Freethought & Rationalism Archive

Jesse · 10-05-2002, 09:21 AM

Jesse:
Van Flandern says one thing when presenting his side, Carlip says something different when presenting his. How can we know who is correct?

Intensity:
It gives me great pleasure to read that beautiful and elegant question. Getting you to this indecisive, somewhat neutral position as concerns this controversy was an uphill task.
Phew! (A while ago, you would have gladly placed your bet on the side with the majority without posing that question)

Sigh. Intensity, I have been emphasizing this all along--that was the whole reason I kept making the point that it would be impossible to gain much new insight without a detailed understanding of the mathematics of GR, a point which you kept disagreeing with me on. Sorry, but until either one of us has this background, there's no way to settle this. We each have our own intuitions--I, for example, think that when dealing with a purely mathematical question about what the equations of GR do or do not predict, the opinion of hundreds of experts in the subject is more likely to be correct than the opinion of one maverick. Obviously your own intuitions, on the other hand, are heavily weighted towards believing Van Flandern for some reason.

Jesse:
the original comment still does not make sense.

Intensity:
Are you accusing him of being incoherent? I think his point was that physicists use analogies that rely on eucledian space and when pressed to explain why a straight spatial distance is not the shortest path, they get confounded and resort to obfuscation instead of underlining the fact that space in GR terms is manifold with a metric" and that there is already a well-defined means of defining paths between points and measuring their distance - not just in terms of lines.

It's possible that is what he meant, although it looked to me like he was talking about a problem with GR itself, not just a problem with physicists' understanding of it. Perhaps you should ask him next time you write to him. In any case, that would still be a pretty silly argument--I doubt you will find many physicists who are "confounded" by the fact that it is spacetime distance (proper time) that is minimized in GR, not spatial distance.

Intensity:
Incidentally he is not the only one.

Read the quote below:

Quote:

Do physicists misunderstand GR? Emphatically yes. Many, perhaps most, learned physicists do not understand GR. The most common interpretation in physicists' minds is that GR predicts that space gets curved by presense of mass. In curved space, straight lines get bent, therefore object fall to earth rather than flying away in a straight line. This is a completely incorrect understanding of GR ... GR does indeed have a spatial curvature component, but it is so miniscule that it couldn't make a speck of dirt fall to the earth. The falling in the GR actually comes from spacetime-curvature, not from space-curvature [Intensity: as the rubber sheet analogy misleads us to think]. And yet there have been interpretations of GR such as "matter tells space how to curve, space tells matter how to move." By claiming that space curvature is the fundamental agent for gravitational movement, this phrase clearly illuminates the fundamental lack of understanding of GR that physicists suffer from.

Says this page

<shrug> I still think it's a silly claim. This guy is another proponent of "alternative" views on relativity and QM, and of course most of them are going to claim that physicists are failing to understand one thing or another. I would like one example of a real relativist who exhibits this sort of elemantary confusion between spatial distance and spacetime distance--that's the only thing that would convince me the argument had some merit.

Jesse:
It seems like sloppy argumentation, just like his comments about what is pulling objects "down" in the rubber sheet model.

Intensity:
You obviously did not understand his argument. Let me first post the salient parts:

Quote:

A gradient, a slope, or a curvature cannot induce a body at rest in a field to begin moving unless a force acts on that body. This is because, without a force to initiate motion, the body has no reason to choose any one direction over any other direction...In the rubber sheet analogy, if the small target body is at rest on the side of a dent caused by a source mass, it will remain at rest forever until some force acts. The rubber sheet analogy works in our imaginations only because we instinctively imagine gravity under the rubber sheet, with its pull providing a meaning to the concept of "downhill". But without pre-existing gravity, the small body has no cause to accelerate, either on the rubber sheet or in the geometrical interpretation of GR. The causality principle is violated.

This causality principle is violated because if the rubber sheet and balls were to be placed in an inertial frame (as it should be), the ball would not move at all (no force to propagate it) so the analogy violates the causality principle (which requires an absolute cause-consequence order of events). I would also add its misleading because one can think that space curvature is what makes the ball go down (which is true but misleading).

But if that�s your interpretation then I think Van Flandern is taking the rubber sheet model too literally, just as I have been saying from the beginning. A ball on a rubber sheet needs a force to move it, but in relativity there are no higher-dimensional forces moving things along spacetime (in fact there is no movement whatsoever, since we are talking about paths in spacetime, not paths in space). It�s just an axiom of relativity that the paths of objects are geodesics in curved space, there is no need for a meta-explanation in terms of "forces" any more than the fact that objects move in straight lines at constant velocities in Newtonian physics needs a meta-explanation too (note that in Aristotelian physics, constant-velocity straight line motion did require a constant force�applying Aristotelian intuitions to classical physics and asking �what force makes them do that?� would be just as inappropriate as applying Newtonian intuitions to GR as Van Flandern does).

If Van Flandern was just saying the rubber-sheet analogy was a misleading pedagogical tool, I�d agree. But from his comments on this subject he clearly indicates that he thinks the analogy demonstrates genuine problems for the theory of GR itself, which to me sounds like nonsense.

Jesse:But since this directly contradicts the Carlip quote I posted above, I suspect that he is contrasting theories of gravity based on GR with theories of gravity that are not, otherwise his comment that "There is a rigorous proof that in GR, no gravitational influence propagates faster than c" would not make sense.

Intensity:
It makes total sense. It means he revised his earlier position. His position has changed, while you are here struggling to understand (or is it "accept"?) that.

His position changed within the span of one paragraph??? Why wouldn�t he go back and edit his earlier comment, then?

Intensity:
He said FTL speeds are possible. End of Story . (His vague allusion to "being possible in some theories" obviously meant FTL speeds are possible with GR and LR but not with SR - his "theres not much more to say" is evocative of a man who has resigned himself to the state of affairs).

Tell you what, Intensity. Why don�t you email Carlip and ask him whether this comment means that gravity can propogate FTL in GR, or if he was referring to non-GR theories. I would be willing to place a large bet on the prediction that he will continue to argue that gravitation propogates at c within GR, period.

Jesse:
The fact that a single equation can have multiple solutions should not be surprising--think of x^2 = 9, for example, which can be solved by either x=3 or x=-3. In GR, multiple solutions would presumably involve different possible shapes of spacetime that satisfy the Einstein field equations as well as whatever other constraints are being considered in the problem at hand

Intensity:
It quite disapointing that you can use a simplistic linear equation of x^2 to state the incorrect proposition that "a single equation can have multiple solutions". This is not what the argument is about. Its not about having multiple solutions. Please, and these equations are non-linear. Note that please.

Now you�re really talking complete nonsense, Intensity. For one thing, x^2 = 9 is a nonlinear equation. Linear equations of one variable all take the form a*x + b = 0�see the note at the bottom of <a href="http://cne.gmu.edu/modules/dau/algebra/equations/linear1_bdy.html" target="_blank">this</a> page, for example.

Also, here�s a page verifying that in some cases the equations of GR have multiple solutions which are nonetheless physically equivalent:

<a href="http://www.lsa.umich.edu/philosophy/Sklar.htm" target="_blank">http://www.lsa.umich.edu/philosophy/Sklar.htm</a>

Quote:

The idea that physical theories ought to be founded upon undefined notions only if those notions are properly "local" is also implicitly at the heart of Einstein�s second great physical theory of spacetime, the theory of general relativity. That this is the case was not so apparent at the inception of the theory. The theory arose out of an attempt to find a relativistically acceptable theory of gravitation. Einstein�s initial motivations came primarily from noting the degree to which gravitational force was equivalent to viewing phenomena from the point of view of an accelerated reference frame, and from the desire to find a theory that would conform to Mach�s relationism. That ideas of "going to the local" were playing a crucial role was only realized later.

Faced with his discovery of the "hole" argument, the fact that the theory seemed to allow multiple possible curved spacetimes in a region devoid of non-gravitational mass-energy for a single fixed set of boundary conditions, Einstein worried that his theory failed, inappropriately, to be deterministic. His way out was to show that the multiple solutions all led to the same intersections of paths of test particles and light rays outside the "hole" and to the same proper time measurements on clocks transported through the hole. [ie the multiple solutions yielded identical physical predictions�Jesse] And, it could be argued, once again, that only these relationships among material test objects "at a point" counted as the physical facts the theory needed to save. The multiple solutions to the equations could then be viewed as simply alternative representations of the true factual contents of the theory.

You can find more discussions of this point if you do a google search for "general relativity" and "hole argument" and "solutions." Apparently the "hole argument" was a proof by Einstein that the stress-energy tensor did not uniquely determine the metric tensor (ie multiple solutions were possible), but he later realized that the different solutions were actually physically identical and only "formally" distinct.

Intensity:
Lets review what Flandern said:

Quote:

Of course, GR has selected one particular form for the field equations, whose solution leads to the Schwarzschild metric or other equivalent metrics, out of a potentially infinite number of possibilities. [Intensity: its about multiple approaches to the solution NOT multiple solutions]

I realize that Van Flandern claims there are multiple possible forms for the field equations (a claim which is disputed by other relativists like Carlip) but I don�t see him commenting one way or another on whether, once you have picked a set of field equations, there can be multiple solutions for the metric tensor given a particular stress-energy tensor.

Quote:

Having made its choice, there are no further adjustable parameters in the GR field equations [Intensity: which is the platform from which Carlip was arguing] . But when Einstein was originally trying to pick the right form for the field equations, he did have many choices [Intensity: but his choices were dictated by what parameters brought the correct precession of the perihelion of mercury (5600 seconds per century?) - which Einstein knew beforehand].

That is Van Flandern�s claim. On the other hand, Carlip says:

Quote:

>The real "postulates" of general relativity have virtually nothing
: >in common with this description. They are (in one version) that
>
: >1) spacetime is a four-dimensional Lorentzian manifold (that is,
: > a four-manifold with a pseudo-Riemannian metric of signature 2);
: >2) there is no "prior geometry," i.e., no background geometric
: > structures apart from those determined by the metric;
: >3) the gravitational Lagrangian is built from the metric and its
: > curvature, and is a density (so the gravitational action is
: > coordinate-independent);
: >4) the action contains no more than two derivatives of the metric.

Carlip is saying that given these assumptions, the form of the Einstein field equations follows uniquely (of course, even once you know the �form� of the equation there may still be adjustable parameters, but according to Carlip the only one in general relativity is G, the gravitational constant).

Quote:

In fact, Alley often quotes Einstein with words to the effect that "The left-hand side of the field equations is like a fine marble; but the right-hand side consists of perishable wood." [Intesity: these are non-linear equations, thats why your x^2 is terribly off the mark]

As I mentioned earlier, this comment is a non-sequitor. I don�t know where you got this idea about x^2=9 being linear or the idea that non-linear equations have a unique solution, but you�re simply wrong on this point.

Quote:

The Yilmaz variant is an example of another choice, in which field stress energy is added to matter stress energy on the right-hand side. [Intensity: this is evidence, if you question it, please find out about the factuality of this claim]

Sure, according to <a href="http://dmoz.org/Science/Physics/Relativity/" target="_blank">this</a> page "Yilmaz discovered an alternative theory of gravitation which surmounts some of the defects of general relativity. The solutions of the field equations contain neither space-time singularities, or, equivalently, there are no black holes." In other words, the "Yilmaz variant" is not a version of GR, it is an alternative to GR. Presumably it would violate one of the 4 postulates that Carlip listed above�according to Carlip, given those postulates, there is only one possible set of equations that will satisfy all of them.

Jesse:
When considering whatever constraints apply to the perihelion-of-Mercury problem, would multiple solutions occur here as well?

Intensity:
Come on Jesse, we dont need Carlip to figure out this one for us. The only thing that I keep asking myself are: did he really know thre precession of the perihelion of mercury? And if he did, how did he know it?
But if we agree that he did know it beforehand, then its clear that whatever metrics, parameters and equations ane mathematical structures he used, he chose those that conformed to what he already knew.

No, that�s not clear at all. It might still be (and in fact I think all mainstream physicists would assert this) that the need to satisfy certain theoretical postulates, such as the equivalence principle, forced him to a unique set of equations. The situation is similar to that of Newton, who already knew Kepler�s "laws" of planetary orbits before coming up with Newtonian gravity and showing that it predicted exactly those laws�you�re not accusing him of "jiggering the equations" too, are you?

Here is another page giving the postulates of GR (seemingly in a slightly less rigorous form than Carlip gave though):

<a href="http://people.hofstra.edu/faculty/Stefan_Waner/diff_geom/Sec13.html" target="_blank">http://people.hofstra.edu/faculty/Stefan_Waner/diff_geom/Sec13.html</a>

Quote:

A1. Locally, M is Minkowski spacetime (so that special relativity holds locally).

A2. Freely falling particles move on timelike geodesics of M.

A3 (Strong Equivalence Principle) All physical laws that hold in flat Minkowski space (ie. "special relativity") are expressible in terms of vectors and tensors, and are meaningful in the manifold M, continue to hold in every frame (provided we replace derivatives by covariant derivatives).

Intensity:
I know he used Riemannian geometry, he tried Eucledian Geometry and Gaussian geometry and with the help of Grossman (his friend and a great mathematician), he developed a general theory of relativity (based on the "Einstein field equations") on what was now called "Gaussian Geometry" .

Sure, but I think the reason he tried different geometries is because he was looking for one that would allow him to satisfy his basic postulates. He wasn�t just trying different geometries so he could fine-tune things to get an accurate perihelion-of-Mercury prediction.

Jesse:
Pauli is saying they would all yield the same prediction for the actual amount of the perihelion we observe, which means that Einstein could not change his prediction by picking a different solution.

Intensity:
NOTE THAT he is talking of the Einstein field equations (a discrete set) not any other equations (of course they have to give "equivalent" predictions - thats why Einsten chose and modeled them!)

Sure, but I thought you were using the Pauli quote to support the idea that within general relativity there are free parameters that can be adjusted, not just that there are other theories of gravity that make different predictions.

And that�s not "why" Einstein chose and modeled them, he chose them because of the basic postulates he was making about things like the equivalence principle and reducing to SR in local regions. In fact the page I mentioned earlier indicates he was almost ready to give up when he found that the "hole argument" seemed to imply solutions to his equations were not unique, until he discovered that these solutions were only formally different but physically equivalent, as Pauli said.

Intensity:
I hope you now have a better understanding of the thrust of this argument concerning the equations and parameters.

Likewise.

[ October 05, 2002: Message edited by: Jesse ]

Jesse · 10-05-2002, 10:16 AM

Quote:

Jesse:
The fact that a single equation can have multiple solutions should not be surprising--think of x^2 = 9, for example, which can be solved by either x=3 or x=-3. In GR, multiple solutions would presumably involve different possible shapes of spacetime that satisfy the Einstein field equations as well as whatever other constraints are being considered in the problem at hand

Intensity:
It quite disapointing that you can use a simplistic linear equation of x^2 to state the incorrect proposition that "a single equation can have multiple solutions". This is not what the argument is about. Its not about having multiple solutions. Please, and these equations are non-linear. Note that please.

Now you�re really talking complete nonsense, Intensity. For one thing, x^2 = 9 is a nonlinear equation. Linear equations of one variable all take the form a*x + b = 0�see the note at the bottom of this page, for example.

Actually my discussion of linear vs. nonlinear there was a little oversimplified. I believe the more general definition of what it means for a function f(x) to be "linear" is this:

f(ax + by) = a*f(x) + b*f(y)

(for any choice of constants 'a' and 'b' this should hold for all values of 'x' and 'y')

The fact remains, though, that contrary to Intensity's assertions the function x^2 is definitely nonlinear, and that it is plainly false that nonlinear equations must have a unique solution.

[ October 05, 2002: Message edited by: Jesse ]

Steven S · 10-05-2002, 04:10 PM

Quote:

Originally posted by Jesse:

Sigh. Intensity, I have been emphasizing this all along--that was the whole reason I kept making the point that it would be impossible to gain much new insight without a detailed understanding of the mathematics of GR, a point which you kept disagreeing with me on. Sorry, but until either one of us has this background, there's no way to settle this. We each have our own intuitions--I, for example, think that when dealing with a purely mathematical question about what the equations of GR do or do not predict, the opinion of hundreds of experts in the subject is more likely to be correct than the opinion of one maverick. Obviously your own intuitions, on the other hand, are heavily weighted towards believing Van Flandern for some reason.

Yes, why are you interested in Van Flandern's work, Intensity? Van Flandern has convinced people who do understand GR that he doesn't know what he's talking about. Wouldn't your time be spent actually learning some relativity from one of the many good books available than trying to convince people of the incorrectness of a subject you do not understand?

Steven S

[ October 05, 2002: Message edited by: Steven S ]

atrahasis · 10-06-2002, 10:00 PM

Van Flandern has convinced people who do understand GR that he doesn't know what he's talking about.
Can you please support this claim!

Wouldn't your time be spent actually learning some relativity from one of the many good books available than trying to convince people of the incorrectness of a subject you do not understand?
What makes you think that those books are always right, what if TVF is right all along....

Jesse · 10-06-2002, 10:15 PM

Van Flandern has convinced people who do understand GR that he doesn't know what he's talking about.
Can you please support this claim!

Note that Steven S's claim is not "Van Flandern doesn't know what he is talking about," but rather "experts in GR are convinced he doesn't know what he is talking about." To verify that claim, just look at any example of a debate between Van Flandern and a GR expert, or check the opinion of any GR expert on questions like the speed of gravity in GR.

Wouldn't your time be spent actually learning some relativity from one of the many good books available than trying to convince people of the incorrectness of a subject you do not understand?
What makes you think that those books are always right, what if TVF is right all along....

Either way the advice is sound. Whether GR is right or wrong, the only way to follow the debates about what GR does or doesn't imply is to have an in-depth understanding of the mathematics involved. Can't get that knowledge without hitting the books.

[ October 06, 2002: Message edited by: Jesse ]

Ted Hoffman · 10-07-2002, 03:08 AM

I have decided to abandon this discussion.

Perhaps I will have to hit the books after all bacause we have dropped Le Sages gravity along the way and are now revolving around controversies.
Thank you all for your contributions. I beleive I have a fairly good grasp of what GR says, I now want to embark on the math - from the basics - what were those introductory books again?

Just a few quick responses though:

Jesse Sorry, but until either one of us has this background, there's no way to settle this.

Intensity In a way, yeah...

Jesse ...But from his comments on this subject he clearly indicates that he thinks the analogy demonstrates genuine problems for the theory of GR itself, which to me sounds like nonsense...

Intensity His contention is that geometric interpretation of GR violates causality principle, and that with the field interpretation, it depends.

Jesse His position changed within the span of one paragraph??? Why wouldn’t he go back and edit his earlier comment, then?

Intensity He needs 10 paragraphs to change his position? Was he even aware that we had examined his earlier comments? No. So there was no need to address whatever he'd said earlier because he was not aware it was in the context of our discussion.

Thanks everyone.

[ October 07, 2002: Message edited by: Intensity ]

Answerer · 10-07-2002, 04:12 AM

Well, I always thought that an object will not move in regard to a stationary frame because of the symmetry of spacetime surrounding it. And that the movement of a stationary object was induced or caused as a result of the asymmetry of spacetime surrounding it( that means there is a curvature of spacetime near it). So guys, am I wrong and if so, whats the actual reason or cause for the induced motion of a stationary object when placed near a gravitional field?

Jesse · 10-07-2002, 04:52 AM

Intensity:
Thank you all for your contributions. I beleive I have a fairly good grasp of what GR says, I now want to embark on the math - from the basics - what were those introductory books again?

One of the best really complete textbooks you could look at would be <a href="http://www.amazon.com/exec/obidos/ASIN/0716703440/internetinfidelsA" target="_blank">Gravitation</a> by Wheeler, Misner, and Thorne. For an introduction you're probably better off with something like <a href="http://www.amazon.com/exec/obidos/ASIN/0521277035/internetinfidelsA" target="_blank">A First Course in General Relativity</a> by Schutz or <a href="http://www.amazon.com/exec/obidos/ASIN/0387942955/internetinfidelsA" target="_blank">A Short Course in General Relativity</a> by Foster or
<a href="http://www.amazon.com/exec/obidos/ASIN/0070334846/internetinfidelsA" target="_blank">Schaum's Outline of Tensor Calculus</a> by Kay or <a href="http://www.amazon.com/exec/obidos/ASIN/020138423X/internetinfidelsA" target="_blank">Exploring Black Holes</a> by Wheeler and Taylor, but I don't think these introductory books cover more advanced issues like the speed of gravitation in GR.

Jesse:
...But from his comments on this subject he clearly indicates that he thinks the analogy demonstrates genuine problems for the theory of GR itself, which to me sounds like nonsense...

Intensity:
His contention is that geometric interpretation of GR violates causality principle, and that with the field interpretation, it depends.

I don't know the details of what is meant by geometric interpretation vs. the field interpretation--do both "interpretations" lead to identical physical predictions? If so how can one violate causality and the other not? Either you can send information faster than light or you can't, there's no gray area (unless Van Flandern means something different by 'causality' than most physicists).

Jesse:
His position changed within the span of one paragraph??? Why wouldn�t he go back and edit his earlier comment, then?

Intensity:
He needs 10 paragraphs to change his position? Was he even aware that we had examined his earlier comments? No. So there was no need to address whatever he'd said earlier because he was not aware it was in the context of our discussion.

I can't understand what you're saying in the paragraph above. What possible "context" could make the comment "There is a rigorous proof that in GR, no gravitational influence propagates faster than c" compatible with the idea that GR can sometimes allow gravitational influence to propogate faster than c?

If you're not going to email Carlip, I will. What's his email address? Feel free to offer any suggestions about how I should phrase the question, just so there's no ambiguity.

Ted Hoffman · 10-07-2002, 07:41 AM

carlip@dirac.ucdavis.edu

Ted Hoffman · 10-07-2002, 08:04 AM

Thanks for the book titles Jesse.

Jesse do both "interpretations" lead to identical physical predictions? .

Intensity I don't think so.

Jesse If so how can one violate causality and the other not? Either you can send information faster than light or you can't,...

Intensity I have already sent Flandern email concerning EPH which he has not responded to since he is busy, so I am a bit reluctant to ask for clarification on this issue. But I think its incorrect to link his argument to FTL speeds issue because that is not what it is based on.

Jesse I can't understand what you're saying in the paragraph above. What possible "context" could make the comment "There is a rigorous proof that in GR, no gravitational influence propagates faster than c" compatible with the idea that GR can sometimes allow gravitational influence to propogate faster than c?

Intensity You quoted from an earlier discussion he participated in. I got info from his most recent email. Since I did not write to him that I am aware that he has been arguing against FTL speeds being possible, obviously he saw no need to make it clear that he had changed his earlier position.
Your "There is a rigorous proof that in GR, no gravitational influence propagates faster than c" is inferior because of its outdatedness. I gave you his most recent position on the matter.

Jesse Feel free to offer any suggestions about how I should phrase the question, just so there's no ambiguity.

Intensity I see no ambiguity as such ( I mean, he made it clear that FTL speeds are possible). Ask him any questions you feel can help clear any issues that have come up in this discussion.
His email is as above.

Answerer So guys, am I wrong and if so, whats the actual reason or cause for the induced motion of a stationary object when placed near a gravitional field?

Intensity It depends on how strong the gravitational field is. Otherwise, if its a stationary object, it will remain stationary. I do not beleive gravitational pull can convert to kinetic energy and pull an object magnet-like. But I could be wrong.

10-06-2002, 10:15 PM	#35
Jesse Moderator - Science Discussions Join Date: Feb 2001 Location: Providence, RI, USA Posts: 9,908	Van Flandern has convinced people who do understand GR that he doesn't know what he's talking about. Can you please support this claim! Note that Steven S's claim is not "Van Flandern doesn't know what he is talking about," but rather "experts in GR are convinced he doesn't know what he is talking about." To verify that claim, just look at any example of a debate between Van Flandern and a GR expert, or check the opinion of any GR expert on questions like the speed of gravity in GR. Wouldn't your time be spent actually learning some relativity from one of the many good books available than trying to convince people of the incorrectness of a subject you do not understand? What makes you think that those books are always right, what if TVF is right all along.... Either way the advice is sound. Whether GR is right or wrong, the only way to follow the debates about what GR does or doesn't imply is to have an in-depth understanding of the mathematics involved. Can't get that knowledge without hitting the books. [ October 06, 2002: Message edited by: Jesse ]</p>

10-07-2002, 03:08 AM	#36
Ted Hoffman Veteran Member Join Date: Feb 2001 Location: "" Posts: 3,863	I have decided to abandon this discussion. Perhaps I will have to hit the books after all bacause we have dropped Le Sages gravity along the way and are now revolving around controversies. Thank you all for your contributions. I beleive I have a fairly good grasp of what GR says, I now want to embark on the math - from the basics - what were those introductory books again? Just a few quick responses though: Jesse Sorry, but until either one of us has this background, there's no way to settle this. Intensity In a way, yeah... Jesse ...But from his comments on this subject he clearly indicates that he thinks the analogy demonstrates genuine problems for the theory of GR itself, which to me sounds like nonsense... Intensity His contention is that geometric interpretation of GR violates causality principle, and that with the field interpretation, it depends. Jesse His position changed within the span of one paragraph??? Why wouldn’t he go back and edit his earlier comment, then? Intensity He needs 10 paragraphs to change his position? Was he even aware that we had examined his earlier comments? No. So there was no need to address whatever he'd said earlier because he was not aware it was in the context of our discussion. Thanks everyone. [ October 07, 2002: Message edited by: Intensity ]</p>

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10-06-2002, 10:00 PM	#34
atrahasis Regular Member Join Date: Mar 2002 Location: South of Sahara Posts: 216	Van Flandern has convinced people who do understand GR that he doesn't know what he's talking about. Can you please support this claim! Wouldn't your time be spent actually learning some relativity from one of the many good books available than trying to convince people of the incorrectness of a subject you do not understand? What makes you think that those books are always right, what if TVF is right all along....

10-07-2002, 04:12 AM	#37
Answerer Veteran Member Join Date: Feb 2002 Location: Singapore Posts: 3,956	Well, I always thought that an object will not move in regard to a stationary frame because of the symmetry of spacetime surrounding it. And that the movement of a stationary object was induced or caused as a result of the asymmetry of spacetime surrounding it( that means there is a curvature of spacetime near it). So guys, am I wrong and if so, whats the actual reason or cause for the induced motion of a stationary object when placed near a gravitional field?

10-07-2002, 04:52 AM	#38
Jesse Moderator - Science Discussions Join Date: Feb 2001 Location: Providence, RI, USA Posts: 9,908	Intensity: Thank you all for your contributions. I beleive I have a fairly good grasp of what GR says, I now want to embark on the math - from the basics - what were those introductory books again? One of the best really complete textbooks you could look at would be <a href="http://www.amazon.com/exec/obidos/ASIN/0716703440/internetinfidelsA" target="_blank">Gravitation</a> by Wheeler, Misner, and Thorne. For an introduction you're probably better off with something like <a href="http://www.amazon.com/exec/obidos/ASIN/0521277035/internetinfidelsA" target="_blank">A First Course in General Relativity</a> by Schutz or <a href="http://www.amazon.com/exec/obidos/ASIN/0387942955/internetinfidelsA" target="_blank">A Short Course in General Relativity</a> by Foster or <a href="http://www.amazon.com/exec/obidos/ASIN/0070334846/internetinfidelsA" target="_blank">Schaum's Outline of Tensor Calculus</a> by Kay or <a href="http://www.amazon.com/exec/obidos/ASIN/020138423X/internetinfidelsA" target="_blank">Exploring Black Holes</a> by Wheeler and Taylor, but I don't think these introductory books cover more advanced issues like the speed of gravitation in GR. Jesse: ...But from his comments on this subject he clearly indicates that he thinks the analogy demonstrates genuine problems for the theory of GR itself, which to me sounds like nonsense... Intensity: His contention is that geometric interpretation of GR violates causality principle, and that with the field interpretation, it depends. I don't know the details of what is meant by geometric interpretation vs. the field interpretation--do both "interpretations" lead to identical physical predictions? If so how can one violate causality and the other not? Either you can send information faster than light or you can't, there's no gray area (unless Van Flandern means something different by 'causality' than most physicists). Jesse: His position changed within the span of one paragraph??? Why wouldn�t he go back and edit his earlier comment, then? Intensity: He needs 10 paragraphs to change his position? Was he even aware that we had examined his earlier comments? No. So there was no need to address whatever he'd said earlier because he was not aware it was in the context of our discussion. I can't understand what you're saying in the paragraph above. What possible "context" could make the comment "There is a rigorous proof that in GR, no gravitational influence propagates faster than c" compatible with the idea that GR can sometimes allow gravitational influence to propogate faster than c? If you're not going to email Carlip, I will. What's his email address? Feel free to offer any suggestions about how I should phrase the question, just so there's no ambiguity.

10-07-2002, 07:41 AM	#39
Ted Hoffman Veteran Member Join Date: Feb 2001 Location: "" Posts: 3,863	carlip@dirac.ucdavis.edu

10-07-2002, 08:04 AM	#40
Ted Hoffman Veteran Member Join Date: Feb 2001 Location: "" Posts: 3,863	Thanks for the book titles Jesse. Jesse do both "interpretations" lead to identical physical predictions? . Intensity I don't think so. Jesse If so how can one violate causality and the other not? Either you can send information faster than light or you can't,... Intensity I have already sent Flandern email concerning EPH which he has not responded to since he is busy, so I am a bit reluctant to ask for clarification on this issue. But I think its incorrect to link his argument to FTL speeds issue because that is not what it is based on. Jesse I can't understand what you're saying in the paragraph above. What possible "context" could make the comment "There is a rigorous proof that in GR, no gravitational influence propagates faster than c" compatible with the idea that GR can sometimes allow gravitational influence to propogate faster than c? Intensity You quoted from an earlier discussion he participated in. I got info from his most recent email. Since I did not write to him that I am aware that he has been arguing against FTL speeds being possible, obviously he saw no need to make it clear that he had changed his earlier position. Your "There is a rigorous proof that in GR, no gravitational influence propagates faster than c" is inferior because of its outdatedness. I gave you his most recent position on the matter. Jesse Feel free to offer any suggestions about how I should phrase the question, just so there's no ambiguity. Intensity I see no ambiguity as such ( I mean, he made it clear that FTL speeds are possible). Ask him any questions you feel can help clear any issues that have come up in this discussion. His email is as above. Answerer So guys, am I wrong and if so, whats the actual reason or cause for the induced motion of a stationary object when placed near a gravitional field? Intensity It depends on how strong the gravitational field is. Otherwise, if its a stationary object, it will remain stationary. I do not beleive gravitational pull can convert to kinetic energy and pull an object magnet-like. But I could be wrong.

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