Jesse

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.

But FTL is what people mean when they talk about a violation of "causality" in the context of relativity. See <a href="http://scienceworld.wolfram.com/physics/CausalityProblem.html" target="_blank">this</a> page, for example. I suppose it's possible that Van Flandern is using his own separate definition of what it means for a physical theory to violate "causality," but this would be a bit like using the term "mass" to refer to momentum.

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.

Perhaps I didn't make this clear enough, but the statement comes from Carlip's email to you, not from some previous discussion.

Quoting from your post:

So, it seems highly unlikely that he was simply contradicting himself within the span of a single email. Therefore his earlier comment about FTL speeds being possible "in some theories" could not have referred to the speed of gravity in GR.

[ October 07, 2002: Message edited by: Jesse ]</p>

Ted Hoffman

Jesse But FTL is what people mean when they talk about a violation of "causality" in the context of relativity. See this page, for example. I suppose it's possible that Van Flandern is using his own separate definition of what it means for a physical theory to violate "causality," but this would be a bit like using the term "mass" to refer to momentum.

Intensity Leonardo Motta (the link you provided) has not defined what he understands by causality (the page in which he is supposed to have defined it is under construction) and he seems to be talking about FTL speeds in the context of SR. But GR has falsified SR in this respect, so his is a non-argument because its based on a different context.
Leonardo Motta is not people and relativity is a very broad term - be specific by stating GR, LR or SR please. Use accurate descriptions.

I have explained why Van Flandern beleives the casuality principle is violated. he said: "In the geometric interpretation, it does [violate the causality principle] because there is no force to induce change in the motion". The geometric interpretation uses gravitational force both as the cause of the acceleration of the body and is also responsible for the curving path of the body. Remember that going by the rubber sheet analogy for example, the small ball goes down the slope and does not resume its course, so in essence, its path is not bent - its sucked into "blackhole" "forever". For its path to be curved (ie for it to resume course after going through the bend) an extra force will have to come into play - presumably the same force that initiated the horizontal motion of the ball. This force is not accounted for in the rubber sheet analogy (hence casuality principle is violated), we use gravity to initiate the rolling but fall short of thinking about what happens after the ball has rolled down the slope.

I am ill-equipped to even address what Flandern is saying and I don't feel comfortable going back to him seeking clarification on everything he says, because that is simply being pesky and he is not at my beck and call. Thats why I thought my best recourse is to hit the books, so, much as this discussion is so entrapping and fascinating to me, I may end up missing the mark or providing incorrect explanations to issues I dont understand very well.

About Carlip, I am looking forward to your update on the response he gives. And I hope it will clear up the issue of FTL speeds. I am reading some more and when I feel I can contribute, I will.