Talk:Speed of gravity
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I read this, and found that Josh Gross had totally removed the content that had been there. I have put back, so information wouldn't be lost. (I hope I haven't stepped on Ben Baker's toes...) Having read the article in the external links section, it was clear Josh was confusing speed of gravity with the speed of gravity waves. (A distinction pointed out in the external link page).
What the heck is the speed of gravity supposed to be, so that we can quantify it at all? Asking because I don't know. --LMS
The speed of gravity is the speed at which changes in the location of an object propagate to all other objects that are affected by the gravity of that object. (essentially the rest of the universe) A gravity wave is a fluctuation in the gravity field around an object, as I understand it.
Why didn't you just put that in the article, if it's correct? :LMS
Okay, I will. but I don't want to get in between a battle between Josh and Ben...
The changes in the location of an object are transmited by the interactions, graviational or electromagnetic. And for most anybody that is the speed of light.
I modified the article not removing the claims, but minimazing its importance (in my years of BS in physics, MS in Astronomy and PhD in Astronomy, never heard of this competing theories, except, I think to remember, as an historic thing). That doesn't by itself mean that it can not be correct, but it is far far away from mainstream science. --AN
Did you read Van Flandern's article ? I'd certainly like to see what flaws you detect in it. He seems to have thought about this a great deal. And yes, he does seem to address your concerns.
I don't think he agrees with you that those changes are transmitted by gravitational waves, nor electromagnetic waves.
I have left in the bit about a supernova, even though he argues they are irrelevant to this issue since the matter distribution from the explosion is symmetric.
Oh, we also need to decide if the link should be to gravitational wave or gravity wave. The article uses both, and they are presumably the same thing.
O.K. I read the article, and I found nothing obviously wrong, what doesn't mean there isn't. I removed the bit about SNe, because the author makes the distinction between gravitational waves (radiation) and the propagation of gravity, and accepts that propagation of radiation has speed c, so that will not prove anything. Still, this seems to be in the fringe of science, the site where the article is hosted has some other weird bits like the one about the "face" on Mars... It is clearly not the most accepted view, and apparently, not one that more that one person accepts. I don't see any of this published in Physics Reviews or any other peer refereed Jornal. This, again, doesn't mean by itself it is wrong, but it can be considered an instance of expert testimony. I will leave the article as it is now, but i'm not sure i want to leave the link in gravity. --AN
Does the electric field even propogate? from what I understand of it, electromagnetic waves broadcast changes in velocity, not position. If acceleration = 0, there are no waves produced, and thus no propogation. The way I found best to think of it was that the electric field does not propogate, it is like a structure, when it changes velocity, however, the relativity induced length expansion/contraction changes the form of the field, and that change needs to propogate out. Sort of like when you stop a car quickly, you get thrown forward because the message of a change in speed needs to reach your body. Not the best analogy, since it isn't exact, but it's what I could come up with. Would gravitational waves work differently? I don't know enough about general relativity to answer, but my gut says they work the same.
They're supposed to work the same. As for the general acceptance of Flandern's work, it didn't take long to find that at one point he was a poster on sci.physics.relativity - many people who disagree with relativity are - and like most of them did not much impress the various people there who actually know what they are talking about. The first thing that came up is this dejanews article (http://groups.google.com/groups?q=+speed+of+gravity+flandern&hl=en&rnum=1&selm=Pine.OSF.4.02A.9908301504070.22451-100000%40goedel1.math.washington.edu) and you are welcome to agree or to disagree with it, but all in all I think the evidence establishes the Flandern does not have any credibility with workers in the field, and his material has not been peer-reviewed. Permission to remove discussion of his material, from special relativity at least? --Josh Grosse
I've come into this discussion cold, but I find it very frustrating that y'all are discussing an article that someone keeps removing the link to. I finally found it after looking through ump-teen versions. (And if they had been purged, I would have had NO way to find it... Would people just leave this link here, for the next soul who has some interest in it ?
http://www.ldolphin.org/vanFlandern/gravityspeed.html
Sigh. Van Flandern has been refuted *many* times. Most notably by Steve Carlip (an expert in General Relativity, and one of the editors of Classical and Quantum Gravity, the leading journal in the field). His ``most recent rebuttal of van Flandern's nonsense is at
http://xxx.lanl.gov/abs/gr-qc/9909087.
You can find tonnes more with a bit of searching: for example the Salon.com article
http://www.salon.com/people/feature/2000/07/06/einstein/index.html
or Chris Hillman's excellent rebuttal
http://math.ucr.edu/home/baez/RelWWW/wrong.html#speed.
Frankly the idea of even having an article on the speed of gravity is a bit silly, at best it deserves a minor mention in a full article on general relativity. -M. Nobes
_____
Well, the question comes down to: Is the speed of gravity c (as a limit), or instantaneous? That should be easy enough to determine experimentally, yet nobody has, that I know of.
Carlip expounding on Van Flandern I'll read, he's a genuine practictioner of this stuff.
Hillman, OTOH, is suspected to be a simulacrum constructed for the purpose of opposing and confounding the undergrads. He's purportedly been graduated from U.W. for something like three years now, hasn't announced gainful employment, which is a nasty state of affairs for most PhD's. If he doesn't announce something -- like a Job -- in the next year or two, I'm going to have to conclude that the 'computer construct' thesis is correct, and that Chris Hillman doesn't -- in human terms -- exist.
-- Stranger
- What's with this gratuitous slam of Chris Hillman? He's a perfectly nice guy and has spent an inordinate amount of time arguing against the crazies on sci.relativity. He left academics after he got his Ph.D. which, as you may know, is allowed. I'm tempted to delete this comment. -- Walt Pohl 02:12, 19 Mar 2004 (UTC)
It doesn't matter whether he is or not, though I've been on the physics newsgroups many times and never saw any evidence of him opposing and confounding undergrads. What matter is whether his arguments are reasonable or not, since from what I can tell, he's made every possible appeal for people to learn the material and check the sources for themselves.
I expounded Van Flandern's argument, and why most physicists who look at it think it's wrong. It's really a subtle and interesting counterintuitive brain-twister. Also, I summarized the rebutals and moved them to the end. Also I removed the attributions to the rebutals since it smelled too much like argument from authority (journal editors have been known to be wrong and it's much more important to give a simple synopsis of *why* Van Flandern appears to be wrong).
I also added a reference to MOND since this is an interesting contrast between non-standard and crankish and non-standard and non-crankish.
Removed this statement
- It is well known that the speed of gravity can't be appreciably less than the speed of light, since that could be detected by changes in the angular momentum of the planets.
It's far from well known, and it's not obvious to me that it is even correct. If it's well known, then it should be easy to find a cite.
Moved the statement about gravity moving less than the speed of light to Van Flandern believes. I've thought about it, and that statement seems to be the very issue that Van Flandern seems to disagree with the scientific consensus. If the "speed of gravity" were small, it would
- not* necessarily violate angular momentum. If it *did* violate
angular momentum there would be a problem with gravity moving at the speed of light since even a small difference is in one direction and the angular momentum would build up and cause the planets to fly off.
This is precisely Van Flandern's argument that gravity moves instantaneously, and most scientists think that he is wrong precisely because they don't believe that finite speed of gravity implies non-conservation of angular momentum.
Where do I go to find the name of the author? And why should anyone have to run the maze with their mouse to find it? Sign the article. Although, in the case of "Speed of Gravity," I can see why the reluctance. Terrible writing, unsupported claims, etc., etc. Compare this to Van Flandern's clear, concise, experimentally supported claims. The fact that there is a universe of competent physicists who can reproduce his results does not exonerate the incompetent who cannot. Frederick George Wilson - "Samizdat"
Just want to posit a thought or two here. Gravitons are one of the most elusive objectives in physics. Anyone ever find one? It seems apparent to me that there's something wrong in the way that we're looking for them, and I personally suspect that it's related to the four-dimension space-time continuum, which I think is probably a wrong -- well, actually insufficient -- construct. I'm not talking about the extra dimensions of, say, string theory, but rather, specifically, extra dimensions of time, and that perhaps time is a dimensional continuum other than, but interacting with, the spacial continuum. What this is all leading up to is that the "speed" of gravity could be simply not what we have assumed that it is; it may be involved with more dimensions of time (possibly less of space?). That would not make it necessarily "instantaneous" but could enable an apparent speed faster than that of light/emr. Food for thought, at least. --user:jaknouse
Where do I go to find the name of the author? And why should anyone have to run the maze with their mouse to find it? Sign the article. Although, in the case of "Speed of Gravity," I can see why the reluctance. Terrible writing, unsupported claims, etc., etc. Compare this to Van Flandern's clear, concise, experimentally supported claims. The fact that there is a universe of competent physicists who can reproduce his results does not exonerate the incompetent who cannot. Frederick George Wilson - "Samizdat"
- Mecklen once said that there is always a
answer that is simple, clear, and wrong. Most every physicist I know of thinks that Von Flandern is just wrong. The problem is that it takes some effort to come up with a picture that explains exactly why he is wrong without a huge amount of mathematics. I've tried, but if the explanation doesn't make any sense, let me know. It's much easier if you have a blackboard.
- Basically the answer is that its not the force that is delayed, its the potential.
Imagine a ball on a rubber sheet. Now imagine the ball moving. If the rubber sheet doesn't transmit the depression immediately, the indentation in the sheet will become distorted, and non-circular. Now look at the line of steepest descent of that indentation. It will point to where the ball is *now* not where the ball was earlier. -- User:Roadrunner
- There is a link in each article called "history". Click on that and each edit ever made to the article, when they made and who made it is all there. This is how we "sign" our work. Welcome BTW. Somebody else will have to answer the other part of your statement. --mav
Whoa, geez, what's going on here guys? This is an ENCYLOPEDIA, not a science debating forum. I don't see the majority of the AIDS article taken up by discussions of the various people who claim its a bioweapon or that HIV doesn't cause it. By the same token, this HUGE section on Van Flandern certainly does NOT belong here, and I am moving it to a different article.
As to his theory specifically, I can't answer because I can't get to the article. *&%&^ surf-blocker software. However one way or the other the description of it here is either weong, or his "theory" is sophmoric. No one believes gravity propagates instantly, in fact, the belief that it does not is one of the best pieces of evidence FOR traditional GR! If Van Flandern's problem actually IS based on the claim that he thinks everyone else believes this, it can be dismissed out of hand.
jaknouse, I will write an article on gravitons for you.
I think it doesnt really matter whos right and wrong; More important that most views should at least have a mention, as to have a complete reference (on the term 'speed of gravity')
On reading some of the discussions above, it seems some the people who are dismissing Tom Van Flandern views have not read what he had to say properly.
on the definition of the speed of gravity
Constant, c, in Maxwell's theory has three different facets: (1) the speed of propagation of electromagnetic waves (light), c_l, (2) the constant linking electric and magnetic fields in Farady's law, c_{em}, (3) the coupling constant between electric current and magnetic field, c{m}, ( subscrpt m stands for Maxwell). These constants could be different, but the experiments prove they are equal to certain degree of accuracy. Thus, one uses a connotation c to denote these constants and call it "the speed of light".
Analogously, the constant, c, in Einstein's theory of relativity has three different facets: (1) the speed of propagation of gravitational waves, c_{gw}, (2) the constant linking gravito-electric and gravito-magnetic fields, c_{gem}, (3) the coupling constant between matter (stress-energy tensor) and gravitational field, c{e}, ( subscrpt e stands for Einstein). These constants could be different in arbitrary theory of gravity, but Einstein postulated that they are equal in General Relativity. I have used connotation c_g to denote these constant and call it "the speed of gravity" following the analogy with Maxwell's theory. General relativity has enormous predictive power to propose experiments in which various facets of c_g could be measured. VLBI experiment I have designed and conducted on September 8, 2002 along with Dr. Ed Fomalont from NRAO uncovered that c_g=c with 20%.
If an alternative theory of gravity rather than General Relativity is used the three facets of c_g are not united and each of c_g-s must be called by its own name. If such an alternative theory of gravity is used for interpretation of the experiment, the accuracy of our experiment sets the upper limit on c_{gem} only.
It is clear that "the speed of light" plays no role in the interpretation of the experiment. Critics of C. Will against my interpretation was based on the postulate that c_{gem}=c. From this he concluded that I and Fomalont measured the speed of light. That postulate is invalid from the first principles. It is our experiment that provides a direct test that c_{gem}=c under assumption that test particles move along geodesics which is described by the same equation as in General Relativity.