Others have made similar arguments, also hoping to avoid the observation-based conclusion that gravitational force appears to propagate much faster than light. Search the USENET archives for: Carlip Van Flandern non central force.
I believe that some of the articles on tvf's Gravity CD also touch on this.
===
TVF points out two problems with this approach:
A) Even though the math works, there are no known phyical phenomena to justify adding these terms to our models. (We're still looking, of course. Absense of evidence is not evidence of absense. But ...)
B) If there were such phenomena they ought to cancel *all* non central forces (for example tidal forces), not just the ones (for example the aberration that would be induced by light speed propagation of a force) that don't support a particular theory.
Regards,
LB
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Jim
1607 Posts |
Posted - 03 Apr 2005 : 16:45:25
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| Why would the speed of gravity have an effect on the orbit of a planet? I'm kind of stupid about this stuff but it seems to me the gravity field would be there for the planet even if gravity was not moving at all. And why would gravity travel anyway? |
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DaveL
USA
15 Posts |
Posted - 03 Apr 2005 : 21:15:16
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Thanks for the response, Larry Burford.
I am looking at this: http://www.lunchwithgeorge.com/lwg_gravity.html
Towards the bottom is very interesting, Carlip's claim that EM forces derived from retarded potentials will point to an extrapolated position rather than the 'instantaneous' position. I'll have to look into that. If that's true, there is no need to postulate either advanced forces or instantaneous ones, at least to the problem of the earth staying in orbit.
I've not found any explicit mention of considering time-advanced forces but I am still reading.
Anyhow, if what Carlip says about retarded EM (Leinard-Weichert) potentials is true, I am going to lose interest in the whole thing, probably, because the credibility of the whole problem will go out the window for me, unless one of the TVF/MR papers has an explicit working out of the force directions under GR, rather than just a diagram, that can show that GR is different than standard EM theroy in this area. I am inclined to take Carlip's word that GR will be the same as EM here, unless somebody with an interest has demonstrated otherwise.
To me it does seem more plausible to have time-advanced forces than instantaneous ones, but any of our feelings about it are irrelevant. All that matters is what works to explain observations.
Its not obvious to me that advanced forces would cancel tidal forces. Is that worked out someplace?
Jim, what I am referring to is described here
http://www.metaresearch.org/cosmology/speed_of_gravity.asp |
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tvanflandern
USA
2793 Posts |
Posted - 04 Apr 2005 : 01:37:55
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quote:
Originally posted by DaveL
... unless one of the TVF/MR papers has an explicit working out of the force directions under GR, rather than just a diagram, that can show that GR is different than standard EM theory in this area. I am inclined to take Carlip's word that GR will be the same as EM here, unless somebody with an interest has demonstrated otherwise.
Figure 3 is an aid to understanding the derivation that follows, which shows that a linear extrapolation is grossly insufficient in the case of gravity. Carlip does not dispute that point, and in fact relies on it in his PLA paper. That is in part what led to my second "speed of gravity" paper, which mentions the Sherwin-Rawcliffe experiment. That experiment (which no one ever bothered to replicate, probably because it would be a career-killer to do so) showed that a linear extrapolation is insufficient even for E&M. -|Tom|-
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DaveL
USA
15 Posts |
Posted - 04 Apr 2005 : 02:51:05
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It did occur to me that linear extrapolation will not fully restore angular momentum conservation, by itself. But it would increase the amount of time required to note an effect. Has anybody calculated how long in that case to have an effect on the earth? You have 1200 years to double the radius if the force points to the past position. That's a very fast-acting effect, in terms of cosmic time. So if linear extrapolation meant it took a thousand or a million times longer to see an effect, it would still be readily apparent.
Anyhow I am not giving Carlip's argument its full due, I suspect. There is more to it than just the fact (according to him) that retarded potentials lead to a linear extrapolated effect, I think. Its going to take me longer than tonight though to get a handle on it. Also he mentions a paper by Damour where I gather this is addressed further. |
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DaveL
USA
15 Posts |
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Jim
1607 Posts |
Posted - 04 Apr 2005 : 19:54:03
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| 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? |
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DaveL
USA
15 Posts |
Posted - 06 Apr 2005 : 15:34:00
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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.
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Jim
1607 Posts |
Posted - 06 Apr 2005 : 19:10:55
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| 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? |
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DaveL
USA
15 Posts |
Posted - 18 Apr 2005 : 01:56:10
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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.
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DaveL
USA
15 Posts |
Posted - 18 Apr 2005 : 03:17:54
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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. |
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Jim
1607 Posts |
Posted - 18 Apr 2005 : 21:07:30
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| 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? |
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DaveL
USA
15 Posts |
Posted - 20 Apr 2005 : 00:33:57
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Jim, there is a mathematical proof that angular momentum must be conserved if the dynamical laws are invariant under spatial rotations. That is Noether's theorem that I spoke of. Actually, though, the angular momentum of the physical bodies need not be conserved if there are radiative fields that carry some of it away. The sun-earth system could decay and still conserve angular momentum, if gravitational radiation carried the angular momentum away. However, its not supposed to do this, unlike the case of electromagnetism, which would. As far as the moon orbiting the sun, it does, but its orbit is highly perturbed by the presence of the earth, if you want to look at this way. Once you have the equations of motion of the system, you can interpret them any number of way. Its just when you are trying to calculate the motion that its convenient to work through starting with the strongest forces first.
Now, back to the fascinating topic of time-advanced forces. I am so far out on a limb that I am going to cut it off myself before somebody else can't resist the temptation. Here's an example of an obvious consequence if it were as easy as I hypothesized to shine a laser beam into the past:
Every time a geosynchronous-orbit communication satellite relayed a message from one ground station to another, the relayed message would be transmitted into both the past and the future. This would result in two messages received at the recieving ground station, one simultaneous with transmission by the transmitting station and one delayed by the round-trip travel time to geo-synch and back. Everybody has probably experienced this latter delay in a satellite phone call, but the advanced one is conspicuously absent.
Price attempts to get around this by supposing that the time-advanced radiation has the opposite effect to time-retarded light. That is, instead of the laser beam heating a spot on the detector, the time-advanced light would cool it. This kind of effect is of course more difficult to detect, though I think not impossible. However, I don't think theory supports that time-advanced radiation would act this way. The modulation on a time-advanced wave changes with time just like on the time retarded wave. The only difference is that it arrives at the reciever before it left the transmitter. This is a trivial consequence of the form of the functional arguments. So why should the same signal at different instants cause cooling in one instance and heating in another? I don't see how it can.
Another way to look at is if we move the transmitter and receiver close together. If the retarded beam is heating (that is, doing positive work on the matter in the detector) while the advanced beam is cooling (which corresponds to doing negative work) then as we approach the laser (say) transmitter the beam actually must become undetectable as the net work done on the detector becomes zero. This also is not observed.
Yet still, it is completely proper to say, current electromagnetic theory predicts that time-advanced radiation exists. This seems to me equally important as Einstein's criticism that quantum theory is not complete, which also has never been adequately answered.
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Jim
1607 Posts |
Posted - 20 Apr 2005 : 11:47:05
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| OK now that is done maybe the real issue can be kicked around a bit. The topic of gravity is much more interesting than the models used to predict how it will effect objects. The moon really does orbit the sun and not the Earth but everyone accepts as fact the moon orbits the Earth. So, what are you saying about force here? |
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Jim
1607 Posts |
Posted - 24 Apr 2005 : 15:41:39
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| The last post by "DaveL" dated the 23rd is not posted. Only the name and time of the post is posted. |
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DaveL
USA
15 Posts |
Posted - 24 Apr 2005 : 17:20:44
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| All I did was fix a spelling error; 'locality" in the initial post was misspelled. |
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makis
Greece
259 Posts |
Posted - 25 Apr 2005 : 18:47:22
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quote:
Originally posted by DaveL
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.
In GR, gravity is the effect of spacetime curvature. Bodies in free fall follow geodesic paths and in 4-D spacetime the resulting motion is force-free uniform motion.
Although GR field equations can be solved for the gravity forces in 3-D, the concept of a gravity force does not exist in GR. Actually, GR was conceived for the purpose of eliminating the concept of a gravity force amongst other things.
Thus, it makes no sense to speak of "the speed of gravity forces" in GR, which by postulation is infinite in Newtonian physics.
The whole issue and debate about the speed of gravity has been founded on gross misconception by all parties involved. First, let's understand what gravity is, because nobody knows and then talk about its speed, if it has one.
Cheers
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Jim
1607 Posts |
Posted - 25 Apr 2005 : 23:10:26
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| One of the problems is we can't understand gravity because no body knows anything about it other than its force. And if that is bogus or infinite then even less is known. How does GR elimate gravity force? |
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tvanflandern
USA
2793 Posts |
Posted - 26 Apr 2005 : 00:37:26
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quote:
Originally posted by makis
The whole issue and debate about the speed of gravity has been founded on gross misconception by all parties involved. First, let's understand what gravity is, because nobody knows and then talk about its speed, if it has one.
Trolling is a nasty occupation. You should get up to speed by reading the peer-reviewed, published papers about the speed of gravity at this web site. They contain the latest, unrefuted words on this subject. Especially, learn the distinction between geometric GR and field GR, even though many schools now teach only the former. When you have read this material, understood it, and verified it, you may want to revise half of what you claimed. Some of the opinions you expressed would have sounded right prior to the 1998 paper, but are no longer defensible. -|Tom|-
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makis
Greece
259 Posts |
Posted - 26 Apr 2005 : 09:41:53
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quote:
Originally posted by tvanflandern
quote:
Originally posted by makis
The whole issue and debate about the speed of gravity has been founded on gross misconception by all parties involved. First, let's understand what gravity is, because nobody knows and then talk about its speed, if it has one.
Trolling is a nasty occupation. You should get up to speed by reading the peer-reviewed, published papers about the speed of gravity at this web site. They contain the latest, unrefuted words on this subject. Especially, learn the distinction between geometric GR and field GR, even though many schools now teach only the former. When you have read this material, understood it, and verified it, you may want to revise half of what you claimed. Some of the opinions you expressed would have sounded right prior to the 1998 paper, but are no longer defensible. -|Tom|-
I agree with you that trolling is a nasty occupation. Since I did not troll, I wonder whose trolling you referred to.
I give you a celebrated example of trolling:
http://www.metaresearch.org/cosmology/gravity/spacetime.asp
"Because this point is of some importance, we will illustrate it physically as well. Consider the geodesic (orbital) path of the Earth with respect to the Sun in Figure 1. If we choose any two points along that path (call them A and B), note that a straight line between A and B (as could be represented by a taut rope) is a shorter path through space than the geodesic path. Precisely the same remarks would be true if the Earth were replaced by a photon whose path is bent with respect to space as it passes the Sun – a taut rope takes a shorter path through space than the photon does. The extra bending is most easily explained as a refraction effect in the space-time or light-carrying medium [[ii],[iii]]. This again illustrates that “curved space-time” geodesic paths do not involve any curvature of space."
HaHaHaHaHaHaHaHa. Who is trolling?
Sci.physics.relativity is waiting for you to defend your claims. HaHaHaHaHa
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Explanation of Paradox
