Re: The Laws of Intelligence Examined

David A. Smith writes:

[DASmith]: I was not discussing "speed of gravity", but your
misrepresentations of GR.

And I was explaining that there is more than one physical
interpretation of GR. You are clearly only familiar with the geometric
interpretation, in which gravity is not a classical force. There is also a
field interpretation of GR used most commonly in celestial mechanics, my
field of specialization, in which gravity remains a classical force. So what
you are calling "misrepresentations" are actually gaps in your familiarity
with how GR is used in relativistic celestial mechanics: as a classical
force representing the gradient of the potential field.

Feynman stopped at saying your preferred geometric
interpretation of GR was "not needed, just marvelous". Vigier and I went
farther and said the geometric interpretation obscures the physics of
gravitation because it provides no cause to initiate motion and no source of
new momentum. ["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)]

So it will get us nowhere for you to keep repeating "That's not
right because that isn't what I was taught." But we can make progress if you
can address the substance of the problems I've mentioned with the geometric
interpretation. And one place where those problems are most evident is in
connection with the "speed of gravity" issue. So let's go over your points
again with these thoughts in mind.

[DASmith]: Einstein, Dirac, and Feynman don't present GR as providing
"force".

These gentlemen did discuss gravitation in Euclidean space, as
is required to make comparisons with observations made in 3-space. For
example, take a look at ["The gravitational equations and the problem of
motion.", A. Einstein, L. Infeld and B. Hoffmann, Ann.Math. 39:65-100
(1938)], or at the book ["Relativity and Cosmology ", H.P. Robertson and
T.W. Noonan, W.B. Saunders Co., Philadelphia (1038)]. These both derive
equations of motion similar to MTW p. 1095. For this discussion, you need to
understand what these equations of motion are all about (i.e., they are
expressions for 3-space acceleration of a target body in the field of one or
more source masses), and also understand that they are the basis for
relativistic celestial mechanics and comparisons of GR with observations. If
these equations of motion are not the full equivalent of GR, then GR is an
untested theory because only the equations of motion have been used in tests
of GR.

Please don't reduce these important distinctions to semantics.
Expressions for 3-space acceleration and expressions for gravitational force
can be made one and the same by just multiplying the former by the target
body's mass to get the latter; i.e., in 3-space, force equals mass times
acceleration. So discussions about GR equations of motion (as appear in the
papers of all those experts you mention) are discussions about the field
interpretation of GR with forces because equations of motion are expressions
for 3-space force or acceleration.

[DASmith]: [In Feynman's quote], gravitation is presented not as a force,
but as a field invoking exchange particles.

"Field" means gravitational potential field. The nature of this
field is still debated. But if momentum-carrying particles are involved (as
the source of the new 3-space momentum transferred to target bodies), they
are applying a force to the target body by definition or 3-space "force":
the time rate of change of momentum. In gravitation, force is also the
gradient of the potential, so "field" (meaning potential field) also implies
force in that sense too.

[TomVF]: So if you were taught only the geometric interpretation, as is
common these days, your understanding of relativity is somewhat
handicapped because that interpretation makes the math easier but the
physics of relativity harder to understand.

[DASmith]: Not really.

Okay, then please explain the physics that causes the 3-space
acceleration of a target body toward a source mass, and the details of the
momentum transfer from source to target. No equations, nothing quantitative.
Just in general physics terms, what happens between a source mass and a
target body to cause a target body at rest in 3-space to commence motion
toward the source mass? (Hint: If you think curvature of space is involved,
you might want to review p. 32 of MTW. Or see my short explanation in "Does
space curve?" at http://metaresearch.org/cosmology/gravity/spacetime.asp.
"Curved spacetime" does not involve any curvature of the Euclidean space
used to make astronomical measurements.)

[TomVF]: ... Because it is obvious that orbiting bodies are continually
changing their 3-space momentum, no one educated in physics can deny that
in 3-space analyses where relativity is important, gravity is a classical
force.

[DASmith]: It is not your choice of tools that is at question, or their
range of validity, but your misrepresentations of GR.

You left it ambiguous whether you do accept that gravity is a
classical force in 3-space relativistic physics, or whether you dispute
this. Elsewhere, you appear to dispute it.

A clear statement here would be helpful. If you accept my
statement, we have made great progress and found some common language. If
you dispute it, you need to say exactly why.

[TomVF]: when you check into this, you will discover that even leading
relativists already know that all aspects of ordinary gravitational force
act without detectible propagation delay.

[DASmith]: Right. What orbits with the bodies is the "spacetime effects"
they individually create. No need for any sort of propagation for
"ordinary effects".

This runs up against the physics-based objection to the
geometric interpretation of GR. Your description requires magic. Not even a
rigid rod can move as a singlet body when a force is applied to some part of
it. The force merely starts a pressure wave traveling at the speed of sound
along the rod (or through the body), and the far end of the rod (or body)
doesn't respond until the pressure wave gets there. The fact that this
occurs too fast to see for most materials should not deceive us into
thinking there can be instantaneous action at a distance, as would be
required by your description.

When a source mass is accelerated (as in a binary pulsar), the
force applied to the mass itself is generally very different from the force
applied to its field because of varying distances and directions to various
field points. Therefore, there is no possible cause except magic for the
field (to vast distances from the source mass) to follow instantly the same
accelerations as its source mass. At best, the source mass should set off a
pressure wave informing the distant field that the source mass accelerated a
while ago.

Physics requires a non-magic explanation for why a target body
accelerates toward where the source mass is now, not toward where it was one
light-time ago. Even the linear extrapolation of where it was one light-time
ago is not a sufficiently good approximation of the real acceleration.

[TomVF]: even the simplest computer experiment with orbits shows that
gravitational interactions between orbiting bodies must be
near-instantaneous.

[DASmith]: So simple computer programs trump both physical reality and
experimental result?

No, they confirm reality and experimental results. They trump
any belief that gravitational equations can be retarded and still represent
reality or experimental results.

[TomVF]: If a light-speed delay is added artificially, angular momentum
conservation is lost and the orbits become open spirals.

[DASmith]: The spacetime curvature that is produced by a body, orbits with
the body. No need for propagation, or at least, with 13 Gy for propagation
to have established itself.

This repeats the magical idea that a "rigid body" can remain in
instant communication with all its parts to vast distances, and force them
to respond instantly to accelerations of the body. It is easy to write a
formula for such a thing, but apparently impossible to explain a physical
process that might allow it.

Your picture lacks that cause-and-effect relationship between a
source mass and its distant field, which is one of the problems with the
physics of the geometric interpretation of GR. But if you think otherwise, I'm
listening.

[TomVF]: General relativity uses instantaneous gravitational force
interactions between bodies.

[TomVF]: [reasoning; experiments; citations]

[DASmith]: Your statement about what GR uses is still entirely false.

No need for strong declarations in lieu of evidence. Your claim
should be trivial to prove if it is true. Just point to any interaction
delay term in GR - in the field equations, in metric solutions to those
equations, or in the equations of motion. All terms involving c^2 and higher
(even) powers are curvature terms. Propagation delay is distance/c to the
first power. Take any GR representation of interactions between bodies and
show me such a delay term.

And if you can't, that indicates there is no propagation delay
in GR because all its interactions are mathematically instantaneous, just as
they had to be in Newtonian gravity. (I showed the math arguing for my point
about GR in section 5 at
http://metaresearch.org/cosmology/gravity/speed_limit.asp.)

[TomVF]: [Carlip's paper] presents a velocity-dependent force as a way to
conserve angular momentum in systems propagating gravitational forces at
the speed of light. Although my later paper with Vigier disagrees with
his solution, at least Carlip recognizes there is a problem needing an
explanation.

[DASmith]: You are speaking of specific solutions, "approximations" if you
will, to GR. Not GR itself. Newtonian gravity is one such approximation.

And your point is ... ? If these "approximations", which may be
made as exact as we please, are not representative of GR, then GR has never
been tested.

[DASmith]: Please stick to describing what you do and what you see, and
leave GR out of it if you don't intend to present it correctly.

This discussion is about the physical interpretation of GR (not
its math), and about whether my discussion of the physics is an improvement
or a regression relative to the standard physical interpretation today. I've
never disputed that your understanding of the geometrical interpretation of
GR is correct. But I very much dispute that geometric GR is physically
viable for the reasons stated. Only the field interpretation of GR can claim
physical viability in the sense of having explicit cause and effect that do
not require miracles.

[DASmith]: Are your arteries up to telling us how "Fat Smitty's" is doing
these days?

The place seems as unique as ever, though my arteries haven't
yet been shocked by the experience. Is this your old stomping
grounds? -|Tom|-


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


.

Relevant Pages

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