Re: The speed of gravity revisited

This replies to Koobee Wublee, Tom Roberts, and Steve Carlip.


"Koobee Wublee" writes:

[Wublee]: As you have correctly pointed out, LR does not manifest the twin's paradox. However, it manifests the time dilation.

You have not understood Lorentzian relativity (LR). See Ref. [1] below. The effect of velocity on clocks represented by the Lorentz transformations is extremely well-verified by GPS, among many other modern experiments. But in LR, the transformations operate in only one direction, from the local gravitational potential field to the moving frame. They do not work in reverse.

The significance of that difference is that velocity affects only the rates of *clocks* moving relative to the local gravitational potential field. Motion has no effect on *time* (the dimension for measuring change). So LR has no time dilation and no paradoxes. Motion simply makes atomic clocks slow down, the way increasing temperature makes pendulum clocks slow down.

That said, LR agrees with all 11 independent experiments that tested special relativity (SR) [see Ref. 6 for list], and can also accept gravity propagating faster than light, which SR cannot do.

You also seem to object to general relativity (GR), another well-verified theory. But once you realize that the math has more than one interpretation, and that the "field interpretation" requires no "curved space-time" or other forms of magic, you will find GR much more acceptable.

Most of what is important about GR can be understood in the context of a light-carrying medium. (Any wave phenomenon requires a medium.) That medium is like an atmosphere filling space. Naturally, the gravity of large masses makes the medium denser as one gets closer, just as it would for an atmosphere comprised of gases. Then as light tries to propagate through a medium with a density gradient, it bends, slows, and redshifts by ordinary, classical refraction in the exact amount the GR equations predict.

Moreover, this interpretation has been known for a long time, back to at least Eddington's 1920 book. See Ref. [2]. A review of the "refraction in an optical medium" model as a simple replacement for Riemann curvature of spacetime, together with citations to many previous authors who knew about this, may be found in Ref. [3].


and "Tom Roberts" writes:

[Prime Mover]: As many published experiments have demonstrated, gravitational forces cannot possibly propagate at c.

[Roberts]: True, _IF_AND_ONLY_IF_ one assumes that they propagate at all.

With the standard classical physics definition of "force" = the time rate of change of (3-space) momentum, it instantly follows that orbiting bodies are experiencing a force by definition. With the further caveat that no magic is allowed in physics, then it follows that the (3-space) momentum transferred to the target body by the force must be the force's own momentum. But the force could not have momentum unless it was propagating. Therefore, gravitational force must propagate. Q.E.D.

The "geometric interpretation" of GR gets around this by changing the definition of "force" and arguing that the new momentum of orbiting target bodies can arise spontaneously from nothing. That is acceptable only to the extent that one is willing to base physics on magic.

[Roberts]: In GR, the gravitational fields "here and now" do not propagate from any source "over there". The fields "here and now" depend ONLY on the fields "right near here a short time ago". So there is no need for propagation from a source.

That claim is factually in error. Ref. [4] proves that the acceleration of a source mass (as in a binary pulsar) is felt by a target body in much less than the light-time between the two stars. Many people have examined this proof in the standard literature and no actual or suspected fault has appeared.

Just as you and others had to give up on the naïve notion that the gravitational force between two bodies might propagate at the speed of light, you must also give up the notion that the gravitational field is projected one light-time ahead along the velocity vector (as is claimed about charge fields), because that still gives orbits that spiral outward. Because you seem disinclined to check the proof for yourself, take note that relativity experts such as Steve Carlip do not dispute this, and accept that your understanding of relativity is incomplete in this important way.

[Prime Mover]: As Tom Van Flandern has said, gravitational potential and gravitational force are two distinct concepts and as such their speeds does not have to be the same.

[Roberts]: I disagree. If "gravitational force" is to be the spatial gradient of "gravitational potential", then they cannot have different "propagation
speeds".

The gravitational potential field can be thought of as an optical medium. Disturbances in it propagate at the speed of light. But forces applied to it, including forces that create a gradient in it, can have any speed. The connection you imagine does not exist.

Consider the gradient of air in Earth's atmosphere. The force creating that gradient is gravity. Disturbances in the gradient (such as winds or sound waves) propagate at speeds that have nothing to do with the speed of gravity. Yet air is analogous to the gravitational potential field, which is likewise shaped by gravitational forces. So the speeds of potential changes and of gravitational forces are unrelated.

[Roberts]: When different people use different meanings of words, the discussion can extend forever, as they talk right past each other. TVF seems to be incapable of recognizing the puns he uses, even when pointed out to him, so he repeats his same invalid claims.

I carefully define my terms to avoid ambiguity. What do you do?

[Prime Mover]: By the way, I read his papers on the subject and I saw no apparent fraud, quite the contrary. They were fairly conducted, in my opinion.

[Roberts]: Including his nonsense about Cydonia? Any "astronomer" who writes such drivel is not trustworthy in my book. I don't mean the fact that he is demonstrably wrong, for that can happen to anybody -- I mean the fact that he claims "proof", and was as strident in claiming that as he is
"speed of gravity".

That cannot be relevant unless there is a flaw in either argument that might apply to the other. If you cannot see or even imagine a flaw in the reasoning, and are left with nothing but ad hominem arguments ("such drivel"), you must decide whether you are a scientist or a "believer at any cost".

I belong to an organization that looks into puzzles and anomalies, helps to eliminate most of them, and focuses on those that remain standing as the areas for potential breakthroughs in physics/astronomy. So it is no coincidence that our rigorous standards have been applied to numerous unrelated anomalies. Although most have mundane explanations, unsurprisingly, more than one has survived this scrutiny.

But there is a definite and obvious logical flaw in your linkage of unrelated anomalies as if the probabilities in one case affected probabilities in the other. So if you cannot justify your claim that one case is "obvious drivel", or even articulate why you think that, then you are exposed as a believer posing as a scientist.

[Albertito]: It is very easy to measure the gravitational force. You can measure a force F, if you know the mass of the body and the acceleration that force exerts on that body.

[Roberts]: That is a measurement of the acceleration, not the force.

You are ignoring definitions again. "Force" in this discussion is the time rate of change of (3-space) momentum. "Momentum" is mass times velocity. For constant mass, the time rate of change of momentum is mass times acceleration, a.k.a. Newton's second law of motion. That is a perfectly valid way of measuring "force" by its standard definition. Only when you change the definition, as in geometric GR, is the force concept suppressed and we would have no means of measuring a true force.

This discussion is about real classical forces operating in Euclidian flat space, where the observations and orbit computations used to test the theories are made. Deal with it, if you are able.


and Steve Carlip writes:

[Tom VF]: no one is disputing that changes in gravitational potential (the subject of the field equations) propagate at the speed of light, c. I am always careful to state that "the speed of gravity" measured by the six available experiments always means the 3-space propagation speed of gravitational force, and has nothing to do with changes in gravitational potential.

[Carlip, summarizing Low's paper]: change the source of gravity in R any way you want. According to GR, nothing at all happens at a point p outside R until the time for a light signal to reach p from R has passed. By any sensible definition of "speed" I can imagine, that means that gravity propagates at the speed of light.

I agree completely with Low's mathematical reasoning that you summarize here, and have never claimed anything different from that. I must also agree that your imagination is limited just as you describe. Indeed, what you describe is nothing more than a wordy description of the retarded potential field in GR, similar to the Lienard-Wiechert potentials. See Ref. [5]. In fact, I just conceded the very point you made in my remarks above that you quoted. Yet you make a big deal about an obvious point that is totally irrelevant to this discussion or to the "speed of gravity" issue in general.

I have no issue with the speed of gravitational potential field updates being c.

Now, consider the 2-body problem with one source mass and one (nearly) massless target body. By construction, the source mass represents Low's R, a collection of smaller masses that are the source of a gravitational potential field. We all agree that changes in the potential field propagate or update at speed c. So there is no issue there. Now let's look at the gravitational force generated outside R, assuming R is a single, fixed mass - the simplest case. Even with nothing changing at the field source, we still have a problem about the force applied to the target body.

The one and only mathematical question of importance here to the speed of gravity issue is this: For a target body with a transverse motion relative to the source mass, should we use the retarded gradient or the instantaneous gradient to get the force? (Here again I stress that the "force" applied to the target body means the time rate of change of its 3-space momentum. And there can be no dispute that the 3-space momentum of target bodies in a gravitational field is changing, so a force exists by definition.)

If this force, or "gravitational influences" (your term), propagates from source mass to target body at speed c, then we must use the retarded gradient, which leads to wrong answers (outward spiraling orbits). If force/gravitational influences propagate >> c, as in GR, then we get the correct orbits.

I have read all the papers you have cited over the years, and read them with understanding. Please do me the courtesy of reading my papers, which have corrected your consistent misunderstanding of the issue. If you had ever computed a real orbit in an n-body simulation, it would be obvious even to someone taught only the geometric interpretation of GR that perturbing forces must be applied to all n bodies instantaneously, without light-time delay. And the reason for that is a physics issue deserving attention and crying out for an explanation.

[Carlip]: If you accept this, then we are merely arguing over semantics ...

If you consider the physics question of whether or not "gravitational influences" propagate FTL to be a semantic one, then you are entitled to your opinion. Nonetheless, the main gravitational influence, the force that keeps planets in their orbits around the Sun, must propagate much faster than the speed of light, according to all experiments sensitive to that speed.

[Carlip]: You just demonstrate that you don't understand the math,
and that you don't know the difference between speed and direction.

That is a non-sequitur and is patronizing in the extreme. And typical of this "hit and run" tactic, you provide no explanation or specifics, so no readers can judge the worth of your claim for themselves. -|Tom|-


REFERENCES:

[1] "Is faster-than-light propagation allowed by the laws of physics? (A primer on Lorentzian relativity)", Infinite Energy 59, 31-33 (2005); also at http://metaresearch.org/cosmology/gravity/LR.asp.

[2] Sir Arthur Eddington, "Space, Time & Gravitation", Cambridge Univ. Press, first published 1920, reprinted 1987, p. 109.

[3] Fernando de Felice, "On the gravitational field acting as an optical medium", Gen.Rel.&Grav. 2#4:347-357 (1971). Einstein himself first suggested the idea that gravitation is equivalent to an optical medium. From the paper's abstract, ". Maxwell's equations may be written as if they were valid in a flat space-time in which there is an optical medium . this medium turns out to be equivalent to the gravitational field. . we find that the language of classical optics for the 'equivalent medium' is as suitable as that of Riemannian geometry."

[4] "The speed of gravity - What the experiments say", Phys.Lett.A 250:1-11 (1998); also at http://metaresearch.org/cosmology/speed_of_gravity.asp. The cited proof appears in the section "Electromagnetic Analogies and Gravitational Radiation / 1. Myth: Gravity from an accelerating source experiences light-time delay".

[5] Misner, Thorne & Wheeler, "Gravitation", Freeman & Co., p. 1080 (1973).

[6] "What the GPS tells us about relativity" in "Open Questions in Relativistic Physics", F. Selleri, ed., Apeiron, Montreal, pp. 81-90 (1998); also at http://metaresearch.org/cosmology/gps-relativity.asp.


Tom Van Flandern - Sequim, WA - see our web site on frontier astronomy research at http://metaresearch.org

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