Re: The speed of gravity revisited

This replies to Steve Carlip, Hans de Vries, Ken S. Tucker, Tom Roberts, and Martin Hogbin. REFERENCES appended at the end.


Steve Carlip writes:

[TomVF]: The direction of the source mass as sensed by an orbiting target body is toward its true instantaneous position when the target body or field point is at rest. And it is toward the source mass's retarded position (retarded by the speed of gravitational force) when the target body is orbiting. That's elementary physics. The exact same statement is equally true if the source mass is moving, then stops (your example). That "move, then stop" distraction just makes a simple problem more complicated.

[Carlip]: Tom, this is simply wrong. According to general relativity, when the source mass stops, the acceleration of the test body will continue to track its "extrapolated" motion, until a time equal to the light-travel time from the source to the test body, at which point the acceleration will rapidly swing back to the actual direction of the source mass. This isn't a guess or an opinion. It's a calculation. Stop talking through your hat, and do the math!

Steve, once again I insist, we have no issues with the math. None. This discussion and the issues of importance are all about the interpretation of the math, the physics behind the math, what the math means for reality. It would be nice if we could begin getting our heads together about this. Can you see your way to acknowledging that general relativity has two different physical interpretations, the geometric and the field? That is not just my opinion, but one well known through the early and middle 20th century, suppressed only following the MTW bid to make the geometric interpretation dominant. But major physicists as recent as Feynman reaffirmed this duality of interpretation, reminding us that the geometric interpretation is not really needed for anything in particular. [Ref. 1] All of relativity can be described with classical forces in a Euclidean 3-space context, which is the field interpretation of general relativity.

Or do you wish to redefine “general relativity” to exclude the field interpretation? :-(

Once we agree on this indisputable fact that the geometric interpretation is not the only way to understand the math of GR, then you will readily see that my statement was true for the field interpretation, and yours for the geometric interpretation. The field equations of GR describe only the gravitational potential field, so the claims you make are true for the potential field. Whether or not they apply also to force or acceleration requires an additional assumption about how force relates to potential. In the geometric interpretation, whatever is true for the field is also true for gravitational force/acceleration (your view). But in the field interpretation, force/acceleration drives the field, not the other way around (my view).

The next step is to notice that what you claim (that gravitational acceleration of a target body will suddenly stop tracking the instantaneous position of the source mass if the source experiences a sudden acceleration) violates the clear evidence from binary pulsars, which show that the acceleration continues to track the instantaneous position of the source mass even when the source mass accelerates by a substantial amount. You apparently wish to claim that there will be a major difference in behavior of the target body if the source mass acceleration experiences a “sudden” (i.e., unanticipated) acceleration instead of just a “substantial” acceleration. But you have no leg to stand on in making such a claim. Not a shred of evidence supports it [Ref. 2]; the lifting of the universal speed limit removes any logical reason to expect it [Ref. 3]; and it is also untrue in electrodynamics, where sudden accelerations of charges are still tracked essentially instantaneously [Ref. 4].

You learned only one way to interpret the math of GR. I learned two ways. If you became more familiar with the way relativistic celestial mechanics operates, I think you will be (pleasantly?) surprised. It opens up a lot of doors to a deeper understanding of gravity, and settles a lot of paradoxes, especially in QM.


and Hans de Vries writes:

[TomVF]: On the contrary, binary pulsars prove that when the source mass accelerates (as in Steve's example), the target body responds almost instantly.

[de Vries]: The EM case is much simpler with the essentially same result. Did you ever try to derive the direction of the E-field from a moving charge?

Of course. That was featured in my joint paper with J.P. Vigier [Ref. 3]. And it is indeed closely parallel with the gravitational case, where even accelerations of charges are tracked instantaneously [Ref. 4], even though many textbooks assert the opposite without any experimental support for that assumption at all. Experimental evidence trumps textbook claims.

[de Vries]: I wrote a derivation of the Lienard-Wiechert potentials in chapter 2 of my book which is step-by-step with comments ...

And I published a detailed explanation of why such retarded potentials describe only the behavior of the potential field, but say nothing about the forces (gravitational or electrodynamic) that shape potential fields. [Ref. 5]

As I said to Steve: Don’t befuddle yourself by making this complicated. Consider a static potential field around an unmoving source mass. If a photon (speed c) and a classical graviton (speed undetermined) leave a source mass at the same instant and travel the same linear path to the same orbiting target body, the photon arrives from the retarded source direction (naturally), whereas the classical graviton arrives from the instantaneous source direction (unexpectedly). The only physical difference available to account for this change in the action direction is that they travel the same path with different speeds.

If you substitute “virtual photon” for “classical graviton” and “charge” for “mass”, the same reasoning applies to electrodynamics. Virtual photons, as you might have heard, are alleged to have infinite speed to “explain” this unexpected behavior.


and Ken S. Tucker writes:

[Tucker]: suppose the Sun was to instantly convert to light energy. Since Earth is in free-fall, that event (aside from the flash) would be undetectable by gravitation except by a tidal variation. ... Either Steve or Tom may predict when that tidal variation could be detected.

My prediction is already on record. Force shuts off instantly (assuming your “light energy” has no mass), whereas the field change takes 8.3 minutes to arrive at Earth. See [Ref. 6].

[Tucker]: Suppose we examine the geodesy of a satellite in a normally circular orbit, orbiting Earth. The gravitational effect of the Sun will cause a tidal effect on the orbit of that satellite, such that the circular orbit will be pulled to an ellipse. The semi-major axis of that elliptical orbit will point to either the "absolute" position of the Sun as Flandern predicts or it will point to the "apparent" position of the Sun as Tucker and Carlip predict. The diff between those two predicted axes is ~ 20" arc.

The solar eclipse experiment already showed precisely what your example suggests as a test, where the Moon is the satellite in question. And the answer is unambiguous and undisputed: the bulge in the Moon’s orbit is toward the instantaneous Sun, not the retarded Sun. [Ref. 2]

[Tucker]: The data base exists, 1st guy to tease out the data accurately gets my nomination for a Nobel.

Thank you.  But I’ll be even happier if you do not change your test in some way, now that it has gone in an unexpected direction for you. That is what “controls against bias” is all about in scientific method – not discounting test results when they go against expectations.

[Tucker]: What I'm pointing out to Steve and Tom is the means to experimentally resolve this ongoing argument, prior to them sitting in an old folks home throwing false teeth at each other.

It was a nice thought. But we still have to find some way to entertain ourselves in the old folks home. And I’ll throw my false teeth at you if you don’t get my last name right: “Van Flandern” (note capital “V”). 


and Tom Roberts writes:

[Roberts]: The actual observations for EVERY ONE of the experiments TVF cites are in accord with the predictions of GR.

Of course. No one has suggested otherwise. [shrug] But you still don’t get what the discussion is about.

[Roberts]: So when he says "binary pulsars prove that when the source mass accelerates (as in Steve's example), the target body responds almost
instantly", you should recognize: ... the source NEVER accelerates "as in Steve's example", the two objects merely orbit each other.

Now you are talking nonsense. Look up the definition of “accelerate” and remember we are having a 3-space discussion. Anything in orbit is accelerating by definition of the word: the time rate of change of (3-space) velocity (a vector).

[Roberts]: In GR, nothing that carries energy, momentum, or information can travel with local speed >c relative to any locally-inertial frame. TVF's "speed of gravity" would violate this, IF IT WERE GR. He is wrong, because he is not actually applying GR. That is Steve's point, and mine.

And you will remain of that unsupportable opinion all your life unless you start to appreciate that there is a difference between math and the physical interpretation of that math. Learn the field interpretation of GR. Study some celestial mechanics. THEN let’s talk.

As to the substance of your point, for the benefit of others uninhibited by being unable to see beyond the geometric interpretation of GR, the universal speed limit in all of physics has now been lifted by the falsification of special relativity (SR) in favor of Lorentzian relativity (LR). [Ref. 3] See [Ref. 7] for a primer on LR.

[Roberts]: If what you say is true, then it cannot be possible that all these statements are true:
A) the "field interpretation" is indeed GR.
B) the "gravitational force" in the "field interpretation" carries
energy and momentum (i.e. it can transfer them from one object
to another).
C) Low's paper has no error, and neither does Carlip's and the
zillions of other papers and textbooks.

A) and B) are clearly true more or less by definitions. Papers of type C) have errors of physical interpretation (despite good math) because they adhere to the now-falsified geometric interpretation of GR.

[Roberts]: You make an unacknowledged and implicit assumption that is false: that "gravitational force" is central (i.e. points directly at the source). In the appropriate approximation to GR in which there is a gravitational force, this is not true.

I make no such assumption and have stressed that I hold the opposite view: gravitational force must have some non-zero aberration, even though that aberration is presently much smaller than light aberration and is presently too small to measure.

The conclusion that gravitational force operates toward the instantaneous source mass location is forced upon us by observations showing clearly that it does. Note: that is a conclusion, not an assumption. Note further: neither Steve nor anyone else in the know disputes that conclusion for all known data, even though Steve proposes a *hypothetical* situation where he thinks it might not hold true. I argued above that his hypothetical is not really viable for the three reasons stated.


and Martin Hogbin writes:

[TomVF]: In this discussion, one of us is answering every point by addressing observations, experiments, citations, or argumentation. And one of us is simply repeating bold claims without any new attempt to justify them. Shall we let the readers decide which of us matches which description?

[Hogbin]: I, for one, think that you (Tom Van Flandern) are the one who is 'simply repeating bold claims without any new attempt to justify them'.

Especially for you, Martin, I have resumed adding references to observations, experiments, argumentation, or citations to back up every important point. You expressed an opinion without saying anything about why you hold it. That adds nothing to the discussion for either side or for onlookers. Speak up -- what troubles you about my position? I think I’ve answered all the objections to the satisfaction of referees, editors, and many physicists, and published repeatedly on this topic. What else would you like to see before you might accept that the speed of gravity really is >> c? -|Tom|-


REFERENCES

[1] Feynman Lectures on Gravitation, R.P. Feynman, Addison-Wesley, New York (1995). Section 8.4, p. 113: “It is one of the peculiar aspects of the theory of gravitation, that it has both a field interpretation and a geometrical interpretation. ... the fact is that a spin-two field has this geometrical interpretation: this is not something readily explainable -- it is just marvelous. The geometrical interpretation is not really necessary or essential to physics.”

[2] “The speed of gravity – What the experiments say”, T. Van Flandern, Phys.Lett.A 250:1-11 (1998). Also at http://metaresearch.org/cosmology/speed_of_gravity.asp.

[3] “Experimental Repeal of the Speed Limit for Gravitational, Electrodynamic, and Quantum Field Interactions”, T. Van Flandern and J.P. Vigier, Found.Phys. 32:1031-1068 (2002). Preprint under title “The speed of gravity – Repeal of the speed limit” available at http://metaresearch.org/cosmology/gravity/speed_limit.asp.

[4] “Electromagnetic mass and the inertial properties of nuclei”, C.W. Sherwin and R.D. Rawcliffe, Report I-92 of March 14, 1960 of the Consolidated Science Laboratory, Univ. of Illinois, Urbana; obtainable from U.S. Department of Commerce’s Clearinghouse for Scientific and Technical Information, document AD 625706.

[5] “Reply to comments on ‘The speed of gravity’”, T. Van Flandern, Phys.Lett.A 262:261-263 (1999).

[6] See animation #6 and its caption at http://metaresearch.org/media%20and%20links/animations/animations.asp

[7] “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.


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

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