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ToSeek
2002-Sep-04, 06:14 PM
We're going to find out (http://www.missouri.edu/%7Enews/releases/gravitymeasuring.html)

Jigsaw
2002-Sep-04, 06:53 PM
Um...is that a joke? How can you measure the speed of gravity? Isn't that kinda like measuring the speed of the color blue?

Not a mathematician, please don't hurt me. /phpBB/images/smiles/icon_biggrin.gif

GrapesOfWrath
2002-Sep-04, 07:07 PM
On 2002-09-04 14:53, Jigsaw wrote:
Isn't that kinda like measuring the speed of the color blue?

Exactly. 299792458 meters per second (http://www.xs4all.nl/~jcdverha/scijokes/11_4.html#6). Red and green are just as fast.

Wiley
2002-Sep-04, 08:04 PM
On 2002-09-04 14:53, Jigsaw wrote:
Um...is that a joke? How can you measure the speed of gravity?


General relativity predicts that gravity waves propagate at the speed of light. But considering we've never directly detected gravity waves, how are they going to measure the speed of them?

Senor Molinero
2002-Sep-04, 11:18 PM
Check out the article on this topic in today's http://www.cosmiverse.com . Jupiter is about to occult two distant quasars and they are going to measure the time difference in the calculated and observed gravitational refractions.

_________________
Pushing the envelope (into the out-tray).

<font size=-1>[ This Message was edited by: Senor Molinero on 2002-09-04 19:19 ]</font>

overrated
2002-Sep-04, 11:47 PM
It seems to me, conceptually at least, that gravity's effects should be instantaneous. Its influence is determined through the square of the distance between masses, but as long as there is a distance and a mass, the influence is calculable, right? So if an object moved farther away or gained mass, why wouldn't the equation adjust at the exact moment that the distance or mass changed?

Please note: I'm not challenging general relativity. I evidently haven't read the part where the theory predicts gravity waves moving at c, and I'm just looking for an explanation for something that seems intuitive to me.

GrapesOfWrath
2002-Sep-04, 11:57 PM
On 2002-09-04 19:47, overrated wrote:
It seems to me, conceptually at least, that gravity's effects should be instantaneous. Its influence is determined through the square of the distance between masses, but as long as there is a distance and a mass, the influence is calculable, right? So if an object moved farther away or gained mass, why wouldn't the equation adjust at the exact moment that the distance or mass changed?

Noted.

Well, the "equation" would adjust, as soon as you found out about the change, which might be a slight delay, maybe at the speed of light. If it ain't instantaneous.

ToSeek
2002-Sep-05, 01:02 AM
Tom Van Flandern begs to differ (http://www.metaresearch.org/home/Viewpoint/Kopeikin.asp)

xriso
2002-Sep-05, 01:10 AM
On 2002-09-04 19:47, overrated wrote:
It seems to me, conceptually at least, that gravity's effects should be instantaneous. Its influence is determined through the square of the distance between masses, but as long as there is a distance and a mass, the influence is calculable, right? So if an object moved farther away or gained mass, why wouldn't the equation adjust at the exact moment that the distance or mass changed?

Please note: I'm not challenging general relativity. I evidently haven't read the part where the theory predicts gravity waves moving at c, and I'm just looking for an explanation for something that seems intuitive to me.



Well, think about electromagnetic effects. The force between two charges depends on the square of the distance between them, but elecromagnetic changes (light) only propogate at the speed of light. So, if the sun suddenly popped out of existence, it would take 8 minutes for us to see it go away, and we would also keep orbiting it for 8 minutes.

Not exactly a proof, but it makes intuitive sense (at least to me).

Silas
2002-Sep-05, 03:16 AM
One simple test would be to cause a mass to "disappear" suddenly, and see what happens.

But... According to those who know the actual GR equations, it is "forbidden" for a mass to "disappear instantly."

(Just as in Newtonian physics, there is no provision for an object to vanish instantly. It simply can't be modeled under Newton's laws.)

I am not able to do tensor math (yet!) but I think that those who argue against Tom Van Flandern have a much stronger case. TVF refuses to show his work, but just waves his hands and insists a lot. That ain't math.

Silas

David Hall
2002-Sep-05, 12:21 PM
From the article, the test of gravity will be a sort of gravitational displacement test. As Jupiter passes by the quasar, it's gravity will bend the light to a certain angle, and that angle will depend on the speed of gravity.

What I don't understand is how this angle depends on the speed of gravitational propigation. Perhaps they also measure how fast the angle changes as the planet approaches?

traztx
2002-Sep-05, 04:17 PM
On 2002-09-05 08:21, David Hall wrote:
What I don't understand is how this angle depends on the speed of gravitational propigation. Perhaps they also measure how fast the angle changes as the planet approaches?


Here is my guess...
The angle depends on how close the light came to Jupiter. Given the angle, you can calculate the "apparent" position of Jupiter at time T. Compare this with the actual position of Jupiter at that instant and you can calculate the speed of gravity.

I could be wrong though.

nebularain
2002-Sep-05, 04:39 PM
Clarification question from the physics-challenged crowd:
I always thought of gravity as an attractive force; the greater the mass, the stronger the force of attraction. How does speed come into the mix?

Wiley
2002-Sep-05, 05:13 PM
On 2002-09-05 12:39, nebularain wrote:
Clarification question from the physics-challenged crowd:
I always thought of gravity as an attractive force; the greater the mass, the stronger the force of attraction. How does speed come into the mix?


We tend to think that gravitational force always points to the center of mass. This is true if the mass is stationary. Now imagine the mass moved. How long would it take an observer to realize that the mass had moved? The moving mass sends out a gravitational wave, kind of a ripple in the gravitational field, and after the wave hits our observer, the observer will note that the mass has moved. The speed of this ripple is the speed of gravity. Most physicists believe it propagates at the speed of light.

GrapesOfWrath
2002-Sep-05, 06:01 PM
On 2002-09-05 13:13, Wiley wrote:
We tend to think that gravitational force always points to the center of mass. This is true if the mass is stationary.

Not really true, but for these purposes, sure. It's only true for spheres.


Now imagine the mass moved. How long would it take an observer to realize that the mass had moved?

If the mass is moving uniformly, the effect is as if the speed was instantaneous. I thought you discussed this somewhere. The gravity wave is going be caused by something accelerated, like a pair of binary stars. I just realized it would be hard to accelerate a star without another star, close by. D'oh.

Wiley
2002-Sep-05, 06:57 PM
On 2002-09-05 14:01, GrapesOfWrath wrote:


On 2002-09-05 13:13, Wiley wrote:
We tend to think that gravitational force always points to the center of mass. This is true if the mass is stationary.

Not really true, but for these purposes, sure. It's only true for spheres.


Dang. You caught me. I was thinking of stars and planets. I should have been more specific.




Now imagine the mass moved. How long would it take an observer to realize that the mass had moved?

If the mass is moving uniformly, the effect is as if the speed was instantaneous. I thought you discussed this somewhere. The gravity wave is going be caused by something accelerated, like a pair of binary stars. I just realized it would be hard to accelerate a star without another star, close by. D'oh.


The effect (of a uniformly moving mass) is not quite the same as if c<sub>g</sub> were infinite. The direction of the force points to the mass, but strength of the force is delayed. This gets into the subtleties of wave mathematics, which I thought best to avoid.

Yes, accerating masses (should) produce gravity waves. (I tried to imply this with the example of a stationary mass which then moves.) The classic example is the binary pulsar PSR 1913+16, for which Hulse and Taylor (http://www.nobel.se/physics/laureates/1993/) showed was losing angular momentum due energy lost by gravitional radiation.

For those seeking more information see
1.) Eric Weisstein's World of Physics (http://scienceworld.wolfram.com/physics/GravitationalWave.html)
2.)Ripples in Spacetime (http://archive.ncsa.uiuc.edu/Cyberia/NumRel/GravWaves.html)

Paul Best
2002-Sep-05, 11:38 PM
Ok, I'm really rather a novice at this sort of thing. But it would seem to me that it would be rather a big coincidence to me if gravity and light both happen to travel at c. Would this suggest that photons are composed of gravitons?

mallen
2002-Sep-06, 12:14 AM
On 2002-09-05 19:38, Paul Best wrote:
Ok, I'm really rather a novice at this sort of thing. But it would seem to me that it would be rather a big coincidence to me if gravity and light both happen to travel at c. Would this suggest that photons are composed of gravitons?


No... it would just imply that they both follow the same law. This would not mean that they are the same, but they would have at least one property in common.

overrated
2002-Sep-06, 12:29 AM
Yeah... sharing a property (such as speed) does not mean two things are identical. For instance, I am a mammal, but I am not a badger. As far as you know, anyway.

As far as gravity, how about this as an experiment: The moon's distance from Earth is measured with lasers bouncing off reflectors on its surface, right? And we can measure the moon's gravitational attraction on Earth, right?

So: We note if the moon is moving (it's slowly moving away, if I remember right) and see whether the distance measurements, which are moving at c, change before, after or at the same time as the resulting change in its gravitational attraction. Does this sound doable?

DALeffler
2002-Sep-06, 02:16 AM
Is the speed of gravity either c or infinite? Could gravity be slower or faster than c?

Doug.

Cloudy
2002-Sep-06, 02:50 AM
From what I remember(mistakenly or not)......

General Relativity forbids transmiting information through space faster than light. If gravity moves through space (or creates space - gravity and spacetime look like one in the same in General Relativisty)at a speed other than the speed of light, our understanding of gravity is not as complete as we currently think it is. If gravity moves faster than light, it is possible in theory to transmit information faster than light. Einstein's theory assumes this is not so.

This much I know....

If this experiment shows gravity to be instantaneous, it would revolutionize physics and make front-page news. It is expected to show that gravity moves the speed of light or at least nearly so. If it is "nearly so" but not exactly - that may be some help in creating a theory that explains how relativity fits in with the rest of physics.

another interesting link re this experiment:

http://spaceflightnow.com/news/n0209/04gravity/


<font size=-1>[ This Message was edited by: Cloudy on 2002-09-05 22:56 ]</font>

<font size=-1>[ This Message was edited by: Cloudy on 2002-09-05 22:59 ]</font>

Wiley
2002-Sep-06, 11:44 PM
On 2002-09-05 22:16, DALeffler wrote:
Is the speed of gravity either c or infinite? Could gravity be slower or faster than c?

Doug.


I glanced at the derivation of the weak field GR equations last night. For the weak field, or linearized theory, the only propagating gravity waves have a zero length worldline. This means that photon propagating in the same direction as the gravity wave would have a constant phase difference, and this implies that the speed of gravity is the same as the speed of light for the weak field approximation. (My knowledge of GR is not advanced enough to determine a general solution to this, if it even exists.)

I very interested in the details of the proposed experiment. It seems that the precision required for this measurement is extreme. This measurement falls under the weak field approximation of GR, which has been fairly well tested. Precession of Mercury and GPS are both weak field successes. I would be very surprised if they measured the speed of gravity anything besides c. It's the strong field part of GR that has only been lightly tested. When we can start testing GR in strong gravitational fields, I think things will get interesting.



<font size=-1>[ This Message was edited by: Wiley on 2002-09-06 19:48 ]</font>

jaydeehess
2002-Sep-08, 05:31 PM
You cannot make mass "dissappear" but you can move it. You cannot move a planet but you can move a large mass quickly.

Try this experiment. In a deep underground chamber in an area of very little seismic activity, set up two masses one large , on the order of a tonne , the other smaller say one kilogram. Both must be on as frictionless a mount as possible. The gravitational effect of Earth on both masses will not change. The two masses also will exert a gravitational attraction between each other, the force of which can be measured. Now with atomic clocks running shift the position of the large mass quickly ( a mechanism such as that used for launching aircraft off carrier decks comes to mind) by 10% of the original distance separating the two and use the atomic clocks to measure the time difference between the start of movement of the large mass and when the difference in gravitational attraction on the other mass occurred. Do the same experiment with the whole experiment turned 90 degrees, 180 degrees and 270 degrees to average out magnetic feild effects. Run the experiment 100 times to reduce experimental errors and write it up for your master's thesis.(lol)

One would have trouble using the Moon- Earth system to measure this since gravity also affects time dialation(according to GR) so your clock on the Moon and your clock on Earth are not in the same time frame of reference.

Gsquare
2002-Sep-15, 10:25 PM
On 2002-09-06 19:44, Wiley wrote:
I very interested in the details of the proposed experiment. It seems that the precision required for this measurement is extreme. This measurement falls under the weak field approximation of GR, which has been fairly well tested.



You are quite correct, Wiley. The precision, if I remember, is on the order of several picoseconds, several orders of magnitude smaller than the Shipiro gravitational/light delay experiments. And of course, as always, the devil is in the details. I'll see if I can get the original report.

G^2

AgoraBasta
2002-Sep-16, 09:43 AM
Well, guys, gravity is a phenomenologically instantaneous force. In the GR formalism, such instantaneous force is reconstructed through retarded potentials; the residual non-instantaneity results in radiation of gravitational waves (in case of point masses).
Kopeikin opts to understand the "speed of gravity" exactly as the "speed" of those retarded potentials. He doesn't even consider the speed of gravitational force for the simple reason that the standard "geometrodynamical" GR formalism does not consider gravity as force.

So what could that experiment possibly "measure"??? Since they analyse in the GR formalism, they'll get a finite speed close enough to c for those potentials which tells us nothing of the speed of the central force. What can sane people infer from their "result"? Any deviation from c would mean that light is losing/gaining energy in interaction with Jupiter's field through some mechanism unaccounted for in the initial analysis of experimental scheme. We could also get an estimation of the effectiveness of residual red/blue shifts by the means of fields of moving objects.

<font size=-1>[ This Message was edited by: AgoraBasta on 2002-09-16 05:48 ]</font>

Gsquare
2002-Sep-16, 06:47 PM
To Wiley: (and To Seek);
Agora brings up a good point - one that I was going to mention to you:

First, this 'experiment' by Kopeikin provides no information about the speed of the gravitational 'force' per se, and as such cannot be used to obviate Van Flandern's hypothesis of greater than c force transfer.

I am rather surprised that so many physicists can't see what Van Flandern is trying to say; maybe they don't "want" to see. I am not implying I agree with him, but let just get all on the same page as to what we are talking about here.

Actually Van Flandern, as with most others, has no problem with 'gravity waves' and other phenomena (derived from gravitational potential) traveling at c. He freely agrees with such. His discrepancy is with the speed of gravity force.

Here are Van Flandern's own words with regards to Kopeikin experiment:

" Kopeikin uses the expression 'the speed of gravity' for the speed of travel of changes in the gravitational potential field responsible for light-bending and radar/radio signal delay, also known as the speed of gravitational waves. No current dispute exists about this speed, which must be the speed of light (c). The Jupiter-quasar appulse may indeed be the first direct measurement of that speed. By contrast, the appulse can provide no information about the propagation speed of gravitational force...... In general relativity, when the solutions to the Einstein equations (which govern the potential) are converted to equations of motion (which describe gravitational acceleration), the assumption of infinite speed of gravitational force is implicitly adopted by setting aberration in the gradient of the potential equal to zero."- Van Flandern

The last statement (in bold), Wiley, you may want to comment upon. Obviously you cannot expect the experiment to provide a rebuttal to Van Flandern when the mechanics upon which the results are based contains the implied assumption which is in question. /phpBB/images/smiles/icon_biggrin.gif

G^2

P.S Agora: please give Wiley a chance to respond.


<font size=-1>[ This Message was edited by: Gsquare on 2002-09-16 14:48 ]</font>

Wiley
2002-Sep-16, 11:23 PM
On 2002-09-16 14:47, Gsquare wrote:
To Wiley: (and To Seek);
Agora brings up a good point - one that I was going to mention to you:

First, this 'experiment' by Kopeikin provides no information about the speed of the gravitational 'force' per se, and as such cannot be used to obviate Van Flandern's hypothesis of greater than c force transfer.

I am rather surprised that so many physicists can't see what Van Flandern is trying to say; maybe they don't "want" to see. I am not implying I agree with him, but let just get all on the same page as to what we are talking about here.

It's not that physicists can't see what Van Flandern is saying, it's that Van Flandern is completely wrong in his interpretation of the mathematics. The relation between force and potential is very simple: force is the gradient of the potential. (This is only true for scalar potentials, electormagnetics requires vector potentials.) We use potentials for the simple reason scalar problems are easier to solve than vector problems.

So we have f = grad P, or force is the gradient of potential. Let's assume we have a potential wave at a distance R from the source at the time tan be written as P(R,t) = P(t - R/v), where v is the velocity of the wave. The term t - R/v is the "retarded time" and represents the time delay of the wave moving from the source to the observation point. Now take the gradient of P(t - R/v), and you'll find that the force f is also a function of retarded time. In other words force and potential travel at the same speed.

One can also use physical intuition as well as mathematical reasoning. Assume force and potential did travel at different speeds. At the source the potential and force would coincide, but the farther we get from the source, the larger the discrepancy between them. Remember potential is just a convenient method to calculate force, this implies a kind of "action at a distance" for the potential. Somehow the potential at one point can "magically" effect the force at another point. Not only are we not allowed magic in our physics, we are also not allowed to use magic in our mathematics.



.... In general relativity, when the solutions to the Einstein equations (which govern the potential) are converted to equations of motion (which describe gravitational acceleration), the assumption of infinite speed of gravitational force is implicitly adopted by setting aberration in the gradient of the potential equal to zero."- Van Flandern


I will really need more time to look at this, but a couple of things strike me as odd immediately. The Einstein equations don't govern potential, they really are the equations of motion. A component, T_<sub>ab</sub>, stress-energy tensor T physically means there is so much momentum in the a direction flowing in the b direction. I have no idea what he means about setting the aberration to zero. When you take the gradient of anything, there are simply no free variables, let alone ones to set to zero. Remember the gradient is a vector derivative, and with any derivative you lose information. Derivatives don't provide extra variables to play with.

AgoraBasta
2002-Sep-17, 10:39 AM
On 2002-09-16 19:23, Wiley wrote:
So we have f = grad P, or force is the gradient of potential. Let's assume we have a potential wave at a distance R from the source at the time tan be written as P(R,t) = P(t - R/v), where v is the velocity of the wave. The term t - R/v is the "retarded time" and represents the time delay of the wave moving from the source to the observation point. Now take the gradient of P(t - R/v), and you'll find that the force f is also a function of retarded time. In other words force and potential travel at the same speed.

I'd presume that you have read the Carlip's attempt at rebuttal to Van Flandern's claims http://www.arxiv.org/pdf/gr-qc/9909087. Then you should know that the things are not as straightforward as attempted in your naiive analysis. When I try to comprehend the implications of the Carlip's analysis, I arrive at a simplified phenomenological interpretation as follows - the laws of conservation conspire to curve the 4D space so that gravity has no aberration even for uniformly accelerating masses, effectively compensating delays by curvature.
Carlip further states that no experiments had been attempted to directly measure the propagation speed of gravity - that's not exactly so since Walker-Dual experiment attempted exactly that. Carlip further rightfully states that gravitational radiation requires quadrupolar momentum of the source. It seems to me intuitively correct to consider the radiation of waves as a measure of non-instantaneity of any specific field configuration. The Walker-Dual experiment deals essentially with a dipolar scheme, so one could expect an instantaneity of sorts.
Furthermore, I often hear that Walker-Dual measured the "phase velocity" rather than group speed. That's totally untrue! The oscillation of the target in their scheme is delivered by varying gravitational field of the source. Here the gravitational field is the carrier and its variation is the modulating signal, thus the target oscillation is the demodulated signal already delivered at the group speed of the carrier. Moreover, secondary modulation of the modulating signal can produce no finite "group" speed of the double-modulated signal because in order to get differing phase and group speeds of such signal we'd need to have different phase speeds of signals of the first order modulation (the subcarrier). But the first order modulation is measured as good as instantaneous! So two different frequencies of the subcarrier have essentially the same infinite speed, and for this very reason they have no spatial dispersion of phase, which immediately means that the group speed of the subcarrier is infinite as well. In plain words - the phase shift between two adjacent components of modulation spectrum is exactly the same as at the source as at any distance from the source, and that phase shift carries information of secondary modulation. So do we have an instance of causal instantaneous signalling? Seems like YES to me...

Any comments?


<font size=-1>[ This Message was edited by: AgoraBasta on 2002-09-17 06:51 ]</font>

Wiley
2002-Sep-17, 06:33 PM
On 2002-09-17 06:39, AgoraBasta wrote:
I'd presume that you have read the Carlip's attempt at rebuttal to Van Flandern's claims http://www.arxiv.org/pdf/gr-qc/9909087. Then you should know that the things are not as straightforward as attempted in your naiive analysis.


Actually, things really are that simple. The purpose of naive analysis was to show that potential and force are intimately and simply related and that they travel at the same speed. The purpose of Carlip's analysis is to show how aberration cancels. You will note that buried in Carlip's analyis is my analysis. Equations (1.6) and (1.7) where he determines the electric field from the Lienard-Wiechart potentials (with the vector potential caveat of my above post) and equation (3.1) the gravitational force from the a scalar potential both use the "gradient" relation between potential and force. All terms of the radiative field depend on the retarded position, velocity, and acceleration, not the instantaneous values.

AgoraBasta
2002-Sep-17, 06:47 PM
Wiley,

Unfortunately, there's nothing for me to argue with in your post.../phpBB/images/smiles/icon_smile.gif

Wiley
2002-Sep-17, 10:56 PM
On 2002-09-17 14:47, AgoraBasta wrote:
Wiley,

Unfortunately, there's nothing for me to argue with in your post.../phpBB/images/smiles/icon_smile.gif


Don't worry. I have issues with your modulation interpretation of the Walker-Dual experiment. I have not completely understood what your trying to say.

AgoraBasta
2002-Sep-17, 11:42 PM
I believe the real question to ponder is not the speed of gravity or real/imaginary subtleties of the only well-developed theory of gravity we're now limited to, but rather if that theory describes the true real entities of our reality and not the apparitions created by the underlying reality. E.g. - are the space and time really tied up inseparably into a 4D space-time, or is there a more basic physical medium that makes us to produce such a logical induction.
So far this question seems far from being answered. A quantum entity closest to the 3D space, and hence to the 4D space-time, is the quantum vacuum. But we have no real bridge between the Quantum Theory and GR. So we have to deal with relativistic effects with quantum corrections or the other way around - quantum effects with relativistic corrections.
The whole situation simply begs for a consistent theory of quantum aether to bridge to a macroscopic 4D space-time. If we chose this particular way, we must look for any possible evidence of break-up of space-time into its probable constituents.
Generalizing my point, I'd say that we need a microscopic theory that would readily connect to the macroscopic one. On this way both those might get modified somewhat. And I believe the first thing to do must be exactly developing of a reasonable theory of physical space, and a lot of experimental work must be done in this almost virgin area.

AgoraBasta
2002-Sep-26, 11:15 PM
On 2002-09-17 14:33, Wiley wrote:
Actually, things really are that simple. The purpose of naive analysis was to show that potential and force are intimately and simply related and that they travel at the same speed.

It's been some time since our discussion died out, and now I have something to say in objection. The force is a true observable, while potential is not. Calculating potential from the force field data is a problem with non-unique solutions.

Consider an analogy - a cavity moving in a superfluid. If an impulse in conveyed to the cavity by a delta-pulse of force, that impulse is actually conveyed to the superfluid; then a pressure wave starts to move through the superfluid, and the motion of the cavity is further governed by the propagation of that wave rather than simple "impulse divided by effective mass of the cavity". Yet if the force acts slowly, the motion is exactly by the "impulse divided by effective mass of the cavity" relation.

Thus, gravity can be effectively constructed off an instantaneous impulse/momentum exchange by (virtual) instantaneous scalar gravitons (attractive force) and further energy/momentum redistribution by (real) "slow" lightspeed gravitons.

Quite obviously, such a mechanism may be absolutely mathematically equivalent to GR, yet central force is exactly instantaneous.

Wiley
2002-Sep-28, 12:27 AM
On 2002-09-26 19:15, AgoraBasta wrote:
It's been some time since our discussion died out, and now I have something to say in objection. The force is a true observable, while potential is not. Calculating potential from the force field data is a problem with non-unique solutions.

This is misleading because we don't calculate potential from the force, we calculate the force from the potential. For those who have taken calculus, the relation between force and potential is analogous to a function and its integral. The integral of a function is not unique, e.g. x<sup>2</sup>/2+1 and x<sup>2</sup>+2 are both integrals of x. Since force is the gradient of the potential, the constant part of the potential is arbitrary. In other words f = grad P = grad (P + constant). Worrying about "non-unique solutions" for the potential is a red herring.



Consider an analogy - a cavity moving in a superfluid. If an impulse in conveyed to the cavity by a delta-pulse of force, that impulse is actually conveyed to the superfluid; then a pressure wave starts to move through the superfluid, and the motion of the cavity is further governed by the propagation of that wave rather than simple "impulse divided by effective mass of the cavity". Yet if the force acts slowly, the motion is exactly by the "impulse divided by effective mass of the cavity" relation.

How does a "delta-pulse of force" act slowly? An impulse by definition must, compared to its environment, act quickly.

I have no doubt a stable theory of gravity could be made that has instantaneous forces. Newtonian gravity is a prime example. All of them, in order to match observation, require special conditions and exceptions. There is no physics guiding these methods; they are more fitting formulae than theory. GR, Brans-Dicke, and other equivalence principle based theories arise from a simple physical principle and require no special excpetions.

AgoraBasta
2002-Sep-28, 08:34 AM
On 2002-09-27 20:27, Wiley wrote:
This is misleading because we don't calculate potential from the force, we calculate the force from the potential. For those who have taken calculus, the relation between force and potential is analogous to a function and its integral

Consider an example - say, we have gradient field that's smooth over a given spacetime area and we have a potential field solution for that gradient field; if the gradient field is arbitrarily modified in only one (or more finite number of) spacetime point(s), the former solution for the potential field stays valid in its unmodified form. But the point-like spacetime events can communicate non-zero impulse/momentum. Furthermore, force is an observable even in a point-like area, the potential field is not.


How does a "delta-pulse of force" act slowly?

A "delta-pulse of force" and a "force acting slowly" are logically juxtaposed in that example.


I have no doubt a stable theory of gravity could be made that has instantaneous forces. .... There is no physics guiding these methods; they are more fitting formulae than theory.

An arbitrary instantaneous force can be superimposed onto any existing metric theory in a way that does not require any modification of such theory. The only requirement is that the effect of such instantaneous force integrates to zero rather fast, i.e. effect of (instantaneous force plus the gradient of retarded potential) is the same as before over long enough "time", say two periods of retardation. Thus, gravitational interaction can be easily constructed of two components - instantaneous transfer of pure momentum (no energy!) by instantaneous gravitons, then energy and momentum redistribution/exchange through slow retarded potential field by lightspeed-slow gravitons.
As to the "physicality" of such modification - I wouldn't insist, but direction of central force as inferred from GR is suspiciously near-instantaneous.
The only thing that I intended to demonstrate is that truly instantaneous force and GR can come along nicely.

Bob
2002-Sep-28, 03:32 PM
I would simply like to acknowlege this promising bulletin board rookie, AgoraBasta, who signed up less than a month ago but is posting at a record 6.84 posts per day. At this rate he will shatter the career records of beskeptical (6.5 posts per day) and Grapes of Wrath (1986 total posts).

<font size=-1>[ This Message was edited by: Bob on 2002-09-28 14:20 ]</font>

AgoraBasta
2002-Sep-28, 03:45 PM
On 2002-09-28 11:32, Bob wrote:
At this rate he will shatter the career records of beskeptical (6.5 posts per day) abd Grapes of Wrath (1986 total posts).

Don't worry too much /phpBB/images/smiles/icon_smile.gif Once I've communicated all I wanted - I'm gone...

Silas
2002-Sep-29, 12:25 AM
On 2002-09-28 11:45, AgoraBasta wrote:
Don't worry too much /phpBB/images/smiles/icon_smile.gif Once I've communicated all I wanted - I'm gone...


Are you interested in learning?

Silas

Wiley
2002-Sep-29, 09:27 PM
On 2002-09-28 04:34, AgoraBasta wrote:
Consider an example - say, we have gradient field that's smooth over a given spacetime area and we have a potential field solution for that gradient field; if the gradient field is arbitrarily modified in only one (or more finite number of) spacetime point(s), the former solution for the potential field stays valid in its unmodified form.

This is aggressively wrong. If the force changes the potential also changes. I suggest reading a text on potential theory.



A "delta-pulse of force" and a "force acting slowly" are logically juxtaposed in that example.

No, they are not. You claim that a slowly acting force can be decribed using an "impulse", this is seriously bad physics.

A force that varies slowly compared to the response time of the system is effectively instantaneous. This is well known. Newtonian gravity works this way. However your claim was that it is "exactly" instantaneous, and this is just horribly wrong.



The only requirement is that the effect of such instantaneous force integrates to zero rather fast, i.e. effect of (instantaneous force plus the gradient of retarded potential) is the same as before over long enough "time", say two periods of retardation.

Integrates to zero, huh? So the force does no work? Not much of a force, is it?



As to the "physicality" of such modification - I wouldn't insist, but direction of central force as inferred from GR is suspiciously near-instantaneous.

That is only true for a uniformly accelerating source, and the magnitude of the force is retarded. As for you not insisting on a physical justification for a modification, I suppose this explains why you are not physicist.


The only thing that I intended to demonstrate is that truly instantaneous force and GR can come along nicely.

Well, as they say, if at first you don't succeed ...

<font size=-1>[ This Message was edited by: Wiley on 2002-09-29 17:30 ]</font>

Wiley
2002-Sep-29, 09:32 PM
On 2002-09-28 20:25, Silas wrote:


On 2002-09-28 11:45, AgoraBasta wrote:
Don't worry too much /phpBB/images/smiles/icon_smile.gif Once I've communicated all I wanted - I'm gone...


Are you interested in learning?

Silas



Empirical evidence does not suggest this conclusion.

AgoraBasta
2002-Sep-29, 11:01 PM
On 2002-09-29 17:27, Wiley wrote:
This is aggressively wrong. If the force changes the potential also changes. I suggest reading a text on potential theory.

This exactly correct, as long as you agree that surface area and volume of a unidimensional entity is zero. I really expected better understanding of basic mathematics from you...

GrapesOfWrath
2002-Sep-30, 12:00 AM
On 2002-09-27 20:27, Wiley wrote:
The integral of a function is not unique, e.g. x<sup>2</sup>/2+1 and x<sup>2</sup>+2 are both integrals of x.
In what sense?

Wiley
2002-Sep-30, 06:16 PM
On 2002-09-29 20:00, GrapesOfWrath wrote:


On 2002-09-27 20:27, Wiley wrote:
The integral of a function is not unique, e.g. x<sup>2</sup>/2+1 and x<sup>2</sup>+2 are both integrals of x.
In what sense?


First, I erred when I wrote the second function. The functions should be

F<sub>1</sub>(x) = x<sup>2</sup>/2 + 1
F<sub>2</sub>(x) = x<sup>2</sup>/2 + 2.

The derivative of these functions are the same, i.e.

f(x) = (d/dx)F<sub>1</sub>(x) = (d/dx)F<sub>2</sub>(x)

Did I answer your question?

Wiley
2002-Sep-30, 06:27 PM
On 2002-09-29 19:01, AgoraBasta wrote:


On 2002-09-29 17:27, Wiley wrote:
This is aggressively wrong. If the force changes the potential also changes. I suggest reading a text on potential theory.

This exactly correct, as long as you agree that surface area and volume of a unidimensional entity is zero. I really expected better understanding of basic mathematics from you...


Again, this is just wrong. Not even a little bit right. If the gradient field is "arbitrarily modified in only one spacetime point" then this will appear as a discontinuity in the potential. You can not modify the force with out modifying the potential. They are linked.

AgoraBasta
2002-Sep-30, 07:11 PM
If the gradient field is "arbitrarily modified in only one spacetime point" then this will appear as a discontinuity in the potential.

Sorry, you're not even close to truth.
An infinitely high gradient in an infinitesimally small area of space and time adds exactly zero energy to the infinitesimally small area of space (notice *no time*). Thus potential field always leaves the possibility for a finite number of singularities in the force field at a given moment in time.
You really ought to understand such basics...

Wiley
2002-Sep-30, 07:35 PM
On 2002-09-30 15:11, AgoraBasta wrote:


If the gradient field is "arbitrarily modified in only one spacetime point" then this will appear as a discontinuity in the potential.

Sorry, you're not even close to truth.
An infinitely high gradient in an infinitesimally small area of space and time adds exactly zero energy to the infinitesimally small area of space (notice *no time*). Thus potential field always leaves the possibility for a finite number of singularities in the force field at a given moment in time.
You really ought to understand such basics...


Hee, hee.

First, I don't consider delta functions basic. Distributions are an advanced topic.

Second, integrate the following from -1 to 1,
f(x) = delta(x)

Third and most importantly, energy is not potential. Your little force spikes cause a branch cut in the potential. Calculating the energy from the potential has now gotten harder.

AgoraBasta
2002-Sep-30, 10:11 PM
On 2002-09-30 15:35, Wiley wrote:
Hee, hee.

GGG


...integrate the following from -1 to 1,
f(x) = delta(x)

Do you expect anything other than "1"?


Third and most importantly, energy is not potential. Your little force spikes cause a branch cut in the potential. Calculating the energy from the potential has now gotten harder.

As soon as you understand how momentum (and impulse=momentum of force)is different from the energy, you get to understanding how force is different from the potential.

As a primitive example, try integrating a smooth function, then modify that function value at one single point to infinity and again integrate - the result is exactly the same as before modification.

Zathras
2002-Oct-01, 03:11 PM
As a primitive example, try integrating a smooth function, then modify that function value at one single point to infinity and again integrate - the result is exactly the same as before modification.


Depends on how you take the point to infinity, and on what it does when it "gets there." Suppose your initial gradient field=
f(x)= 1/(1+x^2).

Change it now so that you have a delta function at the point x=a.
f(x)= 1/(1+x^2) if x does not = a
f(x)= 1/(1+x^2) * delta(x-a) if x=a.
(Note: this is equivalent to defining f(x)=1/(1+x^2)*(1+delta(x-a)) for all x).

The delta function here is defined according to the theory of distributions, not the primitive way of creating a box and making it infinitely high and narrow, so that there is just one point that is altered.

In this example, the integral of the first field equals pi, whereas the integral of the second equals pi + 1/(1+a^2).

Wiley
2002-Oct-01, 04:03 PM
Let's try a different tact.

Suppose the force-field can have these "spikes". They are instantaneous in both time and space, and most importantly, they integrate to zero. Recall that work is the path integral of the force. But the spikes integrate to zero. This means the "force spikes" do no work and impart no energy. These "force spikes" are totally unobservable. They can't do anything. This leads to the question, what's the point?

AgoraBasta
2002-Oct-01, 04:21 PM
On 2002-10-01 11:11, Zathras wrote:
Change it now so that you have a delta function at the point x=a.

Here's exactly the trap you and, obviously, Wiley fall into.
You forget the dimensionality of your delta. To have a non-zero momentum conveyed, it's enough to have delta(x-x0)delta(y-y0)delta(z-z0)delta(t-t0) where (x0,y0,z0,t0) are the coordinates of singularity...
Your example produces a delta-pulse of force on a sphere of radius "a" rather than in a point (a1,a2,a3,t(a)).

AgoraBasta
2002-Oct-01, 04:46 PM
On 2002-10-01 12:03, Wiley wrote:
They can't do anything. This leads to the question, what's the point?

They can do quite a lot. The physical meaning might be as follows - force can still be communicated instantly, yet the motion of the object is retarded by the inertiality of the medium.
Remember my example with a cavity in superfluid - an instant kick leads to retarded reaction; i.e. momentum is spread through the medium after the instant force-pulse. And, more importantly, information about the direction of force may be derived instantly by an intelligent observer who knows the characteristics of the medium. Furthermore, the carriers of the force may be of the instantaneous virtual kind, yet the energy redistribution can happen by slow retarded entities. So GR is correct, TVF is happy, and the "intelligent observer" may have his/her badly needed instantaneous communication; and they all live happily ever after.

Wiley
2002-Oct-01, 05:29 PM
On 2002-10-01 12:46, AgoraBasta wrote:
...force can still be communicated instantly, yet the motion of the object is retarded by the inertiality of the medium.

No, it can't. Communication implies energy transfer. There is no energy transfer and there is no momentum transfer in your theory.



And, more importantly, information about the direction of force may be derived instantly by an intelligent observer who knows the characteristics of the medium.

Since there is no energy transfer, there is no way for an observer to measure the direction of your force-pulse. Without energy, you can't even change the direction of a photon. You force-pulse is not observable in any fashion.



So GR is correct, ...

Rather overreaching, but okay.


... TVF is happy, ...

And I should care because ...


... and the "intelligent observer" may have his/her badly needed instantaneous communication

Nope, you're wrong here again. See above.

Wiley
2002-Oct-01, 05:35 PM
On 2002-10-01 12:21, AgoraBasta wrote:


On 2002-10-01 11:11, Zathras wrote:
Change it now so that you have a delta function at the point x=a.

Here's exactly the trap you and, obviously, Wiley fall into.
You forget the dimensionality of your delta. To have a non-zero momentum conveyed, it's enough to have delta(x-x0)delta(y-y0)delta(z-z0)delta(t-t0) where (x0,y0,z0,t0) are the coordinates of singularity...
Your example produces a delta-pulse of force on a sphere of radius "a" rather than in a point (a1,a2,a3,t(a)).


Don't listen, Zathras. The f(x,y,z,t) = delta(x)*delta(y)*delta(z)*delta(t) has a 4 dimensional domain and a one dimensional range space. Agora apparently uses delta functions that are different than everybody else's.

AgoraBasta
2002-Oct-01, 06:34 PM
On 2002-10-01 13:35, Wiley wrote:
Don't listen, Zathras. .... Agora apparently uses delta functions that are different than everybody else's.

Exactly! Agora uses delta(t) solely, the spatial spike must have zero volume (but not necessarily zero surface), which condition is easy to satisfy.

Wiley, what's so hard to understand? Where is the kinetic energy of an object stored - definitely not in the object itself since changing ref frame changes its value, thus that energy may be stored exactly in the medium (spacetime, vacuum, aether, whatever). You accelerate an object sinking energy into the medium, then you try to stop it and get that energy back from the same medium. It then becomes easy to understand why you can't push your object faster than c - the medium can't take away the energy any faster and the sink's surface grows infinitely. So inertial and gravitational masses may be considered as artifacts of the medium itself.

Wiley
2002-Oct-01, 08:03 PM
On 2002-10-01 14:34, AgoraBasta wrote:
Exactly! Agora uses delta(t) solely, the spatial spike must have zero volume (but not necessarily zero surface), which condition is easy to satisfy.

This will show up in the potential. Remember GR treats time just like a space dimension.



Wiley, what's so hard to understand?

Why you keep going off on tangents.

You're force-pulses can't effect the outcome of any experiment. They are not observable. Again, what's the point?

AgoraBasta
2002-Oct-01, 10:30 PM
On 2002-10-01 16:03, Wiley wrote:
Remember GR treats time just like a space dimension.

Remember GR allows a consistent introduction of "imaginary" time, so we can decompose spacetime...
It's easy to demonstrate that in a free falling ref frame we can write k(t)(F^2)dt^3=dU, (U-potential, F-force, k(t)-a finite function). In a delta singularity, we have finite Fdt for infinitesimal dt, yet we get finite dU/dt, elsewhere in a small enough spatial volume dU/dt=0. Thus we get no discontinuity in potential; furthermore, the further potential changes are only due to the presence/motion of the test mass.

While it must be impossible to send information without energy, it's still possible to encode info into pre-existing energy flux without changing its density. So your former argument about no info without energy cannot stand either.

Zathras
2002-Oct-01, 11:14 PM
While it must be impossible to send information without energy, it's still possible to encode info into pre-existing energy flux without changing its density. So your former argument about no info without energy cannot stand either.


??
No way. I you are not making <u>any</u> changes in the energy density, everything else will stay the same as well. A change in the momentum will change the energy density at different points. What do you think you can change while leaving T(x,y,z,t) (T=stress energy tensor from fundamental GR equation G=T) unchanged by your input of information.

AgoraBasta
2002-Oct-01, 11:19 PM
On 2002-10-01 19:14, Zathras wrote:
??
No way.

Consider modulating phase and polarization of light.
You may also consider encoding into gravity/light aberration - but that's another can of worms.

Wiley
2002-Oct-01, 11:49 PM
On 2002-10-01 19:19, AgoraBasta wrote:
Consider modulating phase and polarization of light.

In both phase and polarization, you will change the energy density. You don't change the average energy, but the instantaneous energy is changed. A phase modulate signal can be written as cos(w*t + p(t)) where w is the carrier frequency and p(t) is the phase modulation. The energy density is then proportional to cos<sup>2</sup>(w*t + p(t)) which is time dependent.


You may also consider encoding into gravity/light aberration - but that's another can of worms.

And one already shown to be bogus.

<font size=-1>[ This Message was edited by: Wiley on 2002-10-01 19:53 ]</font>

Wiley
2002-Oct-02, 12:03 AM
So let's recap:

Your theory of force-pulses has
1.) no observable characteristics
2.) no physical basis
3.) no mathematical basis
And, perhaps most importantly, explains no physical phenomenon.

I hope you see why I and others have a problem with it.



<font size=-1>[ This Message was edited by: Wiley on 2002-10-01 20:03 ]</font>

AgoraBasta
2002-Oct-02, 11:01 AM
On 2002-10-01 19:49, Wiley wrote:
A phase modulate signal can be written as cos(w*t + p(t)) where w is the carrier frequency and p(t) is the phase modulation. The energy density is then proportional to cos<sup>2</sup>(w*t + p(t)) which is time dependent.

Better write it as exp(iwt+ip(t)). The energy density is then constant.


Your theory of force-pulses has
1.) no observable characteristics
2.) no physical basis
3.) no mathematical basis
And, perhaps most importantly, explains no physical phenomenon.

AFAIK, all existing pnenomenology agrees with instantaneous gravitational force. There's no need to change the existing mathematical apparatus. Only the physical interpretation is modified.

Elsewhere on this BB I proposed a simple and cheap experiment to directly measure the speed of gravity. I believe that scheme is not unique in simplicity. What truly amazes me is that nobody even cares to measure the speed of gravitational force, as if the question were of zero importance.

<font size=-1>[ This Message was edited by: AgoraBasta on 2002-10-02 07:02 ]</font>

GrapesOfWrath
2002-Oct-02, 12:44 PM
On 2002-10-02 07:01, AgoraBasta wrote:
[quote]
Elsewhere on this BB I proposed a simple and cheap experiment to directly measure the speed of gravity. I believe that scheme is not unique in simplicity. What truly amazes me is that nobody even cares to measure the speed of gravitational force, as if the question were of zero importance.
I'm surprised. I would have thought that you, of all people, would. /phpBB/images/smiles/icon_smile.gif

Seriously, tests of the general relativity sort have been next to zero importance for over fifty years, I'm not sure why you'd expect that to change.

You're talking about this experiment (http://www.badastronomy.com/phpBB/viewtopic.php?topic=2219&forum=1), right?

AgoraBasta
2002-Oct-02, 01:30 PM
On 2002-10-02 08:44, GrapesOfWrath wrote:
You're talking about this experiment (http://www.badastronomy.com/phpBB/viewtopic.php?topic=2219&forum=1), right?

Quite right. I've proposed to modify the Walker-Dual scheme to coaxial dipole config and use exceptionally effective signal pickup in the quartz oscillators. Speed of gravity could be measured in a simple desktop experiment.

traztx
2002-Oct-02, 02:43 PM
On 2002-10-02 09:30, AgoraBasta wrote:


On 2002-10-02 08:44, GrapesOfWrath wrote:
You're talking about this experiment (http://www.badastronomy.com/phpBB/viewtopic.php?topic=2219&forum=1), right?

Quite right. I've proposed to modify the Walker-Dual scheme to coaxial dipole config and use exceptionally effective signal pickup in the quartz oscillators. Speed of gravity could be measured in a simple desktop experiment.


Since the speed of light depends on the medium, I wonder if the speed of gravity likewise depends on the medium. Would gravity propagate faster in a vacuum than on the desktop?

AgoraBasta
2002-Oct-02, 03:28 PM
On 2002-10-02 10:43, traztx wrote:
Would gravity propagate faster in a vacuum than on the desktop?

I'd like to find out about many other things related to propagation of gravity... /phpBB/images/smiles/icon_frown.gif

Wiley
2002-Oct-02, 05:37 PM
On 2002-10-02 07:01, AgoraBasta wrote:


On 2002-10-01 19:49, Wiley wrote:
A phase modulate signal can be written as cos(w*t + p(t)) where w is the carrier frequency and p(t) is the phase modulation. The energy density is then proportional to cos<sup>2</sup>(w*t + p(t)) which is time dependent.

Better write it as exp(iwt+ip(t)). The energy density is then constant.

Better not. A time domain signal better not have an imaginary component, i.e. it must be completely real. If you wish to work in the frequency domain, that's fine. But you will have to use delta functions. /phpBB/images/smiles/icon_smile.gif


<font size=-1>[ This Message was edited by: Wiley on 2002-10-02 13:40 ]</font>

AgoraBasta
2002-Oct-02, 07:39 PM
On 2002-10-02 13:37, Wiley wrote:
Better not. A time domain signal better not have an imaginary component, i.e. it must be completely real.

If you want it quite real, you have to imagine two real components ping-ponging the energy while carrying it, like E&B are. The outcome is the same, though. Otherwise you have no travelling wave and no energy transport.

Wiley
2002-Oct-02, 09:39 PM
On 2002-10-02 15:39, AgoraBasta wrote:


On 2002-10-02 13:37, Wiley wrote:
Better not. A time domain signal better not have an imaginary component, i.e. it must be completely real.

If you want it quite real,...

I really don't care if you use frequency or time domain, just as long as you use the correct form. I do recommend the time domain for phase modulation since the math is easier. All time domain responses are completely real; imaginary components are artifacts of the Fourier transform (frequency domain representation) which represent the phase advance or delay at a particlular frequency. The expression exp(i*w*t - i*p(t)) is not correct for the time domain nor the frequency domain.



... you have to imagine two real components ping-ponging the energy while carrying it, like E&B are.

No, you don't. Scalar waves are perfectly capable of carrying a signal. In fact I've phase modulated scalar waves many times.



The outcome is the same, though.

I do agree that you will get the same result regardless of whether you use the time or frequency domain.



Otherwise you have no travelling wave and no energy transport.

Ah, so we agree that phase modulation transfers energy.

<font size=-1>[ This Message was edited by: Wiley on 2002-10-02 17:40 ]</font>

AgoraBasta
2002-Oct-02, 10:30 PM
On 2002-10-02 17:39, Wiley wrote:
Scalar waves are perfectly capable of carrying a signal. In fact I've phase modulated scalar waves many times.

Any wave requires exchange between two or more kinds of energy. A longitudinal pressure wave is perfectly scalar, yet there's an exchange between potential energy of compression and kinetic energy of the medium. So full energy flux is not phase-dependent in absence of dissipation.


Ah, so we agree that phase modulation transfers energy.

If you keep amplitude constant - yes, energy flux is modulated slightly. But you always can modulate phase and amplitude jointly so that energy flux is constant. Have an example - modulation of EM-wave by longitudinal deformation of a waveguide.

Wiley
2002-Oct-03, 08:07 PM
On 2002-10-02 18:30, AgoraBasta wrote:
Any wave requires exchange between two or more kinds of energy. A longitudinal pressure wave is perfectly scalar, yet there's an exchange between potential energy of compression and kinetic energy of the medium.

This is true.


So full energy flux is not phase-dependent in absence of dissipation.

This is false. The total energy is phase independent if the phase is constant. This is not true for a phase modulation scheme.

Communication requires energy transfer.

Wiley
2002-Oct-03, 08:14 PM
On 2002-10-02 07:01, AgoraBasta wrote:
AFAIK, all existing pnenomenology agrees with instantaneous gravitational force.

This is true so far as no measurement has successfully determined the speed of gravity. Hopefully this will soon change.


There's no need to change the existing mathematical apparatus. Only the physical interpretation is modified.

This is a contradiction. The best mathematical apparatus says the speed of gravity is the speed of light (at least in the weak field limit). The interpretation of "instantaneous gravitation force" is incorrect for GR.


<font size=-1>[ This Message was edited by: Wiley on 2002-10-03 16:15 ]</font>

AgoraBasta
2002-Oct-03, 09:56 PM
Wiley,

Existing theories require that no energy travels through real space ftl. Momentum can travel ftl (virtual photons). Teleportation/tunneling is possible. Macroscopic quantum systems can transfer information instantly if, and only if, your definition of information allows it. If you manage to build a very long quantum system, you may then send some limited info instantly, but then you wait for the system to regain coherence in no less than light-time. If you have an unlimited number of modes to decohere, you may transmit as much info as you wish while you wait for the used modes to re-cohere. Like I said - all depends on the definition of information.
Are you ready to declare gravity a non-quantum phenomenon?

AgoraBasta
2002-Oct-03, 11:07 PM
On 2002-10-03 16:07, Wiley wrote:
Communication requires energy transfer.

Communication by a real wave requires energy transfer. Yet it doesn't necessarily require modulation of pre-existing flux of energy.

Gravity in this universe is connected with the mass-energy, so something about the gravity is omnipresent and immutable just as much as mass-energy is conserved. In some way, every piece of mass (every quantum particle) is omnipresent through its inherited gravity. I don't see why should two or more omnipresent entities require time to start interacting, while I do understand why it may take time to finalize the "transaction" in the inertial medium...
So my idea is that all interactions start as instantaneous, and only appear retarded because of interference of medium.

Wiley
2002-Oct-03, 11:17 PM
On 2002-10-03 17:56, AgoraBasta wrote:
Are you ready to declare gravity a non-quantum phenomenon?


Don't try to build a strawman.

My claims have not changed. They are

TVF is wrong about distuingishing the speed of gravitational potential and gravitational force in GR. They are intimately linked and travel at the same speed.
The speed of gravity and the speed of information in (the weak field limit of) GR is the speed of light.




<font size=-1>[ This Message was edited by: Wiley on 2002-10-03 19:19 ]</font>

AgoraBasta
2002-Oct-04, 08:09 AM
On 2002-10-03 19:17, Wiley wrote:
My claims have not changed. They are

TVF is wrong about distuingishing the speed of gravitational potential and gravitational force in GR. They are intimately linked and travel at the same speed.
The speed of gravity and the speed of information in (the weak field limit of) GR is the speed of light.


TVF is still in the process of arguing, here are his own words - In our interpretation of GR, both forces and information can be transmitted much faster than light. In the customary interpretation, neither is possible. The new technical paper, “Experimental Repeal of the Speed Limit for Gravitational, Electrodynamic, and Quantum Field Interactions”, by T. Van Flandern and J.P. Vigier, in Foundations of Physics v. 32(#7), pp. 1031-1068 (2002), deals with precisely this issue, and argues for the new interpretation.
As I pointed out to you, force and potential are very weakly connected and need not have the same speed.

Wiley
2002-Oct-04, 09:48 PM
On 2002-10-04 04:09, AgoraBasta wrote:
As I pointed out to you, force and potential are very weakly connected and need not have the same speed.

No they are very much related. Your "force-pulses" are horrendously non-physical. In a homogeneous environment, the force field must be continuous; where do the magical force pulses come from?

Give me one physical example of where force is not equal to the gradient of the potential.

AgoraBasta
2002-Oct-04, 11:00 PM
On 2002-10-04 17:48, Wiley wrote:
Your "force-pulses" are horrendously non-physical.

My force-pulses are a proof of non-unique correspondence between force and potential. Hence, your "force-potential" argument is mathematically impotent.
Potential is "non-physical", it's mathematical. Force is physical. Physically, potential is built out of force; mathematically - it's the other way around. It's possible to introduce potential for a force field while totally neglecting the physical nature of the force carrier and of the medium. This is exactly what is done in GR, but it still works. All the confirming experiments were done for very long distances and time intervals. It's not impossible that shorter time/distance experiments would produce some "internal structure" of the force that is otherwise averaged out.

Wiley
2002-Oct-04, 11:19 PM
On 2002-10-04 19:00, AgoraBasta wrote:


On 2002-10-04 17:48, Wiley wrote:
Your "force-pulses" are horrendously non-physical.

My force-pulses are a proof of non-unique correspondence between force and potential. Hence, your "force-potential" argument is mathematically impotent.

This is quite untrue. I can use potential theory to solve the problem. From the potential obtain the force. And by uniqueness theorem, that solution is the only solution.


Potential is "non-physical", it's mathematical.

I suggest you look up the Aharonov-Bohm effect.

Regardless, you have not given me the example I asked for. Give me one physical example where force is not the gradient of potential.

Gsquare
2002-Oct-05, 01:19 AM
On 2002-10-04 19:19, Wiley wrote:


On 2002-10-04 19:00, AgoraBasta wrote:


On 2002-10-04 17:48, Wiley wrote:
Your "force-pulses" are horrendously non-physical.

My force-pulses are a proof of non-unique correspondence between force and potential. Hence, your "force-potential" argument is mathematically impotent.

This is quite untrue. I can use potential theory to solve the problem. From the potential obtain the force. And by uniqueness theorem, that solution is the only solution.


Potential is "non-physical", it's mathematical.

I suggest you look up the Aharonov-Bohm effect.


Alright already even. You both seem somewhat entrenched, so let me lossen up the dirt a bit.

First, Agora; Wiley is correct in that Aharonov-Bohm shows potential to be 'physically' observable.
Nevertheless, Wiley, by invoking A-B effect are you not, therefore, required to admit a force arising outside of the field and thus agreeing with Agora on the possibility of a non-unique force arising from potential?
G^2



<font size=-1>[ This Message was edited by: Gsquare on 2002-10-04 23:10 ]</font>

AgoraBasta
2002-Oct-05, 08:10 AM
On 2002-10-04 19:19, Wiley wrote:
I suggest you look up the Aharonov-Bohm effect.I would suggest that quantum effects have non-unique explanations. The particle wave function, in the forbidden area containing B-field in that scheme, is not zero but rather is an evanescent wave; moreover, particle's own fields do affect the border conditions of that forbidden area. You may also check this link (http://arxiv.org/abs/quant-ph/9501012), that deals exactly with Aharonov/Bohm and related effects.
Furthermore, the Aharonov/Bohm effect is basically the same thing that works in every electric transformer (ever used half-turn winds?).
Have another example - light appears redshifted by uniform gravitational potential, but the same result comes out of border conditions for the area where real mass generates that potential.
Anyway, the potential and the energy density of the physical medium are fairly interconnected, i.e. potential delivers the component of force that results from the local medium, but that medium is not necessarily the only carrier of interaction.
Regardless, you have not given me the example I asked for. Give me one physical example where force is not the gradient of potential.Have one - force of pressure in isothermal process. Have another - force of a kick in the... you know where /phpBB/images/smiles/icon_biggrin.gif

<font size=-1>[ This Message was edited by: AgoraBasta on 2002-10-05 06:09 ]</font>

<font size=-1>[ This Message was edited by: AgoraBasta on 2002-10-05 12:20 ]</font>

Wiley
2002-Oct-08, 05:19 PM
On 2002-10-04 21:19, Gsquare wrote:
First, Agora; Wiley is correct in that Aharonov-Bohm shows potential to be 'physically' observable.
Nevertheless, Wiley, by invoking A-B effect are you not, therefore, required to admit a force arising outside of the field and thus agreeing with Agora on the possibility of a non-unique force arising from potential?
G^2

Actually, no. In classical electromagnetics, the potential outside the solenoid is constant; hence, the force is zero. Since an electron passing outside the solenoid is deflected, the force must not be zero. It is not a failing of potential theory but a failing of classical electromagnetics. (Off hand I can't think of another macroscopic failing of electromagnetics although there are several at the microscopic level.) Quantum mechanics, which gives potential physical meaning, explains this effect.

Potential theory is a very useful and very common tool for solving gravity problems and electromagnetic problems. Actually potential theory can be used to solve any partial differential equation that involves either the Laplace operator (http://mathworld.wolfram.com/Laplacian.html) or the Helmholtz operator (http://scienceworld.wolfram.com/physics/HelmholtzEquation.html). It doesn't matter if it is the scalar (as in acoustics), or vector (electromagnetics), or tensor (relativity) form of the operators. One of the most important theorems of modern mathematics is the uniqueness theorem. It says there is only one correct answer to these problems, i.e. the solution is unique. So when we use potentials to derive an answer, we know it is the correct answer. This is true of any "well-posed" (http://csep1.phy.ornl.gov/pde/node6.html) partial differential equation.

Agora's "force-pulses" specifically violate the uniqueness theorem. Hence my claim that they have no mathematical basis.

Wiley
2002-Oct-08, 05:22 PM
On 2002-10-05 04:10, AgoraBasta wrote:
Have one - force of pressure in isothermal process.


You will have to be more specific. Are sure you're not getting pressure gradient and potential gradient confused?

AgoraBasta
2002-Oct-08, 07:16 PM
On 2002-10-08 13:22, Wiley wrote:
Are sure you're not getting pressure gradient and potential gradient confused?Sure I'm sure /phpBB/images/smiles/icon_smile.gif The meaning of this example is to demonstrate that, if system is strongly open, potential cannot be generally inroduced; one needs to know how exactly is that system open to develop a potential-like solution from the force data. I used that example for a good reason - that is that an interaction may happen by two or more mediators whose instantaneity/retardation may generally be arbitrary.
I'd propose that you read the link (http://scienceworld.wolfram.com/physics/HelmholtzEquation.html) you posted and think about this part of it - Although, there are two solutions, if the negative exponent is chosen, the potential changes before the charge moves. This is unphysical, so causality is artificially enforced by discarding the advanced solution, and retaining only the retarded one
. (bolding is mine)
I emphasize once more - after sufficient averaging, instantaneous interactions between objects suspended in a medium of high inertiality look exactly the same as retarded interactions.
As to your beloved theorem - it applies to sufficiently smooth functions only. (BTW, Dirac used the same loophole in his speculations on the magnetic monopole)

AgoraBasta
2002-Oct-08, 07:58 PM
Wiley,
While the Aharonov/Bohm matter is still warm /phpBB/images/smiles/icon_smile.gif , I'd like to point out that it's logically quite possible to consider that effect exactly as a proof of non-unique force-to-potential relation /phpBB/images/smiles/icon_biggrin.gif

Wiley
2002-Oct-08, 08:06 PM
On 2002-10-08 15:16, AgoraBasta wrote:
The meaning of this example ....

Well, you really did not give an example.



if system is strongly open, potential cannot be generally inroduced; one needs to know how exactly is that system open to develop a potential-like solution from the force data.

Again, you really have no idea what you are talking about. Potential theory is often used for open systems. All PDE's have boundary conditions (particulary ones called boundary value problems); for open systems one uses a boundary condition like Sommerfled (http://scienceworld.wolfram.com/biography/Sommerfeld.html) developed.

And perhaps your biggest misunderstanding: Force is derived from potential, not the otherway around.


Although, there are two solutions, if the negative exponent is chosen, the potential changes before the charge moves. This is unphysical, so causality is artificially enforced by discarding the advanced solution, and retaining only the retarded one
. (bolding is mine)

This is a completely beside the point. The represent incoming and outgoing waves. The advanced potential does not satisfy the boundary conditions, and thus it is discarded. The retarded potential satisfies all boundary conditions; hence by the uniqueness theorem, it is the correct solution.



I emphasize once more - after sufficient averaging, instantaneous interactions ...

Actually they can't. You already stated you instantaneous interactions carry no energy. So when you average them you must get zero. Sorry, try again.



As to your beloved theorem - it applies to sufficiently smooth functions only. (BTW, Dirac used the same loophole in his speculations on the magnetic monopole)

What? Now you expect the field solutions to be discontinuous in a homogeneous medium? This falls under my criticism of "no physical basis".


<font size=-1>[ This Message was edited by: Wiley on 2002-10-08 16:09 ]</font>

AgoraBasta
2002-Oct-08, 09:20 PM
On 2002-10-08 16:06, Wiley wrote:
Potential theory is often used for open systems.That's when and if they know how exactly are those open, i.e. the boundary conditions must be known. We do not know how the vacuum is open, we only kinda know how that openness averages out.
The represent incoming and outgoing waves.An instantaneous momentum exchange produces an outgoing (advanced incoming for the interaction counterpart) wave.
You already stated you instantaneous interactions carry no energy. So when you average them you must get zero.By the time they average out, retarded energy exchange already starts.
Now you expect the field solutions to be discontinuous in a homogeneous medium?I wouldn't be so sure of homogeneity of a medium derived from quantum vacuum when point-like events are considered...

Wiley
2002-Oct-08, 09:33 PM
I feel like I just read a creationist's response. Remember Agora, inaccurate attacks on mainstream physics is not proof your theories.

Wiley
2002-Oct-08, 09:34 PM
Oh, you still have not provided the specific example where force is not equal to the gradient of the potential.

AgoraBasta
2002-Oct-08, 09:43 PM
On 2002-10-08 17:33, Wiley wrote:
I feel like I just read a creationist's response.I experience about the same at times. Though it's not very often with your posts.

AgoraBasta
2002-Oct-08, 10:21 PM
On 2002-10-08 17:34, Wiley wrote:
Oh, you still have not provided the specific example where force is not equal to the gradient of the potential.A force delivered by motion and impacts of material objects, i.e. gas pressure within small averaging intervals, some types of friction, forces within systems of purely active impedance and so on... You may argue that some "potentials" may be introduced for such forces by mathematical tricks, but that would be totally unphysical.

JS Princeton
2002-Oct-09, 05:23 AM
Potentials are all totally unphysical. That's what makes them potentials. They are all mathematical tricks.

beskeptical
2002-Oct-09, 05:38 AM
I'm glad to see I'm not the only one to get into these wierd discussions. /phpBB/images/smiles/icon_biggrin.gif

Gsquare
2002-Oct-10, 05:55 PM
On 2002-10-08 13:19, Wiley wrote:


On 2002-10-04 21:19, Gsquare wrote:
First, Agora; Wiley is correct in that Aharonov-Bohm shows potential to be 'physically' observable.
Nevertheless, Wiley, by invoking A-B effect are you not, therefore, required to admit a force arising outside of the field and thus agreeing with Agora on the possibility of a non-unique force arising from potential?
G^2

Actually, no. In classical electromagnetics, the potential outside the solenoid is constant; hence, the force is zero. Since an electron passing outside the solenoid is deflected, the force must not be zero.


Actually then your answer is 'yes', a force does arise outside the 'field' of the selenoid.
You are merely restating (in agreement with) what I said in my post. BUT I think you missed my main point altogether. (Excuse the delayed response... I've been out of town.)


It is not a failing of potential theory but a failing of classical electromagnetics....


Now I know you missed my point.
I think you recognize that my question was not referring to whether A-B effect is from classical or 'potential' theory, and apparently you side stepped the issue altogether. It's your assumption about 'uniqueness theorem' that is in question. So let me restate it this way:

Is it not true that the 'uniqueness theorem' (that you refer to in justifying one-to-one correspondence) is conditional and that one of the conditions is that the surface must be (mathematically speaking) 'simply connected'?
Let's be stright forward:
Yes
No
(choose one)
G^2


<font size=-1>[ This Message was edited by: Gsquare on 2002-10-10 14:09 ]</font>

AgoraBasta
2002-Oct-10, 09:56 PM
On 2002-10-10 13:55, Gsquare wrote:
Let's be stright forward:
Yes
No
(choose one)Gsquare,
That's like asking - "And what's going to be your positive answer, dear?" /phpBB/images/smiles/icon_wink.gif

Wiley
2002-Oct-10, 10:03 PM
Wow, there are other people besides myself and Agora reading this thread. Now that I know I'm being watched, I'll try to behave. /phpBB/images/smiles/icon_smile.gif



On 2002-10-10 13:55, Gsquare wrote:
Actually then your answer is 'yes', a force does arise outside the 'field' of the selenoid.
You are merely restating (in agreement with) what I said in my post. BUT I think you missed my main point altogether. (Excuse the delayed response... I've been out of town.)

Let me rephrase my previous post and I hope I clarify things a wee bit. In classical EM you can solve for the field outside the solenoid in (at least) two manners. First, you can calculate the B field directly; second, you can calculate the potential and from the potential calculate the field. Both methods will give the exact same answer, the field outside the solenoid is zero. This is uniqueness at work; potential theory must give exactly the same answer as direct calculation of the B field.

However, experiment shows that the force outside of the solenoid is non-zero. (Recall in EM, force is proportional to |B|.) Thus our answer is wrong. It is unique, but it is also wrong. Thus my previous statement:

It is not a failing of potential theory but a failing of classical electromagnetics....



On 2002-10-10 13:55, Gsquare wrote:
It's your assumption about 'uniqueness theorem' that is in question. So let me restate it this way:

Is it not true that the 'uniqueness theorem' (that you refer to in justifying one-to-one correspondence) is conditional and that one of the conditions is that the surface must be (mathematically speaking) 'simply connected'?
Let's be stright forward:
Yes
No
(choose one)
G^2


I'm not sure exactly sure what "one-to-one correspondence" you are referring to. There is definitely not a one-to-one correspondence between force and potential. Force is the gradient of potential (for the Laplace operator); hence potentials that differ by a constant will yield the same force. I don't claim the potential is unique. I only claim that the force/field is unique.

As to whether the uniqueness theorem holds for mulitply-connected domains, the simple answer is yes, it holds. (I think this is the "no" answer to your question. Questions starting with "Is it not true ..." always confuse me. /phpBB/images/smiles/icon_smile.gif) At least for the Laplace and Helmholtz operators, which are the ones where potentials are used to solve. There are about thirty gazillion different uniqueness theorems. For every problem, people try to develop uniqueness theorems because these theorems not only tell us that there is only one solution, but also what we need to get the solution. For instance, in electromagnetics if the domain is lossy, then to obtain a unique solution you can either specify tangential electric or the tangential magnetic field at the boundary. But if the domain is lossless, you must specify both the tangential electric and the tangential magnetic fields.

I hope this answers your question.


<font size=-1>[ This Message was edited by: Wiley on 2002-10-10 18:07 ]</font>

Wiley
2002-Oct-10, 10:16 PM
On 2002-10-08 18:21, AgoraBasta wrote:A force delivered by motion and impacts of material objects, i.e. gas pressure within small averaging intervals, some types of friction, forces within systems of purely active impedance and so on...

Ah, very good, Grasshopper. Now, why don't we use potentials to solve these types of problems?


You may argue that some "potentials" may be introduced for such forces by mathematical tricks, but that would be totally unphysical.

You've already claimed that potentials are unphysical, so this is an invalid line of argument.

GrapesOfWrath
2002-Oct-11, 12:54 AM
On 2002-10-10 18:03, Wiley wrote:
Wow, there are other people besides myself and Agora reading this thread. Now that I know I'm being watched, I'll try to behave.
Eye nose all, seize all.

AgoraBasta
2002-Oct-11, 09:05 AM
Wiley,

Trivial calculations prove that, arbitrary modification of any existing force/potential solution is a spatial area of lower dimensionality than that of the solution, delivers a solution just as valid. This is absolutely correct if modification area doesn't have common points with the boundary. When intersection with the boundary is desired for a modification area, the same considerations may be applied to the area of modification - i.e. as long as intersection of the boundary with the area of modification is of lower dimensionality than that of both boundary and the area of modification, the modification can be nearly smoothly extended beyond the boundary (by "writedown" of an area of yet smaller dimensionality from the area of modification). Such or similar considerations may be applied to any attempt of proof of any relevant "uniqueness theorem" out there and are a fundamental limitation of applicability of such theorems. Gsquare's question relates to exactly the same kind of limitation.

P.S. You systematically avoid discussing perfectly valid points brought up by me, while I try to address the points of dubious relevance brought up by you. If you still insist on calling me a Grasshopper, you must then assume a Weasel-Grasshopper-Crossbred characteristic of yourself. /phpBB/images/smiles/icon_lol.gif

traztx
2002-Oct-11, 04:41 PM
On 2002-10-11 05:05, AgoraBasta wrote:
If you still insist on calling me a Grasshopper...


Have you ever seen the TV series "Kung Fu"?

AgoraBasta
2002-Oct-11, 05:26 PM
On 2002-10-11 12:41, traztx wrote:
Have you ever seen the TV series "Kung Fu"?What'd be that - Chinese TV? /phpBB/images/smiles/icon_smile.gif

bob7708
2008-Feb-14, 07:17 AM
Xriso:
Well, think about electromagnetic effects. The force between two charges depends on the square of the distance between them, but elecromagnetic changes (light) only propogate at the speed of light. So, if the sun suddenly popped out of existence, it would take 8 minutes for us to see it go away, and we would also keep orbiting it for 8 minutes.

Not exactly a proof, but it makes intuitive sense (at least to me).

Makes sense to me also.

Neverfly
2008-Feb-14, 07:27 AM
Xriso:

Makes sense to me also.

Welcome to BAUT bob7708.

This is one of those questions that keeps popping up.

01101001
2008-Feb-14, 07:28 AM
Note to others: thread revival, one long asleep.


Makes sense to me also.

bob7708: Welcome to the BAUT Forum.

Occams Ghost
2008-Feb-18, 05:22 AM
If there is a fundamental graviton, then shouldn't it move at lightspeed to make sure it can reach massive distances in space and time? I've even heard arguements that it might be a superluminal particle, and such particles are predicted to be very difficult to locate.

CodeSlinger
2008-Feb-18, 06:49 AM
AFAIK, gravitons ARE predicted to travel at the speed of light. Who argues that it might be a superluminal particle? I'd like to see your citations please.

Neverfly
2008-Feb-18, 06:55 AM
Warpage can "move" at faster than light speeds quite easily.
This is part of the reason I find uncertainty in the graviton idea.